Document:

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                                  EXHIBIT 10.19
                   CONFIDENTIAL TREATMENT HAS BEEN REQUESTED
                         FOR THE PORTIONS MARKED [***]

                            AGREEMENT BY AND BETWEEN
                           ARENA PHARMACEUTICALS, INC.
                                       AND
                         TAIGEN BIOTECHNOLOGY CO., LTD.

                                TABLE OF CONTENTS
                                                                           Page
                                                                           ----
<Table>
<Caption>

<S>               <C>                                                      <C>
Article I.        Definitions                                               1
Article II.       GPCR Selection By TaiGen                                  6
Article III.      Arena Activities                                         10
Article IV.       TaiGen Activities                                        11
Article V.        License Grant                                            13
Article VI.       Warranties                                               15
Article VII.      Diligence Is Of The Essence                              16
Article VIII.     TaiGen Royalty Payments                                  17
Article IX.       Arena Royalty Payments                                   18
Article X.        Payment Arrangement                                      19
Article XI.       Confidentiality                                          19
Article XII.      Patent Infringement And Enforcement                      20
Article XIII.     Representations                                          21
Article XIV.      Indemnity                                                22
Article XV.       Termination and Duration                                 22
Article XVI.      Relationship of the Parties                              23
Article XVII.     Miscellaneous Provisions                                 23
Signature Blocks                                                           27
Appendix A        TaiGen Selected GPCRs
Appendix B        Arena Patent Rights
</Table>

                                 --PLEASE NOTE--
            Provisions Within This Agreement Are Deemed "CONFIDENTIAL"
                   in Accordance With The Terms of Article XI.

         Reviewers are advised to confirm with their attorney as to any
       obligations and/or requirements regarding review of this Agreement.

                                   COVER PAGE

<Page>

                                    AGREEMENT

     This Agreement ("Agreement") is effective as defined below by and between
ARENA PHARMACEUTICALS, INC., a Delaware corporation having a place of business
at 6166 Nancy Ridge Drive, San Diego, California, 92121 USA ("Arena"), and
TAIGEN BIOTECHNOLOGY CO., LTD., a corporation incorporated as a company limited
by shares in accordance with the Company law of the Republic of China
("TaiGen").

     WHEREAS, TaiGen is a start-up biopharmaceutical organization focused on the
discovery and development of innovative therapeutics;

     WHEREAS, Arena is a publicly traded biopharmaceutical organization (Nasdaq:
ARNA) focused on the discovery and development of innovative therapeutics;

     WHEREAS, Arena and TaiGen each desires to enter into this Agreement on the
terms and conditions set forth herein.

     NOW, THEREFORE, in consideration of the mutual covenants and agreements
contained herein, Arena and TaiGen hereby agree as follows:

                                    ARTICLE I

                                   DEFINITIONS

     Unless otherwise specifically provided herein, the following terms shall
have the following meanings:

"ACTIVE CAPITAL" means Active Capital Limited, a Cayman Islands company, or its
successor.

"AFFILIATE" when used with reference to a specified person or entity, means any
person or entity directly or indirectly controlling, controlled by or under
common control with the specified person or entity, while "control" in this
context means the direct or indirect ownership of at least 50% of the
outstanding voting securities of a person or entity. For purposes of this
Agreement, the definition of "TaiGen Affiliate" shall include Active Capital.
For purposes of this Agreement, the definition of "Arena Affiliate" shall
specifically exclude: (1) ChemNavigator.com, Inc.; (2) Aressa Pharmaceuticals,
Inc.; and (3) BRL Screening, Inc.

"ANNUAL" means the period between January 1 and December 31, inclusive.

"ARENA ACTIVATION TECHNOLOGY" means an Arena proprietary approach, referred to
by Arena as "CART Technology", to identifying, selecting and altering a
region(s) of a G Protein Coupled Receptor that, when altered, leads to or
enhances ligand-independent constitutive activation of the altered receptor.

"ARENA EXCLUDED GPCR" means any GPCR that (1)[*****************************
**********************************************];

                                       1

                                 CONFIDENTIAL
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(2) [**************************************************************]; or (3)
[*********************************************************************
*************].

"ARENA GPCR" means any GPCR in the possession of Arena during the Term of this
Agreement, BUT NOT INCLUDING any Arena Excluded GPCR.

"ARENA GPCR INFORMATION" means, when and if available, human tissue and cell
line gene expression data and protein sequence Phylogenetic Information for
Arena GPCRs.

"ARENA LICENSING REVENUE" means all financial consideration received by Arena
from a Third Party for the licensing by Arena to such Third Party of a CART
Identified Compound,[********************************************************
********************************************************************].

"ARENA PATENT RIGHTS" means all present and/or future patents (including
inventor's certificates) and all present and/or future applications (including
provisional applications) therefore throughout the world as the case may be, and
substitutions, extensions, reissues, renewals, divisions, continuations, or
continuation-in-part thereof or therefore, owned or controlled (either fully or
partially) by Arena, or under which Arena may grant licenses or sublicenses, to
the extent they are directed to (1) Arena Activation Technology applied to
TaiGen Selected GPCR(s) and/or (2) TaiGen Activated Receptor(s) and/or (3)
Screening Assay(s).

"BEST REASONABLE COMMERCIAL EFFORTS" means efforts to achieve a designated
objective, which efforts are based upon reasonably prudent business factors and
considerations.

"CART IDENTIFIED COMPOUND(S)" means a compound, and/or a Derivative of a CART
Identified Compound, that has been identified as a modulator of a TaiGen
Activated Receptor by TaiGen during the term of this Agreement or by Arena
Screening.

"DERIVATIVE" of a first compound means a compound having the same core structure
as the first compound that has been synthesized or conceived, and then
subsequently reduced to practice by TaiGen during the term of this Agreement.

"DRUG PRODUCT" means a therapeutic product comprising a CART Identified
Compound.

"DRUG PRODUCT REVENUE" means [************************************************
******************************************************************************
******************************************************************************
******************************************************************************
******************************************************************************
******************************************************************************
******************************************************************************
******************************************************************************

                                       2
                                  CONFIDENTIAL

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******************************************************************************
******************************************************************************
************************************************************].

"EFFECTIVE DATE" shall be the First Closing Date as defined in the Stock
Purchase Agreement.

"ENDOGENOUS" means naturally occurring.

"FDA" means the United States Food and Drug Administration.

"G PROTEIN COUPLED RECEPTOR" and "GPCR" means an Endogenous cell-surface
receptor defined by having three (3) intracellular loops, three (3)
extracellular loops, an amino terminus and a carboxy terminus.

"IND" has the same meaning as set forth in 21 C.F.R. Section 312.20,
including any and all amendments, modifications or changes as may be made
thereto in the future, or the equivalent thereof in any applicable country
within the Territory.

"INFORMATION" has the same meaning as set forth in Section 11.1 of this
Agreement.

"INVESTORS' RIGHTS AGREEMENT" means the Investors' Rights Agreement entered into
as of the date of the Stock Purchase Agreement among TaiGen, Arena and certain
other parties.

"LIBRARY COMPOUNDS" means chemical compounds.

"MEASURED RESPONSE" when used in reference to the phrase "TaiGen Activated
Receptor" means a signal measured based upon an assay end-point used to assess
the signal.

"MIGRATE" as used in the definition of "Active Capital" has the same meaning as
set forth in Section 6 of the Investors Rights Agreement.

"NOTICE" has the same meaning as set forth in Section 17.11 of this Agreement.

"PARTY" means either Arena or TaiGen, as the case may be; "Parties" means both
Arena and TaiGen.

"PHASE 1 CLINICAL STUDY" has the same meaning as set forth in 21 C.F.R.
Section 312.21(a), including any and all amendments, modifications or changes
thereto as may be made thereto in the future, or the equivalent thereof, in
any applicable country within the Territory.

"PHASE 2 CLINICAL STUDY" has the same meaning as set forth in 21 C.F.R.
Section 312.21(b), including any and all amendments, modifications or changes
thereto as may be made thereto in the future, or the equivalent thereof, in
any applicable country within the Territory.

"PHASE 3 CLINICAL STUDY" has the same meaning as set forth in 21 C.F.R.
Section 312.21(c), including any and all amendments, modifications or changes
thereto as may be made thereto in the future, or the equivalent thereof, in
any applicable country within the Territory.

                                       3
                                  CONFIDENTIAL

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"PHYLOGENETIC INFORMATION" means analysis of GPCRs for determination of GPCR
subfamilies based upon protein sequence information.

"REGULATORY AGENCY" includes, but is not be limited to, FDA, or similar
regulatory bodies in the Territory.

"SERIES A FINANCING ROUND" means the sale and/or distribution by TaiGen of
[***********************] of TaiGen's Series A preferred stock.

"SERIES B FINANCING ROUND" means the sale and/or distribution by TaiGen of at
least [**********************] of TaiGen's preferred stock after and independent
from the closing of the Series A Financing Round, and where the per share price
of each share of such preferred stock sold in the Series B Financing Round is
greater than the per share price of each share of Series A preferred stock sold
in the Series A Financing Round.

"SCREENING" means the process of contacting a chemical compound with a TaiGen
Activated Receptor.

"SCREENING ACTIVITIES" means the process of contacting a chemical compound with
a GPCR.

"SCREENING ASSAY" means an Arena assay approach for Screening that has been
validated based upon Successful Screening of a TaiGen Activated Receptor.

"SCREENING ASSAY NOTICE" has the same meaning as set forth in Section 3.2 of
this Agreement.

"STOCK PURCHASE AGREEMENT" means the Series A Convertible Preferred Stock
Purchase Agreement entered into as of _____ ___, 2001 among TaiGen, Arena and
certain other parties.

"SUCCESSFUL SCREENING" when used in conjunction with the phrase "TaiGen
Activated Receptor" means that the results of the Screening has been positive
whereby at least one molecule that has been contacted with the TaiGen Activated
Receptor reduces the Measured Response of the TaiGen Activated Receptor by at
least [*********** **********] from the mean response of a screening plate that
includes that compound.

"TAIGEN ACTIVATED RECEPTOR" means a TaiGen Selected GPCR to which the Arena
Activation Technology has been applied.

"TAIGEN ACTIVATED RECEPTOR NOTICE" has the same meaning as set forth in Section
3.1 of this Agreement.

"TAIGEN LICENSING REVENUE" means all financial consideration received by TaiGen
from a Third Party for the licensing by TaiGen to such Third Party of a CART
Identified Compound,[**********************************************************

                                       4
                                  CONFIDENTIAL
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******************************************************************************].

"TAIGEN SELECTED GPCR" has the same meaning as set forth in Section 2.1 of this
Agreement.

"TAIGEN SELECTION REQUEST" means a Notice request by TaiGen for selection of an
Arena GPCR.

"TECHNICAL INFORMATION" means all information, trade secrets, know-how, methods
of manufacture, processes, documents and materials related to TaiGen Activated
Receptor(s) and/or Screening Assay(s), and other proprietary information,
whether patentable or unpatentable, related to TaiGen Activated Receptor(s)
and/or Screening Assay(s), including but not limited to, improvements, that are
owned, possessed by, or licensed to Arena, whether now existing or hereafter
developed or acquired during the term of this Agreement.

"TECHNOLOGY" means Arena Patent Rights and Technical Information.

"TERM" has the same meaning as set forth in Section 15.2 of this Agreement.

"TERRITORY" means the world.

"THIRD PARTY" means any person or entity other than TaiGen and Arena.

                 [THE REST OF THIS PAGE IS INTENTIONALLY BLANK]

                                       5
                                  CONFIDENTIAL
<Page>

                                   ARTICLE II

                            GPCR SELECTION BY TAIGEN

     2.1 Subject to the terms and conditions of this Agreement, within five (5)
business days after the Effective Date, Arena shall provide TaiGen with Arena
GPCR Information available at Arena as of the Effective Date and thereafter when
available, every three (3) months after the Effective Date until the third
anniversary of the Effective Date whereby TaiGen shall be entitled to select and
obtain from Arena up to a maximum of [***] ([**]) Arena GPCRs for inclusion in
this Agreement (each GPCR a "TaiGen Selected GPCR"), with the selection subject
to the following:

          (a)  FIRST [****] ([**]) ARENA GPCRS.

               (i) The selection by TaiGen of up to [***] ([**]) Arena GPCRs may
begin after the Effective Date and must be completed prior to the second
anniversary of the Effective Date.

               (ii) Each Arena GPCR selected by TaiGen in accordance with this
Section 2.1(a) must be made by means of a TaiGen Selection Request, with the
date of the TaiGen Selection Request being the date that the TaiGen Selection
Request is received by Arena.

               (iii) In the event that TaiGen selects a GPCR that, as of the
date of a TaiGen Selection Request, is determined by Arena to be an Arena
Excluded GPCR, Arena shall provide Notice to TaiGen within five business days
after TaiGen Selection Request is received by Arena that the proposed Arena GPCR
cannot be designated as a TaiGen Selected GPCR; in those circumstances where the
GPCR that is the subject of a TaiGen Selection Request is not an Arena Excluded
GPCR, then Arena shall provide Notice to TaiGen within five business days after
TaiGen Selection Request is received by Arena indicating that such Arena GPCR
may be designated as a TaiGen Selected GPCR and at that time, Arena shall
provide to TaiGen the nucleic acid and protein sequence information for such
TaiGen Selected GPCR.

          (b) SECOND [*********] ([**]) ARENA GPCRS.

