<PAGE>
                                                                     EXHIBIT 4.1


                                  EPIMMUNE INC.
                            SHARE PURCHASE AGREEMENT



      This SHARE PURCHASE AGREEMENT (this "AGREEMENT"), is made and entered into
as of December 18, 2001, by and among EPIMMUNE INC., a Delaware corporation (the
"COMPANY"), and the purchasers listed on Schedule A attached hereto
(collectively, the "PURCHASERS" and individually, a "PURCHASER").

1.    AUTHORIZATION OF SALE OF THE SHARES

      Subject to the terms and conditions of this Agreement, the Company has
authorized the sale of up to 2,000,000 shares (the "SHARES") of common stock,
par value $0.001 per share (the "COMMON STOCK"), of the Company.

2.    AGREEMENT TO SELL AND PURCHASE THE SHARES

      2.1   PURCHASE AND SALE

      Subject to the terms and conditions of this Agreement, each Purchaser
severally agrees to purchase, and the Company agrees to sell and issue to each
Purchaser, at the Closing (as defined below) that number of Shares set forth
opposite such Purchaser's name on Schedule A attached hereto.

      2.2   PURCHASE PRICE

      The purchase price of each Share shall be $2.50 (the "PER SHARE PRICE").
The Company shall not, during the period beginning on the date of this Agreement
and ending ninety (90) days after the Closing Date (as defined below), without
adjusting the price per Share hereunder accordingly, sell (i) Shares at a price
per Share of less than the Per Share Price, or (ii) warrants or any other
securities that can be converted into, or otherwise exchanged for, shares of the
Company's common stock at a conversion, exchange or exercise price per Share of
less than the Per Share Price (excluding options under existing stock option
plans provided that the exercise price of each option is not lower than the fair
market value of the underlying common stock on the date of issuance or grant of
such option), or (iii) issue options at a price per share that is less than the
fair market value under the applicable option plan. In the event the Company
shall, during the period beginning on the date of this Agreement and ending
ninety (90) days after the Closing Date, sell any shares of the Company's common
stock at, or any instruments that can be converted into or otherwise exchanged
for the Company's common stock (the "Subsequent Sale") exercisable at, a price
per Share (the "Subsequent Purchase Price") of less than the Per Share Price,
the Company shall, within ten (10) business days of the Subsequent Sale, pay to
the Purchaser a cash amount equal to the number of Shares times the difference
between the Per Share Price and the Subsequent Purchase Price.


                                       1.
<PAGE>
3.    DELIVERY OF THE SHARES AT THE CLOSING

      (a) The completion of the purchase and sale of the Shares (the "CLOSING")
shall occur at the offices of Cooley Godward LLP, counsel to the Company, at
4401 Eastgate Mall, San Diego, CA 92121 at 9:00 a.m. local time on December 18,
2001 assuming the conditions set forth in Sections 3(c) and 3(d) below have been
satisfied or such other time and date as may be agreed by the parties (the
"CLOSING DATE").

      (b) At the Closing, the Company shall authorize its transfer agent (the
"TRANSFER AGENT") to issue to each Purchaser one or more stock certificates
registered in the name of such Purchaser, or in such nominee name(s) as
designated by such Purchaser in writing, representing the number of Shares set
forth on Schedule A and bearing an appropriate legend referring to the fact that
the Shares were sold in reliance upon the exemption from registration provided
by Rule 506 of Regulation D of the Securities Act of 1933, as amended (the
"SECURITIES ACT"). The Company will deliver certificates representing the Shares
(the "CERTIFICATES") against delivery of payment for the Shares by the
Purchasers and in the case of the State of Wisconsin Investment Board, shall
deliver one certificate representing 80% of number of purchased Shares and one
certificate representing 20% of number of purchased Shares. Prior to the
Purchasers' delivery of payment for the Shares, the Company will deliver via
facsimile a copy of the Certificates to be delivered upon Closing to the office
of each Purchaser (at the respective fax numbers indicated on the signature
pages attached hereto).

      (c) The Company's obligation to complete the purchase and sale of the
Shares shall be subject to the following conditions, any one or more of which
may be waived by the Company:

            (i) receipt by the Company from stockholders holding rights to
require the Company to register the sale of any securities owned by such holder
in the Registration Statement (as defined below) of waivers of such rights
(including the waiver of any notice requirements related to such rights);

            (ii) receipt by the Company of same-day funds in the full amount of
the purchase price for the Shares being purchased under this Agreement; and

            (iii) the representations and warranties made by the Purchasers in
this Agreement which are not qualified as to materiality shall be accurate in
all material respects and the representations and warranties of the Purchasers
in this Agreement which are qualified as to materiality shall be true and
accurate as though made as of the Closing Date and the undertaking of the
Purchasers shall have been fulfilled in all material respects on or before the
Closing.

      (d) The Purchasers' obligations to accept delivery of such stock
certificates and to pay for the Shares evidenced by the certificates shall be
subject to the following conditions, any one or more of which may be waived by a
Purchaser with respect to such Purchaser's obligation:

            (i) the representations and warranties made by the Company in this
Agreement which are not qualified as to materiality shall be accurate in all
material respects and the representations and warranties of the Company in this
Agreement which are qualified as to materiality shall be true and accurate as
though made at and as of the Closing Date and the


                                       2.
<PAGE>
undertakings of the Company shall have been fulfilled in all material respects
on or before the Closing and the Purchasers shall have received a certificate
executed by the chairman of the board or president and the chief financial or
accounting officer of the Company, dated the Closing Date, in form and substance
reasonably satisfactory to the Purchasers, to such effect; and

            (ii) the Purchasers shall have received a legal opinion in
substantially the form attached hereto as EXHIBIT A; and

            (iii) the Company shall have obtained gross proceeds of at least
$4,000,000 from the sale of the Shares at the Closing.

4.    REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE COMPANY

      Except as set forth on the Schedule of Exceptions attached hereto as
EXHIBIT B, the Company hereby represents and warrants to the Purchasers as
follows (which representations and warranties shall be deemed to apply, where
appropriate, to each subsidiary of the Company):

      4.1   ORGANIZATION AND QUALIFICATION

      The Company has been duly incorporated and is validly existing as a
corporation in good standing under the laws of the State of Delaware. The
Company has the corporate power and authority to own, lease and operate its
properties and to conduct its business as currently conducted and to enter into
and perform its obligations under this Agreement. The Company is duly qualified
as a foreign corporation to transact business and is in good standing in each
jurisdiction in which such qualification is required, whether by reason of the
ownership or leasing of property or the conduct of business, except where the
failure to so qualify would not, singly or in the aggregate, have a material
adverse effect on the condition, financial or otherwise, or the earnings,
assets, properties, business affairs or business prospects of the Company.

      4.2   CAPITALIZATION

      (a) The authorized capital stock of the Company consists of 25,000,000
shares of Common Stock and 10,000,000 shares of Preferred Stock.

      (b) As of December 18, 2001, the issued and outstanding capital stock of
the Company consists of 10,066,153 shares of Common Stock, 859,666 shares of
Series S Preferred Stock and 549,622 shares of Series S-1 Preferred Stock. The
shares of issued and outstanding capital stock of the Company have been duly
authorized and validly issued, are fully paid and nonassessable and have not
been issued in violation of or are not otherwise subject to any preemptive or
other similar rights. As of December 18, 2001, the 859,666 shares of Series S
Preferred Stock will convert into 1,058,826 shares of Common Stock and the
549,622 shares of Series S-1 Preferred Stock will convert into 549,622 shares of
Common Stock.

      (c) The Company has reserved 1,931,401 shares of Common Stock for issuance
upon the exercise of stock options granted or available for future grant under
the Company's stock option plans.


                                       3.
<PAGE>
      (d) The Company has reserved 245,964 shares of Common Stock for issuance
upon the exercise of outstanding warrants to purchase Common Stock.

      (e) The Company has reserved 200,000 shares of Common Stock for issuance
under the Employee Stock Purchase Plan.

      (f) Peter Allard and Genencor International, Inc. each have the right to
purchase up to 500,000 shares of Common Stock of the Company under certain
conditions.

      (g) Except as set forth on Exhibit B, the Company does not have any direct
or indirect ownership interest in any corporation, partnership, joint venture,
association or other business enterprise.

      With the exception of the foregoing, there are no outstanding
subscriptions, options, warrants, convertible or exchangeable securities or
other rights granted to or by the Company to purchase shares of Common Stock or
other securities of the Company and there are no commitments, plans,
understandings or arrangements to issue any shares of Common Stock or any
security convertible into or exchangeable for Common Stock.

