<PAGE>   1

                                                                   EXHIBIT 10.14









                          SIGNAL PHARMACEUTICALS, INC.



                               SERIES E PREFERRED
                            STOCK PURCHASE AGREEMENT



                                SEPTEMBER 9, 1997







<PAGE>   2

                                TABLE OF CONTENTS

<TABLE>
<S>     <C>    <C>                                                                        <C>
1.             PURCHASE AND SALE OF STOCK...................................................1
        1.1    Sale and Issuance of Series E Preferred Stock................................1
        1.2    Closing......................................................................1
               (a)    First Closing.........................................................1
        1.3    Subsequent Sale of Series E Preferred Stock..................................1
2.             REPRESENTATIONS AND WARRANTIES OF THE COMPANY................................2
        2.1    Organization, Good Standing and Qualification................................2
        2.2    Capitalization and Voting Rights.............................................2
               (a)    Preferred Stock.......................................................2
               (b)    Common Stock..........................................................2
               (c)    Other Rights..........................................................2
        2.3    Subsidiaries.................................................................3
        2.4    Authorization................................................................3
        2.5    Valid Issuance of Preferred and Common Stock.................................3
        2.6    Governmental Consents........................................................4
        2.7    Litigation...................................................................4
        2.8    Proprietary Information Agreements...........................................5
        2.9    Patents and Trademarks.......................................................5
        2.10   Compliance with Other Instruments............................................5
        2.11   Agreements; Action...........................................................6
        2.12   Related-Party Transactions...................................................7
        2.13   Permits......................................................................7
        2.14   Environmental and Safety Laws................................................7
        2.15   Manufacturing and Marketing Rights...........................................8
        2.16   Disclosure...................................................................8
        2.17   Registration Rights..........................................................8
        2.18   Corporate Documents..........................................................8
        2.19   Title to Property and Assets.................................................8
        2.20   Qualified Small Business.....................................................8
        2.21   Financial Statements.........................................................9
        2.22   Changes......................................................................9
        2.23   Employee Benefit Plans......................................................10
        2.24   Tax Returns, Payments and Elections.........................................10
        2.25   Insurance...................................................................10
        2.26   Minute Books................................................................10
        2.27   Labor Agreements and Actions................................................11
        2.28   Section 83(b) Elections.....................................................11
        2.29   Real Property Holding Company...............................................11
</TABLE>



                                       i.
<PAGE>   3
                                TABLE OF CONTENTS
                                  (CONTINUED)

<TABLE>
<S>     <C>    <C>                                                                        <C>
3.             REPRESENTATIONS AND WARRANTIES OF THE INVESTORS.............................12
        3.1    Authorization...............................................................12
        3.2    Purchase Entirely for Own Account...........................................12
        3.3    Disclosure of Information...................................................12
        3.4    Investment Experience.......................................................12
        3.5    Accredited Investor.........................................................13
        3.6    Restricted Securities.......................................................13
        3.7    Further Limitations on Disposition..........................................13
        3.8    Legends.....................................................................13
4.             CALIFORNIA COMMISSIONER OF CORPORATIONS.....................................14
        4.1    Corporate Securities Law....................................................14
5.             CONDITIONS OF EACH INVESTOR'S OBLIGATION AT THE CLOSINGS....................14
        5.1    Representations and Warranties..............................................14
        5.2    Performance.................................................................14
        5.3    Compliance Certificate......................................................14
        5.4    Qualifications..............................................................14
        5.5    Proceedings and Documents...................................................15
        5.6    Board of Directors..........................................................15
        5.7    Opinion of Company Counsel..................................................15
        5.8    Investors' Rights Agreement.................................................15
6.             CONDITIONS OF THE COMPANY'S OBLIGATIONS AT THE CLOSINGS.....................15
        6.1    Representations and Warranties..............................................15
        6.2    Payment of Purchase Price...................................................15
        6.3    Qualification...............................................................15
7.             MISCELLANEOUS...............................................................16
        7.1    Survival of Warranties......................................................16
        7.2    Successors and Assigns......................................................16
        7.3    Governing Law...............................................................16
        7.4    Counterparts................................................................16
        7.5    Expenses....................................................................16
        7.6    Titles and Subtitles........................................................16
        7.7    Notices.....................................................................16
        7.8    Finder's Fee................................................................17
        7.9    Amendments and Waivers......................................................17
</TABLE>


                                      ii.
<PAGE>   4

                                TABLE OF CONTENTS
                                  (CONTINUED)
<TABLE>
<S>     <C>    <C>                                                                        <C>
        7.10   Severability................................................................17
        7.11   Aggregation of Stock........................................................17
        7.12   Regulated Financial Institutions Compliance Obligations.....................17
</TABLE>











                                      iii.
<PAGE>   5

                                TABLE OF CONTENTS


SCHEDULE A  -  Schedule of Investors
SCHEDULE B  -  Schedule of Exceptions

EXHIBIT A - Amended and Restated Articles of Incorporation
EXHIBIT B - Amended and Restated Investors' Rights Agreement
EXHIBIT C - Proprietary Information and Inventions Agreement
EXHIBIT D - Form of Opinion of Cooley Godward LLP














                                      iv.
<PAGE>   6

                            STOCK PURCHASE AGREEMENT

        THIS STOCK PURCHASE AGREEMENT is made as of the 9th day of September,
1997, by and between Signal Pharmaceuticals, Inc., a California corporation (the
"Company"), and the investors listed on Schedule A hereto (individually an
"Investor" and collectively the "Investors").

        THE PARTIES HEREBY AGREE AS FOLLOWS:

1.      PURCHASE AND SALE OF STOCK.

        1.1     SALE AND ISSUANCE OF SERIES E PREFERRED STOCK.

               (a) The Company shall adopt and file with the Secretary of State
of California, and the Secretary of State of California shall accept on or
before the First Closing (as defined below), the Amended and Restated Articles
of Incorporation in the form attached hereto as Exhibit A.

               (b) Subject to the terms and conditions of this Agreement, each
Investor agrees, severally and not jointly, to purchase at a Closing (as defined
below) under this Agreement, and the Company agrees to sell and issue to the
Investors at a Closing, that number of shares of the Company's Series E
Preferred Stock set forth opposite each Investor's name on Schedule A hereto for
a price of $1.91 for each share of Series E Preferred Stock.

        1.2    CLOSING.

               (a) FIRST CLOSING. The initial purchase and sale of the Series E
Preferred Stock shall take place at the offices of Cooley Godward LLP, 4365
Executive Drive, Suite 1100, San Diego, California 92121, at 4:30 P.M. on
September 9, 1997, or at such other time and place as the Company and the
Investors agree orally or in writing (which time and place are designated as the
"First Closing"). At the First Closing and each subsequent Closing, the Company
shall deliver to each Investor purchasing at the First Closing or such
subsequent Closing, a certificate representing the shares of Series E Preferred
Stock which each Investor is purchasing against delivery to the Company by each
Investor of a wire transfer or check in the amount of the purchase price
therefor payable to the Company's order.

