Document:

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                                                                    Exhibit 10.5

FOURTH AMENDMENT TO REGISTRATION RIGHTS AGREEMENT

         This Fourth Amendment to Registration Rights Agreement ("Amendment No.
4") is made and entered into on this 20th day of January, 1994, by and among
Argus Pharmaceuticals, Inc., a Delaware corporation (the "Company"), Vector
Securities International, Inc., a Delaware corporation ("Vector"), and the
persons and entities whose name appear on the signature pages hereof
(collectively, the "Purchasers").

         WHEREAS, the Company and certain of the Purchasers are parties to that
certain Registration Rights Agreement dated August 2, 1989 (as amended, the
"Agreement"), pursuant to which the Company granted such Purchasers certain
registration rights with respect to the shares of common stock, par value $.001
per share ("Common Stock"), that such Purchasers acquired on the conversion of
the Company's Series A and Series B preferred stock, par value $.001 per share;
and

         WHEREAS, the Company, certain of the Purchasers and Pacificorp Credit,
Inc., d/b/a Pacific Venture Finance, Inc. ("Pacificorp"), entered into a Waiver
and First Amendment to the Agreement dated April 18, 1990 ("Amendment No. 1"),
which provided Pacificorp with certain registration rights with respect to the
shares of Common Stock that it is entitled to acquire on the exercise of certain
Common Stock purchase warrants; and

         WHEREAS, the Company, certain of the Purchasers and certain additional
parties entered into a Second Amendment to the Agreement dated October 31, 1991
("Amendment No. 2"), which provided such additional parties certain registration
rights with respect to the shares of Common Stock that were acquired on the
conversion of the Series C preferred stock, par value $.001 per share; and

         WHEREAS, the Company, the Purchasers and Genzyme Corporation, a
Massachusetts corporation ("Genzyme"), entered into a Third Amendment to the
Agreement dated September 10, 1993 ("Amendment No. 3"), which provided Genzyme
with certain registration rights with respect to the shares of Common Stock
acquired by Genzyme pursuant to a Stock Purchase Agreement dated September 10,
1993; and

         WHEREAS, the Company and Vector have negotiated the terms of a Stock
Purchase Warrant (the "Vector Warrant"), and the Company has agreed to grant
Vector certain registration rights with respect to the shares of Common Stock to
be acquired on exercise of such Vector Warrant.

         NOW THEREFORE, in consideration of the premises and certain other good
and valuable consideration, the receipt and sufficiency of which is hereby
acknowledged, the parties hereto agree as follows:

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         1.       The definition of "Holders" included in Section 1 of the
                  Agreement is hereby deleted in its entirety and replaced with
                  the following:

                  "Holders" shall mean the Purchasers, any persons who are
                  granted registration rights by the Company subsequent to the
                  date hereof and any persons who are transferred registration
                  rights pursuant to Section 2.9 hereof.

         2.       The definition of "Purchasers" included in Section 1 of the
                  Agreement is hereby amended to include Vector.

         3.       The term "Registrable Securities" included in Section 1 of the
                  Agreement is hereby amended to include the shares of Common
                  Stock to be issued to Vector from time to time on exercise of
                  the Vector Warrant.

         4.       Section 2.3 of the Agreement is hereby deleted in its entirety
                  and shall read in its entirety as follows:

                  Amendment of Registration Rights. The Company and the Holders
                  acknowledge that the terms of this Agreement were based on the
                  terms of rights to register the Registrable Securities under
                  the Securities Act of 1933, as amended, granted by the Company
                  to other stockholders of the Company ("Registration Rights").
                  The Company hereby agrees that if the Company, after the date
                  hereof, grants to any person Registration Rights that provide
                  for terms that are in any manner more favorable to the holder
                  of such Registration Rights than the equivalent terms of this
                  Agreement (or if the Company amends or waives any provision of
                  any Registration Rights existing on the date hereof to provide
                  for terms that are more favorable to the holder thereof than
                  the equivalent terms of this Agreement), then the Company
                  shall amend this Agreement to provide for any (or all) of such
                  more favorable terms as the Holders shall elect to include
                  herein.

         5.       This Amendment No. 4 may be executed in one or more
                  counterparts and shall be effective when a counterpart
                  signature page is executed by the Company, Vector and the
                  holders of 66-2/3% of the outstanding Registrable Securities,
                  as defined in the Agreement. Except as amended hereby, the
                  Agreement shall remain in full force and effect.