               (i) The selection by TaiGen of up to an additional [*******]
([**]) Arena GPCRs is contingent upon all of the following: (1) completion of
the Series A Financing Round on or before the second anniversary of the
Effective Date; (2) the closing of the Series B Financing Round on or before the
third anniversary of the Effective Date; and (3) securing a per share price for
each share of TaiGen Series B Preferred Stock above the per share price of the
TaiGen Series A Preferred Stock Price:

                    a) in the event that the Series A Financing Round is not
closed by the second anniversary of the Effective Date, then this entire Section
2.1(b) of the Agreement shall be null and void;

                                       6
                                  CONFIDENTIAL
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                    b) in the event that the Series A Financing Round is closed
by the second anniversary of the Effective Date BUT the Series B Financing Round
is not closed on or before the third anniversary of the Effective Date, then as
the third anniversary of the Effective Date, this Section 2.1(b) of the
Agreement shall be null and void and Arena shall have the unilateral right to
request that TaiGen return any TaiGen Selected GPCR and related Technical
Information secured by TaiGen under the provisions of Section 2.1(b)(i) and
TaiGen warrants and represents that it shall be obligated to comply with such
request within thirty (30) days thereof; in such event, the provisions of: (i)
Section 5.1 of this Agreement shall be null and void with respect to any TaiGen
Selected GPCR(s) selected in accordance with the provisions of Section 2.1(b) of
this Agreement, with all such rights automatically reverting back to Arena, and
(ii) Article IX shall not apply with respect to any TaiGen Selected GPCR(s)
selected in accordance with the provisions of Section 2.1(b) of this Agreement.

                    c) In the event that the per share price of the TaiGen
Series B Preferred Stock is:

                         1) [************************************************
******************] then the entire Section 2.1(b) shall be null and
void and Arena shall have the unilateral right to request that TaiGen return any
TaiGen Selected GPCR and related Technical Information secured by TaiGen under
the provisions of Section 2.1(b)(i) and TaiGen warrants and represents that it
shall be obligated to comply with such request within thirty (30) days thereof;
in such event, the provisions of: (i) Section 5.1 of this Agreement shall be
null and void with respect to any TaiGen Selected GPCR(s) selected in accordance
with the provisions of Section 2.1(b) of this Agreement, with all such rights
automatically reverting back to Arena, and (ii) Article IX shall not apply with
respect to any TaiGen Selected GPCR(s) selected in accordance with the
provisions of Section 2.1(b) of this Agreement; OR

                         2) [*************************************************
******************************] (e.g., if the per share price of the TaiGen
Series A Preferred Stock is [****], the per share price of the TaiGen Series B
Preferred Stock must be at least [****] for the provisions of this Section
2.1(b)(i)(b)(2) to apply), the provisions of Section 2.1(b)(i) shall apply; OR

                         3) [*************************************************
*********************************************************************], then
the Parties shall meet to determine the number of additional Arena GPCRs that
may be selected by TaiGen under Section 2.1(b) of this Agreement, with the
proviso that such number shall be at least [***] ([**]) but less than [***]
([**]).

               (ii) Contingent upon the provisions of Section 2.1(b)(i), the
selection by TaiGen of up to [***********] ([**]) Arena GPCRs may begin after
the consummation of the Migration and must be completed prior at or before the
third anniversary of the Effective Date.

                                       7
                                  CONFIDENTIAL
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               (iii) Contingent upon the provisions of Section 2.1(b)(i), each
Arena GPCR selected by TaiGen must be made by means of a TaiGen Selection
Request, with the date of the TaiGen Selection Request being the date that the
TaiGen Selection Request is received by Arena.

               (iv) In the event that TaiGen selects a GPCR that, as of the date
of a TaiGen Selection Request, is determined by Arena to be an Arena Excluded
GPCR, Arena shall provide Notice to TaiGen within five business days after
TaiGen Selection Request is received by Arena that the proposed Arena GPCR
cannot be designated as a TaiGen Selected GPCR; in those circumstances where the
GPCR that is the subject of a TaiGen Selection Request is not an Arena Excluded
GPCR, then Arena shall provide Notice to TaiGen within five business days after
TaiGen Selection Request is received by Arena, indicating that such Arena GPCR
may be designated as a TaiGen Selected GPCR and at that time, Arena shall
provide to TaiGen the nucleic acid and protein sequence information for such
TaiGen Selected GPCR.

     2.2 All TaiGen Selected GPCRs shall be attached hereto as APPENDIX A, which
shall be updated when TaiGen has selected an Arena GPCR that is designated as a
TaiGen Selected GPCR.

     2.3 In consideration of the rights granted under Section 2.1(a), TaiGen
shall provide Arena validly issued and recorded Series A Preferred Stock
Certificates in the name of "Arena Pharmaceuticals, Inc." (including any and all
documents related thereto for signature by Arena, including, but not limited to,
the Stock Purchase Agreement and the Investors' Rights Agreement) for
[************] worth of Series A Preferred Stock in accordance with the terms of
the Stock Purchase Agreement.

     2.4 The consideration for the rights granted under Section 2.1(b) shall be
based upon either Section 2.4(i) or Section 2.4(ii), but not both:

               (i) In consideration of the rights granted under Section 2.1(b),
and in the event that TaiGen has selected [******] ([**]) additional GPCRs that
are designated as TaiGen Selected GPCRs, then at the closing of TaiGen's Series
B Preferred Financing Round, TaiGen shall simultaneously provide Arena a validly
issued and recorded Series B Preferred Stock Certificate in the name of "Arena
Pharmaceuticals, Inc." (including any and all documents related thereto for
signature by Arena, including, but not limited to, a Series B Preferred Stock
purchasing agreement) in an amount sufficient to ensure that at the closing of
TaiGen's Series B Financing Round, and on a fully diluted basis, the Series B
Preferred Stock to be issued to Arena shall be equivalent to [********] percent
([**]%) of the Series B Financing Round.

               (ii) In consideration of the rights granted under Section 2.1(b),
and in the event that TaiGen has selected less than [******] ([**]) additional
GPCRs that are designated as TaiGen Selected GPCRs, then at the closing of
TaiGen's Series B Preferred Financing Round, TaiGen shall simultaneously provide
Arena a validly issued and recorded Series B Preferred Stock Certificate in the
name of "Arena Pharmaceuticals, Inc." (including any and all documents related
thereto for signature by Arena, including,

                                       8
                                  CONFIDENTIAL
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but not limited to, a Series B Preferred Stock purchasing agreement) in an
amount sufficient to ensure that at the closing of TaiGen's Series B Financing
Round, and on a fully diluted basis, the Series B Preferred Stock to be issued
to Arena shall be equivalent to:

          [{each TaiGen Selected GPCR from Section 2.1(b)} X {[****]}]

of the Series B Financing Round, with total percentage being: (a) rounded up to
the next nearest whole number and (b) less than [*******] percent ([**]%).

               (iii) In consideration of the rights granted under Section
2.1(b), Arena shall have the right, but not the obligation, to purchase, PARI
PASSU, TaiGen's Series B Preferred Stock in an amount sufficient to ensure that
at the closing of TaiGen's Series B Financing Round, and on a fully diluted
basis, the combination of the Series A Preferred Stock issued to Arena and the
Series B Preferred Stock to be issued to Arena shall be equivalent to [*****]
percent ([**]%) of all securities issued by TaiGen as of the closing of the
Series B Financing Round, to the extent that shares are available after
allocations have been made to the lead investor and to existing shareholders of
the Company based upon their preemptive rights.

     2.5 TAIGEN REVIEW

          (a) At any time after receipt of Arena GPCR Information by TaiGen,
TaiGen scientists shall have the right to discuss the Arena GPCR Information
with Arena scientist(s) as designated by Arena.

          (b) At any time after receipt of nucleic acid and protein sequence
information regarding a TaiGen Selected GPCR by TaiGen, TaiGen scientists shall
have the right to discuss the nucleic acid and protein sequence information with
Arena scientist(s) as designated by Arena.

          (c) Upon written request by TaiGen, Arena shall provide TaiGen with
the cDNA of a TaiGen Selected Receptor in a suitable expression vector for the
sole and exclusive use by TaiGen to verify Arena GPCR Information. All
information generated by TaiGen regarding such materials shall be provided to
Arena by TaiGen. Subject to the provisions of Section 2.5(d), in the event that
TaiGen is unable to substantially replicate Arena GPCR Information regarding
TaiGen Selected GPCR within [***********] of receipt of the materials from
Arena, then TaiGen may provide Notice to Arena requesting substitution of TaiGen
Selected GPCR with an Arena GPCR.

          (d) In the event that Arena has developed a TaiGen Activated Receptor
and the corresponding Screening Assay using a TaiGen Selected GPCR in accordance
with the provisions of Section 3.1 prior to TaiGen providing Notice to Arena in
accordance with Section 2.5(c), then irrespective of the inability of TaiGen to
replicate Arena GPCR Information regarding such TaiGen Selected GPCR, TaiGen
shall be prohibited from requesting substitution of such TaiGen Selected GPCR
with an Arena GPCR. The maximum number of substitutions that TaiGen may receive
under Section 2.5(c) shall be limited as follows: (1) for the [*********] GPCRs
that may be selected in

                                       9
                                  CONFIDENTIAL
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accordance with the provisions of Section 2.1(a), the maximum number of
substations shall be [****] ([**]); and (2) for the [*****] GPCRs that may be
selected in accordance with the provisions of Section 2.1(b), the maximum number
of substations shall be [***] ([**]).

                                   ARTICLE III

                                ARENA ACTIVITIES

     3.1 DEVELOPMENT OF TAIGEN ACTIVATED RECEPTOR. Subject to the terms and
conditions of this Agreement, Arena agrees to use Best Reasonable Commercial
Efforts to apply Arena Activation Technology to EACH TaiGen Selected GPCR to
establish a TaiGen Activated Receptor within [********] after Arena's Notice to
TaiGen that the Arena GPCR shall be designated as a TaiGen Selected GPCR. Upon
creation of EACH TaiGen Activated Receptor, Arena shall provide Notice to TaiGen
("TaiGen Activated Receptor Notice"), and such TaiGen Activated Receptor Notice
shall include a report comprising all data developed by Arena evidencing that
the TaiGen Activated Receptor is constitutively active.

     3.2 DEVELOPMENT OF SCREENING ASSAY. Subject to the terms and conditions of
this Agreement, Arena agrees to use Best Reasonable Commercial Efforts to
develop a Screening Assay incorporating EACH TaiGen Activated Receptor within
[***********] after TaiGen Activated Receptor Notice is sent by Arena to TaiGen.
Upon development of each Screening Assay, Arena shall provide Notice to TaiGen
("Screening Assay Notice"), and such Screening Assay Notice shall include a
report comprising of data developed by Arena evidencing the Screening Assay.

     3.3 TRANSFER OF TAIGEN ACTIVATED RECEPTOR AND TECHNICAL INFORMATION.

          (a) Subject to the terms and conditions of this Agreement, within
[*******] of each Screening Assay Notice, Arena shall transfer to TaiGen the
applicable TaiGen Activated Receptor and a copy of all additional Technical
Information owned or possessed by Arena then applicable to such TaiGen Activated
Receptor and Screening Assay incorporating such TaiGen Activated Receptor.

          (b) In the event that TaiGen is unable to substantially replicate the
results of the Screening incorporating such TaiGen Selected Receptor in
accordance with the procedures and protocols used by Arena and provided to
TaiGen as part of the applicable Technical Information, Arena shall cooperate
with TaiGen in studying the cause of such issues, and if any defect in the
TaiGen Activated Receptor or the Screening Assay transferred by Arena to TaiGen
is detected and determined to have been caused by or effected by the activities
of Arena, Arena shall, at its costs and expenses, use Best Reasonable Commercial
Efforts to either repair the defect or transfer a substitute TaiGen Activated
Receptor and Screening Assay incorporating such substituted TaiGen Activated
Receptor to TaiGen.

                                       10
                                  CONFIDENTIAL
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     3.4 ADDITIONAL SUPPLY. When requested by TaiGen in writing, Arena shall use
its Best Reasonable Commercial Efforts to supply additional quantity of the
TaiGen Activated Receptor within the time period requested by TaiGen free of
charge.

     3.5 TECHNICAL ASSISTANCE. At any time during the term of this Agreement,
Arena shall use Best Reasonable Commercial Efforts to provide and make available
the following to TaiGen:

          (a) Arena shall make available during its business hours at least one
(1) scientist designated by Arena for consultation with and for responses to any
inquiries of TaiGen relating to TaiGen Selected GPCR, the Technical Information,
Arena GPCR Information, the nucleic acid and amino acid sequences of a TaiGen
Selected GPCR, TaiGen Activated Receptor, Screening Assay, or any other report
and data provided by Arena to TaiGen; such scientist shall be available by means
of telephones, e-mails, written correspondences and any other appropriate means
of communication, PROVIDED HOWEVER, that such scientist shall not be required to
provide more than [***] ([**]) hours' worth of consultation per any twenty (24)
four hour period, and not more than a total of [***] ([**]) hours' worth of
consultation per any five (5) business days. In the event that TaiGen requires
additional consultation time from any Arena scientist, the Parties agree to meet
in good faith to discuss the terms and conditions for a fee-for-service
consulting agreement.

          (b) Upon reasonable advanced notice to Arena, TaiGen shall have the
right to dispatch its personnel to visit the laboratories of Arena during
Arena's normal business hours. Arena shall arrange for a scientist designated by
Arena to accompany TaiGen personnel in their tour of the laboratory and to
answer any questions any such personnel may have over the course of the visit,
PROVIDED HOWEVER, that such scientist shall not be required to provide more than
[**] ([*]) hours' worth of consultation per any twenty (24) four hour period,
and not more than a total of [****] ([**]) hours' worth of consultation per any
five (5) business days. In the event that TaiGen requires additional
consultation time from any Arena scientist, the Parties agree to meet in good
faith to discuss the terms and conditions for a fee-for-service consulting
agreement.

     3.6 PATENT INFORMATION. A list of Arena Patent Rights, and the jurisdiction
of such Arena Patent Rights, relating to any TaiGen Selected GPCR shall be
attached hereto as APPENDIX B, which shall be updated from time to time upon any
new issuance or filings relating thereto.