      4.3   ISSUANCE, SALE AND DELIVERY OF THE SHARES

      (a) The Shares have been duly authorized for issuance and sale to the
Purchasers pursuant to this Agreement and, when issued and delivered by the
Company pursuant to this Agreement against payment of the consideration set
forth in this Agreement, will be validly issued and fully paid and nonassessable
and free and clear of all pledges, liens and encumbrances. The certificates
evidencing the Shares are in due and proper form under Delaware law.

      (b) The issuance and sale of the Shares are not subject to any preemptive
right, co-sale right, right of first refusal, registration right or other
similar rights. No further approval or authority of the stockholders or the
Board of Directors of the Company or others will be required for the issuance
and sale of the Shares to be sold by the Company as contemplated in this
Agreement. There are no stockholders agreements, voting agreements or other
similar agreements with respect to the Common Stock to which the Company is a
party or, to the knowledge of the Company, between or among any of the Company's
stockholders. Except as set forth on Exhibit B, the issuance of the Shares will
not cause any adjustment of the conversion price for any series or class of the
Company's outstanding capital stock.

      (c) Subject to the accuracy of the Purchasers' representations and
warranties in Section 5 of this Agreement, the offer, sale, and issuance of the
Shares in conformity with the terms of this Agreement constitute transactions
exempt from the registration requirements of Section 5 of the Securities Act and
from the registration or qualification requirements of the laws of any
applicable state or United States jurisdiction.

      4.4   FINANCIAL STATEMENTS

      The financial statements and the related notes included (as exhibits or
otherwise) in the Company Documents (as defined below) present fairly the
financial position of the Company as


                                       4.
<PAGE>
of the dates indicated and the results of their operations and cash flows for
the periods specified. Except as otherwise stated in such Company Documents,
such financial statements and the related notes have been prepared in conformity
with generally accepted accounting principles applied on a consistent basis, and
any supporting schedules included with the financial statements present fairly
the information stated in the financial statements at the dates thereof
(subject, in the case of unaudited statements, to normal, recurring
adjustments). The financial and statistical data set forth in the Company
Documents were prepared on an accounting basis consistent with such financial
statements.

      4.5   NO MATERIAL CHANGE

      Since September 30, 2001 there have not been any of the following changes
from that reflected in the Company's Form 10-Q for the period ended September
30, 2001, except changes in the ordinary course of business.

      (a) there has been no material adverse change in the condition, financial
or otherwise, or in the earnings, assets, properties, business affairs or
business prospects of the Company, whether or not arising in the ordinary course
of business;

      (b) there have been no transactions entered into by the Company which are
material with respect to the Company; and

      (c) there has been no dividend or distribution of any kind declared, paid
or made by the Company on any class of its capital stock.

      The Company has no material contingent obligations that are not disclosed
in the Company Documents.

      4.6   ENVIRONMENTAL

      Except as would not, singly or in the aggregate, reasonably be expected to
have a material adverse effect on the condition, financial or otherwise, or the
earnings, assets, properties or business affairs or business prospects of the
Company,

      (a) to its best knowledge, the Company is in compliance with all
applicable Environmental Laws (as defined below);

      (b) to its best knowledge, the Company has all permits, authorizations and
approvals required under any applicable Environmental Laws and is in compliance
with the requirements of such permits authorizations and approvals;

      (c) there are no pending or, to the best knowledge of the Company,
threatened Environmental Claims (as defined below) against the Company; and

      (d) to its best knowledge, under applicable law, there are no
circumstances with respect to any property or operations of the Company that are
reasonably likely to form the basis of an Environmental Claim against the
Company.


                                       5.
<PAGE>
      For purposes of this Agreement, the following terms shall have the
following meanings: "ENVIRONMENTAL LAW" means any United States (or other
applicable jurisdiction's) Federal, state, local or municipal statute, law,
rule, regulation, ordinance, code, policy or rule of common law and any judicial
or administrative interpretation thereof, including any judicial or
administrative order, consent decree or judgment, relating to the environment,
health, safety or any chemical, material or substance, exposure to which is
prohibited, limited or regulated by any governmental authority. "ENVIRONMENTAL
CLAIMS" means any and all administrative, regulatory or judicial actions, suits,
demands, demand letters, claims, liens, notices of noncompliance or violation,
investigations or proceedings relating in any way to any Environmental Law.

      4.7   NO DEFAULTS

      The Company is not in violation of its certificate of incorporation or
bylaws or in default in the performance or observance of any obligation,
agreement, covenant or condition contained in any material contract, indenture,
mortgage, loan agreement, deed, trust, note, lease, sublease, voting agreement,
voting trust, or other instrument or material agreement to which the Company is
a party or by which it may be bound, or to which any of the property or assets
of the Company is subject.

      4.8   LABOR MATTERS

      No labor dispute with the employees of the Company exists or, to the
knowledge of the Company, has been threatened or is imminent.

      4.9   NO ACTIONS

      There is no action, suit or proceeding before or by any court or
governmental agency or body, domestic or foreign, now pending, or, to the
knowledge of the Company, threatened, against or affecting the Company which,
singly or in the aggregate, might result in any material adverse change in the
condition, financial or otherwise, or in the earnings, business affairs or
business prospects of the Company, or which, singly or in the aggregate, might
materially and adversely affect the properties or assets thereof or which might
materially and adversely affect the consummation of this Agreement, nor, to the
knowledge of the Company, is there any reasonable basis therefor. The Company is
not in default with respect to any judgment, order or decree of any court or
governmental agency or instrumentality which, singly or in the aggregate, would
have a material adverse effect on the condition, financial or otherwise, or the
earnings, assets, properties, business affairs or business prospects of the
Company.

      4.10  INTELLECTUAL PROPERTY

      (a) The Company owns, is licensed to use or possesses sufficient legal
rights to all patents, patent rights, patent applications, inventions,
trademarks, trade names, applications for registration of trademarks, service
marks, service mark applications, copyrights, know-how, manufacturing processes,
formulae, trade secrets, licenses and rights in any thereof and any other
intangible property and assets that are material to the business of the Company
as now conducted (in this Agreement called the "PROPRIETARY RIGHTS"), and is
seeking, or will seek, to obtain rights to use such Proprietary Rights that are
material to the business of the Company as proposed to be conducted.


                                       6.
<PAGE>
      (b) The Company does not have any knowledge of, and the Company has not
given or received any written notice of, any pending conflicts with or
infringement of the rights of others with respect to any Proprietary Rights or
with respect to any license of Proprietary Rights which are material to the
business of the Company. The Company has not received any notice of, nor does it
have any knowledge of, infringement by a third party with respect to any
Proprietary Rights of the Company.

      (c) No action, suit, arbitration, or legal, administrative or other
proceeding, or investigation is pending, or, to the knowledge of the Company,
threatened, which involves any Proprietary Rights, nor, to the knowledge of the
Company, is there any reasonable basis therefor.

      (d) The Company is not subject to any judgment, order, writ, injunction or
decree of any court or any Federal, state, local, foreign or other governmental
department, commission, board, bureau, agency or instrumentality, domestic or
foreign, or any arbitrator, which restricts or impairs the use of any such
Proprietary Rights in a manner which would have a material adverse effect on the
use of any of the Proprietary Rights.

      (e) The Company has not entered into any consent, indemnification,
forbearance to sue or settlement agreement with respect to Proprietary Rights
other than in the ordinary course of business. No claims have been asserted by
any person with respect to the validity of the Company's ownership or right to
use the Proprietary Rights and, to the knowledge of the Company, there is no
reasonable basis for any such claim to be successful.

      4.11  PERMITS

      The Company possesses and is operating in compliance with all necessary
licenses, certificates, consents, authorities, approvals and permits from all
state, federal, foreign and other regulatory agencies or bodies necessary to
conduct the businesses now operated by it, and the Company has not received any
notice of proceedings relating to the revocation or modification of any such
permit or any circumstance which would lead it to believe that such proceedings
are reasonably likely which, singly or in the aggregate, if the subject of an
unfavorable decision, ruling or finding, would materially and adversely affect
the condition, financial or otherwise, or the earnings, assets, properties,
business affairs or business prospects of the Company.

