Document:

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                                                                    Exhibit 4.15

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                      SERIES H CONVERTIBLE PREFERRED STOCK
                               PURCHASE AGREEMENT

                                      among

                               ADOLOR CORPORATION

                                       and

                       THE PURCHASERS NAMED IN SCHEDULE I

                            Dated as of July 6, 2000

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                                TABLE OF CONTENTS

ARTICLE I THE PREFERRED SHARES.................................................1
   SECTION 1.01       Issuance, Sale and Delivery of the Preferred Shares......1
   SECTION 1.02       Initial Closing..........................................1
   SECTION 1.03       Additional Closing.......................................2

ARTICLE II REPRESENTATIONS AND WARRANTIES OF THE COMPANY.......................2
   SECTION 2.01       Organization, Qualifications and Corporate Power.........2
   SECTION 2.02       Authorization of Agreements, Etc.........................3
   SECTION 2.03       Validity.................................................3
   SECTION 2.04       Authorized Capital Stock.................................3
   SECTION 2.05       Financial Statements.....................................4
   SECTION 2.06       Events Subsequent to the Date of the Balance Sheet.......5
   SECTION 2.07       Litigation; Compliance with Law..........................5
   SECTION 2.08       Proprietary Information of Third Parties.................6
   SECTION 2.09       Patents, Trademarks, Etc.................................6
   SECTION 2.10       Title to Properties......................................7
   SECTION 2.11       Leasehold Interests......................................7
   SECTION 2.12       Insurance................................................8
   SECTION 2.13       Taxes....................................................8
   SECTION 2.14       Other Agreements.........................................8
   SECTION 2.15       Significant Customers and Suppliers.....................10
   SECTION 2.16       Governmental Approvals..................................10
   SECTION 2.17       Disclosure..............................................10
   SECTION 2.18       Offering of the Preferred Shares........................11
   SECTION 2.19       Brokers.................................................11
   SECTION 2.20       Officers................................................11
   SECTION 2.21       Transactions With Affiliates............................11
   SECTION 2.22       Employees...............................................11
   SECTION 2.23       Environmental Protection................................12
   SECTION 2.24       ERISA...................................................12
   SECTION 2.25       Real Property Holding Corporation.......................14

ARTICLE III REPRESENTATIONS AND WARRANTIES OF THE PURCHASERS..................14

ARTICLE IV CONDITIONS TO THE OBLIGATIONS OF THE PURCHASERS....................15

ARTICLE V COVENANTS OF THE COMPANY............................................16
   SECTION 5.01       Financial Statements, Reports, Etc......................16
   SECTION 5.02       Right of First Refusal..................................17
   SECTION 5.03       Reserve for Conversion Shares...........................19
   SECTION 5.04       Corporate Existence.....................................19
   SECTION 5.05       Properties, Business, Insurance.........................19
   SECTION 5.06       Inspection, Consultation and Advice.....................20
   SECTION 5.07       Restrictive Agreements Prohibited.......................20
   SECTION 5.08       Transactions with Affiliates............................20
   SECTION 5.09       Use of Proceeds.........................................20
   SECTION 5.10       By-laws.................................................20
   SECTION 5.11       Performance of Contracts................................21
   SECTION 5.12       Vesting of Reserved Employee Shares.....................21
   SECTION 5.13       Employee Nondisclosure and Developments Agreements......21
   SECTION 5.14       Compliance with Laws....................................21
   SECTION 5.15       Keeping of Records and Books of Account.................21

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   SECTION 5.16       Rule 144A Information...................................21
   SECTION 5.17       Rule 144A Information...................................22
   SECTION 5.18       Observation Rights......................................22

ARTICLE VI MISCELLANEOUS......................................................22
   SECTION 6.01       Expenses................................................22
   SECTION 6.02       Survival of Agreements..................................22
   SECTION 6.03       Brokerage...............................................23
   SECTION 6.04       Parties in Interest.....................................23
   SECTION 6.05       Notices.................................................23
   SECTION 6.06       Governing Law...........................................23
   SECTION 6.07       Entire Agreement........................................23
   SECTION 6.08       Counterparts............................................23
   SECTION 6.09       Amendments..............................................24
   SECTION 6.10       Severability............................................24
   SECTION 6.11       Titles and Subtitles....................................24
   SECTION 6.12       Certain Defined Terms...................................24
   SECTION 6.13       Prior Agreements........................................24

