<PAGE>
                                                                    Exhibit 1.27

                  SUBSCRIPTION AND DEBENTURE PURCHASE AGREEMENT

To:               ALTAREX CORP.
                  610 Lincoln Street
                  Waltham, Massachusetts 02451
                  USA

A.       SUBSCRIPTION

1.       (a)      The undersigned United Therapeutics Corporation (the
                  "Purchaser") hereby subscribes for and agrees to purchase from
                  AltaRex Corp., a company incorporated under the laws of
                  Alberta, Canada (the "Company"), the following:

                  (A)      a unit consisting of:

                           (i)      4,900,000 common shares of the Company (the
                                    "Common Shares") at a price of $0.50 per
                                    Common Share; and

                           (ii)     a warrant to purchase 3,250,000 common
                                    shares of the Company (the "Warrant") at a
                                    price of $0.50 per share (a "Warrant Share")
                                    in the form attached hereto as Exhibit A,

                           for a total purchase price of $2,450,000 (the
                           "Purchase Price"). The Common Shares represent 11.8%
                           of the pro forma outstanding common shares of the
                           Company on April 17, 2002, after giving effect to the
                           purchase of the Common Shares; and

                  (B)      a convertible debenture in the amount of $50,000 (the
                           "First Debenture") pursuant to the terms of a
                           convertible debenture entered into between the
                           parties, which is attached hereto as Exhibit B-1. The
                           First Debenture will automatically convert into
                           100,000 common shares of the Company (the "First
                           Debenture Shares") at 5:00 p.m. (Toronto time) on
                           August 21, 2002.

         (b)      The Corporation hereby grants to Purchaser the right (the
                  "Debenture Subscription Right") to subscribe for a convertible
                  debenture in the amount of $875,000 (the "Second Debenture")
                  pursuant to the terms of a convertible debenture entered into
                  between the parties dated as of August 20, 2002, which is
                  attached hereto as Exhibit B-2. Pursuant to the terms of the
                  Second Debenture, 883,380 common shares of the Company (the
                  "Second Debenture Shares") will be automatically issued upon
                  the occurrence of certain events at a price of $0.50 per
                  share. The Debenture Subscription Right may be exercised by
                  the Purchaser delivering to the Corporation (i) on or before
                  August 14, 2002, the subscription form, attached hereto as
                  Exhibit B-2A, duly completed and executed, and (ii), on the
                  Debenture Closing Date (as defined herein), with the documents
                  to be delivered by the Purchaser pursuant to Section 2(b);

         all on the terms and conditions set forth below. The Common Shares, the
         Warrant and the First Debenture are collectively referred to herein as
         the "Purchased Securities". The
<PAGE>
                                     - 2 -


         First Debenture and Second Debenture are collectively referred to
         herein as the "Debentures". The First Debenture Shares and Second
         Debenture Shares are collectively referred to herein as the "Debenture
         Shares".

2.       (a)      The closing of the purchase and sale of the Purchased
                  Securities hereunder (the "Initial Closing") shall be held at
                  the offices of McCarthy Tetrault LLP, Toronto Dominion Bank
                  Tower, Toronto Dominion Centre, Toronto, Ontario M5K 1E6, on
                  April 17, 2002 or at such other place upon which the Company
                  and the Purchaser shall agree (the "Initial Closing Date"). At
                  the Closing, the Company will deliver to the Purchaser the
                  Purchased Securities being subscribed for by the Purchaser,
                  registered in the name of the Purchaser, against payment to
                  the Company of the Purchase Price for the Purchased Securities
                  being subscribed for hereunder, by wire transfer, cheque or
                  other method acceptable to the Company and the parties shall
                  execute and deliver the security agreement in the form
                  attached hereto as Exhibit C. All documents, certificates and
                  payments shall be held in escrow until all such documents,
                  certificates and payments have been delivered and the parties
                  hereto have agreed that the escrow is terminated.

         (b)      The closing of the purchase and sale of the Second Debenture
                  hereunder (the "Second Closing") shall be held at 10:00 a.m.
                  (Toronto time) on August 20, 2002 (the "Second Closing Date")
                  at the offices of McCarthy Tetrault LLP at the above-noted
                  address or at such other time and place upon which the Company
                  and the Purchaser shall agree in writing. At the Second
                  Closing, the Company will execute and deliver to the
                  Purchaser, and the Purchaser will execute and deliver to the
                  Company the Second Debenture in the form attached hereto as
                  Exhibit B-2, against payment by the Purchaser to the Company
                  of US$875,000 by wire transfer, cheque or other method
                  acceptable to the Company.

3.       The Company has authorized the sale and issuance of an aggregate of the
         Purchased Securities and of the Second Debenture, Warrant Shares and
         the Debenture Shares to the Purchaser. The Purchaser acknowledges being
         provided with and has read a term sheet (the "Term Sheet") outlining
         the features of the Purchased Securities, a copy of which is attached
         hereto as Exhibit D. The Purchaser acknowledges that the Term Sheet is
         only a summary of the terms and conditions of the Purchased Securities,
         that the Term Sheet may not contain all the information that is
         important to the Purchaser and that the terms and conditions of the
         Purchased Securities and each Debenture shall be governed and subject
         to, in all respects, the terms of such securities.

B.       PURCHASER'S ACKNOWLEDGMENTS AND AGREEMENTS

         The Purchaser understands, acknowledges and agrees that:

1.       This subscription may be accepted or rejected in whole or in part by
         the Company, in its sole discretion.

2.       Except as provided under applicable securities laws, this subscription
         is and shall be irrevocable except that (i) the Purchaser's execution
         and delivery of this Subscription
<PAGE>
                                     - 3 -


         Agreement will not constitute an agreement between the Company and the
         Purchaser until this Subscription Agreement is accepted on behalf of
         the Company and, if not so accepted, the Purchaser's subscription and
         obligations hereunder will terminate and (ii) the Purchaser can, at any
         time prior to acceptance of this Subscription Agreement, request in
         writing that he or it be released from his or its obligations hereunder
         (and the Company may, but need not, in its discretion, elect to release
         the Purchaser from his or its subscription and from such obligations).

         THE COMMON SHARES, THE WARRANT, THE DEBENTURES, THE WARRANT SHARES AND
         THE DEBENTURE SHARES HAVE NOT BEEN RECOMMENDED BY THE SECURITIES AND
         EXCHANGE COMMISSION OR BY ANY STATE OR PROVINCIAL SECURITIES COMMISSION
         OR REGULATORY AUTHORITY. ANY REPRESENTATION TO THE CONTRARY IS A
         CRIMINAL OFFENCE.

3.       Because the Purchased Securities, the Second Debenture, the Warrant
         Shares and the Debenture Shares will not have been registered under the
         Securities Act of 1933, as amended (the "Securities Act"), or
         applicable state securities laws, the Purchaser is aware that any
         resale inconsistent with the Securities Act may create liability on his
         or its part and/or the part of the Company, and agrees not to assign,
         sell, pledge, transfer or otherwise dispose of or transfer the
         Purchased Securities, the Second Debenture, the Warrant Shares and the
         Debenture Shares, except in compliance with the Securities Act and
         applicable state securities laws. The Purchaser is also aware that any
         resale inconsistent with applicable securities laws in the province of
         Ontario ("Canadian Securities Laws") may create liability on his or its
         part and/or the part of the Company, and agrees not to assign, sell,
         pledge, transfer or otherwise dispose of or transfer the Purchased
         Securities, the Second Debenture, the Warrant Shares and the Debenture
         Shares except in compliance with Canadian Securities Laws.

4.       Unless permitted under Canadian Securities legislation, the Purchaser
         shall not trade (i) the Purchased Securities in the Province of Ontario
         before August 18, 2002, and (ii) the Second Debenture or Second
         Debenture Shares before the date that is four months and one day after
         the Second Closing Date.

5.       Each certificate representing the Purchased Securities, the Second
         Debenture, the Warrant Shares and the Debenture Shares will bear a
         legend to the following effect:

         THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
         UNITED STATES SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES
         ACT"). THE HOLDER HEREOF, BY PURCHASING SUCH SECURITIES, AGREES FOR THE
         BENEFIT OF THE COMPANY THAT SUCH SECURITIES MAY BE OFFERED, SOLD OR
         OTHERWISE TRANSFERRED ONLY (A) TO THE COMPANY, (B) OUTSIDE THE UNITED
         STATES IN ACCORDANCE WITH REGULATION S UNDER THE SECURITIES ACT OR (C)
         INSIDE THE UNITED STATES IN ACCORDANCE WITH RULE 144 UNDER THE
         SECURITIES ACT, IF AVAILABLE. DELIVERY OF THIS CERTIFICATE MAY NOT
         CONSTITUTE "GOOD DELIVERY" IN SETTLEMENT OF TRANSACTIONS ON STOCK
         EXCHANGES IN CANADA. UNLESS PERMITTED UNDER CANADIAN
<PAGE>
                                     - 4 -


         SECURITIES LEGISLATION, THE HOLDER OF THE SECURITIES AND THE COMMON
         SHARES ISSUABLE UPON THE EXERCISE OR CONVERSION THEREOF SHALL NOT TRADE
         THE SECURITIES IN THE PROVINCE OF ONTARIO BEFORE [IN THE CASE OF THE
         FIRST DEBENTURE INSERT AUGUST 18, 2002] [IN THE CASE OF THE SECOND
         DEBENTURE AND SECOND DEBENTURE SHARES INSERT DATE THAT IS FOUR MONTHS
         AND ONE DAY AFTER SECOND CLOSING].

         provided that, if any such Purchased Securities, Debenture, the Warrant
         Shares or the Debenture Shares are being sold under Rule 144 under the
         Securities Act, the legend may be removed by delivery to Computershare
         Trust Company of Canada of an opinion of counsel, of recognized
         standing reasonably satisfactory to the Company, to the effect that
         such legend is no longer required under applicable requirements of the
         Securities Act or state securities laws.

6.       Provided that nothing herein shall limit the rights of the Company and
         the Purchaser pursuant to a registration rights agreement to be entered
         into between the Company and the Purchaser (the "Registration Rights
         Agreement") substantially in the form as Exhibit D attached hereto, the
         Purchaser acknowledges that neither the Company nor any other
         representative on behalf of the Company has made any representations
         with respect to registration under the Securities Act or qualification
         under Canadian Securities Laws of the Purchased Securities or the
         Second Debenture Shares, that no such registration is contemplated,
         that there can be no assurance that there will be any market for the
         Purchased Securities or the Second Debenture Shares in the United
         States in the foreseeable future or any liquid market for four months,
         and that, as a result, the Purchaser may be required to bear the
         economic risk of his or its investment for an indefinite period of time
         under the Securities Act, or a period of four months and one day under
         the Canadian Securities Laws.

C.       COMPANY WARRANTIES AND REPRESENTATIONS

         The Company hereby represents and warrants to the Purchaser as follows
and acknowledges that the Purchaser is relying upon the following
representations and warranties in connection with its subscription for the
Purchased Securities and, if applicable, the Debenture:

1.       The Company is a corporation duly amalgamated and organized and validly
         existing under the laws of the Province of Alberta, is duly qualified
         to carry on its business and is in good standing in each jurisdiction
         in which the conduct of its business or the ownership, leasing or
         operation of its property and assets requires such qualification, and
         has all requisite corporate power, authority and capacity to carry on
         its business as now conducted and to own, lease or operate its property
         and assets and to enter into, execute, deliver and perform its
         obligations under this Agreement. The Company is conducting business in
         compliance in all material respects with all applicable laws, rules and
         regulations of each jurisdiction in which its business is carried on,
         and all licenses, registrations and qualifications of the Company are
         valid, subsisting and in good standing, except in respect of matters
         that do not and would not reasonably be expected to have a Material
         Adverse Effect on the Company, and except for a failure to be so
         qualified or the absence of any such license, registration or
         qualification which does not
<PAGE>
                                     - 5 -


         and would not reasonably be expected to have a Material Adverse Effect
         on the Company;

         "Material Adverse Effect" means a material adverse effect on the
         business, operation, results of operations, assets, liabilities or
         financial condition of the Company and the subsidiaries of the Company
         taken as a whole;

2.       (i) The execution and delivery of this Agreement by the Company and the
         consummation by it of the transactions contemplated hereby and thereby
         have been duly authorized by all necessary corporate action and no
         further consent or authorization of the Company or its Directors is
         required, except for the requirement of shareholder approval
         contemplated herein (ii) this Agreement has been duly executed and
         delivered by the Company and is a valid and binding obligation of the
         Company enforceable against the Company in accordance with its terms,
         except as such enforceability may be limited by applicable bankruptcy,
         insolvency, reorganization, moratorium, liquidation, receivership or
         similar laws relating to, or affecting generally the enforcement of,
         creditors' rights and remedies or by other equitable principles of
         general application;

3.       Each of the Company's subsidiaries is a wholly-owned subsidiary
         corporation duly incorporated and organized and validly existing under
         the laws of its jurisdiction of incorporation, is duly qualified to
         carry on its business and is in good standing in each jurisdiction in
         which the conduct of its business or the ownership, leasing or
         operation of its property and assets requires such qualification, and
         has all requisite corporate power, authority and capacity to carry on
         its business as now conducted and to own, lease or operate its property
         and assets, except for the failure to be so qualified that does not and
         would not reasonably be expected to have a Material Adverse Effect on
         the Company. No subsidiaries of the Company own or license any
         intellectual property other than licences for intellectual property
         which are owned by the Company;

4.       As of the date hereof, the authorized capital of the Company consists
         of an unlimited number of Common Shares and an unlimited number of
         preferred shares;

5.       The Company is a "reporting issuer" (or its equivalent), not in
         default, in each of the provinces of British Columbia, Alberta,
         Saskatchewan, Manitoba, Ontario, Quebec, Newfoundland, New Brunswick,
         Nova Scotia and Prince Edward Island;

6.       The issued and outstanding common shares of the Company are listed and
         posted for trading on the Toronto Stock Exchange and no order ceasing
         or suspending trading in any securities of the Company or prohibiting
         the sale of the Purchased Securities or the trading of any of the
         Company's issued securities has been issued;

7.       On April 16, 2002 there are 36,763,556 common shares of the Company
         issued and outstanding;

8.       All necessary corporate action has been taken by and on behalf of the
         Company to authorize the creation, issuance and sale of the Purchased
         Securities and upon receipt of the purchase price therefor, such
         securities shall be issued as fully paid and non-assessable securities
         of the Company; and
<PAGE>
                                     - 6 -


9.       Neither the Company nor any of its subsidiaries has committed an act of
         bankruptcy or sought protection from its creditors from any court or
         pursuant to any legislation, proposed a compromise or arrangement to
         its creditors generally, taken any proceeding with respect to a
         compromise or arrangement, taken any proceeding to have itself declared
         bankrupt or wound up, as the case may be, taken any proceeding to have
         a receiver appointed of any part of its assets, had any encumbrance or
         receiver take possession of any of its property, had an execution or
         distress become enforceable or levied upon any portion of its property
         or had any petition for a receiving order in bankruptcy filed against
         it.

D.       PURCHASER'S REPRESENTATIONS AND WARRANTIES

         The Purchaser hereby represents and warrants to the Company as follows:

1.       (i) The Purchaser has all requisite corporate or other power and
         capacity and has taken all requisite corporate or other action to
         execute and deliver this Agreement, to purchase the Purchased
         Securities, the Second Debenture, the Debenture Shares and the Warrant
         Shares to be purchased by it and to carry out and perform all of its
         obligations under this Agreement; and (ii) this Agreement constitutes
         the legal, valid and binding obligation of the Purchaser, enforceable
         against the Purchaser in accordance with its terms, except as such
         enforceability may be limited by applicable bankruptcy, insolvency,
         reorganization, moratorium, liquidation, receivership or similar laws
         relating to, or affecting generally the enforcement of, creditors'
         rights and remedies or by other equitable principles of general
         application.

2.       The Purchaser has completed or caused to be completed the Purchaser
         Certificates, Private Placement Questionnaire and Undertaking attached
         hereto as Exhibit F and the responses provided therein shall be true
         and correct as of the date hereof and the Closing Date and the
         Debenture Closing Date.

3.       The Purchaser is an "accredited investor" as defined in Rule 501(a) of
         Regulation D under the Securities Act. The Purchaser is aware of the
         Company's business affairs and financial condition and has had access
         to and has acquired sufficient information about the Company to reach
         an informed and knowledgeable decision to acquire the Purchased
         Securities, the Second Debenture, the Debenture Shares and the Warrant
         Shares. Purchaser has such business and financial experience as is
         required to permit it to protect his or its own interests in connection
         with the purchase of the Purchased Securities, the Second Debenture,
         the Debenture Shares and the Warrant Shares. The Purchaser's financial
         condition is such that it is able to bear the risk of holding the
         Purchased Securities, the Second Debenture, the Debenture Shares and
         the Warrant Shares for an indefinite period of time and the risk of
         loss of his or its entire investment.

4.       The Purchaser is an "accredited investor" as defined in Rule 45-501 of
         the Ontario Securities Commission and has delivered the written
         declaration in the form of Exhibit F-3 to the Company and the
         representations, warranties and covenants contained therein shall
         survive the closing of the purchase of the Purchased Securities and the
         Second Debenture, respectively.
<PAGE>
                                     - 7 -


5.       The Purchaser is purchasing the Purchased Securities, the Second
         Debenture, the Debenture Shares and the Warrant Shares for his or its
         own account as principal, for investment purposes only, and not with a
         present view to, or for, the resale distribution thereof, in whole or
         in part, within the meaning of the Securities Act or any state or
         provincial securities laws. Purchaser understands that its acquisition
         of the Purchased Securities, the Second Debenture, the Debenture Shares
         and the Warrant Shares has not been registered under the Securities Act
         or registered or qualified under any state or provincial law in
         reliance on specific exemptions therefrom, which exemptions may depend
         upon, among other things, the bona fide nature of the Purchaser's
         investment intent as expressed herein. Except as contemplated by this
         Agreement, the Purchaser has no present agreement, undertaking,
         arrangement, obligation or commitment providing for the disposition of
         the Purchased Securities, the Second Debenture, the Debenture Shares or
         the Warrant Shares. The Purchaser represents that it has not been
         organized, reorganized or recapitalized specifically for the purpose of
         investing in the Purchased Securities. The Purchaser will not, directly
         or indirectly, offer, sell, pledge, transfer or otherwise dispose of
         (or solicit any offers to buy, purchase or otherwise acquire or take a
         pledge of) any of the Purchased Securities, the Second Debenture, the
         Debenture Shares or the Warrant Shares except in compliance with the
         Securities Act, and the rules and regulations promulgated thereunder
         and applicable state securities laws.

6.       The Purchaser has received and reviewed the Term Sheet. The Purchaser
         is aware that the purchase of the Purchased Securities, the Second
         Debenture, the Debenture Shares or the Warrant Shares may have tax
         consequences both in Canada and the United States.

7.       The foregoing representations and warranties are true as of the date of
         this Agreement and shall be true as of the final date that the Company
         issues and sells the Purchased Securities and the Second Debenture,
         respectively, to the Purchaser and the date the Company issues the
         Warrant Shares or the Debenture Shares, as applicable. If such
         representations and warranties shall not be true in any respect prior
         to any such date, the Purchaser will give prompt written notice of such
         fact to the Company.

E.       CONDITIONS TO CLOSING

         The Company's obligation to sell and issue Purchased Securities at the
Initial Closing and the Second Debenture at the Second Closing, respectively, is
at the option of the Company, subject to the fulfillment or waiver of the
following conditions:

1.       The representations made by the Purchaser and the Company herein shall
         be true and correct when made, and shall be true and correct on the
         Initial Closing Date and the Second Closing Date, respectively, with
         the same force and effect as if they had been made on and as of each
         such date.

2.       The delivery of a legal opinion of Company's legal counsel at the
         Initial Closing satisfactory to the Purchaser.
<PAGE>
                                     - 8 -

3.       All covenants, agreements and conditions contained in this Agreement to
         be performed by the Purchaser on or prior to the applicable Closing
         Date shall have been performed or complied with in all material
         respects.

4.       The Company shall have obtained all necessary blue sky law permits and
         qualifications, or secured exemptions therefrom, required to be filed
         prior to closing by any state or province for the offer and sale of the
         Purchased Securities and the Second Debenture.

5.       All necessary regulatory approvals shall have been obtained by the
         Company.

6.       No proceeding challenging this Agreement or the transactions
         contemplated hereby, or seeking to prohibit, alter, prevent or
         materially delay the Closing, shall have been instituted or be pending
         before any court, arbitrator, governmental body, agency or official.

7.       The sale of Purchased Securities and the Second Debenture by the
         Company shall not be prohibited by any law or governmental order or
         regulation and shall not require any additional consent or approval
         other that the ones obtained by the Company.

8.       The issue and sale of Purchased Securities and the Second Debenture are
         exempt from the requirement to file a prospectus and the requirement to
         deliver an offering memorandum under applicable securities laws.

9.       The delivery of a share certificate representing the Common Shares and
         the Warrant or at the Initial Closing.