                                   ARTICLE IV

                                TAIGEN ACTIVITIES

     4.1 TAIGEN SCREENING.

          (a) Upon receipt of EACH TaiGen Activated Receptor, its corresponding
Screening Assay, and related Technical Information from Arena, TaiGen shall use
Best Reasonable Commercial Efforts to promptly validate the Screening Assay
incorporating such TaiGen Activated Receptor and thereafter initiate Screening.

                                       11
                                  CONFIDENTIAL
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          (b) When requested by TaiGen in writing, Arena shall use its Best
Reasonable Commercial Efforts to assist TaiGen in setting-up a Screening
Assay(s) at TaiGen's facility within the time period(s) requested by TaiGen.
TaiGen agrees to reimburse Arena for the reasonable out-of-pocket costs
associated with the assistance of TaiGen with its Screening.

     4.2 CART IDENTIFIED COMPOUNDS. For each TaiGen Activated Receptor, TaiGen
shall use Best Reasonable Commercial Efforts to develop at least one (1) CART
Identified Compound. In the event that TaiGen determines to develop a CART
Identified Compound into a Drug Product, Arena acknowledges and agrees that
TaiGen has sole and exclusive right to unilaterally determine which CART
Identified Compound is to be developed as Drug Product and whether or not to
continue development of any CART Identified Compound or Drug Product. Except as
otherwise provided for in this Agreement, CART Identified Compounds, Derivatives
or Drug Products developed by TaiGen from any TaiGen Activated Receptor
hereunder shall be the exclusive property of TaiGen and TaiGen shall have sole
ownership and title to such CART Identified Compounds, Derivatives or Drug
Products.

     4.3 PROGRESS REPORTS.

          (a) Beginning from the Effective Date, and throughout the Term of this
Agreement, TaiGen shall provide Arena with written progress reports regarding
the scientific and business-related activities associated with each TaiGen
Selected GPCR. The first report shall be provided by TaiGen to Arena on the
[*********] anniversary of the Effective Date, and each subsequent report shall
be provided by TaiGen to Arena [********] after the previous report was provided
by TaiGen to Arena. Except as otherwise provided for in this Agreement, Arena,
without TaiGen's written consent, shall not utilize any information contained in
such written progress reports.

          (b) For each TaiGen Activated Receptor, such report shall include,
where applicable, information directed to: (1) [******************************];
(2) [***********************************]; (3) [*******************************
************************]; (4)[******************************************
************************************************************************
*********************************************];(5)[***************************
*********************************]; (6)[**************************************
*********************]; and (7)[**********************************************
**********************].

          (c) With respect to EACH TaiGen Activated Receptor, in the event that
[***] ([*]) consecutive progress reports for such TaiGen Activated Receptor
indicate that no significant activity has been taken by TaiGen with respect to
such TaiGen Activated Receptor, then Arena shall provide Notice to TaiGen
regarding such non-progress and TaiGen shall have until the next progress report
to establish that significant activity has been taken by TaiGen with respect to
such TaiGen Activated Receptor. In the event that such activity has not been
established in the next consecutive progress report, then Arena shall have the
unilateral right to request that TaiGen return such TaiGen Selected GPCR

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and related Technical Information and TaiGen warrants and represents that it
shall be obligated to comply with such request within thirty (30) days thereof;
in such event, the provisions of: (i) Section 5.1 of this Agreement shall be
null and void with respect to such TaiGen Selected GPCR, with all such rights
automatically reverting back to Arena, and (ii) Article IX shall not apply with
respect to such TaiGen Selected GPCR.

                                    ARTICLE V

                                  LICENSE GRANT

     5.1 ARENA LICENSE. Subject to the terms and conditions of this Agreement,
Arena hereby grants to TaiGen and its Affiliate Active Capital, effective as of
the Effective Date and continuing for a period of ending upon the expiration of
twenty (20) years from the Effective Date or the date of expiration of the last
to expire patent with claims covering the CART Identified Product(s) and/or Drug
Product(s) derived from such TaiGen Selected Receptor, whichever is longer, the
following with respect ONLY to the TaiGen Activated Receptor that is the subject
of the Screening Assay Notice:

          (a) an exclusive right and license under the Technology, exclusive
even as to Arena, but subject to the provisions of Section 5.3 and, when
applicable, Section 2.1(b) of this Agreement, to use, have used, sell, have
sold, import, have imported, further develop, improve and otherwise exploit in
any manner the TaiGen Activated Receptor, for the purpose of identification of
CART Identified Compound(s) in the Territory, and

          (b) an exclusive right and license under the Technology, exclusive
even as to Arena, to develop, manufacture, have manufactured, promote, market,
sell, distribute and, subject to the provisions of Section 5.3, Section 5.4 and,
when applicable, Section 2.1(b) of this Agreement, license CART Identified
Compound(s) and/or Drug Product(s) in the Territory.

TaiGen acknowledges and agrees that the license granted to it by Arena hereunder
is limited in scope and does not include a license, express or implied, outside
the scope set forth herein, including, but not limited to, use of the Arena
Activation Technology by TaiGen or use of any technology owned or assigned to:
ChemNavigator.com, Inc.; Aressa Pharmaceuticals, Inc.; and/or BRL Screening,
Inc.

     5.2 LIMITED SUBLICENSE RIGHTS. No right is granted by Arena, express or
implied, to TaiGen to sublicense any of the rights granted to TaiGen under
Section 5.1(a) of this Agreement to a Third Party, PROVIDED HOWEVER that if a
Third Party is a licensee of a CART Identified Compound derived by TaiGen from a
TaiGen Selected GPCR, TaiGen shall have the right to sublicense the Screening
Assay related to such CART Identified Compound to such Third Party for the
exclusive and sole use of such Screening Assay for the Third party sublicesee's
own internal research. In all such circumstances, TaiGen shall provide Notice to
Arena after it has sublicensed such Screening Assay to a Third Party and warrant
that the agreement between TaiGen and such Third Party explicitly provides that
(a) the sublicensed rights granted by TaiGen to such Third Party regarding

                                       13
                                  CONFIDENTIAL
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such Screening Assay are limited to internal research use only, and (b) that
such Third party is expressly prohibited from further sublicensing such
Screening Assay to any other Third Party.

     5.3 RIGHT OF FIRST NEGOTIATION. During the Term of this Agreement, and in
the event that TaiGen determines to license any CART Identified Compound, and/or
Derivative, and/or Drug Product to a Third Party, prior to initiating any
discussions related thereto with any Third Party, TaiGen shall first provide
Notice to Arena informing Arena that the subject CART Identified Compound and/or
Drug Product is available for licensing; such Notice shall include a
sufficiently detailed summary of the Technical Information developed by or on
behalf of TaiGen related to such CART Identified Compound and/or Drug Product.
Within [******] ([**]) days of receipt of such Notice, Arena shall provide
Notice to TaiGen informing TaiGen as to whether or not Arena is interested in
entering into negotiations for a license in and to such CART Identified Compound
and/or Drug Product:

          (a) In the event that Arena provides Notice to TaiGen indicating that
Arena will not enter into negotiations for a license in and to such CART
Identified Compound and/or Drug Product, following receipt of such Notice from
Arena, TaiGen may then contact any Third Party regarding the licensing of such
CART Identified Compound and/or Drug Product.

          (b) In the event that Arena provides Notice to TaiGen indicating that
Arena will enter into negotiations for a license in and to such CART Identified
Compound and/or Drug Product, Arena and TaiGen shall each use Best Reasonable
Commercial Efforts to immediately meet and initiate negotiations for a license
regarding such CART Identified Compound and/or Drug Product; each Party agrees
to use Best Reasonable Commercial Efforts to conclude such negotiations within
[***] ([**]) months of the date of the Notice provided by Arena to TaiGen under
this Section 5.3(b). In the event that the Parties are unable to reach mutually
acceptable terms and conditions regarding such a license within such [***]
([**]) month period, TaiGen may then contact any Third Party regarding the
licensing of such CART Identified Compound and/or Drug Product.

     5.4 IMPROVEMENTS.

          (a) TaiGen shall notify Arena, in writing, of any improvement
discovered or developed by TaiGen related to the Technology.

          (b) Arena shall notify TaiGen, in writing, of any improvement
discovered or developed by Arena related to the Technology within one (1) month
of the discovery or development of such improvement.

          (c) The obligations of Sections 5.4(a) and (b) of this Agreement shall
be continuing throughout the Term of this Agreement. During the Term of this
Agreement, TaiGen shall have a royalty-free, exclusive right and license to use
any

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                                  CONFIDENTIAL
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improvement of Arena referred to in Section 5.4(b) in accordance with the
provisions of Sections 5.1 and 5.2 of this Agreement.

          (d) Subject to Section 5.1 and except as specifically provided for in
Section 5.3(e) hereof, Arena shall have a royalty-free, non-exclusive right and
license to use all improvements of TaiGen and/or TaiGen's Licensee(s) referred
to in Section 5.4(a) hereof and to disclose and sublicense the same to its
licensees, if any.

          (e) Both Parties acknowledge and agree that Arena has exclusive
ownership of the Technology existing as of the Effective Date and any
improvement thereof hereafter discovered or developed by Arena and that TaiGen
has exclusive ownership of or control over any improvement of the Technology
hereafter discovered or developed by TaiGen and/or TaiGen's Licensee(s).

                                   ARTICLE VI

                                   WARRANTIES

     6.1 ARENA REPRESENTS AND WARRANTS THAT ARENA HAS VALID TITLE AND OWNERSHIP
OF THE ARENA ACTIVATION TECHNOLOGY, AND THAT AS OF THE EFFECIVE DATE AND TO THE
BEST OF ITS KNOWLEDGE, THE ARENA ACTIVATION TECHNOLOGY WILL NOT VIOLATE THE
PATENT, COPYRIGHT, TRADEMARK OR OTHER PROPRIETARY RIGHTS OF ANY OTHER PERSON.
ARENA FURTHER WARRANTS THAT ARENA HAS FILED FOR PATENT REGISTRATION, OR USING
BEST REASONABLE COMMERCIAL EFFORTS WILL FILE FOR SUCH REGISTRATION, AS SOON AS
POSSIBLE AFTER SELECTION BY TAIGEN, FOR EACH TAIGEN SELECTED GPCR, TAIGEN
ACTIVATED RECEPTOR, SCREENING ASSAY AND ARENA ACTIVATION TECHNOLOGY WITHIN THE
TERRITORY.

     6.2 NEITHER PARTY MAKES ANY REPRESENTATION THAT ANY ARENA GPCR, TAIGEN
SELECTED GPCR, TAIGEN ACTIVATED RECEPTOR OR SCREENING ASSAY TRANSFERRED BY ARENA
TO TAIGEN, WILL NOT INFRINGE ANY PATENT, COPYRIGHT, TRADEMARK OR OTHER
PROPRIETARY RIGHT OF ANY OTHER PERSON. NEITHER PARTY MAKES ANY WARRANTY OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE OR ANY OTHER WARRANTY,
EXPRESS OR IMPLIED, WITH RESPECT TO ANY TAIGEN SELECTED GPCR OR TAIGEN ACTIVATED
RECEPTOR OR SCREENING ASSAY, AS THE CASE MAY BE.

     6.3 NEITHER PARTY MAKES ANY REPRESENTATION TO THE OTHER THAT ANY CART
IDENTIFIED COMPOUND OR ANY DRUG PRODUCT WILL NOT INFRINGE ANY PATENT, COPYRIGHT,
TRADEMARK OR OTHER PROPRIETARY RIGHT OF ANY OTHER PERSON. NEITHER PARTY MAKES
ANY WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE OR ANY OTHER
WARRANTY, EXPRESS OR IMPLIED, TO THE

                                       15
                                  CONFIDENTIAL
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OTHER WITH RESPECT TO ANY CART IDENTIFIED COMPOUND OR ANY DRUG PRODUCT.

                                   ARTICLE VII

                           DILIGENCE IS OF THE ESSENCE

     7.1 TaiGen acknowledges and agrees that upon receipt from Arena of each
TaiGen Activated Receptor and related Technical Information, diligence regarding
the development of a CART Identified Compound and/or Drug Product, and/or
licensing of such CART Identified Compound and/or Drug Product, is of the
essence.

     7.2 The provisions of this Section 7.2 shall apply to a TaiGen Selected
Receptor and corresponding CART Identified Compound and/or Drug Product only if
all three of the premises which give rise to Arena's unilateral right to request
return as set forth in Section 7.2(a), (b) or (c) occur with respect to such
TaiGen Selected Receptor and corresponding CART Identified Compound and/or Drug
Product:

          (a) In the event that TaiGen has not licensed the CART Identified
Compound to a Third Party and/or to Arena prior to the expiration of six (6)
years from the receipt of the corresponding TaiGen Selected Receptor from Arena,
Arena shall have the unilateral right to request that TaiGen return any TaiGen
Selected GPCR, CART Identified Compound(s), Drug Product(s), if any, and related
Technical Information, and TaiGen warrants and represents that it shall be
obligated to comply with such request within thirty (30) days thereof; in such
event, the provisions of: (i) Section 5.1 of this Agreement shall be null and
void with respect to any TaiGen Selected GPCR(s), with all such rights
automatically reverting back to Arena, and (ii) Article IX shall apply with
respect to any such CART Identified Compound(s) and Drug Product(s), if any; OR

          (b) In the event that TaiGen has not filed an IND with a Regulatory
Agency for a Drug Product prior to the expiration of six (6) years from the
receipt of the corresponding TaiGen Selected Receptor from Arena, Arena shall
have the unilateral right to request that TaiGen return any TaiGen Selected
GPCR, CART Identified Compound(s), Drug Product(s), if any, and related
Technical Information, and TaiGen warrants and represents that it shall be
obligated to comply with such request within thirty (30) days thereof; in such
event, the provisions of: (i) Section 5.1 of this Agreement shall be null and
void with respect to any TaiGen Selected GPCR(s), with all such rights
automatically reverting back to Arena, and (ii) Article IX shall apply with
respect to any such CART Identified Compound(s) and Drug Product(s), if any; OR

          (c) In the event that TaiGen has not filed an IND with a Regulatory
Agency for a Drug Product AND the Drug Product has not been introduced into at
least one(1) human volunteer prior to the expiration of six (6) years from the
receipt of the corresponding TaiGen Selected Receptor from Arena, Arena shall
have the unilateral right to request that TaiGen return any TaiGen Selected
GPCR, CART Identified Compound(s), Drug Product(s), if any, and related
Technical Information, and TaiGen warrants and represents that it shall be
obligated to comply with such request within thirty (30) days

                                       16
                                  CONFIDENTIAL
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thereof; in such event, the provisions of: (i) Section 5.1 of this Agreement
shall be null and void with respect to any TaiGen Selected GPCR(s), with all
such rights automatically reverting back to Arena, and (ii) Article IX shall
apply with respect to any such Drug Product.