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                                    iii
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INDEX TO SCHEDULES

SCHEDULE I        Purchasers
SCHEDULE II       Disclosure Schedule
SCHEDULE III      Security Holders
SCHEDULE IV(A)
     AND IV(B)    Agreements
SCHEDULE V        Definition of "qualified institutional buyer"

INDEX TO EXHIBITS

EXHIBIT A         Form of Amendment No. 6 to Registration Rights Agreement
EXHIBIT B         Amended and Restated Certificate of Incorporation
EXHIBIT C         Form of Employee Nondisclosure and Developments
                   Agreement

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     SERIES H CONVERTIBLE PREFERRED STOCK PURCHASE AGREEMENT dated as of July 6,
2000 among Adolor Corporation, a Delaware corporation (the "Company"), and the
several purchasers named in the attached Schedule I (individually a "Purchaser"
and collectively the "Purchasers").

     WHEREAS, the Company wishes to issue and sell to the Purchasers up to an
aggregate of 23,921,569 (the "Preferred Shares") of the authorized but unissued
Series H Convertible Preferred Stock, $.01 par value, of the Company (the
"Series H Convertible Preferred Stock"); and

     WHEREAS, the Purchasers, severally, wish to purchase the Preferred Shares
on the terms and subject to the conditions set forth in this Agreement;

     NOW, THEREFORE, in consideration of the premises and the mutual covenants
contained in this Agreement, the parties agree as follows:

                                    ARTICLE I

                              THE PREFERRED SHARES

     SECTION 1.01 Issuance, Sale and Delivery of the Preferred Shares. The
                  ---------------------------------------------------
Company agrees to issue and sell to each Purchaser, and each Purchaser hereby
agrees to purchase from the Company, the number of Preferred Shares set forth
opposite the name of such Purchaser under the heading "Number of Preferred
Shares to be Purchased" on Schedule I, at the aggregate purchase price set forth
opposite the name of such Purchaser under the heading "Aggregate Purchase Price
for Preferred Shares" on Schedule I.

     SECTION 1.02 Initial Closing. The initial closing shall take place at the
                  ---------------
offices of Dechert Price & Rhoads, 4000 Bell Atlantic Tower, 1717 Arch Street,
Philadelphia, PA 19103, at 10:00 a.m., Philadelphia time, on July 6, 2000, or at
such other location, date and time as may be agreed upon between the Purchasers
and the Company (such closing being called the "Closing" and such date and time
being called the "Closing Date"). At the Closing, the Company shall issue and
deliver to each Purchaser a stock certificate or certificates in definitive
form, registered in the name of such Purchaser, representing the Preferred
Shares being purchased by such Purchaser at the Closing. As payment in full for
the Preferred Shares being purchased by it under this Agreement, and against
delivery of the stock certificate or certificates therefor as aforesaid, on the
Closing Date each Purchaser shall (i) deliver to the Company a check payable to
the order of the Company, in the amount set forth opposite the name of such
Purchaser under the heading "Aggregate Purchase Price for Preferred Shares" on
Schedule I, (ii) transfer such sum to the account of the Company by wire
transfer, or (iii) deliver or transfer such sum to the Company by any
combination of such methods of payments.
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         SECTION 1.03 Additional Closing. After the Closing Date and on or prior
                      ------------------
to July 10, 2000 the Company may hold one or more additional closings (each an
"Additional Closing," and collectively the "Additional Closings") at which the
Company may issue and sell up to the number of Preferred Shares equal to the
difference between 23,921,569 and the aggregate number of Preferred Shares
previously sold on the Closing Date and, as applicable, on the date of any prior
Additional Closing. The sale of Preferred Shares pursuant to this Section 1.03
shall be on the same terms and conditions (including price) as the sale of the
Preferred Shares pursuant to Section 1.02 hereof and shall be effected by the
execution by any investor of a counterpart signature page to this Agreement.
Upon such execution: (i) each such investor shall be deemed to be a Purchaser
for all purposes of this Agreement and Schedule I shall be amended to include
such Purchaser; and (ii) each such Additional Closing shall be deemed to be a
Closing hereunder and the date of each such Additional Closing shall be a
"Closing Date" hereunder. If necessary, the Company shall provide an updated
Disclosure Schedule to Purchasers purchasing in any Additional Closing.