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         IN WITNESS WHEREOF, the parties have executed and delivered the
Amendment No. 4 as of the date first above written.

                                  ARGUS PHARMACEUTICALS, INC.

                                  By:      /s/David M. Leech
                                           David M. Leech, President

                                  VECTOR SECURITIES INTERNATIONAL, INC.

                                  By:      /s/D. Theodore Berghorst
                                           D. Theodore Berghorst
                                  Title:   Chairman and CEO

                                  ALLSTATE INSURANCE COMPANY

                                  By:      /s/Its Authorized Signatories
                                           Its Authorized Signatories
                                  Title:   Chairman and CEO

                                  BFC VENTURES LTD. - 1983

                                  By:      /s/Lloyd M. Butke
                                           Lloyd M. Butke
                                  Title:   General Partner

                                  /s/Luis T. Campos, M.D.
                                  Luis T. Campos, M.D.

                                  R.W. Cunningham, I.R.A.

                                  By:      /s/Authorized Signature
                                           Authorized Signature
                                  Title:   Vice President

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                                 ESSEX VENTURE PARTNER, L.P. FUND I

                                 By:
                                 Title:

                                 ESSEX VENTURE PARTNER, L.P. FUND II

                                 By:
                                 Title:

                                 THE GENESIS FUND, LTD.

                                 By:      /s/Authorized Signature
                                          Authorized Signature
                                 Title:   General Partner

                                 Norman Green

                                 /s/Jeffrey A. Hoffman
                                 Jeffrey A. Hoffman

                                 /s/Miguel Miro-Quesada, M.D.
                                 Miguel Miro-Quesada, M.D.

                                 A.G. Edwards & Sons,
                                 Custodian for Stuart Schube, I.R.A.

                                 By:
                                 Title:

                                 /s/Stuart Schube
                                 Stuart Schube

                                 /s/Authorized Signature
                                 Stuart Schube, Trustee

                                 TRIAD VENTURE LIMITED

                                 By:      /s/Authorized Signature
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                                          Authorized Signature
                                 Title:   General Partner

                                 TRIAD VENTURES LIMITED LIMITED II, L.P.

                                 By:      /s/Authorized Signature
                                          Authorized Signature
                                 Title:   General Partner

                                 UNCO VENTURES, INC.

                                 By:      /s/Authorized Signature
                                          Authorized Signature
                                 Title:   General Partner

                                 THE WOODLANDS VENTURE CAPITAL
                                 COMPANY

                                 By:      /s/Authorized Signature
                                          Authorized Signature
                                 Title:   President

                                 THE WOODLANDS VENTURE FUND, L.P.

                                 By:      /s/Martin P. Sutter
                                          Martin P. Sutter
                                 Title:   Managing General Partner

                                 GENZYME CORPORATION

                                 By:      /s/Authorized Signature
                                          Authorized Signature
                                 Title:   CEO<PAGE>   1
                                                                   Exhibit 10.18

         THIS WARRANT HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933,
         AS AMENDED, OR ANY STATE SECURITIES LAWS. NO SALE OR DISPOSITION MAY BE
         EFFECTED WITHOUT (i) EFFECTIVE REGISTRATION STATEMENTS RELATED THERETO,
         (ii) AN OPINION OF COUNSEL FOR THE HOLDER, REASONABLY SATISFACTORY TO
         THE COMPANY, THAT SUCH REGISTRATIONS ARE NOT REQUIRED, (iii) RECEIPT OF
         NO-ACTION LETTERS FROM THE APPROPRIATE GOVERNMENTAL AUTHORITIES, OR
         (iv) COMPLYING WITH THE PROVISIONS OF RULE 144 OF THE SECURITIES AND
         EXCHANGE COMMISSION.

                          ARONEX PHARMACEUTICALS, INC.

                        WARRANT TO PURCHASE 2,734 SHARES
                                 OF COMMON STOCK

         THIS CERTIFIES THAT, for value received, GATX Capital Corporation is
entitled to subscribe for and purchase 2,734 shares of the fully paid and
nonassessable Common Stock, $.001 par value (as adjusted pursuant to Section 4
hereof, the "Shares") of ARONEX PHARMACEUTICALS, INC., a Delaware corporation
(the "Company"), at the price of $12.00 per share (such price and such other
price as shall result, from time to time, from the adjustments specified in
Section 4 hereof is herein referred to as the "Warrant Price"), subject to the
provisions and upon the terms and conditions hereinafter set forth. As used
herein, (a) the term "Common Stock" shall mean the Company's presently
authorized Common Stock, and any stock into or for which such Common Stock may
hereafter be converted or exchanged, (b) the term "Date of Grant" shall mean
March 1, 1993, and (c) the term "Other Warrants" shall mean any other warrants
issued by the Company in connection with the transaction with respect to which
this Warrant was issued, and any warrant issued upon transfer or partial
exercise of this Warrant. The term "Warrant" as used herein shall be deemed to
include Other Warrants unless the context clearly requires otherwise.