                                  ARTICLE VIII

                            TAIGEN ROYALTY PAYMENTS

     8.1 ROYALTY PAYMENT. In consideration of the right and license granted to
TaiGen hereunder with respect to each TaiGen Activated Receptor, TaiGen shall
provide Arena with a royalty payment based on Annual TaiGen Licensing Revenue
and/or Drug Product Revenue derived from EACH TaiGen Selected Receptor as set
forth below for a period ending upon the expiration of twenty (20) years from
the Effective Date or the date of expiration of the last to expire patent with
claims covering the CART Identified Product(s) and/or Drug Product(s) derived
from such TaiGen Selected Receptor, whichever is longer; such royalty payment
shall be made within three (3) months of December 31 for the Annual period to
which the Annual TaiGen Licensing Revenue applies:

          (a) [**********] ([**]%) of Annual TaiGen Licensing Revenue when the
effective date of a license agreement between TaiGen and a Third Party is prior
to initiation of Phase 1 Clinical Study of a CART Identified Compound.

          (b) [**********] ([**]%) of Annual TaiGen Licensing Revenue when the
effective date of a license agreement between TaiGen and a Third Party is after
initiation of Phase 1 Clinical Study and prior to completion of a Phase 2
Clinical Study of a CART Identified Compound.

          (c) [**********] ([**]%) of Annual TaiGen Licensing Revenue when the
effective date of a license agreement between TaiGen and a Third Party is after
initiation of Phase 2 Clinical Study and prior to completion of a Phase 3
Clinical Study of a CART Identified Compound.

          (d) [**********] ([**]%) of Annual TaiGen Licensing Revenue when the
effective date of a license agreement between TaiGen and a Third Party is after
completion of a Phase 3 Clinical Study of a CART Identified Compound.

          (e) [**********] ([**]%) of Annual Drug Product Revenue.

     8.2 AUDIT. In order to verify the completeness and correctness of TaiGen
Licensing Revenue and/or Drug Product Revenue, TaiGen shall maintain up to date
books and records and Arena shall have the right to conduct, through independent
certified public accountants, at its own cost and at any reasonable time during
business hours, not more often than once each Annual period for not more than
[**] ([*]) previous years, and upon reasonable prior notice, an audit of the
accounting procedures and records of TaiGen in computing and calculating royalty
payment for Annual TaiGen Licensing Revenue and/or Drug Product Revenue due
hereunder. The auditor shall make available

                                       17
                                  CONFIDENTIAL
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to TaiGen and Arena a report enumerating the period covered by the audit of
TaiGen Licensing Revenue and/or Drug Product Revenue computed and calculated by
the auditor. The costs of such audit shall be borne by TaiGen in the event that
a discrepancy of more than [****] per cent ([**]%) is discovered through such
audit.

                                   ARTICLE IX

                             ARENA ROYALTY PAYMENTS

     9.1 ROYALTY PAYMENT. In the event that Arena secures any CART Identified
Compound and/or Drug Product from TaiGen in accordance with the terms and
conditions of Article VII of this Agreement, then for each such CART Identified
Compound and/or Drug Product derived from each TaiGen Selected GPCR, Arena shall
provide TaiGen with a royalty payment based on Annual Arena Licensing Revenue
for a period ending upon the expiration of twenty (20) years from the Effective
Date or the date of expiration of the last to expire patent with claims covering
the CART Identified Product(s) and/or Drug Product(s) derived from such TaiGen
Selected Receptor, whichever is longer; such royalty payment shall be made
within three (3) months of December 31 for the Annual period to which the Annual
Arena Licensing Revenue applies:

          (a) [********] ([**]%) of Annual Arena Licensing Revenue when the
effective date of a license agreement between Arena and a Third Party is prior
to initiation of Phase 1 Clinical Study of a CART Identified Compound.

          (b) [********] ([**]%) of Annual Arena Licensing Revenue when the
effective date of a license agreement between Arena and a Third Party is after
initiation of Phase 1 Clinical Study and prior to completion of a Phase 2
Clinical Study of a CART Identified Compound.

          (c) [********] ([**]%)of Annual Arena Licensing Revenue when the
effective date of a license agreement between Arena and a Third Party is after
initiation of Phase 2 Clinical Study and prior to completion of a Phase 3
Clinical Study of a CART Identified Compound.

          (d) [********] ([**]%) of Annual Arena Licensing Revenue when the
effective date of a license agreement between TaiGen and a Third Party is after
completion of a Phase 3 Clinical Study of a CART Identified Compound.

          (e) [**********] ([**]%) of Annual Drug Product Revenue.

     9.2 AUDIT. In order to verify the completeness and correctness of Arena
Licensing Revenue and/or Drug Product Revenue, Arena shall maintain up to date
books and records and TaiGen shall have the right to conduct, through
independent certified public accountants, at its own cost and at any reasonable
time during business hours, not more often than once each Annual period for not
more than [**] ([*]) previous years, and upon reasonable prior notice, an audit
of the accounting procedures and records of Arena in computing and calculating
royalty payment for Annual Drug Product Revenue due

                                       18
                                  CONFIDENTIAL
<Page>

hereunder. The auditor shall make available to TaiGen and Arena a report
enumerating the period covered by the audit of Arena Licensing Revenue and/or
Drug Product Revenue computed and calculated by the auditor. The costs of such
audit shall be borne by Arena in the event that a discrepancy of more than [**]
per cent ([**]%) is discovered through such audit.

                                    ARTICLE X

                               PAYMENT ARRANGEMENT

     The Parties acknowledge and agree that any and all payments to be made by
TaiGen or Arena to the other under this Agreement are to be (i) in United States
Dollars and (ii) in full as indicated; any income or other tax which one party
is required to pay or withhold on behalf of the other party with respect to
payments payable to other party hereunder shall not be deducted from the amounts
of such payments.

                                   ARTICLE XI

                                CONFIDENTIALITY

     11.1 Each party shall neither disclose to any Third Party any and all of
the information disclosed by the other Party hereunder ("Information"), nor
permit any such Third Party to have access to such Information, nor use such
Information for any purpose other than for purpose of this Agreement, without
the prior written consent of the other Party.

     11.2 The receiving Party's obligations under Section 11.1 hereof shall not
apply, with respect to any of such Information to the extent that the receiving
Party can establish by competent proof that such Information:

          (a) is published, known publicly, or is already in the public domain
at the time of receipt of it by the receiving Party;

          (b) is published, becomes known publicly or becomes a part of the
public domain by publication or otherwise after the time of receipt of it by the
receiving Party, except by breach of this Agreement by the receiving Party;

          (c) is obtained from a Third Party after the receipt of it by the
receiving Party, provided, however, that said Third Party has not obtained it
directly or indirectly from the disclosing Party;

          (d) is in the receiving Party's possession on the date of the receipt
of it and was not acquired directly or indirectly from the disclosing Party; or

          (e) is subsequently developed by the receiving Party independent of
the information received hereunder, as evidenced by competent written records
established by the receiving Party.

                                       19
                                  CONFIDENTIAL
<Page>

     11.3 Notwithstanding anything to the contrary in this Agreement, the
receiving Party shall be entitled to disclose such Information (i) to the extent
required by applicable law or court order provided that the receiving Party
furnishes the disclosing Party with written notice of such request, in advance
of any such disclosure of the Information; (ii) to a government agency,
regulatory authority, clinical research organization, clinical investigator or
other Third Party to whom disclosure is necessary for development of the CART
Identified Compound in connection with drug development, approval or
registration of the CART Identified Compound and/or Drug Product; or (iii) to
the extent required to comply with the reporting requirements of any
governmental agency, including, but not limited to, the U.S. SEC or NASDAQ Stock
market rules, provided that the receiving Party furnishes the disclosing Party
with Notice of such compliance requirement, in advance of any such disclosure of
the Information.

     11.4 All information provided by TaiGen to Arena, including without
limitation, the information provided by TaiGen under Section 4.3, is designated
as "CONFIDENTIAL" by TaiGen; all information provided by Arena to TaiGen under
Sections 2.1, 2.2, 2.5 and Article III is designated as "CONFIDENTIAL" by Arena.

     11.5 The foregoing obligations of confidentiality shall survive for five
(5) years after any termination or expiration of this Agreement.

                                   ARTICLE XII

                       PATENT INFRINGEMENT AND ENFORCEMENT

     12.1 NOTIFICATION OF INFRINGEMENT. Each Party shall promptly provide Notice
to the other of any infringement (of which it becomes aware) of the intellectual
property rights including patent rights on any TaiGen Activated Receptor(s)
and/or Screening Assay(s) and/or CART Identified Compound(s) and/or Drug
Product(s) by any Third Party and shall provide the other with any available
evidence of such infringement of which the Party is aware.

     12.2 SUIT FOR INFRINGEMENT.

          (a) During the Term of this Agreement, Arena shall be responsible for
enforcement of the Arena Patent Rights including, but not limited to, the
bringing of an action for patent infringement, selection of the forum for such
action, and counsel, settlement of any such action, and the costs devoted to
such action. TaiGen agrees to provide reasonable assistance except for financial
assistance to Arena in the enforcement of Arena Patent Rights and TaiGen may
join such action as initiated by Arena with counsel at its own expense and seek
its own damages and other relief. If, within ninety (90) days of TaiGen's giving
notice to Arena of a Third Party infringement in the Territory, Arena fails to
institute the infringement suit that TaiGen reasonably feels is required, TaiGen
may institute such infringement proceedings against said Third Party at its
expense and TaiGen shall have the right to receive all the amounts payable by
said Third Party as a result of such proceedings.

                                       20
                                 CONFIDENTIAL
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          (b) In the event a claim of patent infringement is made against TaiGen
by a Third Party in the Territory by reasons of TaiGen's commercial activities
hereunder, TaiGen and Arena shall meet to analyze the infringement claim and
avoidance of the same. If it is necessary to obtain an appropriate license from
such a Third Party, the Parties shall, in negotiating such a license, make every
efforts to minimize the amount of license fees and/or royalties payable to such
Third Party and (i) in case that such license is related to Arena Activation
Technology, Arena shall be responsible for such license fees and/or royalties,
(ii) in case that such license is related to TaiGen Selected GPCR and/or TaiGen
Activated Receptor and/or Screening Assay, and/or CART Identified Compound,
and/or Drug Product, TaiGen shall be responsible for such license fees and/or
royalties.

                                  ARTICLE XIII

                                 REPRESENTATIONS

     13.1 REPRESENTATIONS AND WARRANTIES OF TAIGEN. TaiGen represents and
warrants to Arena as follows:

          (a) The execution and delivery of this Agreement have been duly and
validly authorized, and all necessary action has been taken to make this
Agreement a legal, valid and binding obligation of TaiGen enforceable in
accordance with its terms.

          (b) The execution and delivery of this Agreement and the performance
by TaiGen of its obligations hereunder will not contravene or result in the
breach of the Articles of Incorporation of TaiGen or result in any material
breach or violation of or material default under any material agreement,
indenture, license, instrument or understanding or, to the best of its
knowledge, result in breach of any law, rule, regulation, statute, order or
decree, to which TaiGen is a party or of which it or any of its property is
subject.

     13.2 REPRESENTATIONS AND WARRANTIES OF ARENA. Arena represents and warrants
to TaiGen as follows:

          (a) The execution and delivery of this Agreement have been duly and
validly authorized, and all necessary action has been taken to make this
Agreement a legal, valid and binding obligation of Arena enforceable in
accordance with its terms.

          (b) The execution and delivery of this Agreement and the performance
by Arena of its obligations hereunder will not contravene or result in the
breach of the Certificate of Incorporation or Bylaws of Arena or result in any
material breach or violation of or material default under any material
agreement, indenture, license, instrument or understanding or, to the best of
its knowledge, result in breach of any law, rule, regulation, statute, order or
decree, to which Arena is a party or of which it or any of its property is
subject.

                                       21
                                  CONFIDENTIAL
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                                   ARTICLE XIV

                                    INDEMNITY

     14.1 INDEMNIFICATION BY TAIGEN. TaiGen will indemnify and hold harmless
Arena and its Affiliates, employees, officers, directors, shareholders and
agents (an "Arena Indemnified Party") from and against all liability, loss,
damages, costs and expenses (including reasonable attorneys' fees) which Arena
Indemnified Party may incur, suffer or be required to pay resulting from or
arising in connection with (i) the breach by TaiGen of any agreement, covenant,
representation or warranty of TaiGen obtained in this Agreement, or (ii)
negligence or omission of TaiGen.

     14.2 INDEMNIFICATION BY ARENA. Arena will indemnify and hold harmless
TaiGen and its Affiliates, TaiGen's Licensees, employees, officers, directors,
shareholders and agents (an "TaiGen Indemnified Party") from and against all
liability, loss, damages, costs and expenses (including reasonable attorneys'
fees) which TaiGen Indemnified Party may incur, suffer or be required to pay
resulting from or arising in connection with (i) the breach by Arena of any
agreement, covenant, representation or warranty of Arena obtained in this
Agreement, or (ii) negligence or omission of Arena.