F.       PRE-EMPTIVE RIGHTS

1.       The Company shall not issue, sell or exchange, agree to issue, sell or
         exchange, or reserve or set aside for issuance, sale or exchange, any
         equity securities of the Company, including common shares and preferred
         shares, any option, warrant or other right to subscribe for, purchase
         or otherwise acquire any equity securities of the Company or any debt
         instrument that is convertible into equity of the Company
         (collectively, the "Offered Securities") to Financial Investors (as
         defined herein) for capital raising purposes unless in each case the
         Company shall have first complied with this Section F. The Company
         shall deliver to the Purchaser a written notice of any proposed or
         intended issuance, sale or exchange of Offered Securities (an "Offer"),
         which shall (i) identify and describe the Offered Securities, (ii)
         describe the price and other terms upon which they are to be issued,
         sold or exchange, and the number or amount of the Offered Securities to
         be issued, sold or exchanged, (iii) identify the persons or entities
         (if known) to which or with which the Offered Securities are to be
         offered, issued, sold or exchanged, and (iv) subject to receipt by the
         Company of applicable regulatory approval, offer to issue and sell to
         or exchange with the Purchaser 19.9% of the Offered Securities on the
         same terms and conditions as the Offered Securities, including without
         limitation the delivery of a legal opinion to anyone in connection with
         the Offering, if delivered. For the purposes of Section F, "Financial
         Investors" are all investors other than employees and directors of, and
         consultants and advisors (including without limitation placement agents
         and underwriters) to the Company in their respective capacities as
         such, other than any person
<PAGE>
                                     - 9 -


         or entity with whom the Company enters into a research and development,
         licensing or partnering agreement. Notwithstanding the foregoing, in
         the event that the Company enters into a research and development,
         licensing or partnering agreement with a third party and in connection
         therewith issues, sells or exchanges securities with a third party, the
         Company shall deliver the Offer to the Purchaser promptly after the
         sale of securities to such third party, and the Purchaser shall have
         the right to purchase an amount of securities equal to 19.9% of the
         securities of the Company issued with respect to such research and
         development, licensing or partnering agreement after the closing of
         such agreement on the same terms and conditions as such securities are
         sold, including, without limitation, the delivery of a legal opinion to
         anyone in connection with the Offering, if delivered.

2.       To accept the Offer, in whole or in part, the Purchaser must deliver to
         the Company, on or prior to 10 business days after the date of delivery
         of the Offer, a written notice of acceptance indicating that the
         Purchaser elects to purchase its share of the Offered Securities.

3.       If the Purchaser does not accept to buy a portion of the Offered
         Securities, the Company will have 120 days from the expiration of the
         period set forth in paragraph 2 above to issue, sell or exchange all or
         any part of the Offered Securities, but only upon terms and conditions
         which are not more favourable, in the aggregate, to the acquiring
         persons or persons or less favourable to the Company than those set
         forth in the Offer.

4.       The rights of the Purchaser under this Section F shall not apply to:

                  (A)      the issuance of any shares as a stock dividend to
                           holders of common shares or upon any subdivision or
                           combination of common shares,

                  (B)      the issuance of any common shares upon conversion of
                           shares of outstanding convertible preferred shares,

                  (C)      the issuance of any common shares upon the exercise
                           of outstanding warrants,

                  (D)      the issuance of securities solely in consideration
                           for the acquisition (whether by merger or otherwise)
                           by the Company or any of its subsidiaries of the
                           shares or assets of any other entity,

                  (E)      the issuance of common shares by the Company pursuant
                           to a firm-commitment underwritten public offering, or

                  (F)      the issuance of common shares or options with respect
                           thereto, issued or issuable to employees, directors
                           or officers of, or consultant to, the Company or any
                           of its subsidiaries pursuant to any plan, agreement
                           or arrangement approved by the Board of Directors of
                           the Company.
<PAGE>
                                     - 10 -


Notwithstanding Section F(4)(v) above, in the event of a firm commitment
underwritten public offering, the Company will use its best efforts to ensure
that the Purchaser is able to purchase 19.9% of the shares offered thereunder.

5.       The provisions in this Section F shall terminate upon the earlier of:

                  (A)      18 months from the date hereof;

                  (B)      the sale of the Company by merger, sale of assets or
                           otherwise, or

                  (C)      the termination of the licence agreement of even date
                           herewith between the Purchaser and the Company.

G.       COVENANTS OF THE COMPANY

1.       The Company covenants to include in its information circular for its
         annual meeting of shareholders an item seeking shareholders approval in
         connection with the issuance of 883,380 Common Shares of the Company to
         the Purchaser (the "Resolution") and such solicitation shall be done in
         compliance with laws which shall include the option for shareholders to
         vote by proxy; and

2.       The Company agrees to insert in its information circular sent to the
         Company's shareholders in connection with the approval of the
         Resolution a recommendation of the board of directors of the Company
         recommending that shareholders vote in favour of the Resolution.

H.       BOARD OBSERVER

         For so long as the Purchaser holds at least 5% of the capital stock of
the Company outstanding and the licence agreement entered into between the
Purchaser and the Company as of the date hereto is in effect, the Purchaser
shall be entitled to receive notice of, to send an Observer and to receive
copies of all materials distributed to the Company's Board of Directors to all
meetings, held in person or by any other means, of the Company's Board of
Directors; provided however, that rights granted pursuant to this section are
not assignable without the consent of the Company and the Company requires as a
condition precedent to such right that each Observer proposing to attend any
meeting of the Board of Directors shall agree to hold in confidence and trust
and to act in a fiduciary manner with respect to all information so received
during such meetings or otherwise; and, provided further, that the Company
reserves the right not to provide information and to exclude the Observer from
any meeting or portion thereof if attendance at such meeting by the Observer
(absent adequate steps to preserve confidentiality) would adversely affect the
attorney-client privilege between the Company and its counsel, is necessary to
protect highly confidential proprietary information or for other similar
reasons.

I.       INDEMNIFICATION

1.       The Company shall indemnify, defend and hold harmless the Purchaser and
         any director, officer, employee, agent or representative of the
         Purchaser (each an "Indemnified Party") from and after the Closing,
         from and against any and all material losses, claims, damages,
<PAGE>
                                     - 11 -


         liabilities, obligations, penalties, judgments, awards, costs,
         reasonable expenses and disbursements (and any and all actions, suits,
         proceedings and investigations in respect thereof and any and all legal
         and other costs, reasonable expenses or disbursements in giving
         testimony or furnishing documents in response to a subpoena or
         otherwise), including, without limitation, the costs, reasonable
         expenses and disbursements as and when incurred, of investigating,
         preparing or defending any such action, suit, proceeding or
         investigation (whether or not such Indemnified Party is a party)
         (together, "Losses") (provided that Losses shall be calculated net of
         the amount of insurance proceed or Third Party contribution or
         indemnification payments actually paid to the Purchaser as
         reimbursement for such Losses), directly or indirectly, caused by,
         relating to, based upon, arising out of or in connection with the
         breach of any representation, warranty, covenant or agreement of the
         Company set forth in this Agreement.

2.       The Purchaser shall indemnify, defend and hold harmless the Company and
         any director, officer, employee, agent or representative of the Company
         (each an "Indemnified Party") from and after the Closing, from and
         against any and all material losses, claims, damages, liabilities,
         obligations, penalties, judgments, awards, costs, reasonable expenses
         and disbursements (and any and all actions, suits, proceedings and
         investigations in respect thereof and any and all legal and other
         costs, reasonable expenses or disbursements in giving testimony or
         furnishing documents in response to a subpoena or otherwise),
         including, without limitation, the costs, reasonable expenses and
         disbursements as and when incurred, of investigating, preparing or
         defending any such action, suit, proceeding or investigation (whether
         or not such Indemnified Party is a party) (together, "Losses")
         (provided that Losses shall be calculated net of the amount of
         insurance proceed or Third Party contribution or indemnification
         payments actually paid to the Company as reimbursement for such
         Losses), directly or indirectly, caused by, relating to, based upon,
         arising out of or in connection with the breach of any representation,
         warranty, covenant or agreement of the Purchaser set forth in this
         Agreement.

3.       An Indemnified Party shall give the Company or the Purchaser, as the
         case may be, (the "Indemnifying Party") prompt, written notice of any
         claim, assertion, event or proceeding concerning any liability or
         damage as to which they may request indemnification from the
         Indemnifying Party hereunder; provided, however, that any failure by an
         Indemnified Party to notify the Indemnifying Party shall not relieve
         the Indemnifying Party from its obligations hereunder except to the
         extent the Indemnifying Party is prejudiced by such failure and shall
         not relieve the Indemnifying Party from any other obligation or
         liability that it may have to any Indemnified Party otherwise than
         under this Article. Upon written notice to such Indemnified Party given
         by the Indemnifying Party after receipt of notice of any such action or
         proceeding, the Indemnifying Party may participate in the defense
         thereof at its own expense with counsel chosen by the Indemnifying
         Party. The Indemnified Party shall not, without the prior written
         consent of the Indemnifying Party, settle or compromise any claim, or
         permit a default or consent to the entry of any judgement in respect
         thereof, which consent shall not be unreasonably withheld. If the
         Indemnifying Party does not participate in the defense of any such
         claim or proceeding pursuant to this section and the Indemnified Party
         proposes to settle such claim or proceeding prior to such a final
         judgement thereon or to forego appeal with respect thereto, then such
         Indemnified Party shall give the Indemnifying Party prompt written
<PAGE>
                                     - 12 -


         notice thereof and the Indemnifying Party shall have the right to
         participate in the settlement proceedings

J.       MISCELLANEOUS

1.       The terms of this Agreement may be waived or amended with the written
         consent of the Company and the Purchaser.

2.       Except for eMedsecurities Inc., for which fees the Purchaser is
         responsible, each of the parties hereto hereby represents that, on the
         basis of any actions and agreements by it, there are no brokers or
         finders entitled to compensation in connection with the sale of
         Purchased Securities, the Second Debenture, the Debenture Shares or the
         Warrant Shares to the Purchaser.

3.       This Agreement shall be governed in all respects by and construed in
         accordance with the laws of the Province of Ontario without any regard
         to conflicts of laws principles.

4.       The representations, warranties, covenants and agreements made in this
         Agreement shall survive any investigation made by the Company or the
         Purchaser and the Closing.

5.       The provisions hereof shall inure to the benefit of, and be binding
         upon, the successors, assigns, heirs, executors and administrators of
         the parties to this Agreement. Notwithstanding the foregoing, the
         Purchaser shall not assign this Agreement or its rights hereunder
         without the prior written consent of the Company.

6.       This Agreement, the Debentures, the Warrant and the Registration Rights
         Agreement constitutes the full and entire understanding and agreement
         between the parties with regard to the subjects thereof.

7.       All notices and other communications required or permitted under this
         Agreement shall be effective upon receipt and shall be in writing and
         may be delivered in person, by telecopy, overnight delivery service or
         registered or certified United States mail addressed to the Company or
         the Purchaser, as the case may be, at their respective addresses set
         forth at the beginning of this Agreement or on the signature page to
         this Agreement or at such other address as the Company or the Purchaser
         shall have furnished to the other party in writing. All notices and
         other communications shall be effective upon the earlier of actual
         receipt thereof by the person to whom notice is directed or (i) in the
         case of notices and communications sent by personal delivery or
         telecopy, one business day after such notice or communication arrives
         at the applicable address or was successfully sent to the applicable
         telecopy number, (ii) in the case of notices and communications sent by
         overnight delivery service, at noon (local time) on the second business
         day following the day such notice or communication was sent, and (iii)
         in the case of notices and communications sent by United States mail
         seven days after such notice or communication shall have been deposited
         in the United States mail.

8.       If any provision of this Agreement shall be judicially determined to be
         invalid, illegal or unenforceable, the validity, legality and
         enforceability of the remaining provisions shall not in any way be
         affected or impaired thereby.
<PAGE>
                                     - 13 -

9.       This Agreement may be executed in any number of counterparts, each of
         which shall be an original, but all of which together shall constitute
         one instrument.

10.      Each party to this Agreement shall do and perform or cause to be done
         and performed all such further acts and things and shall execute and
         deliver all such other agreements, certificates, instruments and
         documents as the other party hereto may reasonably request in order to
         carry out the intent and accomplish the purposes of this Agreement and
         the consummation of the transactions contemplated hereby.

11.      Subject to the provisions of the Registration Rights Agreement, the
         Company and the Purchaser shall each bear its own costs and expenses
         incurred on its behalf with respect to this Agreement and the
         transactions contemplated hereby, including fees of legal counsel.

12.      Except as otherwise specified, all references to "dollars" or "$" in
         this Agreement (including the Exhibits attached hereto) shall be deemed
         to refer to United States dollars.

                            [Signature page follows]
<PAGE>
          SUBSCRIPTION AND DEBENTURE PURCHASE AGREEMENT SIGNATURE PAGE

The foregoing agreement is hereby executed as of the date last below written.

UNITED THERAPEUTICS CORPORATION

Per:   /s/ Paul Mahon
       -----------------------------------------
Name:  Paul Mahon
Title:    Senior Vice President and General Counsel

<TABLE>
<CAPTION>
                 REGISTRATION INSTRUCTIONS                                    DELIVERY INSTRUCTIONS
<S>                                                          <C>
Number of Purchased Securities:                              Name:

4,900,000 Common Shares and a warrant to purchase
3,250,000 Common Shares

Purchase Price:  US$2,450,000                                Address:
Name or Nominee:

Account reference, if applicable:                            Account Reference, if applicable:
                                                             Contact Name:
                                                             Telephone Number:
                                                             Fax Number:
</TABLE>

Agreed to and Accepted:

ALTAREX CORP.

By:    /s/ Richard E. Bagley
    -------------------------------------------------

Title:   President and Chief Executive Officer
      -----------------------------------------------

Date:  April 17, 2002
      -----------------------------------------------
<PAGE>
                                    EXHIBIT A

                                 FORM OF WARRANT
<PAGE>
THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE UNITED
STATES SECURITIES ACT OF 1933, AS AMENDED (THE "U.S. SECURITIES ACT"). THE
HOLDER HEREOF, BY PURCHASING SUCH SECURITIES, AGREES FOR THE BENEFIT OF THE
CORPORATION THAT SUCH SECURITIES MAY BE OFFERED, SOLD OR OTHERWISE TRANSFERRED
ONLY (A) TO THE CORPORATION, (B) OUTSIDE THE UNITED STATES IN ACCORDANCE WITH
REGULATION S UNDER THE U.S. SECURITIES ACT, OR (C) IN ACCORDANCE WITH RULE 144
UNDER THE SECURITIES ACT, IF AVAILABLE. DELIVERY OF THIS CERTIFICATE MAY NOT
CONSTITUTE "GOOD DELIVERY" IN SETTLEMENT OF TRANSACTIONS ON STOCK EXCHANGES IN
CANADA.

UNLESS PERMITTED UNDER CANADIAN SECURITIES LEGISLATION, THE HOLDER OF THE
SECURITIES SHALL NOT TRADE THE WARRANT OR COMMON SHARES ISSUABLE ON EXERCISE
THEREOF IN THE PROVINCE OF ONTARIO BEFORE AUGUST 20, 2002.

                                                               Certificate No. 1

                                  ALTAREX CORP.
                            (AN ALBERTA CORPORATION)

                       Dated this 17th day of April, 2002

                               WARRANT CERTIFICATE

1. UNITED THERAPEUTICS CORPORATION (the "HOLDER") is the holder of a share
purchase warrant (the "WARRANT") of AltaRex Corp. (the "CORPORATION") and is
thereby entitled to purchase at any time during the period commencing on August
14, 2002 and prior to 5:00 p.m. (Toronto time) (the "EXPIRY TIME") on August 20,
2002 (the "EXERCISE PERIOD") three million two-hundred and fifty thousand
(3,250,000) fully paid and non-assessable common shares (the "SHARES") in the
capital of the Corporation as constituted on the date hereof, at a price per
Share equal to US$0.50 (the "SUBSCRIPTION PRICE"), subject, in each case, to
adjustment in the manner set forth herein, by delivering: (i) the exercise form,
attached hereto as Exhibit "I" (the "EXERCISE FORM"), duly completed and
executed; and (ii) a certified cheque, bank draft or wire transfer payable in
lawful money of the United States to or to the order of the Corporation at par
in the amount of US$1,625,000, to the Corporation at its principal offices at
610 Lincoln Street, Waltham, Massachusetts, USA 02451 or such other address as
may be set forth in a written notice to the Holder from the Corporation (the
"CORPORATION'S ADDRESS"). The Corporation will cause to be personally delivered
to the person or persons specified in the Exercise Form the certificate or
certificates for the Shares subscribed for at the respective address or
addresses specified therein within three business days of its receipt of the
duly completed Exercise Form and the aggregate Subscription Price in respect
thereof.

2. The Holder may exercise the Warrant only during the Exercise Period. The
Holder shall not be entitled to exercise the Warrant after the Expiry Time. The
Holder shall not be entitled to
<PAGE>
                                     - 2 -


exercise the Warrant in part only for a number of Shares which is less than the
total number of Shares into which the Warrant may be exercised. No fractional
Shares will be issued.

3. The holding of the Warrant under this Warrant certificate does not make the
Holder a shareholder of the Corporation, nor does it entitle the Holder to any
right or interest except as is expressly provided in this Warrant certificate.

4. Exhibit "I" and Exhibit "II" form part of this Warrant certificate and,
without limitation, the Holder shall be entitled to the additional rights
contained in Exhibit "II".

5. Warrants may not be transferred by the Holder.

6. From time to time the Corporation may, subject to the provisions of this
certificate, execute and deliver by its proper officers, instruments
supplemental hereto, which thereafter shall form part hereof, for any one or
more or all of the following purposes of setting forth adjustments in the
application of Section 2 of Exhibit "II" hereto.

In the case of the consolidation, amalgamation, arrangement or merger
("SUCCESSOR CORPORATION"), the successor corporation shall be bound by all the
provisions hereof including the due and punctual performance of all covenants of
the Corporation and forthwith following the occurrence of such event the
successor corporation resulting from such consolidation, amalgamation,
arrangement, merger or transfer (if not the Corporation) shall expressly assume,
by supplemental certificate satisfactory in form to the Holder and executed and
delivered to the Holder, the due and punctual performance and observance of each
and every covenant and condition of this certificate to be performed and
observed by the Corporation.

7. Time shall be of the essence hereof. This Warrant certificate shall be
governed by and construed in accordance with the laws in force in the Province
of Ontario and the laws of Canada applicable therein and shall be treated in all
respects as an Ontario contract.

8. All dollar amounts herein are expressed in lawful money of the United States.

9. A register shall be kept by the Corporation at the Corporation's Address, and
at such other offices as may be required by law wherein shall be entered the
name, address and description of the Holder and particulars of the Warrant.

10. In case this Warrant certificate shall become mutilated or be lost,
destroyed or stolen, the Corporation shall issue and shall certify and deliver a
new Warrant certificate of like date and tenor as the one mutilated, lost,
destroyed or stolen upon surrender of and in place of and upon cancellation of
the mutilated Warrant certificate or in lieu of and in substitution for the
lost, destroyed or stolen Warrant certificate and the substituted Warrant
certificate shall rank equally in accordance with its terms with the previous
Warrant certificate issued to the Holder.

The applicant for the issue of a new Warrant certificate pursuant to this
section 10 shall bear the cost of the issue thereof and in case of loss,
destruction or theft shall, as a condition precedent to
<PAGE>
                                     - 3 -


the issue thereof, furnish to the Corporation such evidence of ownership and of
the loss, destruction or theft of the Warrant certificate so lost, destroyed or
stolen as shall be satisfactory to the Corporation in its discretion and the
applicant may also be required to furnish an indemnity in amount and form
satisfactory to the Corporation in its discretion, and shall pay the reasonable
charges of the Corporation in connection therewith.

11. This Warrant certificate may, upon compliance with the reasonable
requirements of the Corporation, be exchanged for one or more Warrant
certificates representing Warrants entitling the Holder to acquire an equal
aggregate number of Shares. Warrants may be exchanged only at the Corporation's
Address or at any other place that is designated by the Corporation. Any
Warrants tendered for exchange shall be surrendered to the Corporation and
cancelled. The Corporation shall sign all Warrant certificates necessary to
carry out exchanges as aforesaid and those Warrant certificates shall be
certified by or on behalf of the Corporation.

12. In the event any provision hereof shall be void or unenforceable for any
reason, it shall be severed from the remainder of the provisions hereof and such
remainder shall remain in full force and effect notwithstanding such severance.
Any court with jurisdiction over any dispute with respect to the Warrants may
amend the provisions hereof to the minimum extent required to render the
impugned provision valid and enforceable.

         IN WITNESS WHEREOF, the Corporation has executed this Warrant
certificate.

                                                 ALTAREX CORP.

                                                 Per:
                                                     ---------------------------
                                                            Authorized Signatory
<PAGE>
                               REGISTRATION PANEL
     (No writing on this panel except by the Corporation or other Registrar)
<TABLE>
<CAPTION>
                        DATE OF         NUMBER OF         IN WHOSE NAME       SIGNATURE OF CORPORATION OR OTHER
 CERTIFICATE NO.     REGISTRATION        WARRANTS           REGISTERED                    REGISTRAR
<S>                  <C>                <C>               <C>                 <C>
</TABLE>
<PAGE>
                                   EXHIBIT "I"

                                  EXERCISE FORM

TO:      ALTAREX CORP. (the "Corporation")

         The Holder hereby exercises its right to purchase 3,250,000 Shares (or
such number of other securities or property to which such Warrants entitle the
Holder in lieu thereof or in addition thereto under the provisions set out
elsewhere in the attached Warrant certificate) pursuant to the provisions of
this Warrant certificate at a price of US$0.50 per Share (or such other adjusted
Subscription Price as may be prescribed elsewhere in the attached Warrant
certificate) on the terms specified in this Warrant certificate and encloses and
tenders herewith a certified cheque, bank draft or wire transfer payable in
lawful money of the United States to or to the order of the Corporation at par
for the aggregate Subscription Price of US$1,625,000.

         Capitalized terms used in this Exercise Form have the same meanings
ascribed to them elsewhere in this Warrant certificate.

         The Holder hereby directs that the securities or property be registered
as follows:

<TABLE>
<CAPTION>
NAME(S) IN FULL                        ADDRESS(ES)                         NUMBER(S) OF SHARES
<S>                                    <C>                                 <C>
</TABLE>

(Please print the full name in which certificates representing the Shares are to
be issued. If space is insufficient, attach a separate sheet. If any of the
securities are to be issued to a person or persons other than the Holder, the
Holder must pay all requisite transfer taxes and provide proof of such payments
to the Corporation.)

         The Holder hereby acknowledges that, upon the registration of the
securities purchased herein and except as set out below, the Holder's rights
under this Warrant certificate are hereby terminated and that, if the number of
Warrants exercised is less than the total number of Warrants represented by this
Warrant certificate, the Corporation shall forthwith deliver to the Holder a new
Warrant certificate in respect of the Warrants not then exercised.
<PAGE>
                                     - ii -

         Dated this ____________ day of _________________, ___________.