      4.12  DUE EXECUTION, DELIVERY AND PERFORMANCE

      (a) This Agreement has been duly executed and delivered by the Company and
constitutes a valid and binding obligation of the Company, enforceable against
the Company in accordance with its terms except (a) as limited by applicable
bankruptcy, insolvency, reorganization, moratorium or other laws of general
application affecting enforcement of creditors' rights, (b) general principles
of equity that restrict the availability of equitable remedies, and (c) to the
extent that the enforceability of the indemnification provisions in this
Agreement may be limited by applicable laws.

      (b) The execution, delivery and performance of this Agreement and the
consummation of the transactions contemplated in this Agreement and the
fulfillment of the terms of this Agreement, including the sale, issuance and
delivery of the Shares, (i) have been duly authorized by all necessary corporate
action on the part of the Company, its directors and


                                       7.
<PAGE>
stockholders; (ii) will not conflict with or constitute a breach or violation
of, or default under, or result in the creation or imposition of any lien,
security interest, charge, claim or encumbrance upon any property or assets of
the Company pursuant to, any contract, indenture, mortgage, loan agreement,
deed, trust, note, lease, sublease, voting agreement, voting trust or other
instrument or agreement to which the Company is a party or by which it may be
bound, or to which any of the property or assets of the Company is subject;
(iii) except as set forth on Exhibit B, will not trigger anti-dilution rights or
other rights to acquire additional equity securities of the Company; and (iv)
will not result in any violation of the provisions of the certificate of
incorporation or bylaws of the Company or any applicable statute, law, rule,
regulation, ordinance, decision, directive or order.

      4.13  PROPERTIES

      The Company has good and marketable title to its properties, free and
clear of all material security interests, mortgages, pledges, liens, charges,
encumbrances and claims of record other than (a) those resulting from taxes
which have not yet become delinquent, (b) minor liens and encumbrances which do
not materially detract from the value of the property subject thereto or
materially impair the operations of the Company, and (c) those that have
otherwise arisen in the ordinary course of business. The properties of the
Company are, in the aggregate, in good repair (reasonable wear and tear
excepted), and suitable for their respective uses. Any real property held under
lease by the Company is held under valid, subsisting and enforceable (as to the
Company) leases with such exceptions as are not material and do not interfere
with the conduct of the business of the Company. The Company owns or leases all
such properties as are necessary to its business or operations as now conducted.

      4.14  COMPLIANCE

      The Company has conducted and is conducting its business in compliance
with all applicable Federal, state, local and foreign statutes, laws, rules,
regulations, ordinances, codes, decisions, decrees, directives and orders,
except where the failure to do so would not, singly or in the aggregate, have a
material adverse effect on the condition, financial or otherwise, or on the
earnings, assets, properties, business affairs or business prospects of the
Company.

      4.15  SECURITY MEASURES

      The Company takes all reasonable security measures designed to enable the
Company to assert trade secret protection in its non-patented technology.

      4.16  CONTRIBUTIONS

      The Company has not made any payment of funds of the Company or received
or retained any funds in violation of any law, rule or regulation.

      4.17  USE OF PROCEEDS; INVESTMENT COMPANY

      The Company intends to use the proceeds from the sale of the Shares for
working capital and other general corporate purposes. The Company is not now,
and after the sale of the Shares under this Agreement and under all other
agreements and the application of the net proceeds


                                       8.
<PAGE>
from the sale of the Shares described in the proceeding sentence will not be, an
"investment company" or an entity "controlled" by an "investment company" within
the meaning of the Investment Company Act of 1940, as amended the ("INVESTMENT
COMPANY ACT"), and shall conduct its business in a manner so that it will not
become subject to the Investment Company Act.

      4.18  PRIOR OFFERINGS

      All offers and sales of capital stock of the Company before the date of
this Agreement were at all relevant times duly registered or exempt from the
registration requirements of the Securities Act and were duly registered or
subject to an available exemption from the registration requirements of the
applicable state securities or Blue Sky laws.

      4.19  TAXES

      The Company has filed all material tax returns required to be filed, which
returns are true and correct in all material respects, and the Company is not in
default in the payment of any taxes, including penalties and interest,
assessments, fees and other charges, shown thereon due or otherwise assessed,
other than those being contested in good faith and for which adequate reserves
have been provided or those currently payable without interest which were
payable pursuant to said returns or any assessments with respect thereto.

      4.20  NON-COMPETITION AGREEMENTS

      To the knowledge of the Company, any full-time employee who has entered
into any non-competition, non-disclosure, confidentiality or other similar
agreement with any party other than the Company is not in violation of that
agreement as a result of the business currently conducted by the Company or such
person's performance of his or her obligations to the Company. The Company has
not received written notice that any consultant or scientific advisor of the
Company is in violation of any non-competition, non-disclosure, confidentiality
or similar agreement.

      4.21  TRANSFER TAXES

      On the Closing Date, all stock transfer or other taxes (other than income
taxes) that are required to be paid in connection with the sale and transfer of
the Shares to be sold to the Purchasers under this Agreement will be, or will
have been, fully paid or provided for by the Company and all laws imposing such
taxes will be or will have been fully complied with.

      4.22  INSURANCE

      The Company maintains insurance of the type and in the amount that the
Company reasonably believes is adequate for its business, including, but not
limited to, insurance covering all real and personal property owned or leased by
the Company against theft, damage, destruction, acts of vandalism and all other
risks customarily insured against by similarly situated companies, all of which
insurance is in full force and effect.

      4.23  GOVERNMENTAL/REGULATORY CONSENTS


                                       9.
<PAGE>
      No registration, authorization, approval, qualification or consent with or
required by any court or governmental/ regulatory authority or agency is
necessary in connection with the execution and delivery of this Agreement or the
offering, issuance or sale of the Shares under this Agreement.

      4.24  SECURITIES AND EXCHANGE COMMISSION FILINGS

      The Company has timely filed with the Securities and Exchange Commission
(the "COMMISSION") all documents required to be filed by the Company under the
Securities Exchange Act of 1934, as amended (the "EXCHANGE ACT").

      4.25  ADDITIONAL INFORMATION

      The Company represents and warrants that the information contained in the
following documents (the "COMPANY DOCUMENTS"), which will be provided to the
Purchasers before the Closing, complied in all material respects with the
Commission's requirements as of their respective filing dates and the
information contained therein as of their respective filing dates did not
contain an untrue statement of a material fact or omit to state a material fact
required to be stated therein or necessary to make the statements therein in
light of the circumstances under which they were made not misleading:

      (a) the Company's Annual Report on Form 10-K for the fiscal year ended
December 31, 2000 (the "ANNUAL REPORT");

      (b) the Company's Quarterly Reports on Form 10-Q for the fiscal quarters
ended March 31, 2001, June 30, 2001 and September 30, 2001;

      (c) the Company's Proxy Statement for its 2001 Annual Meeting of
Stockholders; and

      (d) all other documents, if any, filed by the Company with the Commission
since the Annual Report pursuant to the reporting requirements of the Exchange
Act.

      4.26  CONTRACTS

      The contracts described in the Company Documents or incorporated by
reference therein that are described as being ongoing are in full force and
effect (as to the Company) on the date hereof, except for contracts the
termination or expiration of which would not, singly or in the aggregate, have a
material adverse effect on the condition, financial or otherwise, or the
earnings, assets, properties, business affairs or business prospects of the
Company. The Company is not in material breach of or default under any such
contracts.

      4.27  NO INTEGRATED OFFERING

      Neither the Company, nor any of its affiliates, nor any person acting on
its or their behalf, has directly or indirectly made any offers or sales in any
security or solicited any offers to buy any security under circumstances that
would require registration under the Securities Act of the issuance of the
Shares to the Purchasers. The issuance of the Shares to the Purchasers will not
be


                                      10.
<PAGE>
integrated with any other issuance of the Company's securities (past, current or
future) for purposes of the Securities Act or any applicable rules of Nasdaq (or
of any national securities exchange on which the Company's Common Stock is then
traded). The Company will not make any offers or sales of any security (other
than the Shares) that would cause the offering of the Shares to be integrated
with any other offering of securities by the Company for purposes of any
registration requirement under the Securities Act or any applicable rules of
Nasdaq (or of any national securities exchange on which the Company's Common
Stock is then traded).