                                   ARTICLE II

                  REPRESENTATIONS AND WARRANTIES OF THE COMPANY

         The Company represents and warrants to each Purchaser that, as of each
Closing Date on which such Purchaser purchases Preferred Shares hereunder,
except as set forth in the Disclosure Schedule attached as Schedule II, as may
be updated in writing prior to any Additional Closing hereunder, (which
Disclosure Schedule, (as updated, if applicable), makes explicit reference to
the particular representation or warranty as to which exception is taken, which
in each case shall constitute the sole representation and warranty as to which
such exception shall apply):

         SECTION 2.01  Organization, Qualifications and Corporate Power.
                       ------------------------------------------------

                  (a) The Company is a corporation duly incorporated, validly
existing and in good standing under the laws of the State of Delaware and is
duly licensed or qualified to transact business as a foreign corporation and is
in good standing in each jurisdiction in which the nature of the business
transacted by it or the character of the properties owned or leased by it
requires such licensing or qualification. The Company has the corporate power
and corporate authority to own and hold its properties and to carry on its
business as now conducted and as proposed to be conducted, to execute, deliver
and perform this Agreement and Amendment No. 6 to that certain Registration
Rights Agreement by and among the Company and the Purchasers named therein dated
as of November 7, 1994 and as amended by Amendment No. 1 to Registration Rights
Agreement dated as of February 27, 1996, Amendment No. 2 to Registration Rights
Agreement dated as of May 1, 1997, Amendment No. 3 to the Registration Rights
Agreement dated as of December 8, 1998, Amendment No. 4 to the Registration
Rights Agreement dated as of July 22, 1999 and Amendment No. 5 to the
Registration Rights Agreement dated as of January 10, 2000 (as amended, the
"Original Registration Rights Agreement") in the form attached as Exhibit A (the
"Registration Rights Agreement Amendment"), to issue, sell and deliver the
Preferred Shares and to issue and deliver the shares of Common Stock, $.0001 par
value, of the Company ("Common Stock") issuable upon

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conversion of the Preferred Shares (the "Conversion Shares"). The Original
Registration Rights Agreement as amended by the Registration Rights Agreement
Amendment is sometimes referred to herein as the "Registration Rights
Agreement".

     (b) The Company does not (i) own of record or beneficially, directly or
indirectly, (A) any shares of capital stock or securities convertible into
capital stock of any other corporation or (B) any participating interest in any
partnership, joint venture or other non-corporate business enterprise or (ii)
control, directly or indirectly, any other entity.

     SECTION 2.02 Authorization of Agreements, Etc.
                  --------------------------------

     (a) The execution and delivery by the Company of this Agreement and the
Registration Rights Agreement Amendment, the performance by the Company of its
obligations hereunder and thereunder, the issuance, sale and delivery of the
Preferred Shares and the issuance and delivery of the Conversion Shares (as
defined in Section 5.03) have been duly authorized by all requisite corporate
action and will not violate any provision of law, any order of any court or
other agency of government, the Certificate of Incorporation of the Company, as
amended (the "Charter"), or the By-laws of the Company, as amended, or any
provision of any indenture, agreement or other instrument to which the Company
or any of its respective properties or assets is bound, or conflict with, result
in a breach of or constitute (with due notice or lapse of time or both) a
default under any such indenture, agreement or other instrument, or result in
the creation or imposition of any lien, charge, restriction, claim or
encumbrance of any nature whatsoever upon any of the properties or assets of the
Company.

     (b) The Preferred Shares have been duly authorized and, when issued in
accordance with this Agreement, will be validly issued, fully paid and
nonassessable shares of Series H Convertible Preferred Stock with no personal
liability attaching to the ownership thereof and will be free and clear of all
liens, charges, restrictions, claims and encumbrances imposed by or through the
Company except as set forth in the Registration Rights Agreement. The Conversion
Shares have been duly reserved for issuance upon conversion of the Preferred
Shares and, when so issued, will be duly authorized, validly issued, fully paid
and nonassessable shares of Common Stock with no personal liability attaching to
the ownership thereof and will be free and clear of all liens, charges,
restrictions, claims and encumbrances imposed by or through the Company except
as set forth in the Registration Rights Agreement. Neither the issuance, sale or
delivery of the Preferred Shares nor the issuance or delivery of the Conversion
Shares is subject to any preemptive right of stockholders of the Company or to
any right of first refusal or other right in favor of any person which has not
been waived.