                                 ----------------------------------------------
                                 Signature

                                 Print name and address of Holder in full below

                                 Name
                                     -------------------------------------------

                                 Address:
                                         ---------------------------------------


                                 -----------------------------------------------


                                 -----------------------------------------------


                                 -----------------------------------------------




This Exercise Form must be completed and delivered, together with payment for
the Shares purchased, to the Corporation.
<PAGE>
                                  EXHIBIT "II"

1.       DEFINITIONS

         In this Exhibit II:

         (a)      "CURRENT MARKET PRICE" per Share or Participating Share at any
                  date shall be the weighted average price per share for such
                  shares for any 20 consecutive trading days (such 20
                  consecutive trading days being selected by the Corporation)
                  commencing not more than 25 trading days before such date on
                  such stock exchange on which such shares are listed as may be
                  selected for such purpose by the directors of the Corporation
                  or if such shares are not listed on any stock exchange, then
                  on an automated quotation system, such as NASDAQ or the over
                  the counter market. The weighted average price shall be
                  determined by dividing the aggregate sale price of all such
                  shares so sold on the exchange, system or market, as the case
                  may be, during the said 20 consecutive trading days by the
                  total number of such shares so sold. If such shares are not
                  listed on any stock exchange, automated quotation system or
                  traded on an over the counter market, the Current Market Price
                  shall be determined in good faith by the Board of Directors of
                  the Corporation.

         (b)      "DIVIDENDS PAID IN THE ORDINARY COURSE" means cash dividends
                  declared payable on the Shares in any fiscal year of the
                  Corporation to the extent that such cash dividends do not
                  exceed, in the aggregate, the greater of: (i) 125% of the
                  aggregate amount or value of dividends declared payable by the
                  Corporation on the Shares in its immediately preceding fiscal
                  year; and (ii) 50% of the aggregate net earnings of the
                  Corporation, before extraordinary items, for its immediately
                  preceding fiscal year (less the amount or value of all
                  dividends paid or payable in respect of such fiscal year) as
                  shown in the audited consolidated financial statements of the
                  Corporation for such preceding fiscal year or, if there are no
                  audited financial statements with respect to such period,
                  computed in accordance with generally accepted accounting
                  principles consistent with the applications made in
                  preparation of the most recent audited consolidated financial
                  statements of the Corporation, and for such purpose the
                  amounts of any dividend paid in shares shall be the aggregate
                  stated value of such shares and the amount of any dividend
                  paid in other than cash or shares shall be the fair market
                  value of such dividend as declared by resolution passed by the
                  board of directors of the Corporation.

         (c)      "PARTICIPATING SHARE" means a share that carries the right to
                  participate in earnings or in capital on a liquidation or
                  winding-up to an unlimited degree, or which ranks, in terms of
                  priority, equally with the Shares with respect to
                  participation in earnings or in capital on a liquidation or
                  winding-up.
<PAGE>
                                     - ii -

2.       ADJUSTMENT OF SUBSCRIPTION RIGHTS

         The Subscription Price in effect and the number and type of securities
purchasable under the Warrant at any date shall be subject to adjustment from
time to time as follows:

         (a)      If and whenever at any time following April 17, 2002 and prior
                  to the Expiry Time, the Corporation shall (i) subdivide or
                  redivide the outstanding Shares into a greater number of
                  shares, (ii) reduce, combine or consolidate the outstanding
                  Shares into a smaller number of shares, or (iii) issue Shares
                  or other Participating Shares to the holders of all or
                  substantially all of the outstanding Shares by way of a stock
                  dividend, the Subscription Price in effect on the effective
                  date of any such event shall be adjusted immediately after
                  such event or on the record date for such issue of Shares or
                  other Participating Shares by way of stock dividend, as the
                  case may be, so that it shall equal the amount determined by
                  multiplying the Subscription Price in effect immediately prior
                  to such event by a fraction, of which the numerator shall be
                  the total number of Shares and other Participating Shares
                  outstanding immediately prior to such event and of which the
                  denominator shall be the total number of Shares and other
                  Participating Shares outstanding immediately after such event;
                  and the number of Shares which the Holder is entitled to
                  purchase under the terms of this Warrant certificate shall be
                  adjusted at the same time by multiplying the number by the
                  inverse of the aforesaid fraction; such adjustments shall be
                  made successively whenever any event referred to in this
                  subsection (a) shall occur; any such issue of Shares or other
                  Participating Shares by way of a stock dividend shall be
                  deemed to have been made on the record date for the stock
                  dividend for the purpose of calculating the number of
                  outstanding Shares or other Participating Shares immediately
                  after such event under this subsection (a) and subsection (e)
                  of this Section.

         (b)      If and whenever at any time following April 17, 2002 and prior
                  to the Expiry Time the Corporation shall fix a record date for
                  the issuance of rights, options or warrants to all or
                  substantially all of the holders of the outstanding Shares,
                  entitling them, for a period expiring not more than 45 days
                  after such record date, to subscribe for or purchase Shares or
                  other Participating Shares (or securities convertible into or
                  exchangeable for Shares or other Participating Shares) at a
                  price per share (or having a conversion or exchange price per
                  share) less than 95% of the Current Market Price on such
                  record date, the Subscription Price shall be adjusted
                  immediately after such record date so that it shall equal the
                  price determined by multiplying the Subscription Price in
                  effect on such record date by a fraction, of which the
                  numerator shall be the total number of Shares outstanding on
                  such record date plus the number arrived at by dividing the
                  aggregate price of the total number of additional Shares or
                  other Participating Shares offered for subscription or
                  purchase (or the aggregate conversion or exchange price of the
                  convertible or exchangeable securities so offered) by such
                  Current Market Price, and of which the denominator shall be
                  the total number of Shares outstanding on
<PAGE>
                                     - iii -

                  such record date plus the total number of additional Shares or
                  other Participating Shares offered for subscription or
                  purchase (or into which the convertible or exchangeable
                  securities so offered are convertible or exchangeable); any
                  Shares owned by or held for the account of the Corporation or
                  any subsidiary of the Corporation shall be deemed not to be
                  outstanding for the purpose of any such computation; and the
                  number of Shares which the Holder is entitled to purchase
                  under the terms of this Warrant certificate shall be adjusted
                  at the same time by multiplying the number by the inverse of
                  the aforesaid fraction; such adjustment shall be made
                  successively whenever such a record date is fixed; to the
                  extent that any such rights, options or warrants are not so
                  issued or any such rights, options or warrants are, not
                  exercised prior to the expiration thereof, the Subscription
                  Price and the exchange rate shall then be re-adjusted to the
                  Subscription Price and the exchange rate which would then be
                  in effect based upon the number and aggregate price of Shares
                  or other Participating Shares (or securities convertible into
                  or exchangeable for Shares or other Participating Shares)
                  actually issued upon the exercise of such rights, options or
                  warrants, as the case may be.

         (c)      If and whenever at any time following April 17, 2002 and prior
                  to the Expiry Time the Corporation shall fix a record date for
                  the making of a distribution to all or substantially all the
                  holders of its outstanding Shares of (i) shares of any class
                  other than Shares or Participating Shares, other than shares
                  distributed to holders of Shares pursuant to their exercise of
                  options to receive dividends in the form of such shares in
                  lieu of Dividends Paid in the Ordinary Course on the Shares
                  and other than the issue of Shares or other Participating
                  Shares to the holders of all or substantially all of the
                  outstanding Shares by way of a stock dividend, or (ii) rights,
                  options or warrants (excluding rights exercisable for 45 days
                  or less) or (iii) evidences of its indebtedness, or (iv)
                  assets (excluding Dividends Paid in the Ordinary Course),
                  including shares of other corporations, then, in each such
                  case, the Subscription Price shall be adjusted immediately
                  after such record date so that it shall equal the price
                  determined by multiplying the Subscription Price in effect on
                  such record date by a fraction, of which the numerator shall
                  be the total number of Shares outstanding on such record date
                  multiplied by the Current Market Price per Share on such
                  record date, less the fair market value (as determined by the
                  board of directors of the Corporation, acting reasonably,
                  which determination, absent manifest error, shall be
                  conclusive) of such shares or rights, options or warrants or
                  evidences or indebtedness or assets so distributed, and of
                  which the denominator shall be the total number of Shares
                  outstanding on such record date multiplied by such Current
                  Market Price per Share; any Shares owned by or held for the
                  account of the Corporation shall be deemed not to be
                  outstanding for the purpose of any such computation; and the
                  number of Shares which the Holder is entitled to purchase
                  under the terms of this Warrant certificate shall be adjusted
                  at the same time by multiplying the number by the inverse of
                  the aforesaid fraction; such adjustment shall be made
                  successively whenever such a record date is fixed; to the
                  extent that such distribution is not so made, the
<PAGE>
                                     - iv -

                  Subscription Price and the exchange rate shall be re-adjusted
                  to the Subscription Price and the exchange rate which would
                  then be in effect if such record date had not been fixed or to
                  the Subscription Price and the exchange rate which would then
                  be in effect based upon such shares or rights, options or
                  warrants or evidences of indebtedness or assets actually
                  distributed, as the case may be, and in clause (iv) the term
                  "Dividends Paid in the Ordinary Course" shall include the
                  value of any securities or other property or assets
                  distributed in lieu of cash Dividends Paid in the Ordinary
                  Course.

         (d)      If and whenever at any time following April 17, 2002 and prior
                  to the Expiry Time there is a reclassification of the Shares
                  at any time outstanding or a change of the Shares into other
                  shares or a capital reorganization of the Corporation not
                  covered in subsection (a) or a consolidation, amalgamation or
                  merger of the Corporation with or into any other corporation
                  or a sale of the property and assets of the Corporation as or
                  substantially as an entirety to any other person, a Holder
                  holding the Warrant under this Warrant certificate which has
                  not been exercised prior to the effective date of such
                  reclassification, capital reorganization, consolidation,
                  amalgamation, merger or sale shall thereafter, upon the
                  exercise of the Warrant, be entitled to receive and shall
                  accept in lieu of the number of Shares, as then constituted,
                  to which the Holder was previously entitled upon exercise of
                  the Warrant, but for the same aggregate consideration payable
                  therefor, the number of shares or other securities or property
                  of the Corporation or of the corporation resulting from such
                  reclassification, consolidation, amalgamation or merger or of
                  the person to which such sale may be made, as the case may be,
                  that such Holder would have been entitled to receive on such
                  reclassification, capital reorganization, consolidation,
                  amalgamation, merger or sale if, on the effective date
                  thereof, the Holder had been the registered holder of the
                  number of Shares to which the Holder was previously entitled
                  upon due exercise of the Warrant; and in any case, if
                  necessary, appropriate adjustment shall be made in the
                  application of the provisions set forth in this Warrant
                  certificate with respect to the rights and interests
                  thereafter of the Holder to the end that the provisions set
                  forth in this Warrant certificate shall thereafter
                  correspondingly be made applicable, as nearly as may
                  reasonably be, in relation to any shares or securities or
                  property to which the Holder may be entitled upon the exercise
                  of the Warrant thereafter.

         (e)      In any case in which this Warrant certificate shall require
                  that an adjustment shall become effective immediately after a
                  record date for an event referred to herein, the Corporation
                  may defer, until the occurrence of such event, issuing to the
                  Holder of the Warrant if exercised after such record date and
                  before the occurrence of such event the kind and amount of
                  shares, other securities or property to which it would be
                  entitled upon such exercise by reason of the adjustment
                  required by such event; provided, however, that the
                  Corporation shall deliver to the Holder an appropriate
                  instrument evidencing the Holder's right to
<PAGE>
                                      - v -

                  receive the kind and amount of shares, other securities or
                  property to which it would be entitled upon the occurrence of
                  the event requiring such adjustment and the right to receive
                  any distributions made or declared in favour of holders of
                  record of Shares as constituted from time to time on and after
                  such date as the Holder would, but for the provisions of this
                  subsection (e), have received, or become entitled to receive,
                  on such exercise.

         (f)      The adjustments provided for in this Warrant certificate are
                  cumulative and shall apply to successive subdivisions,
                  redivisions, reductions, combinations, consolidations,
                  distributions, issues or other events resulting in any
                  adjustment under the provisions of this Warrant certificate
                  provided that, notwithstanding any other provision of this
                  Section, no adjustment of the Subscription Price or number of
                  Shares, as then constituted, purchasable shall be required
                  unless such adjustment would require an increase or decrease,
                  of at least 1% in the Subscription Price or the number of
                  Shares, as then constituted, purchasable then in effect;
                  provided however, that any adjustments which by reason of this
                  subsection (f) are not required to be made shall be carried
                  forward and taken into account in any subsequent adjustment.

         (g)      In the event of any question arising with respect to the
                  adjustments provided in this Warrant certificate, such
                  question shall, absent manifest error, be conclusively
                  determined by a firm of chartered accountants appointed by the
                  Corporation and acceptable to the Holder (who may be the
                  auditors of the Corporation) with the assistance of legal
                  counsel, who may be legal counsel to the Corporation; such
                  accountants shall have access to all necessary records of the
                  Corporation and such determination shall be binding upon the
                  Corporation and the Holder.

         (h)      As a condition precedent to the taking of any action which
                  would require an adjustment in any of the subscription rights
                  pursuant to the terms of the Warrant, including the number of
                  Shares which are to be received upon the exercise thereof, the
                  Corporation shall take any action which may, in the opinion of
                  legal counsel, be necessary in order that the Corporation may
                  validly and legally issue as fully paid and non-assessable all
                  the Shares which the Holder is entitled to receive on the full
                  exercise thereof in accordance with the provisions hereof.

         (i)      In case the Corporation shall take any action affecting the
                  Shares other than action described in this Warrant
                  certificate, which in the opinion of the board of directors of
                  the Corporation would materially affect the rights of Holder,
                  the Subscription Price and/or the number of Shares which may
                  be acquired upon exercise of the Warrant shall be adjusted in
                  such manner and at such time, by action of the board of
                  directors of the Corporation, in its sole reasonable
                  discretion as it may determine to be equitable in the
                  circumstances, provided that no such adjustment shall be made
                  unless prior approval of any stock exchange on which the
                  Shares
<PAGE>
                                     - vi -

                  are listed for trading has been obtained. Failure of the board
                  of directors of the Corporation to make such an adjustment
                  shall be conclusive evidence that the board of directors of
                  the Corporation have determined that it is equitable to make
                  no adjustment in the circumstances.

3.       NOTICE OF ADJUSTMENT

         At least 21 days prior to the effective date or record date, as the
case may be, of any event referred to in clause 2, the Corporation shall notify
the Holder of the particulars of such event and the estimated amount of any
adjustment required as a result thereof. Promptly after the occurrence of any
event which requires an adjustment in the Subscription Price or in any of the
subscription rights pursuant to the terms of the Warrant pursuant to this
Warrant certificate, including the number of Shares, as then constituted, which
are to be received upon the exercise thereof, the Corporation shall forthwith
deliver to the Holder a certificate of the Corporation specifying the
particulars of such event and the required adjustment and the computation of
such adjustment and give notice to the Holder of the particulars of such event
and the required adjustment in the manner provided in Subsection 5(b) hereof.

4.       GENERAL COVENANTS OF THE CORPORATION

         (a)      The Corporation covenants and agrees that it is duly
                  authorized to enter into and perform its obligations under its
                  Warrant certificate.

         (b)      The Corporation will cause the Shares from time to time
                  subscribed for and the certificates representing the Shares to
                  be duly issued. At all times until the Expiry Time while the
                  Warrant is outstanding, the Corporation shall reserve and
                  there shall remain unissued out of its authorized capital a
                  number of Shares sufficient to satisfy the exercise of the
                  Warrant. All Shares issued upon the due exercise of the
                  Warrant shall be fully paid and non-assessable.

         (c)      The Corporation covenants and agrees that all things necessary
                  have been done and performed to create the Warrant and to make
                  the Warrant and this Warrant certificate legal, valid and
                  binding upon the Corporation with the benefits and subject to
                  the terms of this Warrant certificate. The Corporation will
                  do, execute, acknowledge and deliver or cause to be done,
                  executed, acknowledged and delivered, all other acts, deeds
                  and assurances in law as may be reasonably required for the
                  better accomplishing and effecting of the intentions and
                  provisions of this Warrant certificate.

         (d)      Subject to the express provisions hereof, the Corporation will
                  carry on and conduct and will cause to be carried on and
                  conducted its business in a proper and efficient manner and
                  will cause to be kept proper books of account in accordance
                  with generally accepted accounting practice; and, subject to
                  the express provisions hereof, it will do or cause to be done,
                  all things necessary to preserve and keep in full force and
                  effect its corporate existence, provided, however, that
<PAGE>
                                     - vii -

                  nothing herein contained shall prevent the amalgamation,
                  consolidation, merger, sale, winding up or liquidation of the
                  Corporation or any subsidiary of the Corporation or the
                  abandonment of any rights and franchises of the Corporation or
                  any subsidiary of the Corporation if, in the opinion of the
                  board of directors of the Corporation or officers of the
                  Corporation, it would be advisable and in the best interests
                  of the Corporation or of such subsidiary of the Corporation to
                  do so.

         (e)      The Corporation shall take all such steps and actions and do
                  all such things as may reasonably be necessary to maintain the
                  listing and posting for trading on The Toronto Stock Exchange
                  of those Shares currently listed on The Toronto Stock
                  Exchange.

         (f)      The Corporation will use its reasonable best efforts to ensure
                  that the Shares issuable upon the exercise of the Warrant will
                  be listed and posted for trading on The Toronto Stock Exchange
                  upon their issue or such other stock exchange or automated
                  quotation system on which the Shares may, from time to time,
                  be listed, posted or quoted for trading.

5.       NOTICE

         (a)      Any notice to the Corporation under the provisions hereof
                  shall be valid and effective if delivered by hand or private
                  courier to the Corporation, to the attention of the President
                  at the Corporation's Address and any notice so delivered shall
                  be deemed to be validly given when delivered. The Corporation
                  may from time to time notify the Holder of a change in address
                  which thereafter, until changed by like notice, shall be the
                  address of the Corporation for all purposes of this Warrant
                  certificate.

         (b)      Unless herein otherwise expressly provided, any notice to be
                  given hereunder to a Holder shall be deemed to be validly
                  given if delivered by hand or private courier, addressed to
                  the Holder at its post office address appearing on the records
                  of the Corporation and shall be deemed to have been given when
                  delivered. In determining under any provision hereof the date
                  when notice of any meeting or other event must be given, the
                  date of giving notice shall be excluded and the date of the
                  meeting or other event shall be included. Accidental error or
                  omission in giving notice to the Holder shall not invalidate
                  any action or proceeding founded thereon.
<PAGE>
                                   EXHIBIT B-1

                             FORM OF FIRST DEBENTURE


<PAGE>

THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE UNITED
STATES SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT"). THE HOLDER
HEREOF, BY PURCHASING SUCH SECURITIES, AGREES FOR THE BENEFIT OF THE COMPANY
THAT SUCH SECURITIES MAY BE OFFERED, SOLD OR OTHERWISE TRANSFERRED ONLY (A) TO
THE COMPANY, (B) OUTSIDE THE UNITED STATES IN ACCORDANCE WITH REGULATIONS UNDER
THE SECURITIES ACT OR (C) INSIDE THE UNITED STATES IN ACCORDANCE WITH RULE 144
UNDER THE SECURITIES ACT, IF AVAILABLE. DELIVERY OF THIS CERTIFICATE MAY NOT
CONSTITUTE "GOOD DELIVERY" IN SETTLEMENT OF TRANSACTIONS ON STOCK EXCHANGES IN
CANADA.

UNLESS PERMITTED UNDER CANADIAN SECURITIES LEGISLATION, THE HOLDER OF THE
SECURITIES SHALL NOT TRADE THE SECURITIES OR THE COMMON SHARES ISSUABLE UPON
CONVERSION THEREOF IN THE PROVINCE OF ONTARIO BEFORE AUGUST 18, 2002.


                           FIRST CONVERTIBLE DEBENTURE

         THIS FIRST CONVERTIBLE DEBENTURE ("Agreement") is made and entered into
as of April 17, 2002 by and between UNITED THERAPEUTICS CORPORATION, a Delaware
corporation ("Lender"), and ALTAREX CORP., an Alberta corporation ("Borrower").

                                    RECITALS

         A. Borrower has requested Lender to make a loan to Borrower in an
amount equal to US$50,000 and, subject to the terms and conditions of this
Agreement, Lender is willing to make such loan to Borrower.

         NOW, THEREFORE, the parties hereby mutually agree as follows:

         1.       THE LOAN.

                  1.1 Amount. Subject to the terms and conditions of this
Agreement, Lender agrees to make a loan (the "Loan") to Borrower in an amount
equal to Fifty Thousand United States Dollars (US$50,000).

                  1.2 Advance. The advance of the Loan shall be in the amount of
US$50,000 and shall be made within one (1) Business Day (as defined below) after
the date on which this Agreement is executed by both parties.

                  1.3 Method Of Disbursement. The Loan shall be made by wire
transfer of funds to such account of Borrower as may be specified by Borrower to
Lender in writing.

                  1.4 Maturity. Subject to Section 5, the outstanding principal
amount of the Loan, and all accrued and unpaid interest thereon, shall be due
and payable on April 17, 2005 (the "Maturity Date").



<PAGE>

                                      -2-


                  1.5 Interest. Borrower shall pay interest on the outstanding
principal amount of the Loan at a rate of six percent (6%) simple interest per
annum (the "Applicable Rate").

                  1.6 Interest Computation and Payment. All computations of
interest hereunder shall be made by Lender based on a 360-day year and the
actual number of days elapsed and shall be payable to the Lender in U.S. Dollars
on the first day of April, July, October and January with the first payment date
being July 1, 2002.

                  1.7 Voluntary Prepayment. Borrower may at its option
voluntarily prepay the Loan in whole or in part, without premium or penalty,
upon not less than five (5) days prior written notice of any such prepayment.
Any such notice shall be irrevocable. Borrower shall, concurrently with such
prepayment, pay all accrued but unpaid interest to the date of such prepayment
on the amount prepaid.