      4.28  LISTING OF SHARES

      The Company agrees to promptly secure the listing of the Shares upon each
national securities exchange or automated quotation system upon which shares of
Common Stock are then listed and, so long as any Purchaser owns any of the
Shares, shall maintain such listing of all Shares and comply with all
requirements of any such exchange or automated quotation system. The Company's
Common Stock is registered pursuant to Section 12(g) of the Exchange Act and is
listed on The Nasdaq Stock Market, Inc. National Market (the "NASDAQ NATIONAL
MARKET") and the Company has taken no action designed to, or which is likely to
have the effect of, terminating the registration of the Common Stock under the
Exchange Act or delisting the Common Stock from the Nasdaq National Market, nor
has the Company received any notification that the Commission or the National
Association of Securities Dealers, Inc. (the "NASD") is contemplating
terminating such registration or listing. The consummation of the transactions
contemplated by this Agreement (including, without limitation, the issuance to
the Purchasers of the Shares) does not and will not require any shareholder
approval under Rule 4350(i)(1) of the Nasdaq National Market Marketplace Rules.

      4.29  DISCLOSURE

      The slide presentation given to International Biotechnology Trust PLC on
November 12, 2001 (the "PRESENTATION"), subject to the disclaimer on
forward-looking statements contained on page 2 therein, did not contain an
untrue statement of a material fact or omit to state a material fact required to
be stated therein or necessary to make the statements therein, in light of the
circumstances under which they were made, not misleading.

      4.30  NO MANIPULATION OF STOCK

      The Company has not taken and will not, in violation of applicable law,
take any action outside the ordinary course of business designed to or that
might reasonably be expected to cause or result in stabilization or unlawful
manipulation of the price of the Common Stock to facilitate the sale or resale
of the Shares.

5.    REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE PURCHASERS

      5.1   SECURITIES LAW REPRESENTATIONS AND WARRANTIES

      Each Purchaser severally, but not jointly, represents, warrants and
covenants to the Company as follows:


                                      11.
<PAGE>
      (a) The Purchaser is knowledgeable, sophisticated and experienced in
making, and is qualified to make, decisions with respect to investments in
shares representing an investment decision like that involved in the purchase of
the Shares, including investments in securities issued by the Company, and has
requested, received, reviewed and considered all information it deems relevant
in making an informed decision to purchase the Shares.

      (b) The Purchaser is acquiring the number of Shares set forth on Schedule
A in the ordinary course of its business and for its own account for investment
only, and has no present intention of distributing any of the Shares nor any
arrangement or understanding with any other persons regarding the distribution
of such Shares within the meaning of Section 2(11) of the Securities Act, except
in compliance with Section 5.1(c).

      (c) 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 Shares except in compliance
with the Securities Act, applicable blue sky laws and the rules and regulations
promulgated thereunder (the "RULES AND REGULATIONS").

      (d) The Purchaser has completed or caused to be completed the Stock
Certificate Questionnaire and the Registration Statement Questionnaire, attached
to this Agreement as Appendices I and II, for use in preparation of the
Registration Statement (as defined in Section 7.3 below), and the answers to the
Questionnaires are true and correct as of the date of this Agreement and will be
true and correct as of the effective date of the Registration Statement;
provided that the Purchasers shall be entitled to update such information by
providing notice thereof to the Company at least five business days before the
effective date of such Registration Statement.

      (e) The Purchaser has, in connection with its decision to purchase the
number of Shares set forth on Schedule A, relied solely upon the Company
Documents, the other information delivered to the Purchasers by the Company as
described in Section 5.1(a) above and the representations and warranties of the
Company contained in this Agreement.

      (f) The Purchaser is an "accredited investor" within the meaning of Rule
501 of Regulation D promulgated under the Securities Act.

      (g) The Purchaser represents and warrants to and covenants with the
Company that it has not engaged and will not engage in any short sales of the
Company's Common Stock. Each party hereto acknowledges that each Purchaser is a
third party beneficiary to the representations, warranties and covenants made by
each other Purchaser under this Section 5.1(g).

      5.2   RESALES OF SHARES

      (a) The Purchaser hereby covenants with the Company not to make any sale
of the Shares without satisfying the requirements of the Securities Act and the
Rules and Regulations, including, in the event of any resale under the
Registration Statement, the prospectus delivery requirements under the
Securities Act, and the Purchaser acknowledges and agrees that such Shares are
not transferable on the books of the Company pursuant to a resale under the
Registration Statement unless the certificate submitted to the transfer agent
evidencing the Shares is accompanied by a separate officer's certificate:


                                      12.
<PAGE>
            (i) in the form of Appendix III to this Agreement;

            (ii) executed by an officer of, or other authorized person
designated by, the Purchaser; and

            (iii) to the effect that (A) the Shares have been sold in accordance
with the Registration Statement and (B) the requirement of delivering a current
prospectus has been satisfied.

      (b) The Purchaser acknowledges that there may occasionally be times when
the Company determines, in good faith following consultation with its Board of
Directors or a committee thereof, the use of the prospectus forming a part of
the Registration Statement (the "PROSPECTUS," as further defined in Section
7.3.1 below) should be suspended until such time as an amendment or supplement
to the Registration Statement or the Prospectus has been filed by the Company
and any such amendment to the Registration Statement is declared effective by
the Commission, or until such time as the Company has filed an appropriate
report with the Commission pursuant to the Exchange Act. Subject to clause (c)
below, the Purchaser hereby covenants that it will not sell any Shares pursuant
to the Prospectus during the period commencing at the time at which the Company
gives the Purchaser written notice of the suspension of the use of the
Prospectus and ending at the time the Company gives the Purchaser written notice
that the Purchaser may thereafter effect sales pursuant to the Prospectus. The
Company may, upon written notice to the Purchasers, suspend the use of the
Prospectus for such period as deemed necessary by the Company based on the
reasonable determination of the Company's Board of Directors that there is a
significant business purpose for such determination, such as pending corporate
developments, public filings with the SEC or similar events; provided that the
Company will use best efforts to limit the length of all such periods, in the
aggregate, to less than 45 days in any 365 day period; provided, further, that
in no event shall the Company suspend use of the Prospectus on more than two
occasions in any twelve month period. The Company shall in no event be required
to disclose the business purpose for which it has suspended the use of the
Prospectus if the Company determines in its good faith judgment that the
business purpose should remain confidential. In addition, the Company shall
notify each Purchaser (i) of any request by the SEC for an amendment or any
supplement to such Registration Statement or any related prospectus, or any
other information request by any other governmental agency directly relating to
the offering, and (ii) of the issuance by the SEC of any stop order suspending
the effectiveness of such Registration Statement or of any order preventing or
suspending the use of any related prospectus or the initiation or threat of any
proceeding for that purpose.

      (c) The Purchaser further covenants to notify the Company promptly of the
sale of any of its Shares, other than sales pursuant to a Registration Statement
contemplated in Section 7 of this Agreement or sales upon termination of the
transfer restrictions pursuant to Section 7.4 of this Agreement.

      5.3   DUE EXECUTION, DELIVERY AND PERFORMANCE

      (a) This Agreement has been duly executed and delivered by the Purchaser
and constitutes a valid and binding obligation of the Purchaser, enforceable
against the Purchaser in


                                      13.
<PAGE>
accordance with its terms except (a) as limited by applicable bankruptcy,
insolvency, reorganization, moratorium or other laws of general application
affecting enforcement of creditors' rights, (b) general principles of equity
that restrict the availability of equitable remedies, and (c) to the extent that
the enforceability of the indemnification provisions in this Agreement may be
limited by applicable laws.

      (b) The execution, delivery and performance of this Agreement and the
consummation of the transactions contemplated in this Agreement and the
fulfillment of the terms of this Agreement have been duly authorized by all
necessary corporate, agency or other action on the part of each Purchaser.

      (c) The Purchaser understands that nothing in the Company Documents, this
Agreement or any other materials presented to the Purchaser in connection with
the purchase and sale of the Shares constitutes legal, tax or investment advice.
The Purchaser has consulted such legal, tax and investment advisors as it, in
its sole discretion, has deemed necessary or appropriate in connection with its
purchase of the Shares.

6.    SURVIVAL OF REPRESENTATIONS, WARRANTIES AND AGREEMENTS.

      Notwithstanding any investigation made by any party to this Agreement, all
covenants, agreements, representations and warranties made by the Company and
the Purchasers in this Agreement and in the certificates for the Shares
delivered pursuant to this Agreement shall survive the execution of this
Agreement, the delivery to the Purchasers of the Shares being purchased and the
payment therefor.