        1.3     SUBSEQUENT SALE OF SERIES E PREFERRED STOCK. Subject to the
terms and conditions of this Agreement, the Company may sell, on or before
October 31, 1997, up to the balance of the Company's authorized but unissued
Series E Preferred Stock to such persons as the Company may determine at the
same price per share as the Series E Preferred Stock purchased and sold at the
First Closing. Any such sale (each, a



                                       1.
<PAGE>   7

"Closing") shall be upon the same terms and conditions as those contained
herein, and such persons or entities shall become parties to this Agreement and
that certain Amended and Restated Investors' Rights Agreement of even date
herewith, by and among the Company and the Investors, the form of which is
attached hereto as Exhibit B (the "Investors' Rights Agreement"), and shall have
the rights and obligations of an Investor hereunder and thereunder.

2.      REPRESENTATIONS AND WARRANTIES OF THE COMPANY. The Company hereby
represents and warrants to the Investors that, except as set forth on a Schedule
of Exceptions attached hereto as Schedule B, specifically identifying the
relevant subparagraph hereof, which exceptions shall be deemed to be
representations and warranties as if made hereunder:

        2.1     ORGANIZATION, GOOD STANDING AND QUALIFICATION. The Company is a
corporation duly organized, validly existing and in good standing under the laws
of the State of California and has all requisite corporate power and authority
to carry on its business as now conducted and as proposed to be conducted. The
Company is duly qualified to transact business and is in good standing in each
jurisdiction in which the failure so to qualify would have a material adverse
effect on its business or properties.

        2.2     CAPITALIZATION AND VOTING RIGHTS. The authorized capital of the
Company consists, or will consist prior to the First Closing, of:

               (a) PREFERRED STOCK. 21,747,131 shares of Preferred Stock (the
"Preferred Stock"), of which 2,626,892 shares have been designated Series A
Preferred Stock, all of which are issued and outstanding, of which 2,875,000
shares have been designated Series B Preferred Stock, all of which are issued
and outstanding, of which 8,791,433 shares have been designated Series C
Preferred Stock, of which 8,791,432 are issued and outstanding, of which 250,000
shares have been designated Series C-1 Preferred Stock, all of which are subject
to issued and outstanding warrants, of which 732,601 shares have been designated
Series D Preferred Stock, of which 500,000 shares are issued and outstanding,
and of which 6,471,205 shares have been designated Series E Preferred Stock, up
to all of which will be sold pursuant to this Agreement. The rights, privileges
and preferences of the Series A, Series B, Series C, Series C-1, Series D and
Series E Preferred Stock will be as stated in the Company's Amended and Restated
Articles of Incorporation attached hereto as Exhibit A.

               (b) COMMON STOCK. 30,000,000 shares of common stock ("Common
Stock"), of which 2,437,065 shares are issued and outstanding.

               (c) OTHER RIGHTS. Except for (A) the conversion privileges of the
Series A Preferred Stock, the Series B Preferred Stock, the Series C Preferred
Stock, the



                                       2.
<PAGE>   8

Series C-1 Preferred Stock, the Series D Preferred Stock and the Series E
Preferred Stock sold pursuant to this Agreement, (B) 2,500,000 shares of Common
Stock reserved for issuance pursuant to the Company's 1993 Stock Option Plan, of
which 1,468,391 shares are subject to outstanding options and of which 993,565
shares are issued and outstanding, (C) 550,000 shares of Common Stock reserved
for issuance pursuant to the Company's 1993 Founders' Stock Option Plan, 159,000
of which are subject to outstanding options and of which 373,000 shares are
issued and outstanding, and (D) 1,000,000 shares of Common Stock reserved for
issuance pursuant to the Company's 1997 Stock Option Plan, of which 610,070
shares are subject to outstanding options and none of which are issued and
outstanding, there are not outstanding any options, warrants, rights (including
conversion or preemptive rights) or agreements for the purchase or acquisition
from the Company of any shares of its capital stock. The Company is not a party
or subject to any agreement or understanding, and, to the Company's knowledge,
there is no agreement or understanding between any persons and/or entities,
which affects or relates to the voting or giving of written consents with
respect to any security or by a director of the Company. The only price-based
anti-dilution rights held by any securityholder of the Company are those held by
Tanabe Seiyaku Co., Ltd. ("Tanabe") pursuant to that certain Stock Purchase
Agreement dated March 31, 1996 between the Company and Tanabe, and those set
forth in the Company's Amended and Restated Articles of Incorporation filed
pursuant to Section 1.1(a) hereof.

        2.3 SUBSIDIARIES. The Company does not presently own or control,
directly or indirectly, any interest in any other corporation, association, or
other business entity.

        2.4 AUTHORIZATION. All corporate action on the part of the Company, its
officers, directors and shareholders necessary for the authorization, execution
and delivery of this Agreement, the Investors' Rights Agreement, the performance
of all obligations of the Company hereunder and thereunder and the
authorization, issuance (or reservation for issuance) and delivery of the Series
E Preferred Stock being sold hereunder, the Common Stock issuable upon
conversion of the Series E Preferred Stock has been taken or will be taken prior
to the First Closing, and this Agreement and Investors' Rights Agreement
constitute valid and legally binding obligations of the Company, enforceable in
accordance with their respective terms, except (i) as limited by applicable
bankruptcy, insolvency, reorganization, moratorium, and other laws of general
application affecting enforcement of creditors' rights generally, (ii) as
limited by laws relating to the availability of specific performance, injunctive
relief, or other equitable remedies and (iii) to the extent the indemnification
provisions contained in the Investors' Rights Agreement may be limited by
applicable federal or state securities laws.

        2.5    VALID ISSUANCE OF PREFERRED AND COMMON STOCK.



                                       3.
<PAGE>   9

               (a) The shares of Series E Preferred Stock which are being
purchased by the Investors hereunder, when issued, sold and delivered in
accordance with the terms hereof for the consideration expressed herein, will be
duly and validly issued, and, based in part upon the representations of the
Investors in this Agreement, will be issued in compliance with all applicable
federal and state securities laws and such shares of Series E Preferred Stock
will be fully paid and non-assessable. The Common Stock issuable upon conversion
of the Series E Preferred Stock purchased under this Agreement has been duly and
validly reserved for issuance and, upon issuance in accordance with the terms of
the Amended and Restated Articles of Incorporation, shall be duly and validly
issued, fully paid and nonassessable, and issued in compliance with all
applicable securities laws, as presently in effect, of the United States and
each of the states whose securities laws govern the issuance of any of the
Series E Preferred Stock hereunder.

               (b) The outstanding shares of Series A Preferred Stock, Series B
Preferred Stock, Series C Preferred Stock, Series D Preferred Stock and Common
Stock are all duly and validly authorized and issued, fully paid and
nonassessable, and were issued in compliance with all applicable federal and
state securities laws. The outstanding warrants to purchase Series C-1 Preferred
Stock were duly and validly authorized and issued, and the shares of Series C-1
Preferred Stock underlying such warrants have been duly and validly reserved for
issuance.