                  1.8 Method of Payment. Borrower shall pay all amounts payable
to Lender under this Agreement in U.S. Dollars, in immediately available funds,
not later than 1:00 p.m. eastern standard time on the day on which such payment
is to be made, to such account as Lender may by notice specify to Borrower, or
by such other means as may be acceptable to Lender. All payments made to Lender
under this Agreement shall be applied in such order as Lender may determine.

                  1.9 Business Day. "Business Day" means a day other than a
Saturday, Sunday or legal holiday on which banks are authorized to close in New
York, New York. If the date on which a payment hereunder is due is a day other
than a Business Day, then such payment shall be made on the immediately
preceding Business Day.

         2.       CONVERSION.

                  2.1 The Loan will be automatically converted into 100,000
common shares of the Borrower (the "Common Shares"), being a conversion price of
$0.50 per Common Share (the "Conversion Price") on August 21, 2002. A share
certificate representing the 100,000 Common Shares will be delivered by the
Borrower to the Lender within five Business Days after the issuance of the
100,000 Common Shares.

                  2.2 Exhibit "I" forms part of this Agreement and the Purchaser
shall be entitled to the additional rights set forth therein.

         3.       SECURITY DOCUMENTS.

                  Concurrently with the execution of this Agreement, Borrower
shall execute and deliver to Lender a security agreement in form and substance
satisfactory to Lender (the "Security Agreement").

         4.       REPRESENTATIONS AND WARRANTIES.


<PAGE>

                                      -3-


                  4.1 The representations and warranties given by the Borrower
to the Lender under the Subscription and Debenture Purchase Agreement and the
License Agreement of even date (the "Borrower Representations and Warranties")
are hereby incorporated by reference and the Borrower hereby represents to the
Lender such Borrower Representations and Warranties and acknowledges that the
Lender is relying on such Representations and Warranties in connection with its
advances hereunder.

                  4.2 The representations and warranties given by the Lender to
the Borrower under the Subscription and Debenture Purchase Agreement of even
date (the "Lender Representations and Warranties") are hereby incorporated by
reference and the Lender hereby represents to the Borrower that such Lender
Representations and Warranties are true and that such Representation and
Warranties shall remain true at the time of the issuance of the 100,000 Common
Shares and acknowledges that the Borrower will rely on such Lender
Representations and Warranties in connection with the issuance of such 100,000
Common Shares.

         5.       COVENANTS OF BORROWER.

                  5.1 Use of Loan Proceeds. Borrower shall use the proceeds of
the Loan for research and development expenses, general and administrative
expenses and working capital and other requirements of the Borrower.

                  5.2 Ordinary Course of Business. Borrower shall continue to
operate its business in the ordinary course of business.

                  5.3 Affirmative Covenants

                  So long as any amount owing under this Agreement remains
unpaid and unless written consent is given by the Lender, the Borrower shall:

(a) Deliver to the Lender (i) as soon as practicable and in any event within 60
days after the end of each of the first three financial quarters in each
financial year a consolidated balance sheet of the Borrower as of the end of the
financial quarter, setting forth in comparative form the figures for the
corresponding financial quarter and corresponding portion of the previous
financial year, (ii) as soon as practicable and in any event within 120 days
after the end of each financial year, a copy of the financial statements of the
Borrower for the financial year prepared on consolidated basis reported on by
the Borrower's independent auditors, and (iii) together with each delivery of
financial statements, a compliance certificate in the form attached as Schedule
A hereto;

(b) Pay or cause to be paid when due, (i) all taxes, assessments and
governmental charges or levies imposed upon it or upon its income, sales,
capital or profit or any other property belonging to it, and (ii) all claims
which, if unpaid, might by law become a lien upon the assets, except any such
tax, assessment, charge, levy or claim which is being contested in good faith
and by proper proceedings and in respect of which the Borrower have established
adequate reserves in accordance with Canadian Generally Accepted Accounting
Principles or which are Permitted Liens;

"Permitted Liens" means with respect to any person the following:




<PAGE>

                                      -4-



         (a) liens for taxes, assessments or governmental charges or levies not
at the time due or delinquent;

         (b) undetermined or inchoate liens and charges incidental to current
operations which have not been filed pursuant to law against the Borrower or
which relate to obligations not due or delinquent; and

         (c) title defects or irregularities of a minor nature and which neither
individually nor in the aggregate will materially impair the use of the property
for the purposes for which it is held in the business or the value of such
property.

(c) Deliver to the Lender (i) promptly upon their issuance, copies of all
publicly available notices, reports, press releases, circulars, offering
documents and other publicly available documents filed with, or delivered to,
any stock exchange or the Ontario Securities Commission or a similar
governmental entity in any other jurisdiction, and (ii) such other financial
information respecting the condition or operations, financial or otherwise, of
the Borrower as the Lender may from time to time reasonably request which
information is generally prepared by the Borrower from time to time in the
ordinary course of business;

(d) Comply with the requirements of all applicable laws, judgments, orders,
decisions and awards, non-compliance with which would reasonably be expected to
have a Material Adverse Effect;

"Material Adverse Effect" means a material adverse effect on the business,
operations, results of operations, assets, liabilities or financial condition of
the Borrower and the subsidiaries of the Borrower (the "subsidiaries") taken as
a whole.

(e) At its cost and expense, upon request of the Lender, execute and deliver or
cause to be executed and delivered to the Lender such further instruments and do
and cause to be done such further acts as may be necessary or proper in the
reasonable opinion of the Lender to carry out more effectually the provisions
and purposes of this Agreement.

                  5.4      Negative Covenants

                  So long as any amount owing under this Agreement remains
unpaid and, unless written consent is given by the Lender, the Borrower shall
not:

(a) Create, incur, assume, suffer to exist, or permit any of its subsidiaries to
create, incur, assume, or suffer to exist, any lien on any of their respective
properties or assets, other than a Permitted Lien;

(b) Consumate, or permit any of its subsidiaries (the "Subsidiaries") to
consumate any reorganization, consolidation, amalgamation, arrangement,
winding-up, merger or other similar transaction with any third party, without
providing to the Lender notice of such transaction, as soon as commercially
reasonable;


<PAGE>

                                      -5-


(c) Sell, exchange, lease, release or abandon or otherwise dispose of, or permit
any Subsidiary to sell, exchange, lease, release or abandon or otherwise dispose
of, any of the assets or properties of the Borrower as described in the Security
Agreement to any person without providing to the Lender notice of such sale or
disposition as soon as commercially reasonable other than (i) bona fide sales,
exchanges, leases, abandonments or other dispositions in the ordinary course of
business for the purpose of carrying on the Business or its business, as the
case may be, and at fair market value, (ii) property or assets (other than
shares) which have no material economic value in the business or business or are
obsolete, and (iii) dispositions pursuant to a transaction permitted in 1(c).
Each of the Borrower and Lender acknowledge and agree that for the purposes of
this clause (b) the bona fide licensing by the Borrower of its technology shall
be deemed to be in the ordinary course of its business;

(d) Transactions with Related Parties. Except as otherwise permitted directly or
indirectly, consummate or allow any Subsidiary to consummate any agreement with,
make any financial accommodation for, or otherwise enter into any transaction
with, a non-arm's length third party except in the ordinary course of, and
pursuant to the reasonable requirements of, business or at prices and on terms
not less favourable to the Borrower or the Subsidiary, as the case may be, than
could be obtained in a comparable arm's length transaction with another person;

(e) Distributions. Declare, make or pay or permit any of its Subsidiaries to
declare, make or pay any distributions,

For purposes of this Section 5.4, "Distribution" means with respect to any
person the amount of (i) any dividend or other distribution on issued shares of
the person or any of its subsidiaries, (ii) the purchase, redemption or
retirement amount of any issued shares, warrants or any other options or rights
to acquire shares of the person or any of its subsidiaries redeemed or purchased
by the person or any its subsidiaries, or (iii) any payments whether as
consulting fees, management fees or otherwise to any related party of the person
or any of its subsidiaries. Notwithstanding the foregoing, the Borrower may make
Distributions between and among the Borrower and any wholly-owned subsidiaries
or affiliates;

                  5.5 Security Covenants. So long as any amount owing under this
Agreement remains unpaid, and unless written consent is given by the Lender, the
Borrower shall:

(a) Promptly cure or cause to be cured any defects in the execution and delivery
of this Agreement or any defects in the validity or enforceability of any of the
Security and at its expense, execute and deliver or cause to be executed and
delivered, all such agreements, instruments and other documents (including the
filing of any financing statements or financing change statements) as the Lender
reasonably may consider necessary or desirable to protect or otherwise perfect
the security interest granted under the Security Agreement.


<PAGE>

                                      -6-


         6.       EVENTS OF DEFAULT AND REMEDIES.

                  6.1 Events of Default. The occurrence of any of the following
conditions or events shall each constitute an event of default ("Event of
Default"):

                  (a) failure to repay the Loan or any interest thereon when due
(whether at maturity, by acceleration, or otherwise), and such failure is not
cured within five (5) Business Days after Lender gives to Borrower written
notice of such failure;

                  (b) failure of Borrower to perform or comply with any term,
provision or covenant contained in this Agreement or the Security Agreement or
any other agreement or instrument granting security to or related to security
granted to the Lender (other than with respect to repayment of the Loan or any
interest thereon), and such failure is not cured within thirty (30) days after
Lender gives to Borrower written notice of such failure;

                  (c) the commencement by or against Borrower of a proceeding
under any bankruptcy or similar law of any jurisdiction relating to insolvency
or the relief of debtors, as now or hereafter in effect and the failure by the
Borrower to obtain a dismissal or stay of such proceeding within sixty (60) days
from the date of commencement of such proceeding; the making by Borrower of a
general assignment for the benefit of any of its creditors; the appointment of a
receiver, trustee, custodian or similar officer for Borrower and the failure by
Borrower to secure the discharge of such receiver, trustee, custodian or similar
officer within sixty (60) days from the date of appointment; or the admission in
writing by Borrower of any inability to pay its debts generally as they become
due; or

                  (d) any material breach of a representation, warranty or
certification made by the Borrower in this Agreement, or any document
contemplated by this Agreement, which would reasonably be expected to have a
Material Adverse Effect.

                  6.2 Remedies. Upon the occurrence of an Event of Default,
Lender may, in addition to exercising any other rights or remedies available to
Lender at law, in equity, under this Agreement, the Security Agreement, or
otherwise, declare all principal, interest and other amounts payable under this
Agreement to be immediately due and payable, and, upon reasonable prior notice
to Borrower, offset any amounts owed by Lender to Borrower against the
outstanding amount of the Loan.

                  6.3 Remedies Cumulative. All of Lender's rights, remedies,
powers and privileges are separate and cumulative, and no one of them, whether
exercised or not, shall be or be deemed to be to the exclusion of or to limit or
prejudice any other rights, remedies, powers or privileges Lender may have.


<PAGE>

                                      -7-


         7.       MISCELLANEOUS.

                  7.1 Waiver. No failure by Lender to exercise or delay by
Lender in exercising any right, remedy, power or privilege shall operate as a
waiver thereof nor shall any single or partial exercise of any such right,
remedy, power or privilege preclude any further exercise thereof or of any other
right, remedy, power or privilege.

                  7.2 Governing Law. This Agreement is governed by and shall be
construed in accordance with the laws (other than that body of law relating to
conflicts of law) of the Province of Ontario.

                  7.3 Notices. Any notice, request, demand, statement,
authorization, approval or consent required or permitted under this Agreement
shall be in writing and shall be made by any of the following means, and shall
be deemed effective upon actual receipt: (a) deposit in the mail, postage
prepaid, registered or certified, return receipt requested, such mailing to be
effective upon actual receipt, (b) personal delivery, (c) delivery by a courier
of recognized reputation (such as FedEx) or (d) transmission by telecopier (with
confirmation by mail) as follows, or to such other address and/or such
additional parties as the parties hereto may specify by notice given in
accordance with this Section 7.3:

                  Borrower:            AltaRex Corp.
                                       610 Lincoln Street
                                       Waltham, Massachusetts 02451 USA

                                       Attention:    Edward Fitzgerald
                                       Telephone:    (781) 693-1504
                                       Telecopier:   (781) 466-8740

                  Lender:              United Therapeutics Corporation
                                       1735 Connecticut Avenue, N.W.
                                       Washington, D.C. 20009

                                       Attention: Paul A. Mahon, General Counsel
                                       Telephone: (202) 483-7000
                                       Telecopier:(202) 483-4006

                  7.4 No Third Party Beneficiary. This Agreement creates rights
and duties only among the parties hereto, and no third party shall have any
rights hereunder or in or to the Loan.

                  7.5 Assignment. Borrower shall not assign this Agreement or
any of its rights hereunder without the prior written consent of Lender.

                  7.6 Entire Agreement. This Agreement and the Security
Agreement dated as of the date hereof between the Borrower and the Lender
constitutes the entire agreement of the parties with respect to the subject
matter hereof and shall supersede any prior expressions of



<PAGE>

                                      -8-


intent or understanding with respect to this transaction. This Agreement may
only be amended by a written instrument signed by the parties hereto.

                  7.7 Counterparts. This Agreement may be executed in
counterparts, each of which shall constitute an original, but all of which
together shall constitute a single instrument.

                  IN WITNESS WHEREOF, the parties hereto have duly executed this
Agreement as of the date first above written.


                               UNITED THERAPEUTICS CORPORATION

                               By:      ____________________________________

                               Its:     ____________________________________



                               ALTAREX CORP.

                               By:      ____________________________________

                               Its:     ____________________________________



<PAGE>




                                    Exhibit I

1.       DEFINITIONS

         In this Exhibit I:

         (a)      "CURRENT MARKET PRICE" per Share or Participating Share at any
                  date shall be the weighted average price per share for such
                  shares for any 20 consecutive trading days (such 20
                  consecutive trading days being selected by the Corporation)
                  commencing not more than 25 trading days before such date on
                  such stock exchange on which such shares are listed as may be
                  selected for such purpose by the directors of the Corporation
                  or if such shares are not listed on any stock exchange, then
                  on an automated quotation system, such as NASDAQ or the over
                  the counter market. The weighted average price shall be
                  determined by dividing the aggregate sale price of all such
                  shares so sold on the exchange, system or market, as the case
                  may be, during the said 20 consecutive trading days by the
                  total number of such shares so sold. If such shares are not
                  listed on any stock exchange, automated quotation system or
                  traded on an over the counter market, the Current Market Price
                  shall be determined in good faith by the Board of Directors of
                  the Corporation.

         (b)      "DIVIDENDS PAID IN THE ORDINARY COURSE" means cash dividends
                  declared payable on the Shares in any fiscal year of the
                  Corporation to the extent that such cash dividends do not
                  exceed, in the aggregate, the greater of: (i) 125% of the
                  aggregate amount or value of dividends declared payable by the
                  Corporation on the Shares in its immediately preceding fiscal
                  year; and (ii) 50% of the aggregate net earnings of the
                  Corporation, before extraordinary items, for its immediately
                  preceding fiscal year (less the amount or value of all
                  dividends paid or payable in respect of such fiscal year) as
                  shown in the audited consolidated financial statements of the
                  Corporation for such preceding fiscal year or, if there are no
                  audited financial statements with respect to such period,
                  computed in accordance with generally accepted accounting
                  principles consistent with the applications made in
                  preparation of the most recent audited consolidated financial
                  statements of the Corporation, and for such purpose the
                  amounts of any dividend paid in shares shall be the aggregate
                  stated value of such shares and the amount of any dividend
                  paid in other than cash or shares shall be the fair market
                  value of such dividend as declared by resolution passed by the
                  board of directors of the Corporation.

         (c)      "PARTICIPATING SHARE" means a share that carries the right to
                  participate in earnings or in capital on a liquidation or
                  winding-up to an unlimited degree, or which ranks, in terms of
                  priority, equally with the Shares with respect to
                  participation in earnings or in capital on a liquidation or
                  winding-up.



<PAGE>

                                      -2-



2.       ADJUSTMENT OF SUBSCRIPTION RIGHTS

         The Conversion Price in effect at any date shall be subject to
adjustment from time to time as follows:

         (a)      If and whenever at any time following April 17, 2002 and prior
                  to 5:00 p.m. (Toronto time) on the Maturity Date (the "Expiry
                  Time"), the Corporation shall (i) subdivide or redivide the
                  outstanding Shares into a greater number of shares, (ii)
                  reduce, combine or consolidate the outstanding Shares into a
                  smaller number of shares, or (iii) issue Shares or other
                  Participating Shares to the holders of all or substantially
                  all of the outstanding Shares by way of a stock dividend, the
                  Conversion Price in effect on the effective date of any such
                  event shall be adjusted immediately after such event or on the
                  record date for such issue of Shares or other Participating
                  Shares by way of stock dividend, as the case may be, so that
                  it shall equal the amount determined by multiplying the
                  Conversion Price in effect immediately prior to such event by
                  a fraction, of which the numerator shall be the total number
                  of Shares and other Participating Shares outstanding
                  immediately prior to such event; such adjustment shall be made
                  successively whenever any event referred to in this subsection
                  (a) shall occur; any such issue of Shares or other
                  Participating Shares by way of a stock dividend shall be
                  deemed to have been made on the record date for the stock
                  dividend for the purpose of calculating the number of
                  outstanding Shares or other Participating Shares immediately
                  after such event under this subsection (a) and subsection (e)
                  of this Section.

         (b)      If and whenever at any time following April 17, 2002 and prior
                  to the Expiry Time the Corporation shall fix a record date for
                  the issuance of rights, options or warrants to all or
                  substantially all of the holders of the outstanding Shares,
                  entitling them, for a period expiring not more than 45 days
                  after such record date, to subscribe for or purchase Shares or
                  other Participating Shares (or securities convertible into or
                  exchangeable for Shares or other Participating Shares) at a
                  price per share (or having a conversion or exchange price per
                  share) less than 95% of the Current Market Price on such
                  record date, the Conversion Price shall be adjusted
                  immediately after such record date so that it shall equal the
                  price determined by multiplying the Conversion Price in effect
                  on such record date by a fraction, of which the numerator
                  shall be the total number of Shares outstanding on such record
                  date plus the number arrived at by dividing the aggregate
                  price of the total number of additional Shares or other
                  Participating Shares offered for subscription or purchase (or
                  the aggregate conversion or exchange price of the convertible
                  or exchangeable securities so offered) by such Current Market
                  Price, and of which the denominator shall be the total number
                  of Shares outstanding on such record date plus the total
                  number of additional Shares or other Participating Shares
                  offered for subscription or purchase (or into which the
                  convertible or exchangeable securities so offered are
                  convertible or exchangeable); any Shares owned by or held for
                  the account of the Corporation or any subsidiary of the



<PAGE>

                                      -3-


                  Corporation shall be deemed not to be outstanding for the
                  purpose of any such computation; such adjustment shall be made
                  successively whenever such a record date is fixed; to the
                  extent that any such rights, options or warrants are not so
                  issued or any such rights, options or warrants are, not
                  exercised prior to the expiration thereof, the Conversion
                  Price shall then be re-adjusted to the Conversion Price which
                  would then be in effect based upon the number and aggregate
                  price of Shares or other Participating Shares (or securities
                  convertible into or exchangeable for Shares or other
                  Participating Shares) actually issued upon the exercise of
                  such rights, options or warrants, as the case may be.

         (c)      If and whenever at any time following April 17, 2002 and prior
                  to the Expiry Time the Corporation shall fix a record date for
                  the making of a distribution to all or substantially all the
                  holders of its outstanding Shares of (i) shares of any class
                  other than Shares or Participating Shares, other than shares
                  distributed to holders of Shares pursuant to their exercise of
                  options to receive dividends in the form of such shares in
                  lieu of Dividends Paid in the Ordinary Course on the Shares
                  and other than the issue of Shares or other Participating
                  Shares to the holders of all or substantially all of the
                  outstanding Shares by way of a stock dividend, or (ii) rights,
                  options or warrants (excluding rights exercisable for 45 days
                  or less) or (iii) evidences of its indebtedness, or (iv)
                  assets (excluding Dividends Paid in the Ordinary Course),
                  including shares of other corporations, then, in each such
                  case, the Conversion Price shall be adjusted immediately after
                  such record date so that it shall equal the price determined
                  by multiplying the Conversion Price in effect on such record
                  date by a fraction, of which the numerator shall be the total
                  number of Shares outstanding on such record date multiplied by
                  the Current Market Price per Share on such record date, less
                  the fair market value (as determined by the board of directors
                  of the Corporation, acting reasonably, which determination,
                  absent manifest error, shall be conclusive) of such shares or
                  rights, options or warrants or evidences or indebtedness or
                  assets so distributed, and of which the denominator shall be
                  the total number of Shares outstanding on such record date
                  multiplied by such Current Market Price per Share; any Shares
                  owned by or held for the account of the Corporation shall be
                  deemed not to be outstanding for the purpose of any such
                  computation; such adjustment shall be made successively
                  whenever such a record date is fixed; to the extent that such
                  distribution is not so made, the Conversion Price shall be
                  re-adjusted to the Conversion Price which would then be in
                  effect if such record date had not been fixed or to the
                  Conversion Price which would then be in effect based upon such
                  shares or rights, options or warrants or evidences of
                  indebtedness or assets actually distributed, as the case may
                  be, and in clause (iv) the term "Dividends Paid in the
                  Ordinary Course" shall include the value of any securities or
                  other property or assets distributed in lieu of cash Dividends
                  Paid in the Ordinary Course.