7.    FORM D FILING; REGISTRATION; COMPLIANCE WITH THE SECURITIES ACT; COVENANTS

      7.1   FORM D FILING; REGISTRATION OF SHARES

            7.1.1 REGISTRATION STATEMENT; EXPENSES

      The Company shall:

      (a) file in a timely manner a Form D relating to the sale of the Shares
under this Agreement, pursuant to the Commission Regulation D.

      (b) as soon as practicable after the Closing Date, but in no event later
than the 30th day following the Closing Date, prepare and file with the
Commission a Registration Statement on Form S-3 (or, if the Company is
ineligible to use Form S-3, then on Form S-1) relating to the sale of the Shares
by the Purchasers from time to time on the Nasdaq National Market (or the
facilities of any national securities exchange on which the Company's Common
Stock is then traded) or in privately negotiated transactions (the "REGISTRATION
STATEMENT");

      (c) provide to Purchasers any information required to permit the sale of
the Shares under Rule 144A of the Securities Act;


                                      14.
<PAGE>
      (d) subject to receipt of necessary information from the Purchasers, use
its best efforts to cause the Commission to notify the Company of the
Commission's willingness to declare the Registration Statement effective on or
before 90 days after the Closing Date;

      (e)   notify Purchasers promptly upon the Registration Statement, and any
post-effective amendment thereto, being declared effective by the Commission;

      (f) use its best efforts to prepare and file with the Commission such
amendments and supplements to the Registration Statement and the Prospectus (as
defined in Section 7.3.1 below) and take such other action, if any, as may be
necessary to keep the Registration Statement effective and free from any
material misstatement or omission to state a material fact until the earlier of
(i) the date on which the Shares may be resold by the Purchasers without
registration and without regard to any volume limitations by reason of Rule
144(e) under the Securities Act or any other rule of similar effect or (ii) all
of the Shares have been sold pursuant to the Registration Statement or Rule 144
under the Securities Act or any other rule of similar effect;

      (g) promptly furnish to the Purchasers with respect to the Shares
registered under the Registration Statement such reasonable number of copies of
the Registration Statement, Prospectus and Preliminary Prospectus, including any
supplements to or amendments of the Prospectus, as the Purchasers may request in
order to facilitate the public sale or other disposition of all or any of the
Shares by the Purchasers;

      (h) during the period when copies of the Prospectus are required to be
delivered under the Securities Act or the Exchange Act, will file all documents
required to be filed with the Commission pursuant to Section 13, 14 or 15 of the
Exchange Act within the time periods required by the Exchange Act and the rules
and regulations promulgated thereunder;

      (i) file documents required of the Company for customary Blue Sky
clearance in all states requiring Blue Sky clearance; provided, however, that
the Company shall not be required to qualify to do business or consent to
service of process in any jurisdiction in which it is not now so qualified or
has not so consented;

      (j) bear all expenses in connection with the procedures in paragraphs (a)
through (i) of this Section 7.1.1 and the registration of the Shares pursuant to
the Registration Statement; and

      (k) advise the Purchasers, promptly after it shall receive notice or
obtain knowledge of the issuance of any stop order by the Commission delaying or
suspending the effectiveness of the Registration Statement or of the initiation
or threat of any proceeding for that purpose; and it will promptly use its best
efforts to prevent the issuance of any stop order or to obtain its withdrawal at
the earliest possible moment if such stop order should be issued.

            7.1.2 DELAY IN EFFECTIVENESS OF REGISTRATION STATEMENT

      In the event that the Registration Statement is not declared effective on
or before the 120th day following the Closing Date (the "PENALTY DATE"), the
Company shall pay to each Purchaser liquidated damages in an amount equal to
0.25% of the total purchase price of the


                                      15.
<PAGE>
Shares purchased by such Purchaser pursuant to this Agreement for each week or a
portion of a week after the Penalty Date that the Registration Statement is not
declared effective.

      7.2   TRANSFER OF SHARES AFTER REGISTRATION

      Each Purchaser agrees that it will not effect any disposition of the
Shares or its right to purchase the Shares that would constitute a sale within
the meaning of the Securities Act, except as contemplated in the Registration
Statement referred to in Section 7.1 or as otherwise permitted by law, and that
it will promptly notify the Company of any changes in the information set forth
in the Registration Statement regarding the Purchaser or its plan of
distribution.

      7.3   INDEMNIFICATION

      For the purpose of this Section 7.3, the term "REGISTRATION STATEMENT"
shall include any preliminary or final prospectus, exhibit, supplement or
amendment included in or relating to the Registration Statement referred to in
Section 7.1.

            7.3.1 INDEMNIFICATION BY THE COMPANY

      The Company agrees to indemnify and hold harmless each of the Purchasers,
each affiliate of each such Purchaser, and each person, if any, who controls any
such Purchaser within the meaning of the Securities Act (individually, a
"SELLING STOCKHOLDER" and collectively, the "SELLING STOCKHOLDERS"), against any
losses, claims, damages, liabilities or expenses, joint or several, to which
such Selling Stockholders may become subject, under the Securities Act, the
Exchange Act, or any other federal or state statutory law or regulation, or at
common law or otherwise (including in settlement of any litigation, if such
settlement is effected with the written consent of the Company, which consent
shall not be unreasonably withheld), insofar as such losses, claims, damages,
liabilities or expenses (or actions or proceedings in respect thereof as
contemplated below) arise out of or are based upon any untrue statement or
alleged untrue statement of any material fact contained in the Registration
Statement, including, without limitation, the Prospectus, financial statements
and schedules, and all other documents filed as a part thereof, as amended at
the time of effectiveness of the Registration Statement, including, without
limitation, any information deemed to be a part thereof as of the time of
effectiveness pursuant to paragraph (b) of Rule 430A, or pursuant to Rule 434,
of the Rules and Regulations, or the Prospectus, in the form first filed with
the Commission pursuant to Rule 424(b) of the Regulations, or filed as part of
the Registration Statement at the time of effectiveness if no Rule 424(b) filing
is required (the "PROSPECTUS"), or any amendment or supplement thereto, or arise
out of or are based upon the omission or alleged omission to state in any of
them a material fact required to be stated therein or necessary to make the
statements in any of them, in light of the circumstances under which they were
made, not misleading, or arise out of or are based in whole or in part on any
inaccuracy in the representations and warranties of the Company contained in
this Agreement, or any failure of the Company to perform its obligations under
this Agreement, the Registration Statement or under applicable law, and will
reimburse each Selling Stockholder for any legal and other expenses as such
expenses are reasonably incurred by such Selling Stockholder in connection with
investigating, defending, settling, compromising or paying any such loss, claim,
proceeding, damage, liability, expense or action; provided, however, that the
Company will not be liable in any such case to the extent that any such loss,
claim, damage,


                                      16.
<PAGE>
liability or expense arises out of or is based upon (i) an untrue statement or
alleged untrue statement or omission or alleged omission made in the
Registration Statement, the Prospectus or any amendment or supplement of the
Registration Statement or Prospectus in reliance upon and in conformity with
written information furnished to the Company by or on behalf of the Selling
Stockholder expressly for use in the Registration Statement or the Prospectus,
or (ii) the failure of such Selling Stockholder to comply with the covenants and
agreements contained in Sections 5.2 or 7.2 of this Agreement respecting resale
of the Shares, or (iii) the inaccuracy of any representations made by such
Selling Stockholder in this Agreement or (iv) any untrue statement or omission
of a material fact required to make such statement not misleading in any
Prospectus that is corrected in any subsequent Prospectus that was delivered to
the Selling Stockholder before the pertinent sale or sales by the Selling
Stockholder. The Company shall reimburse each Selling Stockholder for the
amounts provided for herein on demand as such expenses are incurred.