     14.3 CONDITIONS TO INDEMNIFICATION. The obligations of the indemnifying
Party under Sections 14.1 and 14.2 of this Agreement are conditioned upon the
prompt Notice to the indemnifying Party of any of the aforementioned suits or
claims in writing within fifteen (15) days after receipt of notice by the
indemnified Party of such suit or claim. The indemnifying Party shall have the
right to assume the defense of any such suit or claim unless, in the reasoned
judgment of the indemnified Party, such suit or claim involves an issue or
matter which could have a materially adverse effect on the business, operations
or assets of the indemnified Party, in which event the indemnified Party may
participate in the defense of such suit or claim at its sole cost and expense.
The provision for indemnification shall be void and there shall be no liability
against a indemnified Party as to any suit or claim for which settlement or
compromise or an offer of settlement or compromise is made without the prior
consent of the indemnifying Party.

                                   ARTICLE XV

                            TERMINATION AND DURATION

     15.1 BREACH. Failure by either Party to comply with any of its material
obligations contained in this Agreement shall entitle the other Party to give
Notice to the Party in default specifying the nature of the default and
requiring it to cure such default. If such default is not cured within two (2)
months after receipt of such Notice, the notifying Party shall be entitled,
without prejudice to any of its other rights conferred on it by this Agreement,
to terminate this Agreement and the licenses granted to the breaching Party
hereunder with immediate effect by giving notice to such termination. The right
of either Party to terminate this Agreement as herein provided shall not be
affected in any way by its waiver of, or failure to take action with respect to,
any previous default.

                                       22
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     15.2 DURATION OF THIS AGREEMENT.

          (a) This Agreement shall become effective from the Effective Date and
continue to be in effect until expiration of TaiGen's obligation of royalty
payments under Article VIII, or Arena's obligation of royalty payment, if any,
under Article IX, whichever is longer ("Term"). Thereafter, all licenses or
sublicenses granted hereunder shall become fully paid-up irrecoverable license.

          (b) Either Party shall be entitled to terminate this Agreement in the
event of

               (1)  insolvency of the other Party or commencement of bankruptcy
                    proceedings by such Party; or

               (2)  dissolution of the other Party by that Party, or liquidation
                    of such Party by that Party.

          (c) The Parties agree that in the event that TaiGen sublicenses any of
the rights granted to it under this Agreement to a Third Party, such sublicense
shall include provisions whereby if such sublicensee(s) becomes insolvent,
commences bankruptcy proceedings, dissolves, and/or liquidates its assets, any
and all rights granted by TaiGen to such sublicensee(s) shall automatically
revert back to TaiGen.

     15.3 ACCRUED RIGHTS; SURVIVING OBLIGATIONS. Termination or expiration of
this Agreement for any reason shall be without prejudice to any rights that
shall have accrued to the benefit of either Party prior to such termination or
expiration, nor shall such termination or expiration relieve either Party from
obligations that are expressly indicated to survive termination or expiration of
this Agreement.

                                   ARTICLE XVI

                           RELATIONSHIP OF THE PARTIES

     Nothing in this Agreement is intended or shall be deemed to constitute a
partnership, agency, employer-employee, or joint venture relationship between
the Parties. All activities by each Party hereunder shall be provided as an
independent contractor. No Party shall incur any debts or make any commitments
for the other, except to the extent, if at all, specifically provided herein.

                                  ARTICLE XVII

                            MISCELLANEOUS PROVISIONS

     17.1 LIMITATIONS ON ASSIGNMENT. Neither this Agreement nor any interest
hereunder shall be assignable or transferable by TaiGen without the prior
written consent of Arena, which consent may be unilaterally withheld in the sole
and exclusive discretion of Arena, except that Arena agrees that TaiGen may
assign and transfer this Agreement and any interest hereunder to its Affiliate
Active Capital.

                                       23
                                  CONFIDENTIAL
<Page>

     17.2 FURTHER ACTS AND INSTRUMENTS. Each Party hereto agrees to execute,
acknowledge and deliver such further instruments and to do all such other acts
as may be necessary or appropriate to carry out the purpose and intent of this
Agreement.

     17.3 ENTIRE AGREEMENT. This Agreement constitutes and contains the entire
agreement of the Parties and supersedes any and all prior negotiations,
correspondence, understandings, Letters of Intent and agreements between the
Parties respecting the subject matter hereof. This Agreement may be amended or
modified or one or more provisions hereof waived only by a written instrument
signed by the Parties.

     17.4 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 this Agreement shall be interpreted as if such
provision were so excluded.

     17.5 CAPTIONS. The captions to this Agreement are for convenience only and
are to be of no force or effect in construing and interpreting the provisions of
this Agreement.

     17.6 FORCE MAJEURE. Neither Party shall be liable to the other for loss or
damages, or have any right to terminate this Agreement for any default or delay,
attributable to any act of God, flood, fire, explosion, breakdown or plant
strike, lockout, labor dispute, casualty, accident, war, revolution, civil
commotion, act of a public enemy, blockage, embargo, injunction, law, order,
proclamation, regulation, ordinance, demand or requirement of any government or
subdivision, authority or representative of any government, or any other cause
beyond the reasonable control of such Party.

     17.7 NO TRADE NAME OR TRADEMARK LICENSE.

          (a) No right, express or implied, is granted by this Agreement to
TaiGen, TaiGen collaborators or TaiGen's Licensees to use in any manner the name
"Arena," "Arena Pharmaceuticals," "CART" or any trade name or trademark of Arena
in any business dealing which is not directly connected with the performance of
this Agreement; provided, however, that TaiGen shall have the right to use or
disclose the name Arena only to the extent and the manner as may be required by
law.

          (b) No right, express or implied, is granted by this Agreement to
Arena, Arena collaborators or Arena licensees to use in any manner the name
"TaiGen" or any trade name or trademark of TaiGen in any business dealing which
is not directly connected with the performance of this Agreement; provided,
however, that Arena shall have the right to use or disclose the name TaiGen only
to the extent and the manner as may be required by law.

          (c) During the term of this Agreement, the Parties may issue a press
release regarding the acceptance of this Agreement by the Parties with prior
written consent of the other Party on the contents of such release, which
consent shall not be unreasonably withheld (it is not necessary to obtain the
consent of the other Party for

                                       24
                                  CONFIDENTIAL
<Page>

disclosing the information regarding this Agreement which a Party is required by
law to disclose).

     17.8 GOVERNING LAW; CONSENT TO JURISDICTION. This Agreement shall be
governed by and construed under applicable federal law of the United States of
America and the laws of the State of California, excluding any conflict of law
provisions. Each Party hereto hereby voluntarily and irrevocably waives trial by
jury in any action or proceeding brought in connection with this Agreement, any
of the other transaction documents or any of the transactions contemplated
hereby or thereby. Each Party hereby expressly waives any and all rights to
bring any suit, action or other proceeding in or before any court or tribunal
other than arbitration court following the rules of arbitration of the
International Chamber of Commerce and covenants that it shall not seek in any
manner to resolve any dispute other than as set forth in this Section 17.8 or to
challenge or set aside any decision, award or judgment obtained in accordance
with the provisions hereof. Each Party hereby expressly waives any and all
objections it may have to venue, including, without limitation, the
inconvenience of such forum, in any of such courts. In addition, the service of
process regarding the arbitration shall be subject to the rules of arbitration
of the International Chamber of Commerce or applicable laws. The Parties further
agree that any dispute resolution initiated by TaiGen under this Section 17.8
shall take place in San Diego, California (U.S.A.) and any dispute resolution
initiated by Arena under this Section 17.8 shall take place in Taiwan, and in
both situations, the proceedings shall be conducted in the English language.

     17.9 EXPENSES. Except as otherwise provided herein, each Party hereto shall
bear its legal and other expenses incurred in connection with the negotiation,
execution, delivery and performance of this Agreement.

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

     17.11 NOTICE. Unless otherwise provided, any notice required or permitted
under this Agreement shall be given in writing and shall be deemed effectively
given upon personal delivery to the Party to be notified or upon deposit with
the registered or certified mail in the country of residence of the Party giving
the notice, postage prepaid, or upon deposit with an internationally recognized
express courier with proof of delivery, postage prepaid and addressed to the
Party to be notified at the address or addresses indicated below, or upon the
date of fax transmission of such notice (with proof of such fax transmission
established by the sender's fax receipt) using the fax numbers listed below, or
at such other address or fax number as such Party may designate by ten (10)
days' advance written notice to the other Party with copies to be provided as
follows:

                 [THE REST OF THIS PAGE IS INTENTIONALLY BLANK]

                                       25
                                  CONFIDENTIAL
<Page>

IF TO ARENA, ADDRESSED TO:

                     Arena Pharmaceuticals, Inc.
                     6166 Nancy Ridge Drive
                     San Diego, CA  92121 USA
                     Attention:     Jack Lief
                                    President & CEO
                     Fax: (858) 677-0065
                     With a copy to:  General Counsel (at the same address)

IF TO TAIGEN, ADDRESSED TO:
                     TaiGen Biotechnology Co., Ltd.
                     9F, 25 Tung-Hwa South Road, Section 1
                     Taipei, Taiwan, 105
                     R.O.C.
                     Attention:        Dr. Ming-Chu Hsu

                     Fax: 886-2-2570-3107
                     With a copy to:   Jacqueline Fu, Counsel to TaiGen

                                       International Trade Building
                                       Suite 2009, 20th F
                                       333 Keelung Road, Section 1
                                       Taipei, Taiwan 110
                                       R.O.C.

     17.12 SURVIVING RIGHTS AND OBLIGATIONS. The following Articles and Sections
shall survive any termination or expiration of this Agreement: Article I
(Definitions); Article VI (No Warranties); Article X (Payment Arrangement);
Article XI (Confidentiality); Article XII (Patent Infringement and Enforcement);
Article XIII (Representations); Article XIV (Indemnity); Sections 5.3, 5.4(d),
17.8 and 17.11; and any payment otherwise subsequently or otherwise due under
Articles VIII and/or IX. Upon expiration of TaiGen royalty obligation under this
Agreement, all licenses and rights granted to TaiGen hereunder shall become
fully paid-up irrecoverable license.

                            [SIGNATURES ON NEXT PAGE]

                                       26
                                  CONFIDENTIAL
<Page>

     WHEREUPON, the Parties have caused this Agreement to be executed by their
duly authorized agents, as of the dates listed below.

ARENA PHARMACEUTICALS, INC.                 TAIGEN BIOTECHNOLOGY CO., LTD.

By:      /s/  Jack Lief                     By:      /s/  Ming-Chu Hsu
    ----------------------------                   ----------------------

Name:   Jack Lief                           Name:   Ming-Chu Hsu
Title:  President & CEO                     Title:  Chairman

Date:   June 28, 2001                       Date:   June 29, 2001

                           **************************

                                       27
                                  CONFIDENTIAL
<Page>

                                   APPENDIX A

                              TAIGEN SELECTED GPCRS

                                       28
                                  CONFIDENTIAL
<Page>

                                   APPENDIX B

                               ARENA PATENT RIGHTS

                                       29
                                  CONFIDENTIAL<Page>

                                 EXHIBIT 10.20
                   CONFIDENTIAL TREATMENT HAS BEEN REQUESTED
                         FOR THE PORTIONS MARKED [***]

                                                                  CONFIDENTIAL

                            AGREEMENT BY AND BETWEEN
                        IMPERIAL CHEMICAL INDUSTRIES PLC
                                       AND
                           ARENA PHARMACEUTICALS, INC.

                                TABLE OF CONTENTS

<Table>
<Caption>

                                                                            PAGE
<S>               <C>                                                       <C>
Article I.        Definitions                                                 1
Article II.       Feasibility Study                                           4
Article III.      Exclusivity                                                 5
Article IV.       Payments to Arena                                           6
Article V.        Utilization of Data                                         7
Article VI.       Technology License                                          7
Article VII.      Royalty Payments                                            8
Article VIII.     Confidentiality                                            10
Article IX.       Patent Infringement And Enforcement                        11
Article X.        Representations And Warranties                             12
Article XI.       Indemnity                                                  12
Article XII.      Termination                                                13
Article XIII.     Relationship of the Parties                                14
Article XIV.      Miscellaneous Provisions                                   14
Signature Blocks                                                             18
Appendices                                                      A-1 through A-5
</Table>

                                 --PLEASE NOTE--

           Provisions Within This Agreement Are Deemed "CONFIDENTIAL"
                  In Accordance With The Terms of Article VIII.

           Reviewers are advised to confirm with their attorney as to
     any obligations and/or requirements regarding review of this Agreement.

<Page>

                                                                  CONFIDENTIAL

                                   AGREEMENT

     This Agreement ("Agreement") is effective as of June 15, 2001 ("Effective
Date") by and between IMPERIAL CHEMICAL INDUSTRIES PLC, having a place of
business at London SWIP 3JF, United Kingdom ("ICI") and ARENA PHARMACEUTICALS,
INC., having a place of business at 6166 Nancy Ridge Drive, San Diego,
California, 92121 USA ("Arena").

     WHEREAS, ICI is a company which produces specialty products including
fragrances, flavors and food ingredients;

     WHEREAS, Arena is a biopharmaceutical organization focused on the
development and use of its proprietary technology for discovery of modulators of
G protein-coupled receptors;

     WHEREAS, ICI desires to collaborate with Arena to identify broad-spectrum
modulators of specific G protein-coupled receptors to enhance the discovery
processes in the fragrances, flavors and food industry;

     WHEREAS, Arena and ICI each desire to enter into this Agreement on the
terms and conditions set forth herein.