     SECTION 2.03 Validity. This Agreement has been duly executed and delivered
                  --------
by the Company and constitutes the legal, valid and binding obligation of the
Company, enforceable in accordance with its terms. The Registration Rights
Agreement Amendment, when executed and delivered in accordance with this
Agreement, will constitute the legal, valid and binding obligation of the
Company, enforceable in accordance with its terms.

     SECTION 2.04 Authorized Capital Stock. The authorized capital stock of the
                  ------------------------
Company consists of (i) 94,100,667 shares of Preferred Stock, $.01 par value
(the "Preferred Stock"), of

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which 6,000,000 shares have been designated as Series A Convertible Preferred
Stock, 23,107,145 have been designated as Series B Convertible Preferred Stock,
13,814,286 shares have been designated as Series C Convertible Preferred Stock,
960,000 shares have been designated as Series D Convertible Preferred Stock,
11,366,667 shares have been designated as Series E Convertible Preferred Stock,
2,625,000 shares have been designated as Series F Convertible Preferred Stock,
12,306,000 shares have been designated as Series G Convertible Preferred Stock
and 23,921,569 shares have been designated as Series H Convertible Preferred
Stock and (ii) 121,987,903 shares of Common Stock. Immediately prior to the
Closing, 10,137,236 shares of Common Stock, 6,000,000 shares of Series A
Convertible Preferred Stock, 23,107,145 shares of Series B Convertible Preferred
Stock, 13,814,286 shares of Series C Convertible Preferred Stock, 960,000 shares
of Series D Convertible Preferred Stock, 11,366,667 shares of Series E
Convertible Preferred Stock, 2,625,000 shares of Series F Convertible Preferred
Stock and 12,306,000 shares of Series G Convertible Preferred Stock will be
validly issued and outstanding, fully paid and nonassessable with no personal
liability attaching to the ownership thereof, and no shares of Series H
Convertible Preferred Stock will have been issued. The stockholders of record
and holders of subscriptions, warrants, options, convertible securities, and
other rights (contingent or other) to purchase or otherwise acquire equity
securities of the Company, and the number of shares of Common Stock and the
number of such subscriptions, warrants, options, convertible securities, and
other such rights held by each, are as set forth in the attached Schedule III.
                                                                 ------------
The designations, powers, preferences, rights, qualifications, limitations and
restrictions in respect of each class and series of authorized capital stock of
the Company are as set forth in the Charter, a copy of which is attached as
Exhibit B and all such designations, powers, preferences, rights,
---------
qualifications, limitations and restrictions are valid, binding and enforceable
and in accordance with all applicable laws. Except as set forth in the attached
Schedule III, (i) no person owns of record or is known to the Company to own
------------
beneficially any share of Common Stock, (ii) no subscription, warrant, option,
convertible security, or other right (contingent or other) to purchase or
otherwise acquire equity securities of the Company is authorized or outstanding
that has been issued by the Company, and (iii) there is no commitment by the
Company to issue shares, subscriptions, warrants, options, convertible
securities, or other such rights or to distribute to holders of any of its
equity securities any evidence of indebtedness or asset. Except as provided for
in the Charter or as set forth in the attached Schedule III, the Company has no
                                               ------------
obligation (contingent or otherwise) to purchase, redeem or otherwise acquire
any of its equity securities or any interest therein or to pay any dividend or
make any other distribution in respect thereof. Except as set forth in the
attached Schedule III and except for those Management Rights letter agreements
(the "Prior Management Rights Agreements") between the Company and certain of
the Series A Purchasers, the Series B Purchasers and the Series C Purchasers (as
such terms are defined in Section 6.13 hereof), to the best of the Company's
knowledge there are no voting trusts or agreements, stockholders' agreements,
pledge agreements, buy-sell agreements, rights of first refusal, preemptive
rights or proxies relating to any securities of the Company (whether or not the
Company is a party thereto). All of the outstanding securities of the Company
were issued in compliance with all applicable Federal and state securities laws.