            7.3.2 INDEMNIFICATION BY THE PURCHASERS

      Each Purchaser will severally and not jointly indemnify and hold harmless
the Company, each of its directors, each of its officers who signed the
Registration Statement and each person, if any, who controls the Company within
the meaning of Section 15 of the Securities Act, against any losses, claims,
damages, liabilities or expenses, joint or several, to which the Company, each
of its directors, each of its officers who signed the Registration Statement or
controlling person may become subject, under the Securities Act, the Exchange
Act, or any other federal or state statutory law or regulation, or at common law
or otherwise (including in settlement of any litigation, if such settlement is
effected with the written consent of such Purchaser, which consent shall not be
unreasonably withheld) insofar as such losses, claims, damages, liabilities or
expenses (or actions in respect thereof as contemplated below) arise out of or
are based upon (i) any failure on the part of such Purchaser to comply with the
covenants and agreements contained in Sections 5.2 or 7.2 of this Agreement
respecting the sale of the Shares or (ii) the inaccuracy of any representation
made by such Purchaser in this Agreement or (iii) any untrue or alleged untrue
statement of any material fact contained in the Registration Statement, the
Prospectus, or any amendment or supplement to the Registration Statement or
Prospectus, or arise out of or are based upon the omission or alleged omission
to state therein a material fact required to be stated therein or necessary to
make the statements therein not misleading, in each case to the extent, but only
to the extent, that such untrue statement or alleged untrue statement or
omission or alleged omission was made in the Registration Statement, the
Prospectus, or any amendment or supplement thereto, in reliance upon and in
conformity with written information furnished to the Company by or on behalf of
such Purchaser expressly for use therein; provided, however, that the Purchaser
shall not be liable for any such untrue or alleged untrue statement or omission
or alleged omission of which the Purchaser has delivered to the Company in
writing a correction before the occurrence of the transaction from which such
loss was incurred, and the Company was able to correct such untrue or alleged
untrue statement or omission or alleged omission prior to the occurrence of the
transaction from which such loss was incurred and such correction actually
prevented such loss and the Purchaser will reimburse the Company, each of its
directors, each of its officers who signed the Registration Statement or
controlling person for any legal and other expense reasonably incurred by the
Company, each of its directors, each of its officers who signed the Registration
Statement or controlling person in connection with investigating, defending,
settling, compromising or paying any such loss, claim, damage, liability,
expense or


                                      17.
<PAGE>
action, and provided, further, that each Purchaser's obligation to indemnify the
Company shall be limited to the net amount received by each such Purchaser from
the sale of the Shares.

            7.3.3 INDEMNIFICATION PROCEDURE

      (a) Promptly after receipt by an indemnified party under this Section 7.3
of notice of the threat or commencement of any action, such indemnified party
will, if a claim in respect thereof is to be made against an indemnifying party
under this Section 7.3, promptly notify the indemnifying party in writing of the
claim; but the omission so to notify the indemnifying party will not relieve it
from any liability which it may have to any indemnified party for contribution
or otherwise under the indemnity agreement contained in this Section 7.3 or to
the extent it is not prejudiced as a result of such failure.

      (b) In case any such action is brought against any indemnified party and
such indemnified party seeks or intends to seek indemnity from an indemnifying
party, the indemnifying party will be entitled to participate in, and, to the
extent that it elects by delivering a written notice to the indemnified party
promptly after notification from such indemnified party of any such action,
jointly with all other indemnifying parties similarly notified, to assume the
defense thereof with counsel reasonably satisfactory to such indemnified party;
provided, however, if the defendants in any such action include both the
indemnified party and the indemnifying party and the indemnified party shall
have reasonably concluded that there may be a conflict between the positions of
the indemnifying party and the indemnified party in conducting the defense of
any such action or that there may be legal defenses available to it or other
indemnified parties that are different from or additional to those available to
the indemnifying party, the indemnified party or parties shall have the right to
select separate counsel to assume such legal defenses and to otherwise
participate in the defense of such action on behalf of such indemnified party or
parties. Upon receipt of notice from the indemnifying party to such indemnified
party of its election so to assume the defense of such action and approval by
the indemnified party of counsel, the indemnifying party will not be liable to
such indemnified party under this Section 7.3 for any legal or other expenses
subsequently incurred by such indemnified party in connection with the defense
thereof unless:

            (i) the indemnified party shall have employed such counsel in
connection with the assumption of legal defenses in accordance with the proviso
to the preceding sentence (it being understood, however, that the indemnifying
party shall not be liable for the expenses of more than one separate counsel,
approved by such indemnifying party representing all of the indemnified parties
who are parties to such action) or

            (ii) the indemnifying party shall not have employed counsel
reasonably satisfactory to the indemnified party to represent the indemnified
party within a reasonable time after notice of commencement of action, in each
of which cases the reasonable fees and expenses of counsel shall be at the
expense of the indemnifying party. Notwithstanding the provisions of this
Section 7.3, the Purchaser shall not be liable for any indemnification
obligation under this Agreement in excess of the amount of net proceeds received
by the Purchaser from the sale of the Shares.


                                      18.
<PAGE>
      (c) In no event shall any indemnifying party be liable in respect of any
amounts paid in settlement of any action unless the indemnifying party shall
have approved the terms of such settlement; provided, however, that such consent
shall not be unreasonably withheld. No indemnifying party shall, without the
prior written consent of the indemnified party, effect any settlement of any
pending or threatened proceeding in respect of which any indemnified party is or
could have been a party and indemnification could have been sought hereunder by
such indemnified party, unless such settlement includes an unconditional release
of such indemnified party from all liability on claims that are the subject
matter of such proceeding.

            7.3.4 CONTRIBUTION

      If the indemnification provided for in this Section 7.3 is required by its
terms but is for any reason held to be unavailable to or otherwise insufficient
to hold harmless an indemnified party under this Section 7.3 in respect to any
losses, claims, damages, liabilities or expenses (or actions or proceedings in
respect thereof) referred to in this Agreement, then each applicable
indemnifying party shall contribute to the amount paid or payable by such
indemnified party as a result of any losses, claims, damages, liabilities or
expenses (or actions or proceedings in respect thereof) referred to in this
Agreement

      (a) in such proportion as is appropriate to reflect the relative benefits
received by the Company and the Purchaser, as well as any other Selling
Stockholders under such registration statement, from the placement of Common
Stock; or

      (b) if the allocation provided by clause (a) above is not permitted by
applicable law, in such proportion as is appropriate to reflect not only the
relative benefits referred to in clause (a) above but the relative fault of the
Company and the Purchaser, as well as any other Selling Stockholders under such
registration statement, in connection with the statements or omissions, or
inaccuracies in the representations and warranties in this Agreement, that
resulted in such losses, claims, damages, liabilities or expenses (or actions or
proceedings in respect thereof), as well as any other relevant equitable
considerations.

      The respective relative benefits received by the Company on the one hand
and each Purchaser, or other Selling Stockholder, on the other shall be deemed
to be in the same proportion as the amount paid by such Purchaser to the Company
pursuant to this Agreement for the Shares purchased by such Purchaser that were
sold pursuant to the Registration Statement bears to the difference (the
"DIFFERENCE") between the amount such Purchaser paid for the Shares that were
sold pursuant to the Registration Statement and the amount received by such
Purchaser from such sale. The relative fault of the Company and each Purchaser
shall be determined by reference to, among other things, whether the untrue or
alleged statement of a material fact or the omission or alleged omission to
state a material fact or the inaccurate or the alleged inaccurate representation
or warranty relates to information supplied by the Company or by such Purchaser,
or other Selling Stockholder, and the parties' relative intent, knowledge,
access to information and opportunity to correct or prevent such statement or
omission. The amount paid or payable by a party as a result of the losses,
claims, damages, liabilities and expenses referred to above shall be deemed to
include, subject to the limitations set forth in Section 7.3.3, any legal or
other fees or expenses reasonably incurred by such party in connection with
investigating or defending any action or claim. The provisions set forth in


                                      19.
<PAGE>
Section 7.3.3 with respect to the notice of the threat or commencement of any
threat or action shall apply if a claim for contribution is to be made under
this Section 7.3.4; provided, however, that no additional notice shall be
required with respect to any threat or action for which notice has been given
under Section 7.3 for purposes of indemnification. The Company and each
Purchaser agree that it would not be just and equitable if contribution pursuant
to this Section 7.3 were determined solely by pro rata allocation (even if the
Purchaser and other Selling Stockholders were treated as one entity for such
purpose) or by any other method of allocation which does not take account of the
equitable considerations referred to in this paragraph. Notwithstanding the
provisions of this Section 7.3, no Purchaser shall be required to contribute any
amount in excess of the amount by which the Difference exceeds the amount of any
damages that such Purchaser has otherwise been required to pay by reason of such
untrue or alleged untrue statement or omission or alleged omission. No person
guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of
the Securities Act) shall be entitled to contribution from any person who was
not guilty of such fraudulent misrepresentation. The Purchasers' obligations to
contribute pursuant to this Section 7.3 are several and not joint with any other
Purchaser or any other Selling Stockholder.