     NOW, THEREFORE, in consideration of the mutual covenants and agreements
contained herein, and for other good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, ICI and Arena hereby agree as
follows:

                                    ARTICLE I
                                   DEFINITIONS

     Unless otherwise specifically provided herein, the following terms shall
have the following meanings:

"AFFILIATE" when used with reference to a specified person, any person or entity
directly or indirectly controlling, controlled by or under common control with
the specified person, means the direct or indirect ownership of at least 50% of
the outstanding voting securities of an entity.

"ANNUAL" means the period between January 1 and December 31, inclusive.

"ARENA" means Arena and its Affiliates, excluding Aressa Pharmaceuticals, Inc.

"ARENA ACTIVATION TECHNOLOGY" means an Arena proprietary approach to altering a
region of a G Protein-Coupled Receptor to cause, enhance or stabilize
constitutive activation of the altered receptor.

"ARENA PATENT RIGHTS" means all present and/or future patents (including
inventor's certificates) and all present and/or future applications (including
provisional applications) therefor throughout the world as the case may be, and
substitutions, extensions, reissues,

                                       1
                                  CONFIDENTIAL
<Page>

                                                                  CONFIDENTIAL

renewals, divisions, continuations, or continuation-in-part thereof or therefor,
owned or controlled (either fully or partially) by Arena, or under which Arena
may grant licenses or sublicenses, to the extent they are directed to (1) Arena
Activation Technology and/or (2) CART Activated Receptor(s) and/or (3) Screening
Assay(s) and/or (4) CART Identified Ingredient(s). It is understood that ICI
Patent Rights shall be excluded from the scope of Arena Patent Rights. A list of
Arena Patent Rights shall be attached hereto as APPENDIX B1 as of the Effective
Date and Arena shall update APPENDIX B1 from time to time during the terms of
this Agreement by sending ICI such updated APPENDIX B1 and also the updated
APPENDIX B1 as of the end of December in each year within one (1) month after
such date.

"BEST REASONABLE COMMERCIAL EFFORTS" means efforts to achieve a designated
objective, which efforts are based upon reasonably prudent business factors and
considerations.

"CART ACTIVATED RECEPTOR(S)" means a Sensory GPCR to which Arena Activation
Technology has been applied, and which has reached a level of activity such that
its use in an assay results in a signal to noise ratio that predicts its utility
in an Enabled Screening Assay.

"CART IDENTIFIED INGREDIENT(S)" means an ingredient, and/or a Derivative of a
CART Identified Ingredient, that has been identified from ICI Library
Ingredient(s) as a modulator of a CART Activated Receptor(s).

"INGREDIENT DISCOVERED DURING THE FEASIBILITY STUDY" or "INGREDIENT DISCOVERED
DURING THE EXCLUSIVITY PERIOD" means a CART Identified Ingredient.

"DERIVATIVE" and "DERIVATIVES" of a first ingredient means an ingredient having
the same core structure as the first ingredient.

"EC50 Value" means the concentration of an ingredient that inhibits 50% of
measured second messenger signal of a receptor.

 "ENABLED SCREENING ASSAY" means an Arena assay approach for Screening that has
been validated based upon Successful Screening of a CART Activated Receptor.

"ENDOGENOUS" means naturally occurring.

"FIELD" means the [************************************************] uses of
CART Identified Ingredients for modulation of Sensory GPCRs; for purposes of
clarity, the definition of Field includes, but is not limited to,
[**********************].

"G PROTEIN-COUPLED RECEPTOR" and "GPCR" and "GPCRS" each mean Endogenous, human
cell-surface receptor defined by having three (3) intracellular loops, three (3)
extracellular loops, an amino terminus and a carboxy terminus.

"GUSTATORY GPCRS" means GPCRs primarily expressed within the human mouth, throat
and stomach.

                                       2
                                  CONFIDENTIAL
<Page>

                                                                  CONFIDENTIAL

"IC50 VALUE" means the concentration of an ingredient that inhibits 50% of
ligand binding to the receptor.

"ICI" means ICI and its Affiliates.

"ICI LIBRARY INGREDIENTS" shall mean chemical compounds that shall be provided
to Arena by ICI during the term of this Agreement and in accordance with the
requirements set forth in APPENDIX A.

"ICI PATENT RIGHTS" means all present and/or future patents (including
inventor's certificates) and all present and/or future applications (including
provisional applications) therefor throughout the world as the case may be, and
substitutions, extensions, reissues, renewals, divisions, continuations, or
continuation-in-part thereof or therefor, owned or controlled (either fully or
partially) by ICI, or under which ICI may grant licenses or sublicenses, to the
extent they are directed to ICI Library Ingredients It is understood that Arena
Patent Rights shall be excluded from the scope of ICI Patent Rights. A list of
ICI Patent Rights shall be attached hereto as APPENDIX B2 as of the Effective
Date and ICI shall update APPENDIX B2 from time to time during the terms of this
Agreement by sending Arena such updated APPENDIX B2 and also the updated
APPENDIX B2 as of the end of December in each year, within one (1) month after
such date.

"MEASURED RESPONSE" as used in the phrase "Successful Screening" means a signal
measure based upon an assay end-point used to assess the signal.

"OLFACTORY GPCRS" means GPCRs primarily expressed within the olfactory region of
a human.

"PARTY" means either Arena or ICI, as the case may be; "Parties" means both
Arena and ICI.

"REVENUE" means [********************************************************
******************************************************************].

"SCREENING ASSAY" or "RECEPTOR SCREENING ASSAY" means the process of contacting
a chemical compound with a CART Activated Receptor.

"SENSORY GPCRS" means Olfactory GPCRs and Gustatory GPCRs.

"SUCCESSFUL SCREENING" means that the results of the Screening have been
positive whereby at least one molecule from ICI Library Ingredients that has
been Screened reduces or enhances the Measured Response of the CART Activated
Receptor by at least [******************] from the mean response of a screening
plate that includes that compound.

"ARENA TECHNICAL INFORMATION" means all information, trade secrets, know-how,
methods of manufacture, processes, documents and materials (excluding CART
Activated Receptor(s) and Screening Assay(s)), related to CART Activated
Receptor(s) and/or Screening Assay(s), and other proprietary information,
whether patentable or unpatentable, including but not

                                       3
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

limited to, improvements, that are owned or possessed by Arena, whether now
existing or hereafter developed.

"ICI TECHNICAL INFORMATION" means all information, trade secrets, know-how,
methods of manufacture, processes, documents and materials and other proprietary
information, whether patentable or unpatentable, including but not limited to,
improvements, that are owned or possessed by ICI, whether now existing or
hereafter developed, directed to ICI Ingredients.

"TERRITORY" means the world.

"THIRD PARTY" means any person or entity other than ICI, ICI's Licensee(s) and
Arena.

                                   ARTICLE II
                                FEASIBILITY STUDY

     2.1  FEASIBILITY STUDY.

          (a) Subject to the terms and conditions of this Agreement, Arena
agrees to initiate a Feasibility Study within five (5) days after the date that
ICI has made the Initial Payment in accordance with Section 4.1 of this
Agreement. The Feasibility Study shall consist of the activities described in
Sections 2.2 through 2.5 of this Agreement ("Feasibility Study"). The
Feasibility Study will last for a period of six (6) months from the date the
Feasibility Study is initiated by Arena ("Feasibility Period").

          (b) Except as expressly provided for in Article VI, ICI acknowledges
and agrees that nothing in this Agreement provides any right, express or
implied, to ICI to make, have made, use, have used, sell and/or have sold any
CART Activated Receptor(s), any material covered by Arena Patent Rights and/or
any process covered by Arena Patent Rights.

     2.2  ARENA TO SELECT SENSORY GPCRS AND DEVELOP CART ACTIVATED RECEPTOR(S).
Arena, in consultation with ICI, agrees to use Best Reasonable Commercial
Efforts to select at least [******] different Sensory GPCRs ("Selected
Receptors") within thirty (30) days of receipt of the Initial Payment; to use
Best Reasonable Commercial Efforts to apply Arena Activation Technology to
Selected Receptors; and to use Best Reasonable Commercial Efforts to establish
at least [***************************] for each of at least [*******] Selected
Receptors.

     2.3  DEVELOPMENT OF SCREENING ASSAYS. Arena agrees to use Best Reasonable
Commercial Efforts to develop at least [*******] different Screening Assays with
each Screening Assay incorporating at least [***************************]
corresponding to each Selected Receptor ("Enabled Screening Assay").

     2.4  SCREENING ICI LIBRARY. For each Enabled Screening Assay, Arena shall
use Best Reasonable Commercial Efforts for screening up to, but not exceeding,
[*** ****************] ICI Library Ingredients. Arena agrees that it will not
attempt to determine the chemical structure of any ICI Library Ingredients.

                                       4
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

     2.5 CART IDENTIFIED INGREDIENT. For each CART Activated Receptor subject to
screening, Arena shall use Best Reasonable Commercial Efforts to identify at
least [** *****************************] and to determine the IC50 Value and/or
EC50 Value, whichever is applicable, with respect to such CART Identified
Ingredient. Upon identification of each such CART Identified Ingredient, Arena
shall provide Notice to ICI ("Identified Ingredient Notice").

     2.6 TECHNICAL INFORMATION TRANSFER. For each CART Activated Receptor that
has been screened in accordance with Section 2.3 of this Agreement, Arena shall
transfer to ICI a copy of all Arena Technical Information owned or possessed by
Arena then applicable to ICI Library Ingredients subjected to a Screening Assay
and/or with respect to any CART Identified Ingredient of Section 2.5. Arena
agrees that the transfer of such Arena Technical Information to ICI under this
Section 2.6 shall be accomplished within thirty (30) days of each Identified
Ingredient Notice.

                                   ARTICLE III
                                   EXCLUSIVITY

     3.1  EXCLUSIVITY PERIOD.

          (a) For a period of [*****************************] ("Exclusivity
Period"), beginning on the day after the end of the Feasibility Period, ICI
shall have the exclusive right to request that Arena, in consultation with ICI,
select up to, but not exceeding, an additional [******] different Sensory GPCRs
for application thereto of the Arena Activation Technology ("Additional Selected
Receptors"). In addition, during both the Feasibility Period and the Exclusivity
Period, ICI shall have a right of first refusal on Arena projects to develop new
Screening Assays for Sensory GPCRs within the Field, exercisable by sending
written Notice to Arena ("Acceptance Notice") within [*********] days of the
date that Arena gives Notice to ICI of any such projects.

          (b) Except as expressly provided for in Article VI, ICI acknowledges
and agrees that nothing in this Agreement provides any right, express or
implied, to ICI to make, have made, use, have used, sell and/or have sold any
CART Activated Receptor, any material covered by Arena Patent Rights, and/or any
process covered by Arena Patent Rights.

     3.2  ARENA TO SELECT SENSORY GPCRS AND DEVELOP CART ACTIVATED RECEPTOR(S).
Arena, in consultation with ICI, agrees to use Best Reasonable Commercial
Efforts to select the Additional Selected Receptors, within sixty (60) days
after the commencement of the Exclusivity Period; to use Best Reasonable
Commercial Efforts to apply Arena Activation Technology to each Additional
Selected Receptor; and to use Best Reasonable Commercial Efforts to establish at
least [*****************************] for each Additional Selected Receptor.

     3.3  DEVELOPMENT OF SCREENING ASSAYS. Arena agrees to use Best Reasonable
Commercial Efforts to develop at least [*******] different Screening Assays with
each Screening Assay incorporating at least [***************************]
corresponding to each Additional Selected Receptor ("Additional Enabled
Screening Assay").

                                       5
                                  CONFIDENTIAL
<Page>

                                                                  CONFIDENTIAL

     3.4  SCREENING ICI LIBRARY. For each Additional Enabled Screening Assay,
Arena shall use Best Reasonable Commercial Efforts for screening up to, but not
exceeding, [*****************] ICI Library Ingredients. Arena agrees that it
will not attempt to determine the chemical structure of any ICI Library
Ingredients.

     3.5  CART IDENTIFIED INGREDIENT. For each CART Activated Receptor
subject to screening, Arena shall use Best Reasonable Commercial Efforts to
identify at least [** *****************************] and to determine the
IC50 Value and/or EC50 Value, whichever is applicable, with respect to such
CART Identified Ingredient. Upon identification of each such CART Identified
Ingredient, Arena shall provide Identified Ingredient Notice to ICI.

     3.6  TECHNICAL INFORMATION TRANSFER. For each CART Activated Receptor that
has been screened in accordance with Section 3.4 of this Agreement, Arena shall
transfer to ICI a copy of all Arena Technical Information owned or possessed by
Arena then applicable to ICI Library Ingredients subjected to a Screening Assay
and/or with respect to any CART Identified Ingredient of Section 3.5. Arena
agrees that the transfer of such Arena Technical Information to ICI under this
Section 3.6 shall be accomplished within thirty (30) days of each Identified
Ingredient Notice.

                                   ARTICLE IV
                                PAYMENTS TO ARENA

     4.1  INITIAL PAYMENT. Within ten (10) days of the Effective Date, ICI shall
pay to Arena a non-creditable, non-refundable fee in the amount
of [********* **********************] ("Initial Payment").

     4.2  THREE INSTALLMENT PAYMENTS. ICI agrees that it shall pay to Arena
three (3) non-creditable, non-refundable installment payments in the amount
of [******* *********************] each on the following monthly anniversary
dates:

          (a) the [******************] after the Effective Date;

          (b) the [**********************] day after the Effective Date; and

          (c) the [*************************] after the Effective Date.

In the event that any of the foregoing anniversary dates falls on a Saturday,
Sunday or federal holiday ("Non-Business Day"), payment shall be made on the
business day following such Non-Business Day.

     4.3  EXCLUSIVITY PERIOD COSTS. ICI shall fully reimburse Arena for the
cost, including overhead at the rate of [**********************] of costs,
("Reimbursement Fees") associated with all the activities described in
Article III that are performed on behalf of ICI during the Exclusivity
Period. ICI shall make payments to Arena quarterly for such Reimbursement
Fees within thirty (30) days of the receipt of an invoice from Arena for the
previous calendar quarter's Reimbursement Fees.