        2.6     GOVERNMENTAL CONSENTS. No consent, approval, order or
authorization of, or registration, qualification, designation, declaration or
filing with, any federal, state, local or provincial governmental authority on
the part of the Company is required in connection with the consummation of the
transactions contemplated by this Agreement, except for the filing of a Form D
pursuant to Regulation D promulgated under the Securities Act of 1933, as
amended, which filing will be effected within 15 days of the sale of the Series
E Preferred Stock hereunder, and any other filings that must be made after the
Closing, which will be filed in a timely manner.

        2.7     LITIGATION. There is no action, suit, proceeding or
investigation pending or, to the Company's knowledge, currently threatened
against the Company which questions the validity of this Agreement, the
Investors' Rights Agreement, or the right of the Company to enter into either of
them, or to consummate the transactions contemplated hereby or thereby, or which
might result, either individually or in the aggregate, in any material adverse
changes in the assets, condition, affairs or prospects of the Company,
financially or otherwise, or any change in the current equity ownership of the
Company, nor is the Company aware that there is any basis for the foregoing. The
foregoing includes, without limitation, actions pending or threatened involving
the prior employment of any of the Company's employees, their use in connection
with the Company's business of any information or techniques allegedly
proprietary to any of their former employers, or their obligations under any
agreements with prior employers. The



                                       4.
<PAGE>   10

Company is not a party or subject to the provisions of any order, writ,
injunction, judgment or decree of any court or government agency or
instrumentality. There is no action, suit, proceeding or investigation by the
Company currently pending or which the Company intends to initiate.

        2.8 PROPRIETARY INFORMATION AGREEMENTS. Each employee, officer and
consultant of the Company has executed a Proprietary Information and Inventions
Agreement in the form set forth on Exhibit C hereto. The Company, after
reasonable investigation, is not aware that any of its employees, officers or
consultants are in violation thereof, and the Company will use its best efforts
to prevent any such violation.

        2.9 PATENTS AND TRADEMARKS. To the best of its knowledge, the Company
has sufficient title and ownership of all patents, trademarks, service marks,
tradenames, copyrights, trade secrets, information, proprietary rights and
processes necessary for its business as now conducted, and believes it will be
able to develop such intellectual property such that it can operate its business
as proposed to be conducted, without any conflict with or infringement of the
rights of others. The Schedule of Exceptions contains a complete list of patents
and pending patent applications by the Company, as well as all licenses,
collaborative agreements, development agreements, distribution agreements and
similar agreements. The Company has not received any communications alleging,
nor does the Company have any knowledge, that the Company has violated or, by
conducting its business as proposed, would violate any of the patents,
trademarks, service marks, tradenames, copyrights or trade secrets or other
proprietary rights of any other person or entity. The Company is not aware that
any of its employees is obligated under any contract (including licenses,
covenants or commitments of any nature) or other agreement, or subject to any
judgment, decree or order of any court or administrative agency, that would
interfere with the use of his best efforts to promote the best interests of the
Company or that would conflict with the Company's business as proposed to be
conducted. Neither the execution nor delivery of this Agreement, nor the
Investors' Rights Agreement, nor the carrying on of the Company's business by
the employees of the Company, nor the conduct of the Company's business as
proposed, will, to the Company's knowledge, conflict with or result in a breach
of the terms, conditions or provisions of, or constitute a default under, any
contract, covenant or instrument under which any of such employees is now
obligated. The Company does not believe it is or will be necessary to utilize
any inventions of any of its employees (or people it currently intends to hire)
made prior to their employment by the Company.

        2.10 COMPLIANCE WITH OTHER INSTRUMENTS. The Company is not in violation
or default of any provisions of its Amended and Restated Articles of
Incorporation or Bylaws or of any instrument, judgment, order, writ, decree,
license, agreement or contract to which it is a party or by which it is bound
or, to its knowledge, of any provision of federal or state statute, rule or
regulation applicable to the Company. To the Company's



                                       5.
<PAGE>   11

knowledge, no other party to any license, contract or other agreement to which
the Company is a party is in breach, default or violation thereof and, to the
Company's knowledge, no facts exist which could constitute such a breach,
default or violation. The execution, delivery and performance by the Company of
this Agreement or the Investors' Rights Agreement and the consummation of the
transactions contemplated hereby and thereby will not result in any such
violation or be in conflict with or constitute, with or without the passage of
time and giving of notice, either a default under any such provision,
instrument, judgment, order, writ, decree or contract or an event which results
in the creation of any lien, charge or encumbrance upon any assets of the
Company or the suspension, revocation, impairment, forfeiture, or nonrenewal of
any material permit, license, authorization, or approval applicable to the
Company, its business or operations or any of its assets or properties, except
that the foregoing may be limited by (i) applicable bankruptcy, insolvency,
reorganization, moratorium, and other laws of general application affecting
enforcement of creditors' rights generally, (ii) laws relating to the
availability of specific performance, injunctive relief, or other equitable
remedies, and (iii) with respect to indemnification provisions contained in the
Investors' Rights Agreement, applicable federal or state securities laws. The
Company has not performed any act, or failed to perform any act, which would
result in the Company's loss of any right granted under any license,
collaboration, joint venture or other agreement.

        2.11   AGREEMENTS; ACTION.

               (a) Except for agreements explicitly contemplated hereby and by
the Investors' Rights Agreement, there are no agreements, understandings or
proposed transactions between the Company and any of its officers, directors,
affiliates, or any affiliate thereof.

               (b) There are no agreements, understandings, instruments,
contracts, proposed transactions, judgments, orders, writs or decrees to which
the Company is a party or by which it is bound which may involve obligations
(contingent or otherwise) of, or payments to, the Company in excess of $50,000.

               (c) The Company has not (i) declared or paid any dividends, or
authorized or made any distribution upon or with respect to any class or series
of its capital stock, (ii) incurred any indebtedness for money borrowed or any
other liabilities individually in excess of $10,000 (except for trade credit
incurred in the ordinary course of business) or, in the case of indebtedness
and/or liabilities individually less than $10,000, in excess of $50,000 in the
aggregate (except for trade credit incurred in the ordinary course of business),
(iii) made any loans or advances to any person, other than ordinary advances for
travel expenses or (iv) sold, exchanged or otherwise disposed of any of its
assets or rights.



                                       6.
<PAGE>   12

               (d) For the purposes of subsections (b) and (c) above, all
indebtedness, liabilities, agreements, understandings, instruments, contracts
and proposed transactions involving the same person or entity (including persons
or entities the Company has reason to believe are affiliated therewith) shall be
aggregated for the purpose of meeting the individual minimum dollar amounts of
such subsections.

               (e) The Company is not a party to and is not bound by any
contract, agreement or instrument, or subject to any restriction under its
Amended and Restated Articles of Incorporation or Bylaws, which adversely
affects its business as now conducted or as proposed to be conducted, its
properties or its financial condition.