         (d)      If and whenever at any time following April 17, 2002 and prior
                  to the Expiry Time there is a reclassification of the Shares
                  at any time outstanding or a change



<PAGE>

                                      -4-


                  of the Shares into other shares or a capital reorganization of
                  the Corporation not covered in subsection (a) or a
                  consolidation, amalgamation or merger of the Corporation with
                  or into any other corporation or a sale of the property and
                  assets of the Corporation as or substantially as an entirety
                  to any other person, the Purchaser, to the extent that the
                  Loan has not been converted, prior to the effective date of
                  such reclassification, capital reorganization, consolidation,
                  amalgamation, merger or sale shall thereafter, upon conversion
                  of the Loan, be entitled to receive and shall accept in lieu
                  of the number of Shares, as then constituted, to which the
                  Purchaser was previously entitled upon conversion of the Loan,
                  the number of shares or other securities or property of the
                  Corporation or of the corporation resulting from such
                  reclassification, consolidation, amalgamation or merger or of
                  the person to which such sale may be made, as the case may be,
                  that the Purchaser would have been entitled to receive on such
                  reclassification, capital reorganization, consolidation,
                  amalgamation, merger or sale if, on the effective date
                  thereof, the Purchaser had been the registered holder of the
                  number of Shares to which the Purchaser was previously
                  entitled upon due conversion of the Loan; and in any case, if
                  necessary, appropriate adjustment shall be made in the
                  application of the provisions set forth in this Agreement with
                  respect to the rights and interests thereafter of the
                  Purchaser to the end that the provisions set forth in this
                  Agreement shall thereafter correspondingly be made applicable,
                  as nearly as may reasonably be, in relation to any shares or
                  securities or property to which the Purchaser may be entitled
                  upon the conversion of the Loan thereafter.

         (e)      In any case in which this Agreement shall require that an
                  adjustment shall become effective immediately after a record
                  date for an event referred to herein, the Corporation may
                  defer, until the occurrence of such event, issuing to the
                  Purchaser if converted after such record date and before the
                  occurrence of such event the kind and amount of shares, other
                  securities or property to which it would be entitled upon such
                  exercise by reason of the adjustment required by such event;
                  provided, however, that the Corporation shall deliver to the
                  Purchaser an appropriate instrument evidencing the Purchaser's
                  right to receive the kind and amount of shares, other
                  securities or property to which it would be entitled upon the
                  occurrence of the event requiring such adjustment and the
                  right to receive any distributions made or declared in favour
                  of holders of record of Shares as constituted from time to
                  time on and after such date as the Purchaser would, but for
                  the provisions of this subsection (e), have received, or
                  become entitled to receive, on such exercise.

         (f)      The adjustments provided for in this Agreement are cumulative
                  and shall apply to successive subdivisions, redivisions,
                  reductions, combinations, consolidations, distributions,
                  issues or other events resulting in any adjustment under the
                  provisions of this Agreement provided that, notwithstanding
                  any other provision of this Section, no adjustment of the
                  Conversion Price, as then constituted, purchasable shall be
                  required unless such adjustment would require an increase or
                  decrease, of at least 1% in the Conversion Price, then in
                  effect; provided however,



<PAGE>

                                      -5-


                  that any adjustments which by reason of this subsection (f)
                  are not required to be made shall be carried forward and taken
                  into account in any subsequent adjustment.

         (g)      In the event of any question arising with respect to the
                  adjustments provided in this Agreement, such question shall,
                  absent manifest error, be conclusively determined by a firm of
                  chartered accountants appointed by the Corporation and
                  acceptable to the Purchaser (who may be the auditors of the
                  Corporation) with the assistance of legal counsel, who may be
                  legal counsel to the Corporation; such accountants shall have
                  access to all necessary records of the Corporation and such
                  determination shall be binding upon the Corporation and the
                  Purchaser.

         (h)      As a condition precedent to the taking of any action which
                  would require an adjustment in any of the Conversion Price
                  pursuant to the terms of this Agreement, the Corporation shall
                  take any action which may, in the opinion of legal counsel, be
                  necessary in order that the Corporation may validly and
                  legally issue as fully paid and non-assessable all the Shares
                  which the Purchaser is entitled to receive on the full
                  exercise thereof in accordance with the provisions hereof.

         (i)      In case the Corporation shall take any action affecting the
                  Shares other than action described in this Agreement, which in
                  the opinion of the board of directors of the Corporation would
                  materially affect the rights of Purchaser, the Conversion
                  Price shall be adjusted in such manner and at such time, by
                  action of the board of directors of the Corporation, in its
                  sole reasonable discretion as it may determine to be equitable
                  in the circumstances, provided that no such adjustment shall
                  be made unless prior approval of any stock exchange on which
                  the Shares are listed for trading has been obtained. Failure
                  of the board of directors of the Corporation to make such an
                  adjustment shall be conclusive evidence that the board of
                  directors of the Corporation have determined that it is
                  equitable to make no adjustment in the circumstances.

3.       NOTICE OF ADJUSTMENT

         At least 21 days prior to the effective date or record date, as the
case may be, of any event referred to in clause 2, the Corporation shall notify
the Purchaser of the particulars of such event and the estimated amount of any
adjustment required as a result thereof. Promptly after the occurrence of any
event which requires an adjustment in the Conversion Price pursuant to this
Agreement, the Corporation shall forthwith deliver to the Purchaser a
certificate of the Corporation specifying the particulars of such event and the
required adjustment and the computation of such adjustment and give notice to
the Purchaser of the particulars of such event and the required adjustment in
the manner provided in Section 7.3 hereof.

4.       GENERAL COVENANTS OF THE CORPORATION



<PAGE>

                                      -6-


         (a)      The Corporation covenants and agrees that it is duly
                  authorized to enter into and perform its obligations under its
                  Agreement.

         (b)      The Corporation will cause the Shares issuable upon conversion
                  of the Loan and the certificates representing the Shares to be
                  duly issued. At all times until the Expiry Time, the
                  Corporation shall reserve and there shall remain unissued out
                  of its authorized capital a number of Shares sufficient to
                  satisfy the conversion of the Loan. All Shares issued upon the
                  due conversion of the Loan shall be fully paid and
                  non-assessable.

         (c)      The Corporation will do, execute, acknowledge and deliver or
                  cause to be done, executed, acknowledged and delivered, all
                  other acts, deeds and assurances in law as may be reasonably
                  required for the better accomplishing and effecting of the
                  intentions and provisions of Section 2 of this Agreement.

         (d)      Subject to the express provisions hereof, the Corporation will
                  carry on and conduct and will cause to be carried on and
                  conducted its business in a proper and efficient manner and
                  will cause to be kept proper books of account in accordance
                  with generally accepted accounting practice; and, subject to
                  the express provisions hereof, it will do or cause to be done,
                  all things necessary to preserve and keep in full force and
                  effect its corporate existence, provided, however, that
                  nothing herein contained shall prevent the amalgamation,
                  consolidation, merger, sale, winding up or liquidation of the
                  Corporation or any subsidiary of the Corporation or the
                  abandonment of any rights and franchises of the Corporation or
                  any subsidiary of the Corporation if, in the opinion of the
                  board of directors of the Corporation or officers of the
                  Corporation, it would be advisable and in the best interests
                  of the Corporation or of such subsidiary of the Corporation to
                  do so.

         (e)      The Corporation shall take all such steps and actions and do
                  all such things as may reasonably be necessary to maintain the
                  listing and posting for trading on The Toronto Stock Exchange
                  of those Shares currently listed on The Toronto Stock
                  Exchange.

         (f)      The Corporation will use its reasonable best efforts to ensure
                  that the Shares issuable upon conversion of the Loan will be
                  listed and posted for trading on The Toronto Stock Exchange
                  upon their issue or such other stock exchange or automated
                  quotation system on which the Shares may, from time to time,
                  be listed, posted or quoted for trading.


<PAGE>







                                   EXHIBIT B-2

                            FORM OF SECOND DEBENTURE



<PAGE>





THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE UNITED
STATES SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT"). THE HOLDER
HEREOF, BY PURCHASING SUCH SECURITIES, AGREES FOR THE BENEFIT OF THE COMPANY
THAT SUCH SECURITIES MAY BE OFFERED, SOLD OR OTHERWISE TRANSFERRED ONLY (A) TO
THE COMPANY, (B) OUTSIDE THE UNITED STATES IN ACCORDANCE WITH REGULATIONS UNDER
THE SECURITIES ACT OR (C) INSIDE THE UNITED STATES IN ACCORDANCE WITH RULE 144
UNDER THE SECURITIES ACT, IF AVAILABLE. DELIVERY OF THIS CERTIFICATE MAY NOT
CONSTITUTE "GOOD DELIVERY" IN SETTLEMENT OF TRANSACTIONS ON STOCK EXCHANGES IN
CANADA.

UNLESS PERMITTED UNDER CANADIAN SECURITIES LEGISLATION, THE HOLDER OF THE
SECURITIES SHALL NOT TRADE THE SECURITIES OR THE COMMON SHARES ISSUABLE UPON
CONVERSION THEREOF IN THE PROVINCE OF ONTARIO BEFORE [INSERT DATE THAT IS FOUR
MONTHS AND ONE DAY THE DATE OF THE AGREEMENT.]

                          SECOND CONVERTIBLE DEBENTURE

         THIS SECOND CONVERTIBLE DEBENTURE ("Agreement") is made and entered
into as of August   , 2002 by and between UNITED THERAPEUTICS CORPORATION, a
Delaware corporation ("Lender"), and ALTAREX CORP., an Alberta corporation
("Borrower").

                                    RECITALS

         A. Borrower has requested Lender to make a loan to Borrower in an
amount equal to US$875,000 and, subject to the terms and conditions of this
Agreement, Lender is willing to make such loan to Borrower.

         NOW, THEREFORE, the parties hereby mutually agree as follows:

         1.       THE LOAN.

                  1.1 Amount. Subject to the terms and conditions of this
Agreement, Lender agrees to make a loan (the "Loan") to Borrower in an amount
equal to Eight Hundred and Seventy-Five Thousand United States Dollars
(US$875,000).

                  1.2 Advance. The advance of the Loan shall be in the amount of
US$875,000 and shall be made within one (1) Business Day (as defined below)
after the date on which this Agreement is executed by both parties.

                  1.3 Method Of Disbursement. The Loan shall be made by wire
transfer of funds to such account of Borrower as may be specified by Borrower to
Lender in writing.

                  1.4 Maturity. Subject to Section 5, the outstanding principal
amount of the Loan, and all accrued and unpaid interest thereon, shall be due
and payable on [insert date that is 3 years from date of issue of convertible
debenture] (the "Maturity Date").



<PAGE>

                                      -2-


                  1.5 Interest. Borrower shall pay interest on the outstanding
principal amount of the Loan at a rate of six percent (6%) simple interest per
annum (the "Applicable Rate").

                  1.6 Interest Computation and Payment. All computations of
interest hereunder shall be made by Lender based on a 360-day year and the
actual number of days elapsed and shall be payable to the Lender in U.S. Dollars
on the first day of April, July, October and January with the first payment date
being October 1, 2002.

                  1.7 Voluntary Prepayment. Borrower may at its option
voluntarily prepay the Loan in whole or in part, without premium or penalty,
upon not less than five (5) days prior written notice of any such prepayment.
Any such notice shall be irrevocable. Borrower shall, concurrently with such
prepayment, pay all accrued but unpaid interest to the date of such prepayment
on the amount prepaid.

                  1.8 Method of Payment. Borrower shall pay all amounts payable
to Lender under this Agreement in U.S. Dollars, in immediately available funds,
not later than 1:00 p.m. eastern standard time on the day on which such payment
is to be made, to such account as Lender may by notice specify to Borrower, or
by such other means as may be acceptable to Lender. All payments made to Lender
under this Agreement shall be applied in such order as Lender may determine.

                  1.9 Business Day. "Business Day" means a day other than a
Saturday, Sunday or legal holiday on which banks are authorized to close in New
York, New York. If the date on which a payment hereunder is due is a day other
than a Business Day, then such payment shall be made on the immediately
preceding Business Day.

         2.       CONVERSION.

                  2.1 Upon the approval by the shareholders of the Borrower of
the issuance of 883,380 common shares of the Borrower (the "Common Shares") to
the Lender, $441,690 of the principal amount of the Loan will be automatically
converted into 883,380 Common Shares, being a conversion price of $0.50 per
Common Share (the "Conversion Price") on the later of (i) the date that is three
Business Days after such shareholder approval, and (ii) August 20, 2002. For
greater certainty, if such shareholder approval has been obtained prior to
August 20, 2002, such automatic conversion shall occur on August 20, 2002. The
$441,690 will be cancelled from the amount outstanding under this Agreement at
that time. A share certificate representing the 883,380 Common Shares will be
delivered by the Borrower to the Lender within five Business Days after the
issuance of the 883,380 Common Shares.

                  2.2 If at any time, while an amount (whether principal or
interest) remains outstanding under this Agreement, the Lender exercises its
pre-emptive rights to purchase securities of the Borrower (the "Lender's
Pre-emptive Rights") under Section F of the Subscription and Debenture Purchase
Agreement dated as of the date hereof between the Borrower and the Lender, the
Lender shall pay the purchase price for the Borrower's securities in connection
with the exercise of the Lenders Preemptive Rights first by cancelling the
applicable



<PAGE>

                                      -3-


amount outstanding under this Agreement, and then by paying such additional
amount as may be determined.

                  2.3 Exhibit "I" forms part of this Agreement and the Purchaser
shall be entitled to the additional rights set forth therein.

         3.       SECURITY DOCUMENTS.

                  Concurrently with the execution of this Agreement, Borrower
shall execute and deliver to Lender a security agreement in form and substance
satisfactory to Lender (the "Security Agreement").

         4.       REPRESENTATIONS AND WARRANTIES.

                  4.1 The representations and warranties given by the Borrower
to the Lender under the Subscription and Debenture Purchase Agreement and the
License Agreement of even date (the "Borrower Representations and Warranties")
are hereby incorporated by reference and the Borrower hereby represents to the
Lender such Borrower Representations and Warranties and acknowledges that the
Lender is relying on such Representations and Warranties in connection with its
advances hereunder.

                  4.2 The representations and warranties given by the Lender to
the Borrower under the Subscription and Debenture Purchase Agreement of even
date (the "Lender Representations and Warranties") are hereby incorporated by
reference and the Lender hereby represents to the Borrower that such Lender
Representations and Warranties are true and that such Representation and
Warranties shall remain true at the time of the issuance of the 883,380 Common
Shares and acknowledges that the Borrower will rely on such Lender
Representations and Warranties in connection with the issuance of such 883,380
Common Shares.

         5.       COVENANTS OF BORROWER.

                  5.1 Use of Loan Proceeds. Borrower shall use the proceeds of
the Loan for research and development expenses, general and administrative
expenses and working capital and other requirements of the Borrower.

                  5.2 Ordinary Course of Business. Borrower shall continue to
operate its business in the ordinary course of business.

                  5.3 Affirmative Covenants

                  So long as any amount owing under this Agreement remains
unpaid and unless written consent is given by the Lender, the Borrower shall:

(a) Deliver to the Lender (i) as soon as practicable and in any event within 60
days after the end of each of the first three financial quarters in each
financial year a consolidated balance sheet of the Borrower as of the end of the
financial quarter, setting forth in comparative form the figures for the
corresponding financial quarter and corresponding portion of the previous
financial year,



<PAGE>

                                      -4-


(ii) as soon as practicable and in any event within 120 days after the end of
each financial year, a copy of the financial statements of the Borrower for the
financial year prepared on consolidated basis reported on by the Borrower's
independent auditors, and (iii) together with each delivery of financial
statements, a compliance certificate in the form attached as Schedule A hereto.;

(b) Pay or cause to be paid when due, (i) all taxes, assessments and
governmental charges or levies imposed upon it or upon its income, sales,
capital or profit or any other property belonging to it, and (ii) all claims
which, if unpaid, might by law become a lien upon the assets, except any such
tax, assessment, charge, levy or claim which is being contested in good faith
and by proper proceedings and in respect of which the Borrower have established
adequate reserves in accordance with Canadian Generally Accepted Accounting
Principles or which are Permitted Liens;

"Permitted Liens" means with respect to any person the following:

         (a) liens for taxes, assessments or governmental charges or levies not
at the time due or delinquent;

         (b) undetermined or inchoate liens and charges incidental to current
operations which have not been filed pursuant to law against the Borrower or
which relate to obligations not due or delinquent; and

         (c) title defects or irregularities of a minor nature and which neither
individually nor in the aggregate will materially impair the use of the property
for the purposes for which it is held in the business or the value of such
property.

(c) Deliver to the Lender (i) promptly upon their issuance, copies of all
publicly available notices, reports, press releases, circulars, offering
documents and other publicly available documents filed with, or delivered to,
any stock exchange or the Ontario Securities Commission or a similar
governmental entity in any other jurisdiction, and (ii) such other financial
information respecting the condition or operations, financial or otherwise, of
the Borrower as the Lender may from time to time reasonably request which
information is generally prepared by the Borrower from time to time in the
ordinary course of business;

(d) Comply with the requirements of all applicable laws, judgments, orders,
decisions and awards, non-compliance with which would reasonably be expected to
have a Material Adverse Effect;

"Material Adverse Effect" means a material adverse effect on the business,
operations, results of operations, assets, liabilities or financial condition of
the Borrower and the subsidiaries of the Borrower (the "subsidiaries") taken as
a whole.

(e) At its cost and expense, upon request of the Lender, execute and deliver or
cause to be executed and delivered to the Lender such further instruments and do
and cause to be done such further acts as may be necessary or proper in the
reasonable opinion of the Lender to carry out more effectually the provisions
and purposes of this Agreement.



<PAGE>

                                      -5-


                  5.4 Negative Covenants

                  So long as any amount owing under this Agreement remains
unpaid and, unless written consent is given by the Lender, the Borrower shall
not:

(a) Create, incur, assume, suffer to exist, or permit any of its subsidiaries to
create, incur, assume, or suffer to exist, any lien on any of their respective
properties or assets, other than a Permitted Lien;

(b) Consumate, or permit any of its subsidiaries (the "Subsidiaries") to
consumate any reorganization, consolidation, amalgamation, arrangement,
winding-up, merger or other similar transaction with any third party, without
providing to the Lender notice of such transaction, as soon as commercially
reasonable;

(c) Sell, exchange, lease, release or abandon or otherwise dispose of, or permit
any Subsidiary to sell, exchange, lease, release or abandon or otherwise dispose
of, any of the assets or properties of the Borrower as described in the Security
Agreement to any person without providing to the Lender notice of such sale or
disposition as soon as commercially reasonable other than (i) bona fide sales,
exchanges, leases, abandonments or other dispositions in the ordinary course of
business for the purpose of carrying on the Business or its business, as the
case may be, and at fair market value, (ii) property or assets (other than
shares) which have no material economic value in the business or business or are
obsolete, and (iii) dispositions pursuant to a transaction permitted in 1(c).
Each of the Borrower and Lender acknowledge and agree that for the purposes of
this clause (b) the bona fide licensing by the Borrower of its technology shall
be deemed to be in the ordinary course of its business;

(d) Transactions with Related Parties. Except as otherwise permitted directly or
indirectly, consummate or allow any Subsidiary to consummate any agreement with,
make any financial accommodation for, or otherwise enter into any transaction
with, a non-arm's length third party except in the ordinary course of, and
pursuant to the reasonable requirements of, business or at prices and on terms
not less favourable to the Borrower or the Subsidiary, as the case may be, than
could be obtained in a comparable arm's length transaction with another person;

(e) Distributions. Declare, make or pay or permit any of its Subsidiaries to
declare, make or pay any distributions,

For purposes of this Section 5.4, "Distribution" means with respect to any
person the amount of (i) any dividend or other distribution on issued shares of
the person or any of its subsidiaries, (ii) the purchase, redemption or
retirement amount of any issued shares, warrants or any other options or rights
to acquire shares of the person or any of its subsidiaries redeemed or purchased
by the person or any its subsidiaries, or (iii) any payments whether as
consulting fees, management fees or otherwise to any related party of the person
or any of its subsidiaries. Notwithstanding the foregoing, the Borrower may make
Distributions between and among the Borrower and any wholly-owned subsidiaries
or affiliates;

                  5.5 Security Covenants. So long as any amount owing under this
Agreement remains unpaid, and unless written consent is given by the Lender, the
Borrower shall:

<PAGE>

                                      -6-


(a) Promptly cure or cause to be cured any defects in the execution and delivery
of this Agreement or any defects in the validity or enforceability of any of the
Security and at its expense, execute and deliver or cause to be executed and
delivered, all such agreements, instruments and other documents (including the
filing of any financing statements or financing change statements) as the Lender
reasonably may consider necessary or desirable to protect or otherwise perfect
the security interest granted under the Security Agreement.


         6.       EVENTS OF DEFAULT AND REMEDIES.

                  6.1 Events of Default. The occurrence of any of the following
conditions or events shall each constitute an event of default ("Event of
Default"):

                  (a) failure to repay the Loan or any interest thereon when due
(whether at maturity, by acceleration, or otherwise), and such failure is not
cured within five (5) Business Days after Lender gives to Borrower written
notice of such failure;

                  (b) failure of Borrower to perform or comply with any term,
provision or covenant contained in this Agreement or the Security Agreement or
any other agreement or instrument granting security to or related to security
granted to the Lender (other than with respect to repayment of the Loan or any
interest thereon), and such failure is not cured within thirty (30) days after
Lender gives to Borrower written notice of such failure;

                  (c) the commencement by or against Borrower of a proceeding
under any bankruptcy or similar law of any jurisdiction relating to insolvency
or the relief of debtors, as now or hereafter in effect and the failure by the
Borrower to obtain a dismissal or stay of such proceeding within sixty (60) days
from the date of commencement of such proceeding; the making by Borrower of a
general assignment for the benefit of any of its creditors; the appointment of a
receiver, trustee, custodian or similar officer for Borrower and the failure by
Borrower to secure the discharge of such receiver, trustee, custodian or similar
officer within sixty (60) days from the date of appointment; or the admission in
writing by Borrower of any inability to pay its debts generally as they become
due; or

                  (d) any material breach of a representation, warranty or
certification made by the Borrower in this Agreement, or any document
contemplated by this Agreement, which would reasonably be expected to have a
Material Adverse Effect.

                  6.2 Remedies. Upon the occurrence of an Event of Default,
Lender may, in addition to exercising any other rights or remedies available to
Lender at law, in equity, under this Agreement, the Security Agreement, or
otherwise, declare all principal, interest and other amounts payable under this
Agreement to be immediately due and payable, and, upon reasonable prior notice
to Borrower, offset any amounts owed by Lender to Borrower against the
outstanding amount of the Loan.

                  6.3 Remedies Cumulative. All of Lender's rights, remedies,
powers and privileges are separate and cumulative, and no one of them, whether
exercised or not, shall be or
<PAGE>

                                      -7-

be deemed to be to the exclusion of or to limit or prejudice any other rights,
remedies, powers or privileges Lender may have.