         1. Term. The purchase right represented by this Warrant is exercisable,
in whole or in part, at any time and from time to time from the Date of Grant
through seven (7) years after the Date of Grant.

         2. Method of Exercise; Payment; Issuance of New Warrant. Subject to
Section 1 hereof, the purchase right represented by this Warrant may be
exercised by the holder hereof, in whole or in part and from time to time, at
the election of the holder hereof, by the surrender of this Warrant (with the
notice of exercise form attached hereto as Exhibit A duly executed) at the
principal office of the Company and by the payment to the Company, by check, of
an amount equal to the then applicable Warrant Price multiplied by the number of
Shares then being purchased. The person or persons in whose name(s) any
certificate(s) representing shares of Common Stock shall be issuable upon
exercise of this Warrant shall be deemed to have become the holder(s) of record
of, and shall

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be treated for all purposes as the record holder(s) of, the shares represented
thereby (and such shares shall be deemed to have been issued) immediately prior
to the close of business on the date or dates upon which this Warrant is
exercised. In the event of any exercise of the rights represented by this
Warrant, certificates for the shares of stock so purchased shall be delivered to
the holder hereof as soon as possible and in any event within thirty days after
such exercise and, unless this Warrant has been fully exercised or expired, a
new Warrant representing the portion of the Shares, if any, with respect to
which this Warrant shall not then have been exercised shall also be issued to
the holder hereof as soon as possible and in any event within such thirty-day
period.

         3. Stock Fully Paid; Reservation of Shares. All Shares that may be
issued upon the exercise of the rights represented by this Warrant will, upon
issuance pursuant to the terms and conditions herein, be fully paid and
nonassessable, and free from all taxes, liens and charges with respect to the
issue thereof. During the period within which the rights represented by this
Warrant may be exercised, the Company will at all times have authorized, and
reserved for the purpose of the issue upon exercise of the purchase rights
evidenced by this Warrant, a sufficient number of shares of its Common Stock to
provide for the exercise of the rights represented by this Warrant.

         4. Adjustment of Warrant Price and Number of Shares. The number and
kind of securities purchasable upon the exercise of this Warrant and the Warrant
Price shall be subject to adjustment from time to time upon the occurrence of
certain events, as follows:

                  (a) Reclassification or Merger. In case of any
reclassification, change or conversion of securities of the class issuable upon
exercise of this Warrant (other than a change in par value, or from par value to
no par value, or from no par value to par value, or as a result of a subdivision
or combination), or in case of any merger of the Company with or into another
corporation (other than a merger with another corporation in which the Company
is the acquiring and the surviving corporation and which does not result in any
reclassification or change of outstanding securities issuable upon exercise of
this Warrant), or in case of any sale of all or substantially all of the assets
of the Company, the Company, or such successor or purchasing corporation, as the
case may be, shall duly execute and deliver to the holder of this Warrant a new
Warrant (in form and substance satisfactory to the holder of this Warrant), so
that the holder of this Warrant shall have the right to receive, at a total
purchase price not to exceed that payable upon the exercise of the unexercised
portion of this Warrant, and in lieu of the shares of Common Stock theretofore
issuable upon exercise of this Warrant, the kind and amount of shares of stock,
other securities, money and property receivable upon such reclassification,
change or merger by a holder of the number of shares of Common Stock then
purchasable under this Warrant. Such new Warrant shall provide for adjustments
that shall be as nearly equivalent as may be practicable to the adjustments
provided for in this Section 4. The provisions of this subparagraph (a) shall
similarly apply to successive reclassifications, changes, mergers and transfers.

                  (b) Subdivision or Combination of Shares. If the Company at
any time while this Warrant remains outstanding and unexpired shall subdivide or
combine its outstanding shares of Common Stock, the Warrant Price shall be
proportionately decreased in the case of a subdivision

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or increased in the case of a combination, effective at the close of business on
the date the subdivision or combination becomes effective.