                                       6
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

     4.4  ACCOUNTING. Arena shall maintain records in reasonable detail of all
monies paid by Arena for work performed on behalf of ICI during the Exclusivity
Period and shall provide ICI, within thirty (30) days of the end of Exclusivity
Period, with a report stating the expenditures made by Arena to complete the
activities defined in Article III, using Arena's standard project accounting
procedures.

                                    ARTICLE V
                               UTILIZATION OF DATA

     5.1  ICI'S UTILIZATION. ICI shall have the exclusive right, excepting
Arena, to use all data in the Field derived from the Feasibility Study and, when
applicable, during the Exclusivity Period. ICI shall exclusively have and
exclusively retain full right, title and interest in any and all patent rights
to the inventions and/or discoveries in the Field related to the ICI Library
Ingredients, including, but not limited to, CART Identified Ingredient(s).

     5.2  ARENA'S UTILIZATION. Arena shall exclusively have and exclusively
retain the full right, title and interest in any and all data as well as any
patent right to the inventions and/or discoveries derived from the Feasibility
Study and/or during the Exclusivity Period including, but not limited to, Arena
Activation Technology, CART Activated Receptors, Screening techniques and novel
compounds discovered using these techniques.

                                   ARTICLE VI
                               TECHNOLOGY LICENSE

     6.1  ARENA LICENSE. In consideration for the agreement by ICI to make all
applicable payments to Arena pursuant to Article IV, and conditioned upon the
receipt by Arena of all applicable payments under Article IV, Arena grants to
ICI with respect ONLY to the CART Identified Ingredients referred to in Sections
2.5 and 3.5, an exclusive right and license, exclusive even as to Arena, to
make, have made, use, have used, further develop, improve and otherwise exploit
in any manner, including the right to sub-license, the CART Identified
Ingredients within the Field. IN the event that ICI sub-licenses any right
hereunder , such sub-licensee shall agree to be subject to and bound by the
terms and conditions of this Agreement, and ICI shall be responsible for
ensuring that any such sub-licensee is subject to and bound by the terms and
conditions of this Agreement.

     6.2  ARENA TECHNOLOGY. Subject to the terms and conditions of this
Agreement, both Parties acknowledge and agree that Arena has exclusive ownership
of the Arena Patent Rights and Technical Information as of the Effective Date
and any improvement thereof hereafter discovered or developed by Arena.

     6.3  ICI TECHNOLOGY. Subject to the terms and conditions of this Agreement,
both Parties acknowledge and agree that ICI has exclusive ownership of the ICI
Patent Rights and ICI Technical Information as of the Effective Date and any
improvement thereof hereafter discovered or developed by ICI.

                                       7
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

                                   ARTICLE VII
                                ROYALTY PAYMENTS

     7.1  ROYALTY PAYMENT.

          (a) Subject to the provisions of Section 7.1(d), ICI shall pay to
Arena for any and all CART Identified Ingredient(s) of ICI, or any Licensee of
ICI, that incorporates an Ingredient Discovered During the Feasibility Study or
an Ingredient Discovered During the Exclusivity Period, a royalty payment based
on Annual Revenue of such CART Identified Ingredient(s) in the aggregate as set
forth below; such royalty payment shall be made within three (3) months of
December 31 for the Annual period to which the Annual Revenue applies:

               (1)  [**************] of the portion of Annual Revenue between
                    [*******] and [********]; and

               (2)  [**************] of the portion of Annual Revenue between
                    [********] and [********]; and

               (3)  [****************] of the portion of Annual Revenue between
                    [********] and [*********]; and

               (4)  [***************] of the portion of Annual Revenue between
                    [*********] and [*********]; and

               (5)  [***************] of the portion of Annual Revenue between
                    [*********] and [*********]; and

               (6)  [**************] of the portion of Annual Revenue between
                    [*********] and [*********].

The Parties agree that prior to the completion of the Feasibility Study, the
Parties shall meet and shall determine, in good faith, reasonable royalty
payments with respect to (i) Annual Revenue derived form the sale of a CART
Identified Ingredient(s) by ICI to an organization owned and/or controlled by
ICI, including, but not limited to, an ICI Affiliate, and (ii) Annual Revenue
derived form the sale of a product containing a CART Identified Ingredient(s) by
ICI to a Third Party and/or an organization owned and/or controlled by ICI,
including, but not limited to, an ICI Affiliate.

          (b) ANNUAL REVENUE OVER [*********]. The Parties agree to negotiate in
good faith a royalty to be paid by ICI to Arena which will apply to the Annual
Revenue which exceeds [*********] in the aggregate.

          (c) REDUCED ROYALTY FEES. The royalty payment set forth in Section
7.1(a) shall be reduced by [***************] for any Ingredient Discovered
During the Feasibility Study or Ingredient Discovered During the Exclusivity
Period when such

                                       8
                                  CONFIDENTIAL
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                                                                    CONFIDENTIAL

ingredient was (i) in ICI development as of the Effective Date, and (ii) set
forth on APPENDIX C.

          (d) CERTAIN INGREDIENTS NOT SUBJECT TO ROYALTY FEES. The royalty
payment set forth in Section 7.1(a) shall not apply to ICI's existing
ingredients which are listed on APPENDIX D, unless such an ICI existing
ingredient listed on APPENDIX D becomes the subject of a pending or issued
patent that includes Arena Technical Information. In that case, the royalty
payment would be on sales above the existing level at the time of the patent
application was filed.

     7.2  MINIMUM CUMULATIVE SALES. No ICI product that incorporates an
Ingredient Discovered During the Feasibility Study or Ingredient Discovered
During the Exclusivity Period shall be included in calculating the Annual
Revenue until the cumulative sales of such product exceeds
[****************************].

     7.3  ADJUSTMENT OF ROYALTY PAYMENTS. Arena intends that ICI shall not be
commercially disadvantaged from its competitors using similar functional
ingredients arising from parallel technologies or developments as a result of
royalty fee payments to Arena on particular CART Identified Ingredients, and
that the annual ICI Trading Profit on the ingredient shall never be less than
the annual Arena royalty fee on the ingredient.

In the event that ICI believes in good faith that the royalty provisions herein
puts ICI at a commercial disadvantage by virtue of their competitors selling a
similar functional ingredient to a particular CART Identified Ingredient, ICI
may notify Arena and Arena will be open to discussing this issue on a case by
case basis.

     7.4  ROYALTY TERM. ICI shall have a continuing obligation to make all
payments set forth in this Article VII until the later expiration of (i)
[******] years from the date that the cumulative sales by ICI exceeds
[***********************] or (ii) the date that the last patent expires on an
Ingredient Discovered During the Feasibility Study or Ingredient Discovered
During the Exclusivity Period. This Section 7.4 shall survive the earlier
termination of this Agreement.

     7.5  CHANGE OF CONTROL. In the event that ICI or one of its subsidiaries,
disposes of all or part of one of its subsidiaries, businesses, or divisions,
ICI and/or its successor shall guarantee the obligation of ICI and ICI's
Licensee(s) to continue to make the payments to Arena specified under this
Agreement.

     7.6  AUDIT. In order to verify the completeness and correctness of Revenue,
ICI and any ICI Licensee(s) shall maintain up to date books and records and
Arena shall each have the right to conduct, through independent Certified Public
Accountants, at its own cost and at any reasonable time during business hours,
not more often than once each Annual period for not more than [******] previous
years, and upon reasonable prior Notice, an audit of the accounting procedures
and records of ICI and any ICI Licensee(s) used in computing and calculating the
royalty payment for Annual Revenue due hereunder. The auditor shall make
available to ICI and Arena a report enumerating the period covered by the audit
of Revenue computed and calculated by the auditor. The costs of such audit shall
be borne by

                                       9
                                  CONFIDENTIAL
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                                                                    CONFIDENTIAL

ICI in the event that a discrepancy of more than [** **********] is discovered
through such audit.

     7.7  NEITHER PARTY MAKES ANY REPRESENTATION TO THE OTHER THAT ANY CART
IDENTIFIED INGREDIENT AND/OR PRODUCT WILL NOT INFRINGE ANY PATENT, COPYRIGHT,
TRADEMARK OR OTHER PROPRIETARY RIGHT OF ANY OTHER PERSON. NEITHER PARTY MAKES
ANY WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE OR ANY OTHER
WARRANTY, EXPRESS OR IMPLIED, TO THE OTHER WITH RESPECT TO ANY CART IDENTIFIED
INGREIDIENT AND/OR PRODUCT.

                                  ARTICLE VIII
                                 CONFIDENTIALITY

     8.1  CONFIDENTIAL DISCLOSURE AGREEMENT. Each Party shall neither
disclose to any Third Party any or all of the information disclosed by the
other Party hereunder or any and all of the information ("Information")
disclosed by the other Party under the "Agreement for the Disclosure of
Confidential Information" effective on the 1st day of August, 2000 between
Arena and ICI, nor permit any such Third Party to have access to such
Information, nor use such Information for any purpose other than for the
purpose of this Agreement, without the prior written consent of the other
Party.

     8.2  EXCEPTIONS TO CONFIDENTIAL DISCLOSURE AGREEMENT. The receiving Party's
obligations under Article 8.1 hereof shall not apply, with respect to any of
such Information to the extent that the receiving Party can establish by
competent proof that such Information:

          (a) is published, known publicly, or is already in the public domain
at the time of receipt of it by the receiving Party;

          (b) is published, becomes known publicly or becomes a part of the
public domain by publication or otherwise after the time of receipt of it by the
receiving Party, except by breach of this Agreement by the receiving Party;

          (c) is obtained from a Third Party after the receipt of it by the
receiving Party, provided, however, that said Third Party has not obtained it
directly or indirectly from the disclosing Party;

          (d) is in the receiving Party's possession on the date of the receipt
of it and was not acquired directly or indirectly from the disclosing Party; or

          (e) is subsequently developed by the receiving Party independent of
the Information received hereunder, as evidenced by competent written records
established by the receiving Party.

                                       10
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

     8.3  EXCEPTIONS TO CONFIDENTIALITY. Notwithstanding anything to the
contrary in this Agreement, the receiving Party shall be entitled to disclose
Information (i) to the extent required by any applicable law or court order
provided that the receiving Party furnishes the disclosing Party with written
Notice of such request, in advance of any such disclosure of the Information,
or (ii) to a government agency, or (iii) to a regulatory authority or other
Third Party to whom disclosure is necessary for development of the CART
Identified Compound in connection with product development, approval or
registration of the CART Identified Compound and/or Product or (iv) to the
extent required under the U.S. securities laws, NASDAQ Stock Market rules or
stock exchange requirements.

The foregoing obligations of confidentiality shall survive for five (5) years
after any termination or expiration of this Agreement. All terms and conditions
of Article II, Article III, Article IV, Article V Article VI and Article VII of
this Agreement are designated CONFIDENTIAL by Arena and ICI.

                                   ARTICLE IX
                       PATENT INFRINGEMENT AND ENFORCEMENT

     9.1  NOTIFICATION OF INFRINGEMENT. Each Party shall promptly provide Notice
to the other of any infringement (of which it becomes aware) of the intellectual
property rights contemplated by this Agreement, including patent rights on any
CART Activated Receptor(s) and/or Screening Assay(s) and/or CART Identified
Compound(s) and/or Product(s) by any Third Party and shall provide the other
Party with any available evidence of such infringement of which the Party is
aware.

     9.2  SUIT FOR INFRINGEMENT.

          (a) During the term of this Agreement, Arena shall be responsible for
enforcement of the Arena Patent Rights including, but not limited to, the
bringing of an action for patent infringement, selection of the forum for such
action, counsel, settlement of any such action, and the costs devoted to such
action. ICI agrees to provide reasonable assistance, except for financial
assistance to Arena, in the enforcement of Arena Patent Rights and ICI may join
such action as initiated by Arena with counsel at its own expense and seek its
own damages and other relief. If within ninety (90) days of ICI's giving Notice
to Arena of a Third Party infringement in the Territory Arena fails to institute
the infringement suit that ICI reasonably feels is required, ICI may institute
such infringement proceedings against said Third Party at its own expense and
ICI shall have the right to receive all the amounts payable by said Third Party
as a result of such proceedings.

          (b) In the event a claim of patent infringement is made against ICI by
a Third Party in the Territory by reasons of ICI's commercial activities
hereunder, ICI and Arena shall meet to analyze the infringement claim and
avoidance of the same. If it is necessary to obtain an appropriate license from
such a Third Party, the Parties shall, in negotiating such a license, make every
effort to minimize the amount of license fees and/or royalties payable to such
Third Party and (i) in case that such license is related to Arena Activation
Technology, a CART Activated Receptor and/or an Enabled Screening Assay,

                                       11
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

Arena shall be responsible for such license fees and/or royalties, (ii) in case
that such license is related to a CART Identified Compound, and/or Product, ICI
shall be responsible for a for such license fees and/or royalties.

                                    ARTICLE X
                          REPRESENTATION AND WARRANTIES

     10.1 REPRESENTATIONS AND WARRANTIES OF ICI. ICI represents and warrants to
Arena as follows:

          (a) The execution and delivery of this Agreement have been duly and
validly authorized, and all necessary action has been taken to make this
Agreement a legal, valid and binding obligation of ICI enforceable in accordance
with its terms.

          (b) The execution and delivery of this Agreement and the performance
by ICI of its obligations hereunder will not contravene or result in the breach
of the Corporate Charter or Bylaws of ICI or result in any material breach or
violation of or material default under any material agreement, indenture,
license, instrument or understanding or, to the best of its knowledge, result in
any violation of law, rule, regulation, statute, order or decree, to which ICI
is a party or by which it or any of its property is subject.

     10.2 REPRESENTATIONS AND WARRANTIES OF ARENA. Arena represents and warrants
to ICI as follows:

          (a) The execution and delivery of this Agreement have been duly and
validly authorized, and all necessary action has been taken to make this
Agreement a legal, valid and binding obligation of Arena enforceable in
accordance with its terms.