               (f) The Company has not engaged in the past three (3) months in
any discussion (i) with any corporation or corporations regarding the
consolidation or merger of the Company with or into any such corporation or
corporations, (ii) with any corporation, partnership, association or other
business entity or any individual regarding the sale, conveyance or disposition
of all or substantially all of the assets of the Company, (iii) with any
corporation, partnership, association or other business entity or any individual
regarding any transaction or series of related transactions in which more than
fifty percent (50%) of the voting power of the Company would be disposed of, or
(iv) regarding any other form of acquisition, liquidation, dissolution or
winding up of the Company or its assets.

        2.12    RELATED-PARTY TRANSACTIONS. No employee, officer or director of
the Company or member of his or her immediate family is indebted to the Company,
nor is the Company indebted (or committed to make loans or extend or guarantee
credit) to any of them. To the best of the Company's knowledge, none of such
persons has any direct or indirect ownership interest in any firm or corporation
with which the Company is affiliated or with which the Company has a business
relationship, or any firm or corporation that competes with the Company, except
that employees, officers or directors of the Company and members of their
immediate families may own less than 5% of the outstanding stock in publicly
traded companies that may compete with the Company (except that persons or
entities that are affiliated with venture capital investment firms or other
accredited investment institutions shall have no such 5% ownership limitations).
No member of the immediate family of any officer or director of the Company is
directly or indirectly interested in any material contract with the Company.

        2.13    PERMITS. The Company has all franchises, permits, licenses, and
any similar authority necessary for the conduct of its business as now being
conducted by it, the lack of which could materially and adversely affect the
business, properties, prospects, or financial condition of the Company and
believes it can obtain, without undue burden or expense, any similar authority
for the conduct of its business as planned



                                       7.
<PAGE>   13

to be conducted. The Company is not in default in any material respect under any
of such franchises, permits, licenses, or other similar authority.

        2.14 ENVIRONMENTAL AND SAFETY LAWS. To the best of its knowledge, the
Company is not in violation of any applicable statute, law, or regulation
relating to the environment or occupational health and safety, and to the best
of its knowledge, no material expenditures are or will be required in order to
comply with any such existing statute, law or regulation.

        2.15 MANUFACTURING AND MARKETING RIGHTS. The Company has not granted
rights to manufacture, produce, assemble, license, market or sell its products
to any other person and is not bound by any agreement that affects the Company's
exclusive right to develop, manufacture, assemble, distribute, market or sell
its products.

        2.16 DISCLOSURE. The Company has fully provided each Investor with all
the information which such Investor has requested for deciding whether to
purchase the Series E Preferred Stock and all information which the Company
believes is reasonably necessary to enable such Investor to make such decision.
Neither this Agreement, the Investors' Rights Agreement, nor any other
statements or certificates made or delivered in connection herewith or therewith
when taken together contains any untrue statement of a material fact or omits to
state a material fact necessary to make the statements herein or therein not
misleading.

        2.17 REGISTRATION RIGHTS. Except as provided in the Investors' Rights
Agreement, the Company has not granted or agreed to grant any registration
rights, including piggyback rights, to any person or entity.

        2.18 CORPORATE DOCUMENTS. Except for amendments necessary to satisfy
representations and warranties or conditions contained herein (the form of which
amendments has been approved by the Investors), the Amended and Restated
Articles of Incorporation and Bylaws of the Company are in the form previously
provided to the Investors.

        2.19 TITLE TO PROPERTY AND ASSETS. The Company owns its property and
assets free and clear of all mortgages, liens, loans and encumbrances, except
such encumbrances and liens which arise in the ordinary course of business and
do not materially impair the Company's ownership or use of such property or
assets. With respect to the property and assets it leases, the Company is in
compliance with such leases and, to the best of its knowledge, holds a valid
leasehold interest free of any liens, claims or encumbrances.

        2.20 QUALIFIED SMALL BUSINESS. The Company represents and warrants to
the Investors that it qualifies as a "Qualified Small Business" as defined
Section 1202(d) of the Internal Revenue Code of 1986, as amended (the "Code").
The Company covenants



                                       8.
<PAGE>   14

that so long as it reasonably believes that the Series E Preferred Stock (and/or
Conversion Shares) held by Investors or a transferee would qualify as Qualified
Small Business Stock as defined in Section 1202(c) of the Code it will timely
file all reports or filings with the Internal Revenue Service required of a
Qualified Small Business.

        2.21 FINANCIAL STATEMENTS. The Company has delivered to each Investor
its audited Financial Statements at December 31, 1996 and unaudited financial
statements for each of the two quarters ending June 30, 1997 (the "Financial
Statements"). The Financial Statements have been prepared in accordance with
generally accepted accounting principles ("GAAP") applied on a consistent basis
throughout the periods indicated and with each other, except that unaudited
Financial Statements may not contain all footnotes required by GAAP. The
Financial Statements are complete and correct in all material respects and
fairly present the financial condition and results of operations of the Company
as of the dates and for the periods indicated therein, subject in the case of
the unaudited Financial Statements to normal year-end audit adjustments. Except
as set forth in the Financial Statements, the Company has no liabilities,
contingent or otherwise, other than (i) liabilities incurred in the ordinary
course of business subsequent to the date of the Financial Statements and (ii)
obligations under contracts and commitments incurred in the ordinary course of
business and not required under generally accepted accounting principles to be
reflected in the Financial Statements, which, in both cases, individually or in
the aggregate, are not material to the financial condition or operating results
of the Company.

        2.22   CHANGES.  Since June 30, 1997 there has not been:

               (a) any change in the assets, liabilities, financial condition or
operating results of the Company, except changes which have not been, in the
aggregate, materially adverse;

               (b) any damage, destruction or loss, whether or not covered by
insurance, materially and adversely affecting the assets, properties, financial
condition, operating results, prospects or business of the Company (as such
business is presently conducted and as it is proposed to be conducted);

               (c) any waiver by the Company of a valuable right or of a
material debt owed to it;

               (d) any satisfaction or discharge of any lien, claim or
encumbrance or payment of any obligation by the Company, except which is not
material to the assets, properties, financial condition, operating results or
business of the Company (as such business is presently conducted and as it is
proposed to be conducted);



                                       9.
<PAGE>   15

               (e) any change or amendment to a material contract or arrangement
by which the Company or any of its assets or properties is bound or subject;

               (f) any material change in any compensation arrangement or
agreement with any employee or consultant;

               (g) any sale, assignment or transfer of any patents, trademarks,
copyrights, trade secrets or other intangible assets;

               (h) any resignation or termination of employment of any officer,
key employee or consultant of the Company; and the Company does not know of any
impending resignation or termination of employment of any such officer, employee
or consultant; or

               (i) to the Company's knowledge, any other event or condition of
any character which might materially and adversely affect the assets,
properties, financial condition, operating results or business of the Company
(as such business is presently conducted and as it is proposed to be conducted).