                  (c) Stock Dividends and Other Distributions. If the Company at
any time while this Warrant is outstanding and unexpired shall (i) pay a
dividend with respect to Common Stock payable in Common Stock, or (ii) make any
other distribution with respect to Common Stock (except any distribution
specifically provided for in the foregoing subparagraphs (a) and (b)) of Common
Stock, then the Warrant Price shall be adjusted, from and after the date of
determination of shareholders entitled to receive such dividend or distribution,
to that price determined by multiplying the Warrant Price in effect immediately
prior to such date of determination by a fraction (i) the numerator of which
shall be the total number of shares of Common Stock outstanding immediately
prior to such dividend or distribution, and (ii) the denominator of which shall
be the total number of shares of Common Stock outstanding immediately after such
dividend or distribution.

                  (d) Adjustment of Number of Shares. Upon each adjustment in
the Warrant Price, the number of Shares of Common Stock purchasable hereunder
shall be adjusted, to the nearest whole share, to the product obtained by
multiplying the number of Shares purchasable immediately prior to such
adjustment in the Warrant Price by a fraction, the numerator of which shall be
the Warrant Price immediately prior to such adjustment and the denominator of
which shall be the Warrant Price immediately thereafter.

         5. Notice of Adjustments. Whenever the Warrant Price or the number of
Shares purchasable hereunder shall be adjusted pursuant to Section 4 hereof, the
Company shall make a certificate signed by its chief financial officer setting
forth, in reasonable detail, the event requiring the adjustment, the amount of
the adjustment, the method by which such adjustment was calculated, and the
Warrant Price and the number of Shares purchasable hereunder after giving effect
to such adjustment, shall be mailed (without regard to Section 13 hereof, by
first class mail, postage prepaid) to the holder of this Warrant.

         6. Fractional Shares. No fractional shares of Common Stock will be
issued in connection with any exercise hereunder, but in lieu of such fractional
shares the Company shall make a cash payment therefor based on the fair market
value of the Common Stock on the date of exercise as reasonably determined in
good faith by the Company's Board of Directors.

         7. Compliance with Securities Act; Disposition of Warrant or Shares of
Common Stock.

                  (a) Compliance with Securities Act. The holder of this
Warrant, by acceptance hereof, agrees that this Warrant, and the shares of
Common Stock to be issued upon exercise hereof are being acquired for investment
and that such holder will not offer, sell or otherwise dispose of this Warrant,
or any shares of Common Stock to be issued upon exercise hereof except under
circumstances which will not result in a violation of the Securities Act of
1933, as amended (the "Act"). Upon exercise of this Warrant, unless the Shares
being acquired are registered under the Act or an exemption from such
registration is available, the holder hereof shall confirm in writing, by

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executing the form attached as Schedule 1 to Exhibit A hereto, that the shares
of Common Stock so purchased are being acquired for investment and not with a
view toward distribution or resale. This Warrant and all shares of Common Stock
issued upon exercise of this Warrant (unless registered under the Act) shall be
stamped or imprinted with a legend in substantially the following form:

         "THE SECURITIES EVIDENCED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
         SECURITIES ACT OF 1933, AS AMENDED, OR ANY STATE SECURITIES LAWS. NO
         SALE OR DISPOSITION MAY BE EFFECTED WITHOUT (i) EFFECTIVE REGISTRATION
         STATEMENTS RELATED THERETO, (ii) AN OPINION OF COUNSEL FOR THE HOLDER,
         REASONABLY SATISFACTORY TO THE COMPANY, THAT SUCH REGISTRATIONS ARE NOT
         REQUIRED, (iii) RECEIPT OF NO-ACTION LETTERS FROM THE APPROPRIATE
         GOVERNMENTAL AUTHORITIES, OR (iv) COMPLYING WITH THE PROVISIONS OF RULE
         144 OF THE SECURITIES AND EXCHANGE COMMISSION."

         In addition, in connection with the issuance of this Warrant, the
holder specifically represents to the Company by acceptance of this Warrant as
follows:

         (1) The holder is aware of the Company's business affairs and financial
condition, and has acquired information about the Company sufficient to reach an
informed and knowledgeable decision to acquire this Warrant. The holder is
acquiring this Warrant for its own account for investment purposes only and not
with a view to, or for the resale in connection with, any "distribution" thereof
for purposes of the Act.