          (b) The execution and delivery of this Agreement and the performance
by Arena of its obligations hereunder will not contravene or result in the
breach of the Certificate of Incorporation or Bylaws of Arena or result in any
material breach or violation of or material default under any material
agreement, indenture, license, instrument or understanding or, to the best of
its knowledge, result in any violation of law, rule, regulation, statute, order
or decree, to which Arena is a party or by which it or any of its property is
subject.

                                   ARTICLE XI
                                    INDEMNITY

     11.1 INDEMNIFICATION BY ICI. ICI will indemnify and hold harmless Arena and
its Affiliates, employees, officers, directors, shareholders and agents (an
"Arena Indemnified Party") from and against all liability, loss, damages, costs
and expenses (including reasonable attorneys' fees) which an Arena Indemnified
Party may incur, suffer or be required to pay resulting from or arising in
connection with (i) the breach by ICI of any agreement, covenant, representation
or warranty of ICI contained in this Agreement, or (ii) negligence or omission
of ICI.

                                       12
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

     11.2 INDEMNIFICATION BY ARENA. Arena will indemnify and hold harmless ICI
and its Affiliates, employees, officers, directors, shareholders and agents (an
"ICI Indemnified Party") from and against all liability, loss, damages, costs
and expenses (including reasonable attorneys' fees) which an ICI Indemnified
Party may incur, suffer or be required to pay resulting from or arising in
connection with (i) the breach by Arena of any agreement, covenant,
representation or warranty of Arena contained in this Agreement, or (ii)
negligence or omission of Arena.

     11.3 CONDITIONS TO INDEMNIFICATION. The obligations of the indemnifying
Party under Sections 11.1 and 11.2 of this Agreement are conditioned upon the
prompt Notice to the indemnifying Party of any of the aforementioned suits or
claims in writing within fifteen (15) days after receipt of notice by the
indemnified Party of such suit or claim. The indemnifying Party shall have the
right to assume the defense of any such suit or claim unless, in the reasoned
judgment of the indemnified Party, such suit or claim involves an issue or
matter which could have a materially adverse effect on the business, operations
or assets of the indemnified Party, in which event the indemnified Party may
participate in the defense of such suit or claim at its sole cost and expense.
The provision for indemnification shall be void and there shall be no liability
against a Party as to any suit or claim for which settlement or compromise or an
offer of settlement or compromise is made without the prior consent of the
indemnifying Party.

                                   ARTICLE XII
                                   TERMINATION

     12.1 BREACH. Failure by either Party to comply with any of its material
obligations contained in this Agreement shall entitle the other Party to give
Notice to the Party in default specifying the nature of the default and
requiring it to cure such default. If such default is not cured within one (1)
month after receipt of such Notice, the notifying Party shall be entitled,
without prejudice to any of its other rights conferred on it by this Agreement,
to terminate this Agreement and the licenses granted to the breaching Party
hereunder with immediate effect by giving Notice of such termination. The right
of either Party to terminate this Agreement as herein provided shall not be
affected in any way by its waiver of, or failure to take action with respect to,
any previous default.

     12.2 DURATION OF THIS AGREEMENT.

          (a) This Agreement shall become effective from the Effective Date and
continue to be in effect until expiration of ICI's obligation of royalty payment
hereunder. Thereafter, all licenses or sublicenses granted hereunder shall
become fully paid-up irrecoverable license.

          (b) Either Party shall be entitled to terminate this Agreement in the
event of:

               (1)  insolvency of the other Party or commencement of bankruptcy
                    proceedings by such Party; or

                                       13
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

               (2)  dissolution of the other Party by that Party, or liquidation
                    of such Party by that Party.

          (c) The Parties agree that in the event that ICI sublicenses any of
the rights granted to it under this Agreement to a Third Party, such sublicense
shall include provisions whereby if such sublicensee(s) becomes insolvent,
commences bankruptcy proceedings, dissolves, and/or liquidates its assets, any
and all rights granted by ICI to such sublicensee(s) shall automatically revert
back to ICI.

     12.4 ACCRUED RIGHTS; SURVIVING OBLIGATIONS. Termination or expiration of
this Agreement for any reason shall be without prejudice to any rights which
shall have accrued to the benefit of either Party prior to such termination or
expiration, nor shall such termination or expiration relieve either Party from
obligations which are expressly indicated to survive termination or expiration
of this Agreement.

                                  ARTICLE XIII
                           RELATIONSHIP OF THE PARTIES

     Nothing in this Agreement is intended or shall be deemed to constitute a
partnership, agency, employer-employee, or joint venture relationship between
the Parties. All activities by each Party hereunder shall be provided as an
independent contractor. No Party shall incur any debts or make any commitments
for the other, except to the extent, if at all, specifically provided herein.

                                   ARTICLE XIV
                            MISCELLANEOUS PROVISIONS

     14.1 The Parties acknowledge and agree that any and all payments to be made
by ICI to Arena under this Agreement are to be (i) in United States Dollars and
(ii) in full as indicated. The Parties acknowledge and agree that the Product
Revenue rates and terms in Article VII under this Agreement are in terms of
United States Dollars.

     14.2 LIMITATIONS ON ASSIGNMENT. Neither this Agreement nor any interest
hereunder shall be assignable or transferable by ICI without the prior written
consent of Arena, which consent shall not be unreasonably withheld.

     14.3 FURTHER ACTS AND INSTRUMENTS. Each Party hereto agrees to execute,
acknowledge and deliver such further instruments and to do all such other acts
as may be necessary or appropriate to carry out the purpose and intent of this
Agreement.

     14.4 ENTIRE AGREEMENT. This Agreement constitutes and contains the entire
agreement of the Parties and supersedes any and all prior negotiations,
correspondence, understandings, letters of intent, term sheets and agreements
between the Parties respecting the subject matter hereof. This Agreement may be
amended or modified or one or more provisions hereof waived only by a written
instrument signed by the Parties.

                                       14
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

     14.5 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.

     14.6 INTERPRETATION. The Parties expressly and intentionally waive all
rights and benefits which they now have or in the future may have under the
principle of contra proferentem, which provides that "the language of a contract
should be interpreted most strongly against the party who caused the uncertainty
to exist," as stated in California Civil Code Section 1654. Moreover, the
Parties agree that this entire Agreement, and each provision hereof, shall be
deemed to have been drafted jointly by the Parties. This contract shall be
construed as a whole and in accordance with its fair meaning. In interpreting
this Agreement, any gender shall be deemed to include the other gender, the
singular includes the plural, and vice versa, as the context may require. The
headings and captions to this Agreement are for convenience only and are to be
of no force or effect in construing or interpreting the provisions of this
Agreement.

     14.7 FORCE MAJEURE. Neither Party shall be liable to the other Party for
loss or damages, or have any right to terminate this Agreement for any default
or delay, attributable to any act of God, flood, fire, explosion, breakdown or
plant strike, lockout, labor dispute, casualty, accident, war, revolution, civil
commotion, act of a public enemy, blockage, embargo, injunction, law, order,
proclamation, regulation, ordinance, demand or requirement of any government or
subdivision, authority or representative of any government, or any other cause
beyond the reasonable control of such Party.

     14.8 NO TRADE NAME OR TRADEMARK LICENSE.

          (a) No right, express or implied, is granted by this Agreement to ICI,
ICI collaborators or ICI's Licensees to use in any manner the name "Arena,"
"Arena Pharmaceuticals," "CART" or any trade name or trademark of Arena in any
business dealing which is not directly connected with the performance of this
Agreement; provided, however, that ICI shall have the right to use or disclose
the name Arena only to the extent and the manner as may be required by law.

          (b) No right, express or implied, is granted by this Agreement to
Arena, Arena collaborators or Arena licensees to use in any manner the name
"ICI" or any trade name or trademark of ICI in any business dealing which is not
directly connected with the performance of this Agreement; provided, however,
that Arena shall have the right to use or disclose the name ICI only to the
extent and the manner as may be required by law.

          (c) During the term of this Agreement, the Parties may issue a press
release regarding the acceptance of this Agreement by the Parties with prior
written consent of the other Party on the contents of such release, which
consent shall not be unreasonably withheld (it is not necessary to obtain the
consent of the other Party for disclosing the information regarding this
Agreement which a Party is required by law to disclose).

     14.9 GOVERNING LAW; CONSENT TO JURISDICTION. This Agreement shall be
governed by and construed under applicable federal law of the United States of
America and the laws of the State of California, excluding any conflict of law
provisions. Each Party consents to

                                       15
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

the service of process by personal service or any manner in which Notices may be
delivered hereunder in accordance with Section 14.12. The Parties further agree
that any dispute resolution under this Section 14.9 shall take place in San
Diego, California (U.S.A.). Each Party hereby expressly waives any and all
objections it may have to venue, including, without limitation, the
inconvenience of such forum, in any of such courts.

     14.10 EXPENSES. Except as otherwise provided herein, each Party hereto
shall bear its legal and other expenses incurred in connection with the
negotiation, execution, delivery and performance of this Agreement.

     14.11 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.

     14.12 NOTICE. Unless otherwise provided, any notice required or permitted
under this Agreement shall be given in writing and shall be deemed effectively
given upon personal delivery to the Party to be notified or upon deposit with
the United States Post Office registered or certified mail, postage prepaid, or
upon deposit with an internationally recognized express courier with proof of
delivery, postage prepaid and addressed to the Party to be notified at the
address or addresses indicated below, or upon the date of fax transmission of
such notice (with proof of such fax transmission established by the sender's fax
receipt) using the fax numbers listed below, or at such other address or fax
number as such Party may designate by ten (10) days' advance written notice to
the other Party with copies to be provided as follows:

          IF TO ICI, ADDRESSED TO:
                  Imperial Chemical Industries PLC
                  ICI Group Headquarters
                  London, SWIP 3JF, UK
                  Attention:
                  Fax: (202) 293-4198
                  with a copy to:
                  Address:

                  Fax:

                 [THE REST OF THIS PAGE IS INTENTIONALLY BLANK]

                                       16
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

          IF TO ARENA, ADDRESSED TO:
                  Arena Pharmaceuticals, Inc.
                  6166 Nancy Ridge Drive
                  San Diego, CA 92121 USA
                  Attention:       Jack Lief
                                   President & CEO
                  Fax: (858) 453-7210
                  with a copy to:  General Counsel
                  Address:         same as above
                  Fax:             same as above

     14.13 APPENDICES AND EXHIBITS. All appendices and exhibits to which
reference is made are deemed incorporated in full in this Agreement, whether or
not actually attached.

     14.14 SURVIVING OBLIGATIONS. The following Articles and Sections shall
survive any termination or expiration of this Agreement: Article I
(Definitions); Article V (Utilization of Data) Article VIII (Confidentiality);
Article IX (Patent Infringement and Enforcement); Article X (Representations and
Warranties); Article XI (Indemnity); and Sections 6.2, 7.4, 7.7, 14.2, 14.3,
14.8, 14.9, 14.12 and 14.14.

                 [THE REST OF THIS PAGE IS INTENTIONALLY BLANK]

                                       17
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

     WHEREUPON, the Parties have caused this Agreement to be executed by their
duly authorized agents, as of the dates listed below.

<Table>
<Caption>

------------------------------------------------- -------------------------------------------
IMPERIAL CHEMICAL INDUSTRIES PLC                        ARENA PHARMACEUTICALS, INC.
<S>                                                     <C>
------------------------------------------------- -------------------------------------------
By:  /s/ Robert Wynne Toleman Turner                    By:   /s/ Richard P. Burgoon, Jr.
     -------------------------------                          --------------------------
Name:    Robert Wynne Toleman Turner                    Name:     Richard P. Burgoon, Jr.
Title:   Duly Authorized Signatory                      Title:    Sr. Vice President, Operations
                                                                  General Counsel & Secretary

Date:  24 May 2001                                      Date:  13 June 2001
       -----------                                             -------------

------------------------------------------------- -------------------------------------------
</Table>

                                       18
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

                                   APPENDIX A
                      ICI LIBRARY INGREDIENTS REQUIREMENTS

                                      A-1
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

                                      A-2
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

                                   APPENDIX B1
                               ARENA PATENT RIGHTS

                                    ATTACHED

                                      A-3
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

                               ARENA PATENT RIGHTS

<Table>
<Caption>

US SERIAL NO.                       FILING DATE               PUBLICATION
<S>                                 <C>                       <C>

09/060,188                          4/14/98                   WO 98/46995

09/170,496                          10/13/98                  WO 00/22129

09/364,425                          7/30/99                   WO 00/06597

U.S. PATENT NO.                     FILING DATE               ISSUE DATE

5,462,856                           7/16/91                   10/31/95

5,601,992                           9/9/94                    2/11/97

6,051,386                           6/7/95                    4/18/00

EP PATENT NO.                       FILING DATE               ISSUE DATE

0539518                             7/17/91                   3/21/01
</Table>

                                      A-4
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

                                   APPENDIX B2
                                ICI PATENT RIGHTS

                                    ATTACHED

                                      A-5
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

CASE #                 FILE DATE          PATENT NUMBER*
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]

                                      A-6
                                  CONFIDENTIAL
<Page>

                                                                    CONFIDENTIAL

[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
[*******]        [*****************]           [***************]
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[*******]        [*****************]           [***************]
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* Priority filing with equivalents in other countries

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                                   APPENDIX C
                     ICI LIBRARY INGREDIENTS IN DEVELOPMENT
                            AS OF THE EFFECTIVE DATE

                                    ATTACHED

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                                   APPENDIX D
                                 ICI INGREDIENTS

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o   [******************************************]
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o   [*************************************************************
     *******************]
o   [**********************************************************************]
o   [******************************************]****************************
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               The terms on this page shall have the same meanings
                         as they have in the Agreement.

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