        2.23 EMPLOYEE BENEFIT PLANS. The Company does not have any Employee
Benefit Plan as defined in the Employee Retirement Income Security Act of 1974.

        2.24 TAX RETURNS, PAYMENTS AND ELECTIONS. The Company has filed all tax
returns and reports as required by law. These returns and reports are true and
correct in all material respects. The Company has paid all taxes and other
assessments due, except those contested by it in good faith which are listed in
the Schedule of Exceptions. The provisions for taxes as set forth in the
Financial Statements is adequate for taxes due or accrued as of the date
thereof. The Company has not elected pursuant to the Code, to be treated as a
Subchapter S corporation or a collapsible corporation pursuant to Section 341(f)
or Section 1362(a) of the Code, nor has it made any other elections pursuant to
the Code (other than elections which relate solely to methods of accounting,
depreciation or amortization) which would have a material effect on the Company,
its financial condition, its business as presently conducted or proposed to be
conducted or any of its properties or material assets. Since the date of the
Financial Statements, the Company has made adequate provisions on its books of
account for all taxes, assessments and governmental charges with respect to its
business, properties and operations for such period.

        2.25 INSURANCE. The Company has in full force and effect fire and
casualty insurance policies, with extended coverage, sufficient in amount
(subject to reasonable deductibles) to allow it to replace any of its properties
that might be damaged or destroyed. The Company has in full force and effect
errors and omissions insurance in amounts customary for companies similarly
situated.



                                      10.
<PAGE>   16

        2.26 MINUTE BOOKS. The minute books of the Company contain a complete
summary of all meetings of directors and shareholders since the time of
incorporation and reflect all transactions referred to in such minutes
accurately in all material respects.

        2.27 LABOR AGREEMENTS AND ACTIONS. The Company is not bound by or
subject to (and none of its assets or properties is bound by or subject to) any
written or oral, express or implied, contract, commitment or arrangement with
any labor union, and no labor union has requested or, to the knowledge of the
Company, has sought to represent any of the employees, representatives or agents
of the Company. There is no strike or other labor dispute involving the Company
pending, or to the knowledge of the Company threatened, which could have a
material adverse effect on the assets, properties, financial condition,
operating results, or business of the Company (as such business is presently
conducted and as it is proposed to be conducted), nor is the Company aware of
any labor organization activity involving its employees. The Company is not
aware that any officer or key employee, or that any group of key employees,
intends to terminate their employment with the Company, nor does the Company
have a present intention to terminate the employment of any of the foregoing.
Subject to general principles related to wrongful termination of employees, the
employment of each officer and employee of the Company is terminable at the will
of the Company. The Company has complied in all material respects with all
applicable state and federal equal employment opportunity and other laws related
to employment.

        2.28 SECTION 83(B) ELECTIONS. To the best of the Company's knowledge,
elections and notices pursuant to Section 83(b) of the Code and any analogous
provisions of applicable state tax laws have been timely filed by all
individuals who have purchased shares of the Company's Common Stock which are
subject to vesting or other risk of forfeiture.

        2.29 REAL PROPERTY HOLDING COMPANY. The Company is not a "United States
real property holding corporation" (as that term is defined in Treasury
Regulation section 1.897-2(b)). If at any time in the future the Company shall
become a "United States real property holding corporation," the Company shall,
as promptly as practicable, notify each foreign Investor of such change in
status. Within 30 days after receipt of a request from a foreign investor, the
Company shall prepare and deliver to such foreign Investor the statement
required under Treasury Regulation section 1.897-2(h)(l)(i) and either or both
of the following documents: (i) an affidavit in conformance with the
requirements of Section 1445(b)(3) of the Code or (ii) a notarized statement,
executed by an officer having actual knowledge of the facts, that the shares of
Company stock held by such foreign Investor are of a class that is regularly
traded on an established securities market, within the meaning of Section
1445(b)(6) of the Code. If the Company is unable to provide either document
described in (i) or (ii) above, if requested, it shall promptly notify such
foreign Investor in writing of the reasons for such inability. Finally, upon the



                                      11.
<PAGE>   17

request of a foreign Investor and without regard to whether either document
described in (i) or (ii) above has been requested, the Company shall cooperate
fully with the efforts of such foreign Investor to obtain a "qualifying
statement," within the meaning of Section 1445(b)(4) of the Code, or such other
documents as would excuse a transferee of a foreign Investor's interest from
withholding of income tax imposed pursuant to Section 1445(a) of the Code.

3.      REPRESENTATIONS AND WARRANTIES OF THE INVESTORS. Each Investor,
severally and not jointly, hereby represents and warrants that:

        3.1 AUTHORIZATION. This Agreement constitutes its valid and legally
binding obligation, enforceable in accordance with its terms, except (i) as
limited by applicable bankruptcy, insolvency, reorganization, moratorium, and
other laws of general application affecting enforcement of creditors' rights
generally, (ii) as limited by laws relating to the availability of specific
performance, injunctive relief, or other equitable remedies and (iii) to the
extent the indemnification provisions contained in the Investors' Rights
Agreement may be limited by applicable federal or state securities laws.

        3.2 PURCHASE ENTIRELY FOR OWN ACCOUNT. This Agreement is made with each
Investor in reliance upon such Investor's representation to the Company, which
by such Investor's execution of this Agreement Investors hereby confirm that the
Series E Preferred Stock to be received by Investors will be acquired for
investment for Investor's own account, not as a nominee or agent, and not with a
view to the resale or distribution of any part thereof, and that such Investor
has no present intention of selling, granting any participation in, or otherwise
distributing the same. By executing this Agreement, each Investor further
represents that such Investor does not have any contract, undertaking, agreement
or arrangement with any person to sell, transfer or grant participations to such
person or to any third person, with respect to any of the Series E Preferred
Stock. Each Investor represents that it has full power and authority to enter
into this Agreement.

        3.3 DISCLOSURE OF INFORMATION. Each Investor believes it has received
all the information it considers necessary or appropriate for deciding whether
to purchase the Series E Preferred Stock. Each Investor further represents that
it has had an opportunity to ask questions and receive answers from the Company
regarding the terms and conditions of the offering of the Series E Preferred
Stock. The foregoing, however, does not limit or modify the representations and
warranties of the Company in Section 2 of this Agreement or the right of each
Investor to rely thereon.

        3.4 INVESTMENT EXPERIENCE. Each Investor is an investor in securities of
companies in the development stage and acknowledges that it is able to fend for
itself, can bear the economic risk of its investment and has such knowledge and
experience in financial or business matters that it is capable of evaluating the
merits and risks of the



                                      12.
<PAGE>   18

investment in the Series E Preferred Stock. If other than an individual, each
Investor also represents it has not been organized for the purpose of acquiring
the Series E Preferred Stock.

        3.5 ACCREDITED INVESTOR. Each Investor is an "accredited investor"
within the meaning of SEC Rule 501 of Regulation D under the Act, as presently
in effect.