         (2) The holder understands that this Warrant has not been registered
under the Act in reliance upon a specific exemption therefrom, which exemption
depends upon, among other things, the bona fide nature of the holder's
investment intent as expressed herein. In this connection, the holder
understands that, in the view of the SEC, the statutory basis for such exemption
may be unavailable if the holder's representation was predicated solely upon a
present intention to hold the Warrant for the minimum capital gains period
specified under tax statutes, for a deferred sale, for or until an increase or
decrease in the market price of the Warrant, or for a period of one year or any
other fixed period in the future.

         (3) The holder further understands that this Warrant must be held
indefinitely unless subsequently registered under the Act and any applicable
state securities laws, or unless exemptions from registration are otherwise
available. Moreover, the holder understands that the Company is under no
obligation to register this Warrant.

         (4) The holder is aware of the provisions of Rule 144 and 144A,
promulgated under the Act, which, in substance, permit limited public resale of
"restricted securities" acquired, directly or indirectly, from the issuer
thereof (or from an affiliate of such issuer), in a non-public offering subject
to the satisfaction of certain conditions, if applicable, including, among other
things: the

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availability of certain public information about the Company; the resale
occurring not less than two years after the party has purchased and paid for the
securities to be sold; the sale being made through a broker in an unsolicited
"broker's transaction" or in transactions directly with a market maker (as said
term is defined under the Securities Exchange Act of 1934, as amended); and the
amount of securities being sold during any three-month period not exceeding the
specified limitations stated therein.

         (5) The holder further understands that at the time it wishes to sell
this Warrant there may be no public market upon which to make such a sale, and
that, even if such a public market then exists, the Company may not be
satisfying the current public information requirements of Rule 144 and 144A, and
that, in such event, the holder may be precluded from selling this Warrant under
Rule 144 and 144A even if the two-year minimum holding period had been
satisfied.

         (6) The holder further understands that in the event all of the
requirements of Rule 144 and 144A are not satisfied, registration under the Act,
compliance with Regulation A, or some other registration exemption will be
required; and that, notwithstanding the fact that Rule 144 and 144A are not
exclusive, the Staff of the SEC has expressed its opinion that persons proposing
to sell private placement securities other than in a registered offering and
otherwise than pursuant to Rule 144 and 144A will have a substantial burden of
proof in establishing that an exemption from registration is available for such
offers or sales, and that such persons and their respective brokers who
participate in such transactions do so at their own risk.

                  (b) Disposition of Warrant or Shares. With respect to any
offer, sale or other disposition of this Warrant or any shares of Common Stock
acquired pursuant to the exercise of this Warrant prior to registration of such
Warrant or shares, the holder hereof and each subsequent holder of this Warrant
agrees to give written notice to the Company prior thereto, describing briefly
the manner thereof, together with a written opinion of such holder's counsel, if
requested by the Company, to the effect that such offer, sale or other
disposition may be effected without registration or qualification (under the Act
as then in effect or any federal or state law then in effect) of this Warrant or
such shares of Common Stock and indicating whether or not under the Act
certificates for this Warrant or such shares of Common Stock to be sold or
otherwise disposed of require any restrictive legend as to applicable
restrictions on transferability in order to ensure compliance with such law.
Promptly upon receiving such written notice and reasonably satisfactory opinion,
if so requested, the Company, as promptly as practicable, shall notify such
holder that such holder may sell or otherwise dispose of this Warrant or such
shares of Common Stock, all in accordance with the terms of the notice delivered
to the Company. If a determination has been made pursuant to this subsection (b)
that the opinion of counsel for the holder is not reasonably satisfactory to the
Company, the Company shall so notify the holder promptly after such
determination has been made and shall specify in detail the legal analysis
supporting any such conclusion. Notwithstanding the foregoing, this Warrant or
such shares of Common Stock may, as to such federal laws, be offered, sold or
otherwise disposed of in accordance with Rule 144 or 144A under the Act,
provided that the Company shall have been furnished with such information as the
Company may reasonably request to provide a reasonable assurance that the
provisions of Rule 144 or 144A have been satisfied. Each

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certificate representing this Warrant or the shares of Common Stock thus
transferred (except a transfer pursuant to Rule 144 or 144A) shall bear a legend
as to the applicable restrictions on transferability in order to ensure
compliance with such laws, unless in the aforesaid opinion of counsel for the
holder, such legend is not required in order to ensure compliance with such
laws. The Company may issue stop transfer instructions to its transfer agent in
connection with such restrictions.