     SECTION 2.05 Financial Statements. The Company has furnished to the
                  --------------------
Purchasers the audited consolidated balance sheet of the Company as of December
31, 1999 and the unaudited consolidated balance sheet of the Company as of April
30, 2000 (the latter of which is herein

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referred to as the "Balance Sheet"), and the related audited consolidated
statements of income and cash flows of the Company for the twelve months ended
December 31, 1999 and the unaudited consolidated statements of income and cash
flows for the Company for the four months ended April 30, 2000. All such
financial statements have been prepared in accordance with generally accepted
accounting principles consistently applied (except that the unaudited financial
statements do not contain all of the required footnotes) and fairly present in
all material respects the consolidated financial position of the Company as of
December 31, 1999 and April 30, 2000, respectively, and the consolidated results
of its operations and cash flows for the twelve months ended December 31, 1999
and the four months ended April 30, 2000 respectively. Since the date of the
Balance Sheet, (i) there has been no change in the assets, liabilities or
financial condition of the Company (on a consolidated basis) from that reflected
in the Balance Sheet except for changes in the ordinary course of business which
in the aggregate have not been materially adverse and (ii) none of the business,
prospects, financial condition, operations, property or affairs of the Company
(on a consolidated basis) has been materially adversely affected by any
occurrence or development, individually or in the aggregate, whether or not
insured against.

     SECTION 2.06 Events Subsequent to the Date of the Balance Sheet. Since the
                  --------------------------------------------------
date of the Balance Sheet, the Company has not (i) issued any stock, bond or
other corporate security, except pursuant to the exercise of stock options
outstanding as of the date of the Balance Sheet, (ii) borrowed any amount or
incurred or become subject to any liability (absolute, accrued or contingent),
except current liabilities incurred and liabilities under contracts entered into
in the ordinary course of business, (iii) discharged or satisfied any lien or
encumbrance or incurred or paid any obligation or liability (absolute, accrued
or contingent) other than current liabilities shown on the Balance Sheet and
current liabilities incurred since the date of the Balance Sheet in the ordinary
course of business, (iv) declared or made any payment or distribution to
stockholders or purchased or redeemed any share of its capital stock or other
security, (v) mortgaged, pledged, encumbered or subjected to lien any of its
assets, tangible or intangible, other than liens of current real property taxes
not yet due and payable, (vi) sold, assigned or transferred any of its tangible
assets except in the ordinary course of business, or canceled any debt or claim,
(vii) sold, assigned, transferred or granted any exclusive license with respect
to any patent, trademark, trade name, service mark, copyright, trade secret or
other intangible asset, (viii) suffered any loss of property or waived any right
of substantial value whether or not in the ordinary course of business, (ix)
made any change in officer compensation except in the ordinary course of
business and consistent with past practice, (x) made any material change in the
manner of business or operations of the Company, (xi) entered into any
transaction except in the ordinary course of business or as otherwise
contemplated hereby or (xii) entered into any commitment (contingent or
otherwise) to do any of the foregoing.

     SECTION 2.07 Litigation; Compliance with Law. There is no (i) action, suit,
                  -------------------------------
claim, proceeding or investigation pending or, to the best of the Company's
knowledge, threatened against or affecting the Company, at law or in equity, or
before or by any Federal, state, municipal or other governmental department,
commission, board, bureau, agency or instrumentality, domestic or foreign, (ii)
arbitration proceeding relating to the Company pending under collective
bargaining agreements or otherwise or (iii) governmental inquiry pending or, to
the best of the Company's knowledge, threatened against or affecting the Company
(including