         7. MISCELLANEOUS.

                  7.1 Waiver. No failure by Lender to exercise or delay by
Lender in exercising any right, remedy, power or privilege shall operate as a
waiver thereof nor shall any single or partial exercise of any such right,
remedy, power or privilege preclude any further exercise thereof or of any other
right, remedy, power or privilege.

                  7.2 Governing Law. This Agreement is governed by and shall be
construed in accordance with the laws (other than that body of law relating to
conflicts of law) of the Province of Ontario.

                  7.3 Notices. Any notice, request, demand, statement,
authorization, approval or consent required or permitted under this Agreement
shall be in writing and shall be made by any of the following means, and shall
be deemed effective upon actual receipt: (a) deposit in the mail, postage
prepaid, registered or certified, return receipt requested, such mailing to be
effective upon actual receipt, (b) personal delivery, (c) delivery by a courier
of recognized reputation (such as FedEx) or (d) transmission by telecopier (with
confirmation by mail) as follows, or to such other address and/or such
additional parties as the parties hereto may specify by notice given in
accordance with this Section 7.3:

                  Borrower:         AltaRex Corp.
                                    610 Lincoln Street
                                    Waltham, Massachusetts 02451 USA

                                    Attention:    Edward Fitzgerald
                                    Telephone:    (781) 693-1504
                                    Telecopier:   (781) 466-8740

                  Lender:           United Therapeutics Corporation
                                    1735 Connecticut Avenue, N.W.
                                    Washington, D.C. 20009

                                    Attention:    Paul A. Mahon, General Counsel
                                    Telephone:    (202) 483-7000
                                    Telecopier:   (202) 483-4006

                  7.4 No Third Party Beneficiary. This Agreement creates rights
and duties only among the parties hereto, and no third party shall have any
rights hereunder or in or to the Loan.

                  7.5 Assignment. Borrower shall not assign this Agreement or
any of its rights hereunder without the prior written consent of Lender.
<PAGE>
                                      -8-

                  7.6 Entire Agreement. This Agreement and the Security
Agreement dated as of the date hereof between the Borrower and the Lender
constitutes the entire agreement of the parties with respect to the subject
matter hereof and shall supersede any prior expressions of intent or
understanding with respect to this transaction. This Agreement may only be
amended by a written instrument signed by the parties hereto.

                  7.7 Counterparts. This Agreement may be executed in
counterparts, each of which shall constitute an original, but all of which
together shall constitute a single instrument.

                  IN WITNESS WHEREOF, the parties hereto have duly executed this
Agreement as of the date first above written.


                                     UNITED THERAPEUTICS CORPORATION

                                     By:
                                           ------------------------------------

                                     Its:
                                           ------------------------------------


                                     ALTAREX CORP.

                                     By:
                                           ------------------------------------

                                     Its:
                                           ------------------------------------
<PAGE>
                                    Exhibit I

1.       DEFINITIONS

         In this Exhibit I:

         (a)      "CURRENT MARKET PRICE" per Share or Participating Share at any
                  date shall be the weighted average price per share for such
                  shares for any 20 consecutive trading days (such 20
                  consecutive trading days being selected by the Corporation)
                  commencing not more than 25 trading days before such date on
                  such stock exchange on which such shares are listed as may be
                  selected for such purpose by the directors of the Corporation
                  or if such shares are not listed on any stock exchange, then
                  on an automated quotation system, such as NASDAQ or the over
                  the counter market. The weighted average price shall be
                  determined by dividing the aggregate sale price of all such
                  shares so sold on the exchange, system or market, as the case
                  may be, during the said 20 consecutive trading days by the
                  total number of such shares so sold. If such shares are not
                  listed on any stock exchange, automated quotation system or
                  traded on an over the counter market, the Current Market Price
                  shall be determined in good faith by the Board of Directors of
                  the Corporation.

         (b)      "DIVIDENDS PAID IN THE ORDINARY COURSE" means cash dividends
                  declared payable on the Shares in any fiscal year of the
                  Corporation to the extent that such cash dividends do not
                  exceed, in the aggregate, the greater of: (i) 125% of the
                  aggregate amount or value of dividends declared payable by the
                  Corporation on the Shares in its immediately preceding fiscal
                  year; and (ii) 50% of the aggregate net earnings of the
                  Corporation, before extraordinary items, for its immediately
                  preceding fiscal year (less the amount or value of all
                  dividends paid or payable in respect of such fiscal year) as
                  shown in the audited consolidated financial statements of the
                  Corporation for such preceding fiscal year or, if there are no
                  audited financial statements with respect to such period,
                  computed in accordance with generally accepted accounting
                  principles consistent with the applications made in
                  preparation of the most recent audited consolidated financial
                  statements of the Corporation, and for such purpose the
                  amounts of any dividend paid in shares shall be the aggregate
                  stated value of such shares and the amount of any dividend
                  paid in other than cash or shares shall be the fair market
                  value of such dividend as declared by resolution passed by the
                  board of directors of the Corporation.

         (c)      "PARTICIPATING SHARE" means a share that carries the right to
                  participate in earnings or in capital on a liquidation or
                  winding-up to an unlimited degree, or which ranks, in terms of
                  priority, equally with the Shares with respect to
                  participation in earnings or in capital on a liquidation or
                  winding-up.
<PAGE>
                                      -2-

2.       ADJUSTMENT OF SUBSCRIPTION RIGHTS

         The Conversion Price in effect at any date shall be subject to
adjustment from time to time as follows:

         (a)      If and whenever at any time following April 17, 2002 and prior
                  to 5:00 p.m. (Toronto time) on the Maturity Date (the "Expiry
                  Time"), the Corporation shall (i) subdivide or redivide the
                  outstanding Shares into a greater number of shares, (ii)
                  reduce, combine or consolidate the outstanding Shares into a
                  smaller number of shares, or (iii) issue Shares or other
                  Participating Shares to the holders of all or substantially
                  all of the outstanding Shares by way of a stock dividend, the
                  Conversion Price in effect on the effective date of any such
                  event shall be adjusted immediately after such event or on the
                  record date for such issue of Shares or other Participating
                  Shares by way of stock dividend, as the case may be, so that
                  it shall equal the amount determined by multiplying the
                  Conversion Price in effect immediately prior to such event by
                  a fraction, of which the numerator shall be the total number
                  of Shares and other Participating Shares outstanding
                  immediately prior to such event; such adjustment shall be made
                  successively whenever any event referred to in this subsection
                  (a) shall occur; any such issue of Shares or other
                  Participating Shares by way of a stock dividend shall be
                  deemed to have been made on the record date for the stock
                  dividend for the purpose of calculating the number of
                  outstanding Shares or other Participating Shares immediately
                  after such event under this subsection (a) and subsection (e)
                  of this Section.

         (b)      If and whenever at any time following April 17, 2002 and prior
                  to the Expiry Time the Corporation shall fix a record date for
                  the issuance of rights, options or warrants to all or
                  substantially all of the holders of the outstanding Shares,
                  entitling them, for a period expiring not more than 45 days
                  after such record date, to subscribe for or purchase Shares or
                  other Participating Shares (or securities convertible into or
                  exchangeable for Shares or other Participating Shares) at a
                  price per share (or having a conversion or exchange price per
                  share) less than 95% of the Current Market Price on such
                  record date, the Conversion Price shall be adjusted
                  immediately after such record date so that it shall equal the
                  price determined by multiplying the Conversion Price in effect
                  on such record date by a fraction, of which the numerator
                  shall be the total number of Shares outstanding on such record
                  date plus the number arrived at by dividing the aggregate
                  price of the total number of additional Shares or other
                  Participating Shares offered for subscription or purchase (or
                  the aggregate conversion or exchange price of the convertible
                  or exchangeable securities so offered) by such Current Market
                  Price, and of which the denominator shall be the total number
                  of Shares outstanding on such record date plus the total
                  number of additional Shares or other Participating Shares
                  offered for subscription or purchase (or into which the
                  convertible or exchangeable securities so offered are
                  convertible or exchangeable); any Shares owned by or held for
                  the account of the Corporation or any subsidiary of the
<PAGE>
                                      -3-

                  Corporation shall be deemed not to be outstanding for the
                  purpose of any such computation; such adjustment shall be made
                  successively whenever such a record date is fixed; to the
                  extent that any such rights, options or warrants are not so
                  issued or any such rights, options or warrants are, not
                  exercised prior to the expiration thereof, the Conversion
                  Price shall then be re-adjusted to the Conversion Price which
                  would then be in effect based upon the number and aggregate
                  price of Shares or other Participating Shares (or securities
                  convertible into or exchangeable for Shares or other
                  Participating Shares) actually issued upon the exercise of
                  such rights, options or warrants, as the case may be.

         (c)      If and whenever at any time following April 17, 2002 and prior
                  to the Expiry Time the Corporation shall fix a record date for
                  the making of a distribution to all or substantially all the
                  holders of its outstanding Shares of (i) shares of any class
                  other than Shares or Participating Shares, other than shares
                  distributed to holders of Shares pursuant to their exercise of
                  options to receive dividends in the form of such shares in
                  lieu of Dividends Paid in the Ordinary Course on the Shares
                  and other than the issue of Shares or other Participating
                  Shares to the holders of all or substantially all of the
                  outstanding Shares by way of a stock dividend, or (ii) rights,
                  options or warrants (excluding rights exercisable for 45 days
                  or less) or (iii) evidences of its indebtedness, or (iv)
                  assets (excluding Dividends Paid in the Ordinary Course),
                  including shares of other corporations, then, in each such
                  case, the Conversion Price shall be adjusted immediately after
                  such record date so that it shall equal the price determined
                  by multiplying the Conversion Price in effect on such record
                  date by a fraction, of which the numerator shall be the total
                  number of Shares outstanding on such record date multiplied by
                  the Current Market Price per Share on such record date, less
                  the fair market value (as determined by the board of directors
                  of the Corporation, acting reasonably, which determination,
                  absent manifest error, shall be conclusive) of such shares or
                  rights, options or warrants or evidences or indebtedness or
                  assets so distributed, and of which the denominator shall be
                  the total number of Shares outstanding on such record date
                  multiplied by such Current Market Price per Share; any Shares
                  owned by or held for the account of the Corporation shall be
                  deemed not to be outstanding for the purpose of any such
                  computation; such adjustment shall be made successively
                  whenever such a record date is fixed; to the extent that such
                  distribution is not so made, the Conversion Price shall be
                  re-adjusted to the Conversion Price which would then be in
                  effect if such record date had not been fixed or to the
                  Conversion Price which would then be in effect based upon such
                  shares or rights, options or warrants or evidences of
                  indebtedness or assets actually distributed, as the case may
                  be, and in clause (iv) the term "Dividends Paid in the
                  Ordinary Course" shall include the value of any securities or
                  other property or assets distributed in lieu of cash Dividends
                  Paid in the Ordinary Course.

         (d)      If and whenever at any time following April 17, 2002 and prior
                  to the Expiry Time there is a reclassification of the Shares
                  at any time outstanding or a change
<PAGE>
                                      -4-

                  of the Shares into other shares or a capital reorganization of
                  the Corporation not covered in subsection (a) or a
                  consolidation, amalgamation or merger of the Corporation with
                  or into any other corporation or a sale of the property and
                  assets of the Corporation as or substantially as an entirety
                  to any other person, the Purchaser, to the extent that the
                  Loan has not been converted, prior to the effective date of
                  such reclassification, capital reorganization, consolidation,
                  amalgamation, merger or sale shall thereafter, upon conversion
                  of the Loan, be entitled to receive and shall accept in lieu
                  of the number of Shares, as then constituted, to which the
                  Purchaser was previously entitled upon conversion of the Loan,
                  the number of shares or other securities or property of the
                  Corporation or of the corporation resulting from such
                  reclassification, consolidation, amalgamation or merger or of
                  the person to which such sale may be made, as the case may be,
                  that the Purchaser would have been entitled to receive on such
                  reclassification, capital reorganization, consolidation,
                  amalgamation, merger or sale if, on the effective date
                  thereof, the Purchaser had been the registered holder of the
                  number of Shares to which the Purchaser was previously
                  entitled upon due conversion of the Loan; and in any case, if
                  necessary, appropriate adjustment shall be made in the
                  application of the provisions set forth in this Agreement with
                  respect to the rights and interests thereafter of the
                  Purchaser to the end that the provisions set forth in this
                  Agreement shall thereafter correspondingly be made applicable,
                  as nearly as may reasonably be, in relation to any shares or
                  securities or property to which the Purchaser may be entitled
                  upon the conversion of the Loan thereafter.

         (e)      In any case in which this Agreement shall require that an
                  adjustment shall become effective immediately after a record
                  date for an event referred to herein, the Corporation may
                  defer, until the occurrence of such event, issuing to the
                  Purchaser if converted after such record date and before the
                  occurrence of such event the kind and amount of shares, other
                  securities or property to which it would be entitled upon such
                  exercise by reason of the adjustment required by such event;
                  provided, however, that the Corporation shall deliver to the
                  Purchaser an appropriate instrument evidencing the Purchaser's
                  right to receive the kind and amount of shares, other
                  securities or property to which it would be entitled upon the
                  occurrence of the event requiring such adjustment and the
                  right to receive any distributions made or declared in favour
                  of holders of record of Shares as constituted from time to
                  time on and after such date as the Purchaser would, but for
                  the provisions of this subsection (e), have received, or
                  become entitled to receive, on such exercise.

         (f)      The adjustments provided for in this Agreement are cumulative
                  and shall apply to successive subdivisions, redivisions,
                  reductions, combinations, consolidations, distributions,
                  issues or other events resulting in any adjustment under the
                  provisions of this Agreement provided that, notwithstanding
                  any other provision of this Section, no adjustment of the
                  Conversion Price, as then constituted, purchasable shall be
                  required unless such adjustment would require an increase or
                  decrease, of at least 1% in the Conversion Price, then in
                  effect; provided however,
<PAGE>
                                      -5-


                  that any adjustments which by reason of this subsection (f)
                  are not required to be made shall be carried forward and taken
                  into account in any subsequent adjustment.

         (g)      In the event of any question arising with respect to the
                  adjustments provided in this Agreement, such question shall,
                  absent manifest error, be conclusively determined by a firm of
                  chartered accountants appointed by the Corporation and
                  acceptable to the Purchaser (who may be the auditors of the
                  Corporation) with the assistance of legal counsel, who may be
                  legal counsel to the Corporation; such accountants shall have
                  access to all necessary records of the Corporation and such
                  determination shall be binding upon the Corporation and the
                  Purchaser.

         (h)      As a condition precedent to the taking of any action which
                  would require an adjustment in any of the Conversion Price
                  pursuant to the terms of this Agreement, the Corporation shall
                  take any action which may, in the opinion of legal counsel, be
                  necessary in order that the Corporation may validly and
                  legally issue as fully paid and non-assessable all the Shares
                  which the Purchaser is entitled to receive on the full
                  exercise thereof in accordance with the provisions hereof.

         (i)      In case the Corporation shall take any action affecting the
                  Shares other than action described in this Agreement, which in
                  the opinion of the board of directors of the Corporation would
                  materially affect the rights of Purchaser, the Conversion
                  Price shall be adjusted in such manner and at such time, by
                  action of the board of directors of the Corporation, in its
                  sole reasonable discretion as it may determine to be equitable
                  in the circumstances, provided that no such adjustment shall
                  be made unless prior approval of any stock exchange on which
                  the Shares are listed for trading has been obtained. Failure
                  of the board of directors of the Corporation to make such an
                  adjustment shall be conclusive evidence that the board of
                  directors of the Corporation have determined that it is
                  equitable to make no adjustment in the circumstances.

3.       NOTICE OF ADJUSTMENT

         At least 21 days prior to the effective date or record date, as the
case may be, of any event referred to in clause 2, the Corporation shall notify
the Purchaser of the particulars of such event and the estimated amount of any
adjustment required as a result thereof. Promptly after the occurrence of any
event which requires an adjustment in the Conversion Price pursuant to this
Agreement, the Corporation shall forthwith deliver to the Purchaser a
certificate of the Corporation specifying the particulars of such event and the
required adjustment and the computation of such adjustment and give notice to
the Purchaser of the particulars of such event and the required adjustment in
the manner provided in Section 7.3 hereof.

4.       GENERAL COVENANTS OF THE CORPORATION
<PAGE>
                                      -6-

         (a)      The Corporation covenants and agrees that it is duly
                  authorized to enter into and perform its obligations under its
                  Agreement.

         (b)      The Corporation will cause the Shares issuable upon conversion
                  of the Loan and the certificates representing the Shares to be
                  duly issued. At all times until the Expiry Time, the
                  Corporation shall reserve and there shall remain unissued out
                  of its authorized capital a number of Shares sufficient to
                  satisfy the conversion of the Loan. All Shares issued upon the
                  due conversion of the Loan shall be fully paid and
                  non-assessable.

         (c)      The Corporation will do, execute, acknowledge and deliver or
                  cause to be done, executed, acknowledged and delivered, all
                  other acts, deeds and assurances in law as may be reasonably
                  required for the better accomplishing and effecting of the
                  intentions and provisions of Section 2 of this Agreement.

         (d)      Subject to the express provisions hereof, the Corporation will
                  carry on and conduct and will cause to be carried on and
                  conducted its business in a proper and efficient manner and
                  will cause to be kept proper books of account in accordance
                  with generally accepted accounting practice; and, subject to
                  the express provisions hereof, it will do or cause to be done,
                  all things necessary to preserve and keep in full force and
                  effect its corporate existence, provided, however, that
                  nothing herein contained shall prevent the amalgamation,
                  consolidation, merger, sale, winding up or liquidation of the
                  Corporation or any subsidiary of the Corporation or the
                  abandonment of any rights and franchises of the Corporation or
                  any subsidiary of the Corporation if, in the opinion of the
                  board of directors of the Corporation or officers of the
                  Corporation, it would be advisable and in the best interests
                  of the Corporation or of such subsidiary of the Corporation to
                  do so.

         (e)      The Corporation shall take all such steps and actions and do
                  all such things as may reasonably be necessary to maintain the
                  listing and posting for trading on The Toronto Stock Exchange
                  of those Shares currently listed on The Toronto Stock
                  Exchange.

         (f)      The Corporation will use its reasonable best efforts to ensure
                  that the Shares issuable upon conversion of the Loan will be
                  listed and posted for trading on The Toronto Stock Exchange
                  upon their issue or such other stock exchange or automated
                  quotation system on which the Shares may, from time to time,
                  be listed, posted or quoted for trading.

<PAGE>

                                  EXHIBIT B-2A

                      FORM OF DEBENTURE SUBSCRIPTION FORM
<PAGE>
                                  EXHIBIT B-2A

                       SECOND DEBENTURE SUBSCRIPTION FORM



To:      AltaRex Corp.

Re:      Subscription and Debenture Purchase Agreement dated as of April 17,
         2002 (the "Subscription Agreement") between AltaRex Corp. and United
         Therapeutics Corporation

         Capitalized terms used, but not otherwise defined, herein have the
meanings given to them in the Subscription Agreement.

         Pursuant to Section A.1(b) of the Subscription Agreement, United
Therapeutics Corporation hereby irrevocably exercises the Debenture Subscription
Right and hereby subscribes for and agrees to purchase from the Company the
Second Debenture all on terms and conditions set forth in the Subscription
Agreement.

         DATED                      , 2002.
               ---------------------

                                          UNITED THERAPEUTICS CORPORATION

                                          Per:
                                                --------------------------------
                                                   Name:
                                                   Title:



<PAGE>

                                   EXHIBIT C

                           FORM OF SECURITY AGREEMENT
<PAGE>
                               SECURITY AGREEMENT

         THIS SECURITY AGREEMENT is made as of April 17, 2002 by and between
ALTAREX CORP., an Alberta corporation ("Debtor"), and UNITED THERAPEUTICS
CORPORATION, a Delaware corporation ("Secured Party").

                                    RECITALS

         A. Secured Party and Debtor have entered into (i) a convertible
debenture of even date herewith (the "First Convertible Debenture"), pursuant
to, and subject to the terms and conditions of which, Secured Party will make a
loan to Debtor in an amount equal to US$50,000, and (ii) a subscription and
debenture purchase agreement of even date herewith pursuant to which the Debtor
has granted to the Secured Party the right to subscribe for a convertible
debenture (the "Second Convertible Debenture" and together with the First
Convertible Debenture, the "Convertible Debenture") pursuant to, and subject to
the terms and conditions of which, Secured Party will make a loan to Debtor in
an amount equal to US$875,000.

         B. Debtor has executed this Agreement to induce Secured Party to enter
into the Convertible Debenture.

         NOW, THEREFORE, for valuable consideration received, the receipt and
sufficiency of which are hereby acknowledged, Debtor and Secured Party hereby
agree as follows:

         1.       DEFINITIONS.

                  1.1      Definitions.

                  "Collateral" means all of Debtor's right, title and interest
in, to and under any and all of its intellectual property, including but not
limited to, all worldwide patents, patent rights, patent applications,
trademarks, trademark rights, trademark applications, service marks, service
mark rights, service mark applications, trade names, domain names, copyrights,
copyright applications, software rights, database rights, moral rights,
inventions, know-how, trade secrets, procedures, techniques, test results,
documentation and other similar intangible personal property whether presently
existing or hereafter acquired, including without limitation the intellectual
property listed on the attached Schedule 1, and all proceeds of the foregoing.

                  "Secured Obligations" means all of the indebtedness and
obligations of Debtor to the Secured Party under the First Convertible Debenture
and the Second Convertible Debenture, including without limitation the
obligation of Debtor to pay principal and interest under the Convertible
Debenture.

                  "PPSA" means the Personal Property Security Act (Alberta), as
amended from time to time and any Act substituted therefore and all amendments
thereto;
<PAGE>
                                      -2-

                  1.2 PPSA Terms. Unless otherwise defined herein, or unless the
context otherwise requires, all terms used herein which are defined in the PPSA
shall have the meanings given in the PPSA.