                  (c) Excepted Transfers. Neither any restrictions of any legend
described in this Warrant nor the requirements of Section 7(b) above shall apply
to any transfer without any additional consideration of, or grant of a security
interest in, this Warrant or any part hereof to not more than two (2) partners
of the holder if the holder is a partnership; provided however, in any such
transfer, the transferee shall on the Company's request agree in writing to be
bound by the terms of this Warrant as if an original signatory hereto.

                  (d) Removal of Legend. The restrictions on transfer imposed by
this Section 7 shall cease and terminate as to this Warrant and the Shares when
(i) such securities shall have been effectively registered under the Act and
sold by the holder thereof in accordance with such registration, (ii) the
Company receives an acceptable opinion as described in Paragraph 7(b) hereof or
an SEC "no action" letter stating that future transfers of such securities by
the transferor or the contemplated transferees would be exempt from registration
under the Act, or (iii) such securities may be sold in accordance with the
provisions of Rule 144(k) promulgated under the Act. When the restrictions on
transfer contained in this Section 7 have terminated as provided above, the
holder of the securities as to which such restrictions shall have terminated or
the transferee of such holder shall be entitled to receive promptly from the
Company, without expense to such holder, new certificates not bearing the legend
set forth on page 1 of this Warrant and in Paragraph 7(b) hereof.

         8. Rights as Shareholders; Information. No holder of this Warrant, as
such, shall be entitled to vote or receive dividends or be deemed the holder of
Common Stock or any other securities of the Company which may at any time be
issuable on the exercise hereof for any purpose, nor shall anything contained
herein be construed to confer upon the holder of this Warrant, as such, any of
the rights of a shareholder of the Company or any right to vote for the election
of directors or upon any matter submitted to shareholders at any meeting
thereof, or to receive notice of meetings, or to receive dividends or
subscription rights or otherwise until this Warrant shall have been exercised
and the Shares purchasable upon the exercise hereof shall have become
deliverable, as provided herein.

         9.       Registration Rights.  Intentionally Omitted.

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         10.      Additional Rights.

                  10.1     Right to Convert Warrant into Common Stock; Net
                           Issuance.

                           (a)      Right to Convert.  In addition to and
without limiting the rights of the holder under the terms of this Warrant, the
holder shall have the right to convert this Warrant or any portion thereof (the
"Conversion Right") into shares of Common Stock as provided in this Section 10.1
at any time or from time to time during the term of this Warrant. Upon exercise
of the Conversion Right with respect to a particular number of shares subject to
this Warrant (the "Converted Warrant Shares"), the Company shall deliver to the
holder (without payment by the holder of any exercise price or any cash or other
consideration) (X) that number of shares of fully paid and nonassessable Common
Stock equal to the quotient obtained by dividing the value of this Warrant (or
the specified portion hereof) on the Conversion Date (as defined in subsection
(b) hereof), which value shall be determined by subtracting (A) the aggregate
Warrant Price of the Converted Warrant Shares immediately prior to the exercise
of the Conversion Right from (B) the aggregate fair market value of the
Converted Warrant Shares issuable upon exercise of this Warrant (or the
specified portion hereof) on the Conversion Date (as herein defined) by (Y) the
fair market value of one share of Common Stock on the Conversion Date (as herein
defined).

         Expressed as a formula, such conversion shall be computed as follows:

         X= B-A
             Y

         Where:   X = the number of shares Common Stock that may be issued to
                       holder

                  Y = the fair market value (FMV) of one share of Common Stock

                  A = the aggregate Warrant Price (i.e., Converted Warrant
                      Shares x Warrant Price)

                  B = the aggregate FMV (i.e., FMV x Converted Warrant Shares)

         No fractional shares shall be issuable upon exercise of the Conversion
Right, and, if the number of shares to be issued determined in accordance with
the foregoing formula is other than a whole number, the Company shall pay to the
holder an amount in cash equal to the fair market value of the resulting
fractional share on the Conversion Date (as hereinafter defined).

                           (b)      Method of Exercise.  The Conversion Right
may be exercised by the holder by the surrender of this Warrant at the principal
office of the Company together with a written statement specifying that the
holder thereby intends to exercise the Conversion Right and indicating the
number of shares subject to this Warrant which are being surrendered (referred
to in subsection (a) hereof as the Converted Warrant Shares) in exercise of the
Conversion Right. Such conversion

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<PAGE>   8
shall be effective upon receipt by the Company of this Warrant together with the
aforesaid written statement, or on such later date as is specified therein (the
"Conversion Date"). Certificates for the shares issuable upon exercise of the
Conversion Right and, if applicable, a new warrant evidencing the balance of the
shares remaining subject to this Warrant, shall be issued as of the Conversion
Date and shall be delivered to the holder within thirty (30) days following the
Conversion Date.