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without limitation any inquiry as to the qualification of the Company to hold or
receive any license or permit). The Company is not in default with respect to
any order, writ, injunction or decree known to or served upon the Company of any
court or of any Federal, state, municipal or other governmental department,
commission, board, bureau, agency or instrumentality, domestic or foreign. There
is no action or suit by the Company pending or threatened against others. The
Company has complied with all laws, rules, regulations and orders applicable to
its business, operations, properties, assets, products and services, the Company
has all necessary permits, licenses and other authorizations required to conduct
its business as conducted and as proposed to be conducted, and the Company has
been operating its business pursuant to and in compliance with the terms of all
such permits, licenses and other authorizations except where any instance or
instances of noncompliance do not, individually or in the aggregate, have a
material adverse effect on the Company's business, prospects, financial
condition, operations, property or affairs. There is no existing law, rule,
regulation or order, and the Company after due inquiry is not aware of any
proposed law, rule, regulation or order, whether Federal, state, county or
local, which would prohibit or restrict the Company from, or otherwise
materially adversely affect the Company in, conducting its business in any
jurisdiction in which it is now conducting business or in which it proposes to
conduct business.

     SECTION 2.08 Proprietary Information of Third Parties. To the best of the
                  ----------------------------------------
Company's knowledge, no third party has claimed or has reason to claim that any
person employed by or affiliated with the Company has (a) violated or may be
violating any of the terms or conditions of his or her employment,
non-competition or non-disclosure agreement with such third party, (b) disclosed
or may be disclosing or utilized or may be utilizing any trade secret or
proprietary information or documentation of such third party or (c) interfered
or may be interfering in the employment relationship between such third party
and any of its present or former employees. To the best of the Company's
knowledge, no third party has requested information from the Company which
suggests that such a claim might be contemplated. To the best of the Company's
knowledge, no person employed by or affiliated with the Company has employed or
proposes to employ any trade secret or any information or documentation
proprietary to any former employer, and to the best of the Company's knowledge,
no person employed by or affiliated with the Company has violated any
confidential relationship which such person may have had with any third party,
in connection with the development, manufacture or sale of any product or
proposed product or the development or sale of any service or proposed service
of the Company, and the Company has no reason to believe there will be any such
employment or violation. To the best of the Company's knowledge, none of the
execution or delivery of this Agreement, or the carrying on of the business of
the Company as officers, employees or agents by any officer, director or key
employee of the Company, or the conduct or proposed conduct of the business of
the Company, will conflict with or result in a breach of the terms, conditions
or provisions of or constitute a default under any contract, covenant or
instrument under which any such person is obligated.

     SECTION 2.09 Patents, Trademarks, Etc.. Set forth in Schedule II is a list
                  -------------------------               -----------
and brief description of all domestic and foreign patents, patent rights, patent
applications, trademarks, trademark applications, service marks, service mark
applications, trade names and copyrights, and all applications for such which
are in the process of being prepared, owned by or registered in the name of the
Company, or of which the Company is a licensor or licensee or in which the

                                       6
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Company has any right, and in each case a brief description of the nature of
such right. The Company owns or possesses adequate licenses or other rights to
use all patents, patent applications, trademarks, trademark applications,
service marks, service mark applications, trade names, copyrights, manufacturing
processes, formulae, trade secrets, customer lists and know how (collectively,
"Intellectual Property") necessary to the conduct of its business as conducted
and as proposed to be conducted, and no claim is pending or, to the best of the
Company's knowledge, threatened to the effect that the operations of the Company
infringe upon or conflict with the asserted rights of any other person under any
Intellectual Property, and the Company reasonably believes that there is no
basis for any such claim (whether or not pending or threatened). No claim is
pending or, to the best of the Company's knowledge, threatened to the effect
that any such Intellectual Property owned or licensed by the Company, or which
the Company otherwise has the right to use, is invalid or unenforceable by the
Company, and the Company reasonably believes that there is no basis for any such
claim (whether or not pending or threatened). All prior art known to the Company
which may be or may have been pertinent to the examination of any United States
patent or patent application listed in Schedule II has been cited to the United
                                       -----------
States Patent and Trademark Office. To the best of the Company's knowledge, all
technical information developed by and belonging to the Company which has not
been patented has been kept confidential. The Company has not granted or
assigned to any other person or entity any right to manufacture, have
manufactured, assemble or sell the products or proposed products or to provide
the services or proposed services of the Company.