        3.6 RESTRICTED SECURITIES. Each Investor understands that the shares of
Series E Preferred Stock it is purchasing are characterized as "restricted
securities" under the federal securities laws inasmuch as they are being
acquired from the Company in a transaction not involving a public offering and
that under such laws and applicable regulations such securities may be resold
without registration under the Securities Act of 1933, as amended (the "Act"),
only in certain limited circumstances. In this connection, each Investor
represents that it is familiar with SEC Rule 144, as presently in effect, and
understands the resale limitations imposed thereby and by the Act.

        3.7 FURTHER LIMITATIONS ON DISPOSITION. Without in any way limiting the
representations set forth above, Investors further agree not to make any
disposition of all or any portion of the Series E Preferred Stock unless:

                (a)     the disposition is made as part of a firmly underwritten
public offering of the Company's stock, or

                (b)     until the transferee has agreed in writing for the
benefit of the Company to be bound by this Section 3 and Section 7 hereof, and
the Investors' Rights Agreement and the (i) Investors shall have notified the
Company of the proposed disposition and shall have furnished the Company with a
detailed statement of the circumstances surrounding the proposed disposition,
and (ii) if reasonably requested by the Company, Investors shall have furnished
the Company with an opinion of counsel, reasonably satisfactory to the Company,
that such disposition will not require registration of such shares under the
Act. It is agreed that the Company will not require opinions of counsel for
transactions made pursuant to Rule 144 except in unusual circumstances.

                (c)     Notwithstanding the provisions of (a) and (b) above, no
such public offering or opinion of counsel shall be necessary for a transfer by
an Investor which is a partnership or limited liability company to a partner of
such partnership, a retired partner of such partnership who retires after the
date hereof or a member of such limited liability company, or to the estate of
any such partner, retired partner or member, or the transfer by gift, will or
intestate succession of any partner or member to his spouse or to the siblings,
lineal descendants or ancestors of such partner or member or his spouse, if the
transferee agrees in writing to be subject to the terms hereof to the same
extent as if he were an original Investor hereunder.



                                      13.
<PAGE>   19

        3.8 LEGENDS. It is understood that the certificates evidencing the
Series E Preferred Stock may bear the following legend:

                "These securities have not been registered under the Securities
        Act of 1933, as amended (the "Act"). They may not be sold, offered for
        sale, pledged or hypothecated in the absence of a registration statement
        in effect with respect to the securities under such Act or an opinion of
        counsel reasonably satisfactory to the Company that such registration is
        not required or unless sold pursuant to Rule 144 of such Act."

4.      CALIFORNIA COMMISSIONER OF CORPORATIONS.

        4.1 CORPORATE SECURITIES LAW. THE SALE OF THE SECURITIES WHICH ARE THE
SUBJECT OF THIS AGREEMENT HAS NOT BEEN QUALIFIED WITH THE COMMISSIONER OF
CORPORATIONS OF THE STATE OF CALIFORNIA AND THE ISSUANCE OF SUCH SECURITIES OR
THE PAYMENT OR RECEIPT OF ANY PART OF THE CONSIDERATION FOR SUCH SECURITIES
PRIOR TO SUCH QUALIFICATION IS UNLAWFUL, UNLESS THE SALE OF SECURITIES IS EXEMPT
FROM SUCH QUALIFICATION. THE RIGHTS OF ALL PARTIES TO THIS AGREEMENT ARE
EXPRESSLY CONDITIONED UPON SUCH QUALIFICATION BEING OBTAINED, UNLESS THE SALE IS
SO EXEMPT.

5.      CONDITIONS OF EACH INVESTOR'S OBLIGATION AT THE CLOSINGS. The
obligations of each Investor under subsection 1.1(b) of this Agreement are
subject to the fulfillment on or before the First Closing and each subsequent
Closing, as applicable, of each of the following conditions, the waiver of which
shall not be effective against any Investor who does not consent in writing
thereto:

        5.1 REPRESENTATIONS AND WARRANTIES. The representations and warranties
of the Company contained in Section 2 shall be true on and as of the First
Closing and each subsequent Closing, as applicable, with the same effect as
though such representations and warranties had been made on and as of the date
of such closing.

        5.2 PERFORMANCE. The Company shall have performed and complied with all
agreements, obligations and conditions contained in this Agreement that are
required to be performed or complied with by it on or before such closing.

        5.3 COMPLIANCE CERTIFICATE. The President of the Company shall deliver
to each Investor at the First Closing or such subsequent Closing, as applicable,
a certificate certifying that the conditions specified in Sections 5.1, 5.2,
5.4, 5.6 and 5.8 have been fulfilled and stating that there shall have been no
adverse change in the business, affairs, operations, properties, assets or
condition of the Company since the date of the Financial Statements.



                                      14.
<PAGE>   20

        5.4 QUALIFICATIONS. All authorizations, approvals, or permits, if any,
of any governmental authority or regulatory body of the United States or of any
state that are required in connection with the lawful issuance and sale of the
Series E Preferred Stock pursuant to this Agreement shall be duly obtained and
effective as of the First Closing or any subsequent Closing, as applicable
(except for such as may be properly obtained subsequent to such closing).

        5.5 PROCEEDINGS AND DOCUMENTS. All corporate and other proceedings in
connection with the transactions contemplated at the First Closing and each
subsequent Closing, as applicable, and all documents incident thereto shall be
reasonably satisfactory in form and substance to each Investor, and the Investor
shall have received all such counterpart original and certified or other copies
of such documents as they may reasonably request.

        5.6 BOARD OF DIRECTORS. At the time of the First Closing and each
subsequent Closing, the directors of the Company shall be John Walker, Brook
Byers, Luke Evnin, Harry Hixson, Jr., Patrick Latterell, Alan Lewis and Arnold
Oronsky.

        5.7 OPINION OF COMPANY COUNSEL. Each Investor shall have received from
Cooley Godward LLP, counsel for the Company, an opinion, dated as of the First
Closing and each subsequent Closing, as applicable, in the form attached hereto
as Exhibit D.

        5.8 INVESTORS' RIGHTS AGREEMENT. The Company and Investors shall have
entered into the Investors' Rights Agreement in the form attached hereto as
Exhibit B.

6.      CONDITIONS OF THE COMPANY'S OBLIGATIONS AT THE CLOSINGS. The obligations
of the Company to each Investor under this Agreement are subject to the
fulfillment on or before the First Closing and each subsequent Closing, as
applicable, of each of the following conditions by the Investors purchasing at
such Closing:

        6.1 REPRESENTATIONS AND WARRANTIES. The representations and warranties
of each Investor contained in Section 3 shall be true on and as of the First
Closing and any subsequent Closing, as applicable, with the same effect as
though such representations and warranties had been made on and as of such
closing.