                           (c)      Determination of Fair Market Value.
For purposes of this Section 10.1, "fair market value" of a share of Common
Stock as of a particular date (the "Determination Date") shall mean:

                                    (i)     If traded on a securities exchange
         or the NASDAQ National Market System), the fair market value of the
         Common Stock shall be deemed to be the average of the closing prices of
         the Common Stock on such exchange over the 30-day period ending five
         business days prior to the Determination Date;

                                    (ii) If traded over-the-counter (other than
         on the NASDAQ National Market System), the fair market value of the
         Common Stock shall be deemed to be the average of the closing bid
         prices of the Common Stock over the 30-day period ending five business
         days prior to the Determination Date; and

                                    (iii) If there is no public market for the
         Common Stock, then fair market value shall be determined in good faith
         by the Board of Directors of the Company.

         11. Modification and Waiver. This Warrant and any provision hereof may
be changed, waived, discharged or terminated only by an instrument in writing
signed by the party against which enforcement of the same is sought.

         12. Notices. Any notice, request, communication or other document
required or permitted to be given or delivered to the holder hereof or the
Company shall be delivered, or shall be sent by certified or registered mail,
postage prepaid, to each such holder at its address as shown on the books of the
Company or to the Company at the address indicated therefor on the signature
page of this Warrant.

         13. Binding Effect on Successors. This Warrant shall be binding upon
any corporation succeeding the Company by merger, consolidation or acquisition
of all or substantially all of the Company's assets, and all of the obligations
of the Company relating to the Common Stock issuable upon the exercise or
conversion of this Warrant shall survive the exercise, conversion and
termination of this Warrant and all of the covenants and agreements of the
Company shall inure to the benefit of the successors and assigns of the holder
hereof.

         14. Lost Warrants or Stock Certificates. The Company covenants to the
holder hereof that, upon receipt of evidence reasonably satisfactory to the
Company of the loss, theft, destruction or mutilation of this Warrant or any
stock certificate and, in the case of any such loss, theft or

                                      -8-
<PAGE>   9
destruction, upon receipt of an indemnity reasonably satisfactory to the
Company, or in the case of any such mutilation upon surrender and cancellation
of such Warrant or stock certificate, the Company will make and deliver a new
Warrant or stock certificate, of like tenor, in lieu of the lost, stolen,
destroyed or mutilated Warrant or stock certificate.

         15. Descriptive Headings. The descriptive headings of the several
paragraphs of this Warrant are inserted for convenience only and do not
constitute a part of this Warrant.

         16. Governing Law. This Warrant shall be construed and enforced in
accordance with, and the rights of the parties shall be governed by, the laws of
the State of California.

         17. Survival of Representations; Warranties and Agreements. All
representations and warranties of the Company and the holder hereof contained
herein shall survive the Date of Grant, the exercise or conversion of this
Warrant (or any part hereof) or the termination or expiration of rights
hereunder. All agreements of the Company and the holder hereof contained herein
shall survive indefinitely until, by their respective terms, they are no longer
operative.

         18. Remedies. In case any one or more of the covenants and agreements
contained in this Warrant shall have been breached, the holders hereof (in the
case of a breach by the Company), or the Company (in the case of a breach by a
holder), may proceed to protect and enforce their or its rights either by suit
in equity and/or by action at law, including, but not limited to, an action for
damages as a result of any such breach and/or an action for specific performance
of any such covenant or agreement contained in this Warrant.

         19. Acceptance. Receipt of this Warrant by the holder hereof shall
constitute acceptance of and agreement to the foregoing terms and conditions.

         20. No Impairment of Rights. The Company will not, by amendment of its
Charter or through any other means, avoid or seek to avoid the observance or
performance of any of the terms of this Warrant, but will at all times in good
faith assist in the carrying out of all such terms and in the taking of all such
action as may be necessary or appropriate in order to protect the rights of the
holder of this Warrant against impairment.

                                           ARONEX PHARMACEUTICALS, INC.

                                           By: /s/Geoffrey F. Cox, Ph.D.
                                                  Geoffrey F. Cox, Ph.D.
                                                  Chief Executive Officer

                                           Address: 8707 Technology Forest Place
                                                      The Woodlands, Texas 77381

Dated effective as of June 28, 1993

                                      -9-
<PAGE>   10
                                    EXHIBIT A

                               NOTICE OF EXERCISE

To: ARONEX PHARMACEUTICALS, INC.