     SECTION 2.10 Title to Properties. The Company has good, clear and
                  -------------------
marketable title to its properties and assets reflected on the Balance Sheet or
acquired by it since the date of the Balance Sheet (other than properties and
assets disposed of in the ordinary course of business since the date of the
Balance Sheet), and all such properties and assets are free and clear of
mortgages, pledges, security interests, liens, charges, claims, restrictions and
other encumbrances (including without limitation, easements and licenses),
except for liens for or current taxes not yet due and payable and minor
imperfections of title, if any, not material in nature or amount and not
materially detracting from the value or impairing the use of the property
subject thereto or impairing the operations or proposed operations of the
Company, including without limitation, the ability of the Company to secure
financing using such properties and assets as collateral. To the best of the
Company's knowledge, there are no condemnation, environmental, zoning or other
land use regulation proceedings, either instituted or planned to be instituted,
which would adversely affect the use or operation of the Company's and its
subsidiaries' properties and assets for their respective intended uses and
purposes, or the value of such properties, and the Company has not received
notice of any special assessment proceedings which would affect such properties
and assets.

     SECTION 2.11 Leasehold Interests. Each lease or agreement to which the
                  -------------------
Company is a party under which it is a lessee of any property, real or personal,
is a valid and subsisting agreement, duly authorized and entered into, without
any default of the Company thereunder and, to the best of the Company's
knowledge, without any default thereunder of any other party thereto. No event
has occurred and is continuing which, with due notice or lapse of time or both,
would constitute a default or event of default by the Company under any such
lease or agreement or, to the best of the Company's knowledge, by any other
party thereto. The Company's

                                       7
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possession of such property has not been disturbed and, to the best of the
Company's knowledge, no claim has been asserted against the Company adverse to
its rights in such leasehold interests.

     SECTION 2.12 Insurance. The Company holds valid policies covering all of
                  ---------
the insurance required to be maintained by it under Section 5.05.

     SECTION 2.13 Taxes. The Company has filed all tax returns, Federal, state,
                  -----
county and local, required to be filed by it, and the Company has paid all taxes
shown to be due by such returns as well as all other taxes, assessments and
governmental charges which have become due or payable, including without
limitation all taxes which the Company is obligated to withhold from amounts
owing to employees, creditors and third parties. The Company has established
adequate reserves for all taxes accrued but not yet payable. The Federal income
tax returns of the Company have never been audited by the Internal Revenue
Service. No deficiency assessment with respect to or proposed adjustment of the
Company's Federal, state, county or local taxes is pending or, to the best of
the Company's knowledge, threatened. There is no tax lien, whether imposed by
any Federal, state, county or local taxing authority, outstanding against the
assets, properties or business of the Company. Neither the Company nor any of
its present or former stockholders has ever filed an election pursuant to
Section 1362 of the Internal Revenue Code of 1986, as amended (the "Code"), that
the Company be taxed as an S corporation.

     SECTION 2.14 Other Agreements. Except as set forth in the attached Schedule
                  ----------------                                      --------
IV(A), (which Schedule IV(A) may be updated in writing prior to any Additional
-----         --------------
Closing hereunder) as of each Closing Date hereunder, the Company reasonably
believes after due investigation that it is not a party to or otherwise bound by
any written or oral agreement, instrument, commitment or restriction which
individually or in the aggregate could materially adversely affect the business,
prospects, financial condition, operations, property or affairs of the Company.
Except as set forth in the attached Schedule IV(B), the Company is not a party
                                    --------------
to or otherwise bound by any written or oral:

          (a) distributor, dealer, manufacturer's representative or sales agency
agreement which is not terminable on less than ninety (90) days' notice without
cost or other liability to the Company (except for agreements which, in the
aggregate, are not material to the business of the Company);

          (b) sales agreement which entitles any customer to a rebate or right
of set-off, to return any product to the Company after acceptance thereof or to
delay the acceptance thereof, or which varies in any material respect from the
Company's standard form agreements;

          (c) agreement with any labor union (and, to the knowledge of the
Company, no organizational effort is being made with respect to any of its
employees);

          (d) agreement with any supplier containing any provision permitting
any party other than the Company to renegotiate the price or other terms, or
containing any pay-back or other similar provision, upon the occurrence of a
failure by the Company to meet its obligations under the agreement when due or
the occurrence of any other event;

                                       8
<PAGE>

          (e) agreement for the future purchase of fixed assets or for the
future purchase of materials, supplies or equipment in excess of its normal
operating requirements;