                  1.3 Attachment. The Secured Party and the Debtor have not
agreed to postpone the time for attachment of the security interest granted
hereby.

         2. SECURITY INTEREST.

                  2.1 Grant of Security Interest. In order to secure the full
and punctual payment and performance of the Secured Obligations, Debtor hereby
grants to Secured Party a security interest in the Collateral.

                  2.2 No Obligations Undertaken. Nothing contained in this
Agreement shall relieve Debtor of, or impose on Secured Party, any obligation or
liability for, under or in respect of the Collateral.

         3. REPRESENTATIONS AND WARRANTIES OF DEBTOR.

                  Debtor hereby represents and warrants to Secured Party as
follows:

                  3.1 Organization and Powers of Debtor. Debtor is a corporation
duly organized, validly existing and in good standing under the laws of the
Province of Alberta, and has all requisite power and authority to conduct its
business and own and operate its properties, and to enter into and execute the
Convertible Debenture and this Agreement and to grant the security interests
provided for herein.

                  3.2 Corporate Action of Debtor. Debtor has taken all
appropriate and necessary corporate action to authorize the execution, delivery
and performance of the Convertible Debenture and this Agreement, the
representatives of Debtor executing the Convertible Debenture and this Agreement
are fully authorized to do so, and the Convertible Debenture and this Agreement
have been duly executed and delivered by the Debtor to the Secured Party.

                  3.3 Enforceability. The Convertible Debenture and this
Agreement are the legal, valid and binding obligations of Debtor, enforceable in
accordance with their respective terms.

                  3.4 No Conflict. The execution, delivery and performance of
the Convertible Debenture and this Agreement are not in contravention of or in
conflict with the Certificate of Incorporation or Bylaws of Debtor, or any
agreement, indenture or undertaking to which Debtor is a party or by which
Debtor or any of its property may be bound or affected, and will not cause any
security interest, lien or other encumbrance to be created or imposed upon any
such property by reason thereof, except for the security interest created in
favor of Secured Party pursuant to this Agreement.

                  3.5 Consents and Approvals. No consent, approval, permit,
license, authorization, filing, registration or other action of, with, or by any
governmental authority or other
<PAGE>
                                      -3-

person or entity is required in connection with the execution, delivery and
performance of the Convertible Debenture and this Agreement, except for the
filing of a financing statement and the recording of appropriate documents in
the United States Patent and Trademark Office and the Canadian Intellectual
Property Office as applicable relating to the grant of the security interest.

                  3.6 Perfection. Upon the filing of a financing statement at
the Alberta Personal Property Registry and the recording of appropriate security
documents in the United States Patent and Trademark Office and the Canadian
Intellectual Property Office the Secured Party will have a valid and perfected
security interest in all of the Collateral.

                  3.7 No Other Liens. The Collateral is owned by or licensed to
the Debtor free of all security interests, mortgages, liens, claims, charges and
other encumbrances.

         4. COVENANTS OF DEBTOR.

                  Debtor hereby covenants and agrees to and with Secured Party
as follows:

                  4.1 Existence. Debtor shall do or cause to be done all things
necessary to preserve and maintain its existence as a corporation under the laws
of the Province of Alberta and all rights and franchises material to its
business and operations.

                  4.2 Notice of Litigation. Debtor shall give or cause to be
given prompt written notice to the Secured Party of any action, suit or
proceeding instituted against Debtor or claim asserted relating to any of the
Collateral which could have a material adverse effect upon the business, assets,
value or condition (financial or otherwise) of Debtor.

                  4.3 Disposition of Collateral. Except as contemplated in the
license agreement to be entered into between the parties, the Debtor shall not
dispose of any Collateral without the prior written consent of Secured Party and
shall not, without the prior written consent of the Secured Party, create or
permit to exist any security interest, mortgage, lien, claim, charge and other
encumbrance against any of the Collateral.

                  4.4 Maintenance of Security Interests. Debtor hereby
authorizes Secured Party to file financing statements and continuation
statements with respect to the Collateral to the fullest extent permitted by
law. In addition, Debtor shall from time to time execute and deliver to Secured
Party such financing statements, continuation statements or other filings or
documents as Secured Party may reasonably require to further assure to Secured
Party its rights under this Agreement.

                  4.5 Defend Collateral. Debtor shall use commercially
reasonable efforts to protect and preserve the Collateral, and shall defend the
Collateral for the benefit of the Secured Party against the claims and demands
of all other persons.

                  4.6 Expenses. Debtor shall forthwith pay all costs, charges,
expenses and legal fees and disbursements (on a solicitor and his own client
basis) which may be incurred by the Secured Party in:
<PAGE>
                                      -4-

         (a)      taking, recovering, keeping possession of and insuring the
                  Collateral;

         (b)      connection with any disclosure requirements under the PPSA;
                  and

         (c)      all other actions and proceedings taken in connection with the
                  preservation of the Collateral and the confirmation,
                  perfection and enforcement of this Security Agreement and of
                  any other security held by the Secured Party as security for
                  the Obligations.

         5. REMEDIES.

                  5.1 Remedies. Upon the occurrence of an Event of Default (as
defined in the Convertible Debenture):

         (a)      Secured Party shall have, in addition to any other rights or
                  remedies Secured Party may have at law or in equity or
                  otherwise, the right to accelerate all indebtedness
                  outstanding under the Convertible Debenture and to declare
                  such indebtedness to be immediately due and payable, with or
                  without notice to Debtor; and

         (b)      the security hereby constituted will immediately become
                  enforceable.

                  5.2 Enforcement. To enforce and realize on the security
constituted by this Security Agreement, the Secured Party may take any action
permitted by law or in equity, as it may deem expedient, and in particular,
without limiting the generality of the foregoing, the Secured Party may do any
one or more of the following:

         (a)      appoint by instrument a receiver, receiver and manager or
                  receiver-manager (the person so appointed is herein called the
                  "Receiver") of the Collateral, with or without bond as the
                  Secured Party may determine, and from time to time in its sole
                  discretion remove such Receiver and appoint another in its
                  stead;

         (b)      preserve, protect and maintain the Collateral as the Secured
                  Party may deem advisable;

         (c)      sell, lease or otherwise dispose of or concur in selling,
                  leasing or otherwise disposing of all or any part of the
                  Collateral, whether by public or private sale or lease or
                  otherwise, in such manner, at such price as can be reasonably
                  obtained therefor and on such terms as to credit and with such
                  conditions of sale and stipulations as to title or conveyance
                  or evidence of title or otherwise as to the Secured Party may
                  seem reasonable, provided that the Debtor will not be entitled
                  to be credited with the proceeds of any such sale, lease or
                  other disposition until the monies therefor are actually
                  received; and

         (d)      exercise all of the rights and remedies of a secured party
                  under the PPSA.
<PAGE>
                                      -5-

                  5.3 Proceeds. Subject to applicable law, all amounts realized
from the disposition of the Collateral pursuant to this Security Agreement will
be applied as the Secured Party, in its sole discretion, may direct as follows:

         FIRSTLY:          in or toward payment of all costs, charges and
                           expenses (including legal fees and disbursements on a
                           solicitor and his own client basis) incurred by the
                           Secured Party in connection with or incidental to:

                  (1)      the exercise by the Secured Party of all or any of
                           the powers granted to it pursuant to this Security
                           Agreement; and

                  (2)      the appointment of the Receiver and the exercise by
                           the Receiver of all or any of the powers granted to
                           the Receiver pursuant to this Security Agreement,
                           including the Receiver's reasonable remuneration and
                           all outgoings properly payable by the Receiver;

         SECONDLY:         in or toward payment to the Secured Party of all
                           principal and other monies (except interest) due in
                           respect of the Secured Obligations;

         THIRDLY:          in or toward payment to the Secured Party of all
                           interest remaining unpaid in respect of the Secured
                           Obligations;

         FOURTHLY:         any surplus will be paid to the Debtor.


                  5.4 Waivers. Secured Party may exercise any of its rights and
remedies without demand, advertisement or notice other than as may be required
by law. To the fullest extent permitted by law, Debtor waives demand, notice,
protest, notice of acceptance of this Agreement or other action taken in
reliance hereon and all other demands and notices of any description.

                  5.5 Deficiency. If the amounts realized from the disposition
of the Collateral are not sufficient to pay the Secured Obligations in full to
the Secured Party, the Debtor will immediately pay to the Secured Party the
amount of such deficiency.

                  5.6 Rights Cumulative. All rights and remedies of the Secured
Party set out in this Security Agreement are cumulative and no right or remedy
contained herein is intended to be exclusive but each will be in addition to
every other right or remedy contained herein or in any existing or future
security agreement or now or hereafter existing at law or in equity or pursuant
to any other agreement between the Debtor and the Secured Party that may be in
effect from time to time.

         6. LIABILITY OF SECURED PARTY.

                  6.1 Obligations and Covenants of Debtor. The Secured Party
shall not be bound to do, observe or perform or to see to the observance or
performance by the Debtor of any
<PAGE>
                                      -6-

obligations or covenants imposed upon the Debtor nor shall the Secured Party be
obliged to keep any of the Collateral identifiable.

                  6.2 Duty to Inquire. The Secured Party shall not be obliged to
inquire into the right of any person purporting to be entitled under the PPSA to
information and materials from the Secured Party by making a demand upon the
Secured Party for such information and materials and the Secured Party shall be
entitled to comply with such demand and shall not be liable for having complied
with such demand notwithstanding that such person may in fact not be entitled to
make such demand.

                  6.3 Waiver. The Debtor hereby waives any applicable provision
of law permitted to be waived by it which imposes higher or greater obligations
upon the Secured Party than provided in this Security Agreement.

         7. MISCELLANEOUS.

                  7.1 Performance of Obligations. If the Debtor fails to perform
any of its Obligations hereunder, the Secured Party may, but shall not be
obliged to, perform any or all of such Obligations without prejudice to any
other rights and remedies of the Secured Party hereunder, and any payments made
and any costs, charges, expenses and legal fees and disbursements (on a
solicitor and his own client basis) incurred in connection therewith shall be
payable by the Debtor to the Secured Party forthwith with interest until paid at
the highest rate borne by any of the Secured Obligations and such amounts shall
form part of the Secured Obligations and constitute a charge upon the Collateral
in favour of the Secured Party prior to all claims subsequent to this Security
Agreement.

                  7.2 Notices. Any notice, request, demand, statement,
authorization, approval or consent required or permitted under this Agreement
shall be in writing and shall be made by, and deemed duly given upon, (a)
deposit in the United States mail, postage prepaid, registered or certified,
return receipt requested, such mailing to be effective upon receipt, (b)
personal delivery, (c) 24 hours after delivery to a courier of recognized
reputation (such as FedEx) or (d) transmission by facsimile with a copy sent
separately by mail, as follows or to such other address and/or such additional
parties as either party may specify by notice given in accordance with this
Section 7.2:

                  Debtor:           AltaRex Corp.
                                    610 Lincoln Street
                                    Waltham, Massachusetts 02451
                                    USA

                                    Attention:    Edward Fitzgerald
                                    Telephone:    (781) 693-1504
                                    Telecopier:   (781) 466-8740


                  Secured Party:    United Therapeutics Corporation
                                    1735 Connecticut Avenue, N.W.
                                    Washington, D.C. 20009
<PAGE>
                                      -7-

                                    Attention:    Paul A. Mahon, General Counsel
                                    Telephone:    (202) 483-7000
                                    Telecopier:   (202) 483-4006

                  7.3 Waiver, Amendment or Modification. No waiver, amendment or
modification of any provision hereof or of any right or remedy hereunder shall
be effective unless in writing and signed by the party against whom such waiver,
amendment or modification is sought to be enforced. No failure by Secured Party
to exercise, and no delay by Secured Party in exercising, any right, power or
remedy granted hereunder shall operate as a waiver of any such right, power or
remedy. A waiver of any right or remedy by Secured Party on any one occasion
shall not be construed as a bar to or waiver of any right or remedy on any
future occasion.

                  7.4 Assignment. Secured Party may assign any of its rights or
delegate any of its duties under this Agreement without the prior written
consent of Debtor. Debtor may not assign any of its rights or delegate any of
its duties under this Agreement without the prior written consent of Secured
Party.

                  7.5 Governing Law. This Agreement is governed by and shall be
construed in accordance with the laws of the Province of Alberta.

                  7.6 Severability. If any term, covenant or provision of this
Agreement shall be held to be invalid, illegal or unenforceable in any respect,
this Agreement shall be construed without such term, covenant or provision.

                  7.7 Construction of Agreement. The titles and headings of the
sections of this Agreement have been inserted for convenience of reference only
and are not intended to summarize or otherwise describe the subject matter of
such paragraphs and shall not be given any consideration in the construction of
this Agreement.

                  7.8 No Merger. This Security Agreement shall not operate so as
to create any merger or discharge of any Secured Obligations, or any assignment,
transfer, guarantee, lien, contract, promissory note, bill of exchange or
security in any form held or which may hereafter be held by the Secured Party
from the Debtor or from any other person whomsoever. The taking of a judgment
with respect to any of the Secured Obligations will not operate as a merger of
any of the terms, conditions, covenants, agreements or provisos contained in
this Security Agreement.

                  7.9 Further Assurances. At the Secured Party's request at any
time and from time to time, the Debtor shall execute and deliver such further
and other security, documents and instruments to further mortgage, charge and
grant an assignment to the Secured Party in and to the Collateral, whether in
any other jurisdiction or otherwise, and do all other acts and things as the
Secured Party reasonably requires in order to give effect to this Security
Agreement or such other mortgage, charge or assignment against the Collateral or
to confirm and perfect, and maintain perfection of, the security constituted by
this Security Agreement or such other mortgage, charge or assignment in favour
of the Secured Party.
<PAGE>
                                      -8-

                  7.10 Financing Statement. The Debtor hereby:

         (a)      acknowledges receiving a copy of this Security Agreement; and

         (b)      waives all rights to receive from the Secured Party a copy of
                  any financing statement, financing change statement or
                  verification statement filed at any time or from time to time
                  respect of this Security Agreement.

                  7.11 Additional Security. This Security Agreement and the
security constituted hereby are in addition to and not in substitution for any
other security which the Secured Party may now or from time to time hold or take
from the Debtor or from any other person whomsoever.

                  IN WITNESS WHEREOF, the parties hereto have executed this
Agreement as of the day and year first above written.

                           ALTAREX CORP.
                           ("Debtor")

                           By:
                                --------------------------------------

                           Its:
                                --------------------------------------

                           UNITED THERAPEUTICS CORPORATION

                           ("Secured Party")


                           By:
                                --------------------------------------

                           Its:
                                --------------------------------------




<PAGE>

                                   EXHIBIT D

                          REGISTRATION RIGHTS AGREEMENT
<PAGE>
                          REGISTRATION RIGHTS AGREEMENT

         THIS AGREEMENT is made as of April 17, 2002;

BETWEEN:

                  ALTAREX CORP., a corporation incorporated under the laws of
                  Alberta (hereinafter referred to as the "Company")

                                     - and -

                  UNITED THERAPEUTICS CORPORATION, a corporation incorporated
                  under the laws of Delaware (hereinafter referred to as
                  "United")

         WHEREAS:

         (a)      United wishes to purchase Purchased Securities from the
                  Company;

         (b)      the Company has agreed to issue the Purchased Securities to
                  United; and

         (c)      this Agreement is being entered into as a condition to the
                  completion of the purchase of the Purchased Securities;

         NOW THEREFORE THIS AGREEMENT WITNESSES that in consideration of the
premises and the covenants and agreements herein contained, the parties hereto
agree as follows:

                           ARTICLE 1 - INTERPRETATION

1.1      DEFINITIONS

         In this Agreement, unless something in the subject matter or context is
inconsistent therewith:

         "affiliate" and "associate" have the respective meanings assigned
         thereto in the Securities Act (Ontario) as at the date hereof;

         "Agreement" means this agreement and all amendments made hereto by
         written agreement between the parties hereto;

         "business day" means a day other than a Saturday, Sunday or statutory
         or civic holiday in Toronto, Ontario;

         "Canadian Securities Laws" means the securities laws of any province or
         territory of Canada;

         "Common Shares" means the common shares of the Company;
<PAGE>
                                      -2-

         "Convertible Debentures" means, collectively, the First Convertible
         Debenture and Second Convertible Debenture;

         "Convertible Debenture Shares" means, collectively, the 100,000 Common
         Shares issued upon conversion of the First Convertible Debenture and
         the 883,380 Common Shares issued upon the conversion of US $441,690 of
         the Second Convertible Debenture;

         "Expenses" means all expenses incident to the Company's performance of
         or compliance with this Agreement (including, without limitation, all
         filing fees, expenses of compliance with securities and "blue sky"
         laws, printing expenses and reasonable fees and disbursements of the
         Company's counsel, independent accountants, underwriters (but excluding
         underwriting discounts and selling commissions) and other persons
         retained by the Company) other than the Company's internal expenses
         (including, without limitation, all salaries and expenses of its
         officers and employees performing legal or accounting duties) which
         shall continue to be paid by the Company in any event;

         "First Convertible Debenture" means the First Convertible Debenture
         dated as of the date hereof between the Company and United;

         "Piggyback Qualification" has the meaning ascribed thereto in Section
         3.1;

         "Prospectus" means a prospectus filed by the Company with a Canadian
         commission for a public offering and sale of securities of the Company;

         "Purchased Securities" means collectively the 4,900,000 Common Shares
         subscribed for by United pursuant to the Subscription and Debenture
         Purchase Agreement and the Convertible Debenture Shares, if any;

         "Qualifiable Shares" means (i) the Convertible Debenture Shares
         issuable upon conversion of the Convertible Debentures, as applicable,
         and (ii) any other Common Shares issuable in respect of such shares
         (because of share splits, share dividends, reclassifications,
         recapitalizations or similar events);

         "Second Convertible Debenture" means the Second Debenture (as such term
         is defined in the Subscription and Debenture Purchase Agreement);

         "Securities Acts" means the securities legislation of each of the
         provinces of Canada and all regulations, policy statements, orders,
         rules, rulings, communiques and interpretation notes issued thereunder
         or in relation thereto, as amended, re-enacted or replaced from time to
         time; and

         "Subscription and Debenture Purchase Agreement" the subscription and
         debenture purchase agreement dated as of the date hereof between the
         Company and United.

1.2      HEADINGS

         The division of this Agreement into Articles and Sections and the
insertion of headings are for convenience of reference only and shall not affect
the construction or interpretation of
<PAGE>
                                      -3-

this Agreement. The terms "this Agreement", "hereof", "hereunder" and similar
expressions refer to this Agreement and not to any particular Article, Section
or other portion hereof and include any agreement supplemental hereto. Unless
something in the subject matter or context is inconsistent therewith, references
herein to Articles and Sections are to Articles and Sections of this Agreement.

1.3      EXTENDED MEANINGS

         In this Agreement words importing the singular number only shall
include the plural and vice versa, words importing the masculine gender shall
include the feminine and neuter genders and vice versa and words importing
persons shall include individuals, partnerships, associations, trusts,
unincorporated organizations and corporations.

                       ARTICLE 2 - QUALIFICATION REQUEST

2.1      REQUEST FOR QUALIFICATION

         Subject to Section 2.2, United may at any time after the issue and sale
of the applicable Convertible Debenture, by written notice to the Company,
request (a "Qualification Request") the Company to (i) qualify for distribution
Qualifiable Shares issuable upon conversion of such Convertible Debenture under
the Securities Acts, or (ii) in the event the Company is offering Common Shares
to the public by way of a prospectus, to sell on a secondary basis the Purchased
Securities, such notice to specify the number of Common Shares requested to be
qualified or sold, as the case may be, and the intended method of disposition of
such Common Shares.

         Promptly upon receipt of a Qualification Request, the Company shall use
its best efforts to effect, as expeditiously as possible, the qualification for
distribution or resale under the Securities Acts of the Common Shares that the
Company has been requested to qualify by United to the extent necessary to
permit the disposition of the Common Shares to be qualified in accordance with
the intended methods of distribution.

2.2      RESTRICTIONS ON QUALIFICATION REQUEST

         The Company shall not be obligated to effect:

         (a)      any Qualification Request that requires the Company to qualify
                  the Qualifiable Shares in any jurisdiction outside of Canada;
                  or

         (b)      any Qualification Request unless United has requested either
                  (i) the qualification for distribution of not less than 100%
                  of the total number of Qualifiable Shares owned by United on
                  the date of any such Qualification Request or (ii) the sale on
                  a secondary basis of not less than 100% of the total number of
                  Purchased Securities owned by United on the date of such
                  Qualification Request.

2.3       QUALIFICATION REQUEST EXPENSES

         The Company will pay all Expenses relating to any Qualification
Request:
<PAGE>
                                      -4-

         (a)      provided that with respect to the sale of Common Shares
                  pursuant to a public offering in Canada, United shall pay its
                  proportionate share (based on the number of Common Shares that
                  United is selling of the total number of Common Shares being
                  qualified for sale) of the Expenses to the extent required to
                  be paid by United under Securities Acts and for greater
                  certainty the expenses for which United would be responsible
                  hereunder are limited to its proportionate proportion of the
                  Canadian Securities Commission's regulated fees;

         (b)      provided, however, that if a qualification under Section 2.1
                  is withdrawn at the request of United (other than a withdrawal
                  within 10 days of the United becoming aware of, and as a
                  result of, information concerning the business or financial
                  condition of the Company which is made known to United after
                  the date on which such qualification was requested), United
                  shall pay its proportionate share (based on the number of
                  Common Shares that United is selling of the total number of
                  Common Shares being qualified for sale) of the Expenses to the
                  extent required to be paid by United under Securities Laws.

2.4      UNDERWRITTEN OFFERINGS

         If any Qualification Request is an underwritten offering:

         (a)      United shall provide notice of this fact as part of its
                  Qualification Registration;

         (b)      the Company, even in the case of an offering (a "Secondary
                  Distribution") including United as a secondary offeror, shall
                  have the right to select the investment banker(s) and
                  manager(s) to underwrite any offering contemplated by a
                  Qualification Request, subject, in the case of a Secondary
                  Distribution, to the approval of United, such approval not to
                  be unreasonably withheld; and

         (c)      United shall bear or pay its proportionate share of the
                  underwriting discounts and selling commissions determined on
                  the basis of the proportion that the number of United's Common
                  Shares included in the Qualification Request bears to the
                  total number of Common Shares qualified for distribution.