      7.4   TERMINATION OF CONDITIONS AND OBLIGATIONS

      The restrictions imposed by Section 5 or this Section 7 upon the
transferability of the Shares shall cease and terminate as to any particular
Shares (i) when such Shares have been sold or otherwise disposed of in
accordance with the intended method of disposition set forth in the Registration
Statement covering such Shares, (ii) upon the passage of two years from the
Closing Date or (iii) at such time as an opinion of counsel satisfactory in form
and substance to the Company shall have been rendered to the effect that such
conditions are not necessary in order to comply with the Securities Act.

      7.5   INFORMATION AVAILABLE

      From the date of this Agreement through the date the Registration
Statement covering the resale of Shares owned by any Purchaser is no longer
effective, the Company will furnish to such Purchaser:

      (a) as soon as practicable after available (but in the case of the
Company's Annual Report to Stockholders, within 120 days after the end of each
fiscal year of the Company), one copy of

            (i) its Annual Report to Stockholders (which Annual Report shall
contain financial statements audited in accordance with generally accepted
accounting principles by a national firm of certified public accountants);

            (ii)  if not included in substance in the Annual Report to
Stockholders, its Annual Report on Form 10-K;

            (iii) its quarterly reports on Form 10-Q; and

            (iv) a full copy of the particular Registration Statement covering
the Shares (the foregoing, in each case, excluding exhibits);


                                      20.
<PAGE>
      (b) upon the request of such Purchaser, a reasonable number of copies of
the Prospectus to supply to any other party requiring the Prospectus; and the
Company, upon the reasonable request of each Purchaser, will meet with the
Purchaser or a representative thereof at the Company's headquarters to discuss
all information relevant for disclosure in the Registration Statement covering
the Shares and will otherwise cooperate with such Purchaser in conducting an
investigation for the purpose of reducing or eliminating such Purchaser's
exposure to liability under the Securities Act, including the reasonable
production of information at the Company's headquarters; provided, that the
Company shall not be required to disclose any confidential information to, or
meet at its headquarters with, any Purchaser until and unless such Purchaser
shall have entered into a confidentiality agreement in form and substance
reasonably satisfactory to the Company with the Company with respect thereto.

      7.6   RULE 144 INFORMATION

      Until the earlier of (i) the date on which the Shares may be resold by the
Purchasers without registration and without regard to any volume limitations by
reason of Rule 144(e) under the Securities Act or any other rule of similar
effect or (ii) all of the Shares have been sold pursuant to the Registration
Statement or Rule 144 under the Securities Act or any other rule of similar
effect, the Company shall file all reports required to be filed by it under the
Securities Act, the Rules and Regulations and the Exchange Act and shall take
such further action to the extent required to enable the Purchasers to sell the
Shares pursuant to Rule 144 under the Securities Act (as such rule may be
amended from time to time).

       7.7  STOCK OPTION MATTERS AND PROHIBITION ON TOXICS

      The Company shall, within thirty (30) days of the Closing Date, adopt such
amendments to, with respect to (i) and (ii) below, the Company's stock option
plans and By-laws, and, with respect to (iii) and (iv) below, the Company's
By-laws (together, the "Stock Option Plan and By-law Amendments") to provide
that, unless approved by the holders of a majority of the shares present and
entitled to vote at a duly convened meeting of stockholders, the Company shall
not:

            (i) grant any stock option, including stock appreciation right, with
an exercise price that is less than 100% of the fair market value of the
underlying stock on the date of grant;

            (ii) reduce the exercise price of any stock option, including stock
appreciation right, outstanding or to be granted in the future; cancel and
re-grant options at a lower exercise price (including entering into any "6 month
and 1 day" cancellation and re-grant scheme), whether or not the cancelled
options are put back into the available pool for grant; replace underwater
options with restricted stock in an exchange, buy-back or other scheme; or
replace any options with new options having a lower exercise price or
accelerated vesting schedule in an exchange, buy-back or other scheme;

            (iii) sell or issue any security of the Company convertible,
exercisable or exchangeable into shares of common stock of the Company, having a
conversion, exercise or exchange price per share which is subject to downward
adjustment based on the market price of the common stock at the time of
conversion, exercise or exchange of such security into common


                                      21.
<PAGE>
stock (except for appropriate adjustments made to give effect to any stock
splits or stock dividends); or

            (iv) enter into (a) any equity line or similar agreement or
arrangement; or (b) any agreement to sell common stock of the Company (or any
security convertible, exercisable or exchangeable into shares of common stock
("Common Stock Equivalent")) at a per share price (or, with respect to a Common
Stock Equivalent, at a conversion, exercise or exchange price, as the case may
be ("Equivalent Price")) that is fixed after the execution date of the
agreement, whether or not based on any predetermined price-setting formula or
calculation method. Notwithstanding the foregoing, however, a price protection
clause shall be permitted in an agreement for sale of common stock or Common
Stock Equivalent, if such clause provides for an adjustment to the price per
share of common stock or, with respect to a Common Stock Equivalent, to the
Equivalent Price (provided that such price or Equivalent Price is fixed on or
before the execution date of the agreement)(the "Fixed Price") in the event that
the Company, during the period beginning on the date of the agreement and ending
no later than 90 days after the closing date of the transaction, sells shares of
common stock or Common Stock Equivalent to another investor at a price or
Equivalent Price, as the case may be, below the Fixed Price.

      The Stock Option Plan and By-law Amendments may not be further amended or
repealed without the affirmative vote of the holders of a majority of the shares
present and entitled to vote at a duly convened meeting of stockholders. Upon
the adoption of the Stock Option Plan and By-law Amendments, the Company shall
promptly furnish a copy of such amendments to the Purchasers. The Company agrees
that, prior to the adoption of the Stock Option Plan and By-law Amendments by
all necessary corporate action of the Company as described above, the Company
shall not conduct any of the actions specified in (i), (ii), (iii) or (iv) above
of this Section 0.

8.    LEGAL FEES AND OTHER TRANSACTION EXPENSES

      At the Closing, the Company agrees to pay a flat fee of $5,000 to the
State of Wisconsin Investment Board, $2,000 to Peter Allard and $2,000 to
Genencor International, Inc. for their legal and other transaction expenses
(whether internal or external) arising in connection with the transactions
contemplated by this Agreement. In addition, the Company will pay up to $20,000
to International Biotechnology Trust PLC for their actual legal and other
transaction expenses (whether internal or external, including without
limitation, any due diligence fees and expenses)

9.    BROKER'S FEE

      Each of the parties to this Agreement 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 the Shares to the Purchasers. The
Company shall indemnify and hold harmless the Purchasers from and against all
fees, commissions or other payments owing by the Company to any person or firm
acting on behalf of the Company hereunder and the Purchasers shall severally,
and not jointly, indemnify and hold harmless the Company from and against all
fees, commissions or other payments owing by the Purchasers to any person or
firm acting on behalf of the Purchasers hereunder.


                                      22.
<PAGE>
10.   NOTICES

      All notices, requests, consents and other communications under this
Agreement shall be in writing, shall be mailed by first-class registered or
certified airmail, confirmed facsimile or nationally recognized overnight
express courier postage prepaid, and shall be delivered as addressed as follows:

      (a)   if to the Company, to:

      Epimmune Inc.
      5820 Nancy Ridge Drive, Suite 100
      San Diego, CA 92121
      Attn:  Chief Financial Officer

or to such other person at such other place as the Company shall designate to
the Purchaser in writing; and

      (b) if to a Purchaser, at its address as set forth on the signature page
to this Agreement, or at such other address or addresses as may have been
furnished to the Company in writing.

      Such notice shall be deemed effectively given upon confirmation of receipt
by facsimile, one business day after deposit with such overnight courier or
three days after deposit of such registered or certified airmail with the U.S.
Postal Service, as applicable.

11.   MODIFICATION; AMENDMENT OR WAIVER

      This Agreement may not be modified or amended, and no provision hereof
shall be waived, except pursuant to an instrument in writing signed by the
Company and each of the Purchasers.

12.   TERMINATION

      This Agreement may be terminated as to any Purchaser, at the option of
such Purchaser, if the Closing has not occurred on or before thirty (30) days
from the date of this Agreement.

13.   HEADINGS

      The headings of the various sections of this Agreement have been inserted
for convenience of reference only and shall not be deemed to be part of this
Agreement.

14.   SEVERABILITY

      If any provision contained in this Agreement should be held to be invalid,
illegal or unenforceable in any respect, the validity, legality and
enforceability of the remaining provisions contained in this Agreement shall not
in any way be affected or impaired thereby.