        6.2 PAYMENT OF PURCHASE PRICE. Each Investor shall have delivered the
purchase price specified in Section 1.1(b).

        6.3 QUALIFICATION. All authorizations, approvals, or permits, if any, of
any governmental authority or regulatory body of the United States or of any
state that are required in connection with the lawful issuance and sale of the
Series E Preferred Stock pursuant to this Agreement shall be duly obtained and
effective as of such closing (except for such as may be properly obtained
subsequent to such closing).



                                      15.
<PAGE>   21

7.      MISCELLANEOUS.

        7.1 SURVIVAL OF WARRANTIES. The warranties, representations and
covenants of the Company and each Investor contained in or made pursuant to this
Agreement shall survive the execution and delivery of this Agreement and the
First Closing and all subsequent Closings hereunder and shall in no way be
affected by any investigation of the subject matter thereof made by or on behalf
of the Investors or the Company.

        7.2 SUCCESSORS AND ASSIGNS. Except as otherwise provided herein, the
terms and conditions of this Agreement shall inure to the benefit of and be
binding upon the respective successors and assigns of the parties (including
transferees of any shares of Series E Preferred Stock sold hereunder or any
Common Stock issued upon conversion of such Series E Preferred Stock). Nothing
in this Agreement, express or implied, is intended to confer upon any party
other than the parties hereto or their respective successors and assigns any
rights, remedies, obligations, or liabilities under or by reason of this
Agreement, except as expressly provided in this Agreement.

        7.3 GOVERNING LAW. This Agreement shall be governed by and construed
under the laws of the State of California as applied to agreements among
California residents entered into and to be performed entirely within
California.

        7.4 COUNTERPARTS. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

        7.5 EXPENSES. If the Closing is effected, the Company shall, at the
First Closing, reimburse the reasonable fees of special counsel for the
Investors, not to exceed $15,000.00, and shall, upon receipt of a bill therefor,
reimburse the reasonable out of pocket expenses of such counsel. In addition, at
each subsequent Closing, the Company shall reimburse the reasonable fees of
special counsel to the Investors, provided all such fees for the First Closing
and any subsequent Closings in the aggregate do not exceed $15,000.00. If any
action at law or in equity is necessary to enforce or interpret the terms of
this Agreement, the Investors' Rights Agreement or the Amended and Restated
Articles of Incorporation, the prevailing party shall be entitled to reasonable
attorneys' fees, costs and necessary disbursements in addition to any other
relief to which such party may be entitled.

        7.6 TITLES AND SUBTITLES. The titles and subtitles used in this
Agreement are used for convenience only and are not to be considered in
construing or interpreting this Agreement.

        7.7 NOTICES. Unless otherwise provided, any notice required or permitted
under this Agreement shall be given in writing and shall be deemed effectively
given upon



                                      16.
<PAGE>   22

personal delivery or delivery by courier to the party to be notified or upon
deposit with the United States Post Office, by registered or certified mail,
postage prepaid and addressed to the party to be notified at the address
indicated for such party on Schedule A hereto, or at such other address as such
party may designate by ten (10) days' advance written notice to the other
parties.

        7.8 FINDER'S FEE. Each party represents that it neither is nor will be
obligated for any finders' fee or commission in connection with this
transaction, except for the Company's obligation to pay to Lehman Brothers Inc.,
Hambrecht & Quist LLC and Robertson, Stephens & Company LLC (collectively, the
"Agents") a placement fee of 9% of the gross proceeds received by the Company
from the sale of the Series E Preferred Stock, of which two-thirds will be
payable in cash and one-third will be payable in Series E Preferred Stock at the
same price as the Series E Preferred Stock sold at the Closing (the "Placement
Fee"). The Company agrees to indemnify and hold harmless each Investor from any
liability for any commission or compensation in the nature of a finders' fee
(and the costs and expenses of defending against such liability or asserted
liability) for which the Company or any of its officers, employees or
representatives is responsible, including, but not limited to, the Placement Fee
to be paid to the Agents.

               Each Investor, severally and not jointly, agrees to indemnify and
to hold harmless the Company from any liability for any commission or
compensation in the nature of a finders' fee (and the costs and expenses of
defending against such liability or asserted liability) for which such Investor
or any of its officers, partners, employees, or representatives is responsible.

        7.9 AMENDMENTS AND WAIVERS. Except as provided in Section 5 of this
Agreement, any term of this Agreement may be amended and the observance of any
term of this Agreement may be waived (either generally or in a particular
instance and either retroactively or prospectively), only with the written
consent of the Company and the holders of a majority of the Common Stock issued
or issuable upon conversion of the Series E Preferred Stock. Any amendment or
waiver effected in accordance with this paragraph shall be binding upon each
holder of any securities purchased under this Agreement at the time outstanding
(including securities into which such securities are convertible), each future
holder of all such securities, and the Company.

        7.10 SEVERABILITY. If one or more provisions of this Agreement are held
to be unenforceable under applicable law, such provision shall be excluded from
this Agreement and the balance of the Agreement shall be interpreted as if such
provision were so excluded and shall be enforceable in accordance with its
terms.



                                      17.
<PAGE>   23

        7.11 AGGREGATION OF STOCK. All shares of the Series E Preferred Stock
held or acquired by affiliated entities or persons shall be aggregated together
for the purpose of determining the availability of any rights under this
Agreement.

        7.12 REGULATED FINANCIAL INSTITUTIONS COMPLIANCE OBLIGATIONS. Nothing
contained in this Agreement shall diminsh the continuing obligations of any
financial institution to comply with applicable requirements of law that such
financial institution maintain responsibility for the disposition of, and
control over, its admitted assets, investments and property, including (without
limiting the generality of the foregoing) the provisions of Section 1411(b) of
the New York Insurance Law, as amended, and as hereinafter from time to time in
effect.





                                      18.
<PAGE>   24

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

                                        SIGNAL PHARMACEUTICALS, INC.

                                        By:
                                             -----------------------------------
                                             Alan J. Lewis, President


                                        INVESTORS:

                                        ARES-SERONO S.A.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        BIOCENTIVE


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        FINSBURY WORLDWIDE PHARMACEUTICAL
                                        TRUST, PLC


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        LOMBARD ODIER IMMUNOLOGY FUND


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------


                   SIGNATURE PAGE TO STOCK PURCHASE AGREEMENT



<PAGE>   25

                                        NEUROSCIENCE PARTNERS LIMITED
                                        PARTNERSHIP


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        NEW YORK LIFE INSURANCE COMPANY


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        PHARMA/WHEALTH


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        THE HEALTH CARE AND BIOTECHNOLOGY
                                        VENTURE FUND


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        BAYVIEW INVESTORS LTD.