                      ARTICLE 3 - PIGGYBACK QUALIFICATION

3.1      RIGHT TO PIGGYBACK

         If the Company proposes to (i) qualify for distribution any Common
Shares under any of the Securities Acts (other than in connection with any
securities exchange offer, dividend reinvestment plan or stock option or other
employee benefit plan) or (ii) sell by way of prospectus commons shares out of
treasury ("Offering"), the Company shall give prompt written notice to United of
its intention to do so and, subject to Section 3.4, shall include in such
qualification all Qualifiable Shares and shall include as part of such offering
on a secondary basis the Purchased Securities (a "Piggyback Qualification") with
respect to which the Company has received from United a written request for
inclusion therein within ten Business Days after
<PAGE>
                                      -5-

the date of such notice. The Company has the right to postpone or withdraw any
qualification in its sole discretion.

3.2      PIGGYBACK EXPENSES

         The Company will pay all Expenses relating to any Piggyback
Qualification.

3.3      UNDERWRITTEN OFFERINGS

         If any Piggyback Qualification is an underwritten offering:

         (a)      the Company shall be entitled to select the investment
                  banker(s) and manager(s) to underwrite the offering
                  contemplated thereby;

         (b)      United shall enter into an underwriting agreement in customary
                  form with the underwriter or underwriters selected for the
                  underwriting by the Company; and

         (c)      United shall bear or pay its proportionate share of the
                  underwriting discounts and selling commissions determined on
                  the basis of the proportion that the number of United's Common
                  Shares included in the Piggyback Qualification bears to the
                  total number of Common Shares qualified for distribution.

3.4      PRIORITY ON PRIMARY QUALIFICATION

         If any Piggyback Qualification includes an underwritten offering and
the managing underwriters advise the Company that the number of Common Shares
that the Company and United have requested to be included in such distribution
pursuant to Section 3.1 exceeds the number (in this Section 3.4, the "Maximum
Secondary Offering Size") which can be sold on a secondary basis in an orderly
manner in such offering within a price range acceptable to the Company (the
"Assessment"), the Company shall include in such distribution as many of the
Common Shares proposed to be sold by United on a secondary basis as will not
cause the offering to exceed the Maximum Secondary Offering Size which can be
sold on a secondary basis in an orderly manner in such offering within a price
range acceptable to the Company.

If United disagrees with the Assessment, the matter shall be decided by an
arbitrator mutually agreeable to the parties within ten calendar days whose
decision shall be final and binding upon the parties.

Subject to the foregoing, the Company shall use its reasonable efforts to
arrange for the underwriters to include the Piggyback Qualification as part of
the Common Shares to be distributed by or through such underwriters.
<PAGE>
                                      -6-

                      ARTICLE 4 - QUALIFICATION PROCEDURES

4.1      OBLIGATIONS OF THE COMPANY

         Whenever the Company receives a request for a Qualification Request or
a Piggyback Qualification, the Company shall use its best efforts to effect such
qualification for distribution and pursuant thereto the Company shall, as
expeditiously as possible, and to the extent necessary by virtue of the
requirements of the jurisdictions in which such qualification is to be effected:

         (a)      promptly prepare and file a preliminary prospectus in the
                  relevant jurisdictions and such other related documents as may
                  be necessary or appropriate relating to the proposed
                  distribution or qualification and shall, as soon as possible
                  after any comments of the relevant regulatory authorities have
                  been satisfied with respect thereto, prepare and file under
                  the Securities Acts a prospectus (provided that before filing
                  a preliminary prospectus, prospectus or any amendments or
                  supplements thereto, the Company shall furnish to United
                  copies of such documents for their review) and obtain receipts
                  therefor and shall take all other steps and proceedings that
                  may be necessary in order to qualify the Common Shares for
                  distribution under such Securities Acts by registrants who
                  comply with the relevant provisions of such Securities Acts;

         (b)      prepare and file with the relevant regulatory authorities such
                  amendments and supplements to such preliminary prospectus,
                  prospectus as may be necessary to comply with the provisions
                  of the Securities Acts with respect to the distribution of all
                  Common Shares covered thereby until all of such Common Shares
                  have been distributed in accordance with the intended method
                  or methods of disposition;

         (c)      furnish to United such number of copies of such preliminary
                  prospectus, prospectus and any amendment and supplement
                  thereto and such other relevant documents as such shareholder
                  may reasonably request in order to facilitate the disposition
                  of its Common Shares;

         (d)      furnish to United an opinion of counsel for the Company in a
                  form that is customary at such time addressed to United and
                  the underwriters and dated the effective date of such (final)
                  prospectus;

         (e)      notify, as soon as possible, United of any circumstance or the
                  happening of any event as a result of which the preliminary
                  prospectus, prospectus as then in effect would include an
                  untrue statement of material fact or would omit any fact that
                  is required to be stated or that is necessary to make any
                  statement therein not misleading, and at the request of United
                  prepare and furnish to United a reasonable number of
                  commercial copies of a supplement to or an amendment of the
                  preliminary prospectus, prospectus as may be necessary so
                  that, as thereafter delivered to the purchasers of such Common
                  Shares, such document shall not include an untrue statement of
                  material fact or omit to state any fact that is
<PAGE>
                                      -7-

                  required to be stated or that is necessary to make any
                  statement therein not misleading;

         (f)      otherwise comply with all applicable Securities Acts during
                  the course of the distribution;

         (g)      enter into such customary agreements, including underwriting
                  agreements, containing such representations and warranties by
                  the Company and such other terms and provisions as are
                  customary therein including, without limitation, rights of
                  indemnity and contribution;

         (h)      in the event of the issuance of any order or ruling suspending
                  the effectiveness of a prospectus receipt or of any order
                  suspending or preventing the use of any prospectus or
                  suspending the qualification of any Common Shares qualified by
                  such prospectus for sale in any jurisdiction, use its best
                  efforts promptly to obtain the withdrawal of such order or
                  ruling;

         (i)      otherwise use its reasonable efforts to facilitate the
                  offering of the Common Shares including, without limitation,
                  causing management of the Company to participate in any road
                  shows, sales meetings or other activities arranged by the
                  underwriters;

         (j)      if the Company has delivered a Prospectus to United and after
                  having done so the Prospectus is amended to comply with the
                  requirements of the Securities Act, the Company shall promptly
                  notify United and, if requested, United shall immediately
                  cease making offers of Qualifiable Shares or Purchased
                  Securities and return all Prospectuses to the Company. The
                  Company shall promptly provide United with revised
                  Prospectuses and, following receipt of the revised
                  Prospectuses or United shall be free to resume making offers
                  of the Qualifiable Shares or Purchased Securities; and

         (k)      in the event that, in the judgement of the Company, it is
                  advisable to suspend use of a Prospectus due to pending
                  material developments or other events that have not yet been
                  publicly disclosed and as to which the Company believes public
                  disclosure would be detrimental to the Company, the Company
                  shall notify United to such effect, and, upon receipt of such
                  notice, United shall immediately discontinue any sales of
                  Qualifiable Shares or Purchased Securities pursuant to such
                  Prospectus until United has received copies of a supplemented
                  or amended Prospectus or until United is advised in writing by
                  the Company that the then current Prospectus may be used and
                  has received copies of any additional or supplemental filings
                  that are incorporated or deemed incorporated by reference in
                  such Prospectus. Notwithstanding anything to the contrary
                  herein, the Company shall not exercise its rights under this
                  Section 4.1(k) to suspend sales of Qualifiable Shares or
                  Purchased Securities for a number of days in excess of 120
                  days in any 365-day period.
<PAGE>
                                      -8-

4.2      OBLIGATIONS OF UNITED

(1) If in the reasonable opinion of counsel to the Company it is necessary or
appropriate in order to comply with any applicable Securities Acts, the
obligations of the Company under Articles 2 and 3 shall be conditional upon
United and any underwriter participating in such offering executing and
delivering to the Company an appropriate agreement, in a form reasonably
satisfactory to counsel for the Company, that such person shall comply with all
prospectus delivery requirements of all relevant Securities Acts and with
anti-stabilization, manipulation and similar provisions of the relevant
Securities Acts and shall furnish to the Company information about sales made in
such offering.

(2) United shall not (until further notice) effect sales of any Common Shares
qualified by or included in a prospectus or deliver any prospectus in respect of
such sale after notification by the Company of any order or ruling suspending
the effectiveness of the receipt for such prospectus.

4.3      PREPARATION; REASONABLE INVESTIGATION

         In connection with the preparation and filing of any preliminary
prospectus, prospectus, or similar document pursuant to a qualification request,
United and the underwriters, if any, and its counsel, auditors and other
representatives, shall be given the opportunity to participate in the
preparation of such documents and each amendment thereof or supplement thereto
and there shall be inserted therein such material as is required under the
applicable Securities Acts or which in the reasonable judgment of United and its
counsel should be included. United and the underwriters shall also be given such
access to the books and records of the Company and such opportunities to discuss
the business of the Company with its officers and auditors as shall be necessary
in their respective opinions or in the opinion of their respective counsel, and
to conduct all due diligence which any of the foregoing persons may reasonably
require in order to conduct a reasonable investigation for purposes of
establishing a due diligence defence as contemplated by the Securities Acts and
in order to enable United and underwriters to execute the certificates required
to be executed by them pursuant to the Securities Acts for inclusion in each
such document.

4.4      EXPENSES

(1) To the extent permitted under applicable Securities Acts, all Expenses shall
be borne by the Company. The Company shall pay its internal expenses (including,
without limitation, all salaries and expenses of its officers and employees
performing legal or accounting duties).

(2) To the extent Expenses are not permitted by law to be paid by the Company,
United shall pay those Expenses allocable to the distribution or qualification
of the Qualified Shares or Purchased Securities owned by it as provided herein.
<PAGE>
                                      -9-


             ARTICLE 5 - ARTICLE - INDEMNIFICATION AND CONTRIBUTION

5.1      INDEMNIFICATION

(1) The Company shall indemnify and hold United, each of its officers,
directors, legal counsel and accountants and each person which owns or controls
United from and against any and all expenses, claims, losses, damages and
liabilities (or actions, proceedings or settlements in respect thereof)
including costs of investigation and reasonable fees and expenses of legal
counsel arising out of or based upon:

         (a)      any liability pursuant to a provision of (including any
                  indemnity in) any underwriting agreement, purchase agreement
                  or other document relating to any Qualification Request or
                  Piggyback Qualification and directly or indirectly arising out
                  of or based upon any misrepresentation, breach of warranty,
                  untrue statement or alleged untrue statement, whether of a
                  material fact or otherwise, or any omission or alleged
                  omission to state a fact, material or not, required to be
                  stated or necessary to make a statement therein not
                  misleading, in light of the circumstances in which it is made;
                  or

         (b)      any misrepresentation or alleged misrepresentation, breach of
                  warranty or untrue statement or alleged untrue statement,
                  whether of a material fact or otherwise contained in any
                  preliminary prospectus, prospectus registration statement or
                  similar document (including any amendment or supplement
                  thereto) relating to any Qualification Request or Piggyback
                  Qualification, or in any underwriting agreement, purchase
                  agreement or other document relating thereto, or arising out
                  of or based upon any omission or alleged omission to state in
                  any such preliminary prospectus, prospectus, registration
                  statement or similar document (including any amendment or
                  supplement thereto), or any such underwriting agreement,
                  purchase agreement or other document, a fact, material or not,
                  required to be stated therein or necessary to make a statement
                  therein not misleading, in light of the circumstances in which
                  it is made.

(2) In the event of any qualification of any of the Qualifiable Shares or
Purchased Securities under the Securities Act pursuant to this Agreement, United
will indemnify and hold harmless the Company, each of its directors and officers
and each underwriter (if any) and each person, if any, who controls the Company
or any such underwriter within the meaning of the Securities Act, against any
losses, claims, damages or liabilities, joint or several, to which the Company,
such directors and officers, underwriter or controlling person may become
subject under Securities Act, or otherwise, insofar as such losses, claims,
damages or liabilities (or actions in respect thereof) arise out of or are based
upon any untrue statement or alleged untrue statement of a material fact
contained in any preliminary or final Prospectus under which such Qualifiable
Shares or Purchased Securities were qualified, or any amendment or supplement to
the Prospectus, or arise out of or are based upon any omission or alleged
omission to state a material fact required to be stated therein or necessary to
make the statements therein not misleading, if the statement or omission was
made in reliance upon and in conformity with information relating to United
furnished in writing to the Company by or on behalf of United specifically for
use in connection with the preparation of such Prospectus, amendment or
supplement; provided,
<PAGE>
                                      -10-

however, that the obligations of United hereunder shall be limited to an amount
equal to the net proceeds to United of Qualifiable Shares or Purchased
Securities sold in connection with such qualification.

(3) Each party entitled to indemnification under this Section 5.1 (the
"Indemnified Party") shall give notice to the Company or United, as the case may
be, promptly after such Indemnified Party has actual knowledge of any claim as
to which indemnity may be sought, and shall permit the Company or United, as the
case may be, to assume the defence of any such claim or any litigation resulting
therefrom, provided that counsel for the Company or United, as the case may be,
who shall conduct the defence of such claim or any litigation resulting
therefrom, shall be approved by the Indemnified Party, and the Indemnified Party
may participate in such defence, and provided further that the failure of any
Indemnified Party to give notice as provided herein shall not relieve the
Company of its obligations under this Section 5.1. The Company or United, as the
case may be, shall not, in the defence of any such claim or litigation, except
with the consent of each Indemnified Party, consent to entry of any judgment or
enter into any settlement which does not include as an unconditional term
thereof the giving by the claimant or plaintiff to such Indemnified Party of a
release from all liability in respect to such claim or litigation.

5.2      CONTRIBUTION

         If the indemnification provided for in Section 5.1 is unavailable or
insufficient to hold harmless the Indemnified Parties in respect of any
expenses, claims, losses, damages or liabilities or actions in respect thereof,
then the Company or United, as the case may be, shall in lieu of indemnifying
the Indemnified Parties contribute to the amount paid or payable by the
Indemnified Parties as a result of such expenses, claims, losses, damages,
liabilities or actions in such proportion as is appropriate to reflect the
relative fault of the parties thereto, in connection with the statements or
omissions which resulted in such expenses, claims, losses, damages, liabilities
or actions as well as any other relevant equitable considerations; provided,
however, that no person guilty of fraudulent misrepresentation within the
meaning of Section 11(f) of the United States Securities Act of 1933, as
amended) shall be entitled to contribution from any person who was not guilty of
such fraudulent misrepresentation. The parties hereto agree that it would not be
just and equitable if contribution pursuant to this Section 5.2 were determined
by pro rata allocation or by any other method of allocation which did not take
account of the equitable considerations referred to above in this Section 5.2.
The amount paid or payable by the Indemnified Parties as a result of the
expenses, claims, losses, damages, liabilities or actions in respect thereof
referred to above in this Section 5.2 shall be deemed to include any legal or
other expenses reasonably incurred by the Indemnified Parties in connection with
investigating or defending any such action or claim.

                              ARTICLE 6 - GENERAL

6.1      INJUNCTIVE RELIEF

         The Company acknowledges and agrees that damages would be inadequate to
compensate for the breach of any of its obligations contained in this Agreement
and that other parties hereto would be seriously and irreparably injured if any
provision of this Agreement is
<PAGE>
                                      -11-

not performed by it in accordance with the specific terms and conditions of this
Agreement. Accordingly, the Company agrees, without prejudice to any additional
or alternative remedies the other parties hereto may have hereunder, that such
other parties shall be entitled:

         (a)      to an injunction to prevent any breach of this Agreement by
                  the Company;

         (b)      to enforce specifically the terms and provisions hereof and
                  any obligation in favour of such other parties, or any of
                  them, contained in this Agreement; and

         (c)      to declaratory relief or injunctive relief in respect of
                  anything done in breach of an obligation in favour of such
                  other parties, or any of them, contained in this Agreement.

6.2      TERMINATION

         This Agreement shall terminate (i) in the case of the Common Shares
constituting a portion of the Purchased Securities, four months from the date
hereof, and (ii) in the case of the Qualifiable Shares, four months from the
date of issue and sale of the Convertible Debentures.

6.3      FURTHER ASSURANCES

         Each of the parties hereto shall from time to time execute and deliver
all such further documents and instruments and do all acts and things as the
other parties may reasonably require to effectively carry out or better evidence
or perfect the full intent and meaning of this Agreement.

6.4      TIME OF THE ESSENCE

         Time shall be of the essence of this Agreement.

6.5      BENEFIT OF THE AGREEMENT

         This Agreement shall enure to the benefit of and be binding upon the
respective successors and permitted assigns of the parties hereto.

6.6      ENTIRE AGREEMENT

         This Agreement, the Subscription and Debenture Purchase Agreement and
the Convertible Debenture constitutes the entire agreement between the parties
hereto with respect to the subject matter hereof and cancels and supersedes any
other understandings and agreements between the parties hereto with respect
thereto. There are no representations, warranties, terms, conditions,
undertakings or collateral agreements, express, implied or statutory, between
the parties other than as expressly set forth in this Agreement.

6.7      AMENDMENTS AND WAIVER

         No modification of or amendment to this Agreement shall be valid or
binding unless set forth in writing and duly executed by the parties hereto and
no waiver of any breach of any term
<PAGE>
                                      -12-

or provision of this Agreement shall be effective or binding unless made in
writing and signed by the party purporting to give the same and, unless
otherwise provided, shall be limited to the specific breach waived.

6.8      ASSIGNMENT

         This Agreement may not be assigned by any party hereto without the
written consent of the other parties hereto.

6.9      NOTICES

         Any demand, notice or other communication to be given in connection
with this Agreement shall be given in writing and shall be given by personal
delivery or by facsimile transmission addressed to the recipient as follows:

         (a)      to the Company:

                  AltaRex Corp.
                  610 Lincoln Street
                  Waltham, Massachusetts 02451

                  Telecopier:       781-466-8740
                  Attention:        Edward Fitzgerald

         (b)      to United:

                  United Therapeutics Corporation
                  1735 Connecticut Avenue, N.W
                  Washington, D.C.  20009

                  Telecopier:       (202) 483-7000
                  Attention:        Paul A. Mahon, General Counsel


or to such other address, individual or facsimile number as may be designated by
notice given by any party to the others. Any demand, notice or other
communication given by personal delivery shall be conclusively deemed to have
been given on the day of actual delivery thereof and, if given by facsimile
transmission, on the day of transmittal thereof.

6.10     GOVERNING LAW

         This Agreement shall be governed by and construed in accordance with
the laws of the Province of Ontario and the laws of Canada applicable therein.

6.11     ATTORNMENT

         In relation to any legal proceedings or action to enforce this
Agreement, the courts of the Province of Ontario shall have jurisdiction to
entertain any action arising under this Agreement.
<PAGE>
                                      -13-

Each of the parties hereto hereby irrevocably attorns to the jurisdiction of the
courts of the Province of Ontario and waives any objections to proceedings in
such courts on the grounds of venue or on the grounds that the proceedings have
been brought in an inconvenient forum. This Section 6.10 shall not affect the
right of any party to take proceedings in any other jurisdiction nor shall the
taking of proceedings in any jurisdiction preclude any party from taking
proceedings in any other jurisdiction.

6.12     COUNTERPARTS

         This Agreement may be executed in any number of counterparts, each of
which shall be deemed to be an original and all of which shall constitute one
and the same agreement.

         IN WITNESS WHEREOF the parties have executed this Agreement.

                                    ALTAREX CORP.

                                    Per:
                                         --------------------------------------


                                    Per:
                                         --------------------------------------



                                    UNITED THERAPEUTICS CORPORATION


                                    Per:
                                        ---------------------------------------


                                    Per:
                                        ---------------------------------------





<PAGE>

                                    EXHIBIT E
                                    ---------

Unless otherwise defined herein, all capitalised terms shall have the meanings
ascribed thereto in the Subscription Agreement.

                                  ALTAREX CORP.
                                   TERM SHEET

ISSUER:             AltaRex Corp. ("AltaRex" or the "Company").

ISSUE:              A unit consisting of 4,900,000 Common Shares and a warrant
                    to purchase 3,250,000 common shares at a price of $0.50; a
                    convertible debenture in the principal amount of $50,000;
                    and a right to subscribe for a convertible debenture in the
                    principal amount of $875,000.

PURCHASE
PRICE:              U.S. $0.50 per Common Share for a purchase price of
                    $2,450,000

COMMON
SHARES
OUTSTANDING:                                    PRE-ISSUE            POST-ISSUE
                                              ------------           ----------
                    Basic:                    36.7 million          45.0 million
                    Fully-diluted:            49.2 million          58.4 million

RESTRICTED
SECURITIES:         The Purchased Securities will not be registered under the
                    Securities Act, or applicable state securities laws, and may
                    not be transferred except in compliance with the Securities
                    Act and applicable state securities laws. Each certificate
                    representing the Purchased Securities will bear a legend to
                    the foregoing effect.

USE OF
PROCEEDS:           Research and development expenses, general and
                    administrative expenses and working capital and other
                    requirements of the Company

LISTING:            The Common Shares are listed on the TSE under the
                    symbol "AXO".

ANTICIPATED
CLOSING  DATE:      April 17, 2002.



<PAGE>


                                   EXHIBIT F-1
                                   -----------

                                  ALTAREX CORP.

                         PRIVATE PLACEMENT QUESTIONNAIRE
                         -------------------------------



<PAGE>


                                   EXHIBIT F-2
                                   -----------

                                  ALTAREX CORP.

                    CERTIFICATE FOR CORPORATION, PARTNERSHIP,
                     TRUST, FOUNDATION AND JOINT PURCHASERS
                    -----------------------------------------

<PAGE>

                                  EXHIBIT F-3
                                  -----------

                   DECLARATION OF ACCREDITED INVESTOR STATUS
<PAGE>


                                   EXHIBIT F-4
                                   -----------
                           THE TORONTO STOCK EXCHANGE