         1. The undersigned hereby elects to purchase _____ shares of Common
Stock, $.001 par value, of ARONEX PHARMACEUTICALS, INC., pursuant to the terms
of the attached Warrant, and tenders herewith payment of the purchase price of
such shares in full.

         2. Please issue a certificate or certificates representing said shares
in the name of the undersigned or in such other name or names as are specified
below:

                            -------------------------
                                     (Name)

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

                            -------------------------
                                    (Address)

         3. The undersigned represents that the aforesaid shares are being
acquired for the account of the undersigned for investment and not with a view
to, or for resale in connection with, the distribution thereof and that the
undersigned has no present intention of distributing or reselling such shares.
In support thereof, the undersigned has executed an Investment Representation
Statement attached hereto as Schedule 1.

                                                    ---------------------------
                                                    (Signature)

-------------------
(Date)
<PAGE>   11
                                   Schedule 1

                       INVESTMENT REPRESENTATION STATEMENT

Purchaser:

Company:          ARONEX PHARMACEUTICALS, INC.

Security:         Common Stock, $.001 par value

Amount:

Date:

         In connection with the purchase of the above-listed securities (the
"Securities"), the undersigned (the "Purchaser") represents to the Company as
follows:

         (a) The Purchaser is aware of the Company's business affairs and
financial condition, and has acquired sufficient information about the Company
to reach an informed and knowledgeable decision to acquire the Securities. The
Purchaser is purchasing the Securities for its own account for investment
purposes only and not with a view to, or for the resale in connection with, any
"distribution" thereof for purposes of the Securities Act of 1933, as amended
(the "Act").

         (b) The Purchaser understands that the Securities have not been
registered under the Securities Act in reliance upon a specific exemption
therefrom, which exemption depends upon, among other things, the bona fide
nature of the Purchaser's investment intent as expressed herein. In this
connection, the Purchaser understands that, in the view of the Securities and
Exchange Commission ("SEC"), the statutory basis for such exemption may be
unavailable if the Purchaser's representation was predicated solely upon a
present intention to hold these Securities for the minimum capital gains period
specified under tax statutes, for a deferred sale, for or until an increase or
decrease in the market price of the Securities, or for a period of one year or
any other fixed period in the future.

         (c) The Purchaser further understands that the Securities must be held
indefinitely unless subsequently registered under the Act or unless an exemption
from registration is otherwise available. Moreover, the Purchaser understands
that the Company is under no obligation to register the Securities. In addition,
the Purchaser understands that the certificate evidencing the Securities will be
imprinted with the legend referred to in the Warrant under which the Securities
are being purchased unless Paragraph 7(d) of such Warrant provides otherwise.

<PAGE>   12
         (d) The Purchaser is aware of the provisions of Rule 144 and 144A,
promulgated under the Act, which, in substance, permit limited public resale of
"restricted securities" acquired, directly or indirectly, from the issuer
thereof (or from an affiliate of such issuer), in a non-public offering subject
to the satisfaction of certain conditions, if applicable, including, among other
things: The availability of certain public information about the Company, the
resale occurring not less than two years after the party has purchased and paid
for the securities to be sold; the sale being made through a broker in an
unsolicited "broker's transaction" or in transactions directly with a market
maker (as said term is defined under the Securities Exchange Act of 1934, as
amended) and the amount of securities being sold during any three-month period
not exceeding the specified limitations stated therein.

         (e) The Purchaser further understands that at the time it wishes to
sell the Securities there may be no public market upon which to make such a
sale, and that, even if such a public market then exists, the Company may not be
satisfying the current public information requirements of Rule 144 and 144A, and
that, in such event, the Purchaser may be precluded from selling the Securities
under Rule 144 and 144A even if the two-year minimum holding period had been
satisfied.

         (f) The Purchaser further understands that in the event all of the
requirements of Rule 144 and 144A are not satisfied, registration under the Act,
compliance with Regulation A, or some other registration exemption will be
required; and that, notwithstanding the fact that Rule 144 is not exclusive, the
Staff of the SEC has expressed its opinion that persons proposing to sell
private placement securities other than in a registered offering and otherwise
than pursuant to, Rule 144 will have a substantial burden of proof in
establishing that an exemption from registration is available for such offers or
sales, and that such persons and their respective brokers who participate in
such transactions do so at their own risk.

                                                     Purchaser:

                                                     Date:             , 20

                                       -2-

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