15.   GOVERNING LAW; JURISDICTION


                                      23.
<PAGE>

      This Agreement shall be governed by and construed in accordance with the
laws of the state of Delaware and the federal law of the United States of
America. Nothing herein shall affect the right of the Purchaser to serve process
in any manner permitted by law or limit the right of the Purchaser to bring
proceedings against the Company in the competent courts of any other
jurisdiction or jurisdictions.

16.   NO CONFLICTS OF INTEREST

The Company represents, warrants, and covenants that, to its knowledge, no
trustee or employee of the State of Wisconsin Investment Board identified on the
attached list, either directly or indirectly (a) currently holds, except as may
be specifically set forth below, a personal interest in the Company or any of
its affiliates (together, the "ENTITY") or the Entity's property or securities,
or (b) will, in connection with the investment made pursuant to this Agreement,
receive (i) a personal interest in the Entity or the Entity's property or
securities or (ii) anything of substantial economic value for his or her private
benefit from the Entity or anyone acting on its behalf. As to ownership of an
interest in the Entity's publicly traded securities, "knowledge" hereunder is
based solely on an examination of a list of record holders provided by American
Stock Transfer & Trust Company to the Company (and also to the State of
Wisconsin Investment Board) and actual knowledge of the undersigned.

17.   COUNTERPARTS

      This Agreement may be executed in two or more counterparts, each of which
shall constitute an original, but all of which, when taken together, shall
constitute but one instrument, and shall become effective when one or more
counterparts have been signed by each party to this Agreement and delivered to
the other parties.

18.   ENTIRE AGREEMENT

      This Agreement and other documents delivered pursuant hereto, including
the exhibits, constitute the full and entire understanding and agreement between
the parties with regard to the subjects hereof and thereof.

19.   SUCCESSORS AND ASSIGNS

      Except as otherwise expressly provided herein, the provisions of this
Agreement shall inure to the benefit of, and be binding upon, the successors,
assigns, heirs, executors and administrators of the parties hereto.


                                      24.
<PAGE>


      IN WITNESS WHEREOF, the parties to this Agreement have caused this
Agreement to be executed by their duly authorized representatives as of the day
and year first above written.

                                    EPIMMUNE INC.



                                    By:  /s/  Robert De Vaere
                                         ----------------------------------

                                    Name:   Robert De Vaere
                                           ------------------------------

                                    Its:    VP, Finance and CFO
                                          --------------------------------

                                    Address:

                                    5820 Nancy Ridge Drive, Suite 100
                                    San Diego, CA 92121
                                    Facsimile: (858) 860-2600



                                    STATE OF WISCONSIN INVESTMENT BOARD

                                    By:    /s/  John F. Nelson
                                         -----------------------------------

                                    Name:    John F. Nelson
                                           --------------------------------

                                    Title:    Investment Director
                                            ---------------------------------

                                    Address:

                                    121 East Wilson Street
                                    Madison, WI 53702
                                    Facsimile: (608) 266-2436

                                    GENENCOR INTERNATIONAL, INC.

                                    By:    /s/  Stuart L. Melton
                                         ------------------------------------

                                    Name:    Stuart L. Melton
                                           -------------------------------

                                    Title:   Senior VP and General Counsel
                                            ---------------------------------

                                    Address:

                                    925 Page Mill Road
                                    Palo Alto, CA 94304


                            SHARE PURCHASE AGREEMENT
                                 SIGNATURE PAGE
<PAGE>
                                    GREENE FAMILY TRUST

                                    By:    /s/  Ted Greene
                                         ------------------------------------
                                          Ted Greene
                                          Trustee

                                    Address:    P.O. Box 8984
                                                Rancho Santa Fe, CA 92067



                                    PETER ALLARD

                                    By:    /s/ Peter Allard
                                         -------------------------------------

                                    Address:    Seaview
                                                Chancery Lane
                                                Christ Church
                                                Barbados



                                    INTERNATIONAL BIOTECHNOLOGY TRUST PLC

                                    By:     /s/  Tom Daniel
                                         ----------------------------------

                                    Name:      Tom Daniel
                                           -----------------------------

                                    Title:    IBT Signatory
                                            -------------------------------

                                    Address:

                                    31 Gresham Street
                                    London
                                    EC2V 7QA
                                    United Kingdom
                                    Attention: Steve Cunningham
                                    Facsimile: 44(0)20 7658 2422

                                    With a copy to:

                                    Testa, Hurwitz & Thibeault, LLP
                                    125 High Street
                                    Boston, MA 02110
                                    Attention: Mitchell S. Bloom, Esq.
                                    Facsimile: (617) 248-7100
<PAGE>
                                    SCHEDULE A

                              SCHEDULE OF PURCHASERS

<TABLE>
<CAPTION>
               INVESTOR                      SHARES             AMOUNT
               --------                      ------             ------

<S>                                         <C>              <C>
International Biotechnology Trust PLC         800,000        $2,000,000.00
Genencor International, Inc.                  173,950        $  434,875.00
State of Wisconsin Investment Board           706,050        $1,765,125.00
Peter Allard                                  300,000        $  750,000.00
Greene Family Trust                            20,000        $   50,000.00

TOTALS                                      2,000,000        $5,000,000.00
</TABLE>
<PAGE>
                                     EXHIBIT A
<PAGE>
                                    EXHIBIT B


                             SCHEDULE OF EXCEPTIONS
<PAGE>
                                    EXHIBIT C


               TRUSTEES OF THE STATE OF WISCONSIN INVESTMENT BOARD


Jon D. Hammes
John Petersen III
Eric O. Stanchfield
Andrea Steen Crawford
James A. Senty

Wayne McCaffery
George F. Lightbourn
James R. Nelsen
William R. Sauey


          RELEVANT EMPLOYEES OF THE STATE OF WISCONSIN INVESTMENT BOARD


All Transactions

Patricia Lipton         Executive Director
Jean Ledford            Chief Investment Officer - Equities
Jon Traver              Chief Investment Officer - Fixed Income
Keith Johnson           Chief Legal Counsel
Teresa Lau              Assistant Legal Counsel
Cynthia Richson         Assistant Legal Counsel


For Small Cap Portfolio Direct Placements

John Nelson             Investment Director
Chad Neumann            Assistant Portfolio Manager
Jackie Doeler           Securities Analyst
Mark Traster            Securities Analyst
Dan Kane                Securities Analyst
<PAGE>
                                   APPENDIX I

                                  EPIMMUNE INC.

                          STOCK CERTIFICATE QUESTIONNAIRE



      Pursuant to Section 3 of the Agreement, please provide us with the
following information:

1.    The exact name that your Shares are to be registered in (this is the name
      that will appear on your stock certificate(s)). You may use a nominee name
      if appropriate:


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

2.    The relationship between the Purchaser of the Shares and the Registered
      Holder listed in response to item 1 above:


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

3.    The mailing address of the Registered Holder listed in response to item 1
      above:


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

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

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

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


4.    The Social Security Number or Tax Identification Number of the Registered
      Holder listed in response to item 1 above:


      -----------------------------------
<PAGE>
                                   APPENDIX II

                                  EPIMMUNE INC.

                      REGISTRATION STATEMENT QUESTIONNAIRE



      In connection with the preparation of the Registration Statement, please
provide us with the following information:

5.    Pursuant to the "Selling Stockholder" section of the Registration
      Statement, please state your or your organization's name exactly as it
      should appear in the Registration Statement:

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

6.    Please provide the number of shares that you or your organization will own
      immediately after Closing, including those Shares purchased by you or your
      organization pursuant to this Purchase Agreement and those shares
      purchased by you or your organization through other transactions:

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

7.    Have you or your organization had any position, office or other material
      relationship within the past three years with the Company or its
      affiliates?

                    Yes                 No
             ------              ------


      If yes, please indicate the nature of any such relationships below:


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

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

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

      -------------------------------------------------------
<PAGE>
                                  APPENDIX III

                   PURCHASER'S CERTIFICATE OF SUBSEQUENT SALE



      The undersigned, an officer of, or other person duly authorized by


               [fill in official name of individual or institution]

hereby certifies that he/she/it is the Purchaser of the shares evidenced by the
attached certificate, and as such, sold such shares on ________________, 200__
in accordance with Registration Statement number 333-________________, and
complied with the requirement of delivering a current prospectus in connection
with such sale.

PRINT OR TYPE:

Name of Purchaser (Individual or Institution):


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

Name of Individual representing Purchaser (if an Institution)


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

Title of Individual representing Purchaser (if an Institution):


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

SIGNATURE:

Individual Purchaser or Individual representing Purchaser:


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