                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------


                  SIGNATURE PAGE TO STOCK PURCHAASE AGREEMENT



<PAGE>   26

                                        VENROCK ASSOCIATES


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        VENROCK ASSOCIATES II, L.P.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        KLEINER PERKINS CAUFIELD & BYERS VI


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        ACCEL INVESTORS `93 L.P.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        ACCEL IV L.P.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        ACCEL JAPAN L.P.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------


                   SIGNATURE PAGE TO STOCK PURCHASE AGREEMENT



<PAGE>   27

                                        ACCEL KEIRETSU L.P.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        ELLMORE C. PATTERSON PARTNERS


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        PROSPER PARTNERS


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        INTERWEST INVESTORS V


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        INTERWEST PARTNERS V


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        OXFORD BIOSCIENCE PARTNERS (ADJUNCT)
                                        L.P.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------


                   SIGNATURE PAGE TO STOCK PURCHASE AGREEMENT



<PAGE>   28

                                        OXFORD BIOSCIENCE PARTNERS (BERMUDA)
                                        L.P.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        OXFORD BIOSCIENCE PARTNERS L.P.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        SECOND VENTURES II, L.P.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        U.S. VENTURES PARTNERS IV, L.P.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        USVP ENTREPRENEUR PARTNERS II, L.P.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------


                   SIGNATURE PAGE TO STOCK PURCHASE AGREEMENT



                                      19.
<PAGE>   29

                                        HARRY F. HIXSON, JR. SEPARATE PROPERTY
                                        TRUST, DATED DECEMBER 15, 1995


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------



                                        VERTICAL FUND ASSOCIATES, L.P.


                                        By:
                                             -----------------------------------
                                        Its:
                                             -----------------------------------








                   SIGNATURE PAGE TO STOCK PURCHASE AGREEMENT



<PAGE>   30

                                   SCHEDULE A

                              Schedule of Investors

<TABLE>
<CAPTION>
                                                                                  NUMBER
                   INVESTORS                          AMOUNT INVESTED            OF SHARES
<S>                                                   <C>                        <C>
Ares-Serono S.A.                                        $1,883,836.82              986,302
c/o Ares Services S.A.
Attn:  Leon R. Bushara
15bis Chemin des Mines
1202 Geneva
Switzerland

BioCentive                                                $999,999.60              523,560
c/o Mesco Ltd.
Attn:  Joel R. Mesznik
122 East 42nd Street
49th Floor
New York, NY  10168

Finsbury Worldwide                                        $999,999.60              523,560
Pharmaceutical Trust, plc
c/o Finsbury Asset Management
Attn:  Barbara Macaulay
Alderman's House
Alderman's Walk
London EC2M 3XR
England

Lombard Odier Immunology Fund                           $3,000,000.71            1,570,681
c/o Lombard Odier & Cie
Attn:  Mlle. Laurence Mauron
11, rue de la Corraterie
1204 Geneva
Switzerland
</TABLE>



                                       1.
<PAGE>   31

<TABLE>
<CAPTION>
                                                                         NUMBER
                   INVESTORS                       AMOUNT INVESTED     OF SHARES
<S>                                                <C>                 <C>
Neuroscience Partners Limited Partnership              $999,999.60       523,560
Attn:  Michael Callaghan
100 International Boulevard
Etobicoke, Ontario
Canada  M9W 6J6

New York Life Insurance Company                      $1,499,999.40       785,340
Attn:  Dominique Semon
51 Madison Avenue
New York, NY  10010

PHARMA/wHEALTH                                         $999,999.60       523,560
c/o. Mehta and Isaly
Attn:  Mr. Samuel D. Isaly
41 Madison Avenue
40th Floor
New York, NY  10010

The Health Care and Biotechnology Venture Fund         $499,999.80       261,780
Attn:  Micahel Callaghan
100 International Boulevard
Etobicoke, Ontario
Canada  M9W 6J6

Bayview Investors Ltd.                                 $116,160.47        60,817
c/o Robertson Stephens & Co.
Attn: John Coquia
555 California Street, Suite 2600
San Francisco, CA  94104

Venrock Associates                                    $  83,027.70        43,470
Venrock Associates II, L.P.                             110,059.93        57,623
Attn:  Patrick F. Latterell
One Maritime Plaza, Suite 1919
San Francisco, CA  94111
</TABLE>



                                       2.
<PAGE>   32

<TABLE>
<CAPTION>
                                                                                  NUMBER
                   INVESTORS                          AMOUNT INVESTED            OF SHARES
<S>                                                   <C>                        <C>
Kleiner Perkins Caufield & Byers VI                     $  193,087.63              101,093
Attn:  Brook H. Byers
2750 Sand Hill Road
Menlo Park, CA  94025

Accel Investors `93 L.P.                               $     7,143.40                3,740
Accel IV L.P.                                              161,614.65               84,615
Accel Japan L.P.                                            15,448.08                8,088
Accel Keiretsu L.P.                                          3,476.20                1,820
Ellmore C. Patterson Partners                                4,247.84                2,224
Prosper Partners                                             1,157.46                  606
Attn:  Luke B. Evnin, Ph.D.
One Embarcadero Center, Suite 3820
San Francisco, CA  94111

InterWest Investors V                                 $        947.36                  496
InterWest Partners V                                       150,525.19               78,809
Attn:  Arnold Oronsky, Ph.D.
3000 Sand Hill Road
Building 3, Suite 255
Menlo Park, CA  94025

Oxford Bioscience Partners (Adjunct) L.P.                $  20,196.34               10,574
Oxford Bioscience Partners (Bermuda) L.P.                   17,543.35                9,185
Oxford Bioscience Partners L.P.                             63,242.01               33,111
Attn:  Edmund M. Olivier
650 Town Center Drive, Suite 810
Costa Mesa, CA  92626

Second Ventures II, L.P.                                 $  10,602.41                5,551
U.S. Venture Partners IV, L.P.                              87,350.03               45,733
USVP Entrepreneur Partners II, L.P.                          3,029.26                1,586
Attn:  Philip M. Young
2180 Sand Hill Road, Suite 300
Menlo Park, CA  94025
</TABLE>



                                       3.
<PAGE>   33

<TABLE>
<CAPTION>
                                                                                  NUMBER
                   INVESTORS                          AMOUNT INVESTED            OF SHARES
<S>                                                   <C>                        <C>
Harry F. Hixson, Jr. Separate Property Trust,            $  42,058.20               22,020
Dated December 15, 1995
Attn:  Harry F. Hixson, Ph.D.
c/o Neuroscience Biosciences Inc.
3050 Science Park Road
San Diego, CA  92121-1102

Vertical Fund Associates, L.P.                           $  25,244.47               13,217
Attn:  John Runnells
18 Bank Street
Summit, NJ  07901

TOTAL                                                   11,999,997.11            6,282,721
</TABLE>








                                       4.
<PAGE>   34

                                   SCHEDULE B

                             Schedule of Exceptions














<PAGE>   35

                                    EXHIBIT A

                 Amended and Restated Articles of Incorporation












<PAGE>   36

                                    EXHIBIT B

                Amended and Restated Investors' Rights Agreement












<PAGE>   37

                                    EXHIBIT C

                Proprietary Information and Inventions Agreement












<PAGE>   38

                                    EXHIBIT D

                      Form of Opinion of Cooley Godward LLP