          (f) agreement for the employment of any officer, employee or other
person on a full-time or consulting basis which is not terminable by the Company
at will without liability to the Company, except pursuant to severance and
accrued vacation pay policies applicable to all employees of the Company;

          (g) bonus, pension, profit-sharing, retirement, hospitalization,
insurance, stock purchase, stock option or other plan, agreement or
understanding pursuant to which benefits are provided to any employee of the
Company (other than group insurance plans applicable to employees generally);

          (h) agreement relating to the borrowing of money or to the mortgaging
or pledging of, or otherwise placing a lien or security interest on, any asset
of the Company;

          (i) guaranty of any obligation for borrowed money or otherwise;

          (j) voting trust or agreement, stockholders' agreement, pledge
agreement, buy-sell agreement or first refusal or preemptive rights agreement
relating to any securities of the Company (other than this Agreement, the
Registration Rights Agreement Amendment, and the Prior Management Rights
Agreements), the Series A Agreement (as defined in section 6.13 hereof), the
Series B Agreement (as defined in Section 6.13 hereof), the Series C Agreement
(as defined in Section 6.13 hereof), the Stock Purchase Agreement, dated as of
November 5, 1997, between the Company and Kwang Dong Pharmaceutical Company (the
"Series D Agreement"), the Series E Agreement (as defined in Section 6.13
hereof), the Series F Agreement (as defined in Section 6.13 hereof) and the
Series G Agreement (as defined in Section 6.13 hereof);

          (k) agreement, or group of related agreements with the same party or
any group of affiliated parties, under which the Company has advanced or agreed
to advance money or has agreed to lease any property as lessee or lessor;

          (l) agreement or obligation (contingent or otherwise) to issue, sell
or otherwise distribute or to repurchase or otherwise acquire or retire any
share of its capital stock or any of its other equity securities other than
pursuant to its Charter as in effect on any Closing Date hereunder;

          (m) assignment, license or other agreement with respect to any form of
intangible property;

          (n) agreement under which it has granted any person any registration
rights, other than the Registration Rights Agreement;

          (o) agreement under which it has limited or restricted its right to
compete with any person in any respect; and

                                       9
<PAGE>

          (p) other agreement or group of related agreements with the same party
involving more than $40,000 or continuing over a period of more than six months
from the date or dates thereof (including renewals or extensions optional with
another party), which agreement or group of agreements is not terminable by the
Company without penalty upon notice of thirty (30) days or less, but excluding
any agreement or group of agreements with a customer of the Company for the
sale, lease or rental of the Company's products or services if such agreement or
group of agreements was entered into by the Company in the ordinary course of
business.

The Company, and to the best of the Company's knowledge, each other party
thereto have in all material respects performed all the obligations required to
be performed by them to date (or each non-performing party has received a valid,
enforceable and irrevocable written waiver with respect to its non-performance),
have received no notice of default and are not in default (with due notice or
lapse of time or both) under any agreement, instrument, commitment, plan or
arrangement to which the Company is a party or by which it or its property may
be bound. The Company has no present expectation or intention of not fully
performing all its obligations under each such agreement, instrument,
commitment, plan or arrangement, and the Company has no knowledge of any breach
or anticipated breach by the other party to any agreement, instrument,
commitment, plan or arrangement to which the Company is a party. The Company is
in full compliance with all of the terms and provisions of its Charter and
By-laws, as amended.

     SECTION 2.15 Significant Customers and Suppliers. No customer or supplier,
                  -----------------------------------
or group of two or more thereof (whether or not affiliated) which was material
to the Company, individually or in the aggregate, during the period covered by
the financial statements referred to in Section 2.05 or which has been material
to the Company thereafter, has terminated, materially reduced or threatened to
terminate or materially reduce its or their purchases from or provision of
products or services to the Company, as the case may be.

     SECTION 2.16 Governmental Approvals. Subject to the accuracy of the
                  ----------------------
representations and warranties of the Purchasers set forth in Article III, no
registration or filing with, or consent or approval of or other action by, any
Federal, state or other governmental agency or instrumentality is or will be
necessary for the valid execution, delivery and performance by the Company of
this Agreement or the Registration Rights Agreement Amendment, the issuance,
sale and delivery of the Preferred Shares or, upon conversion thereof, the
issuance and delivery of the Conversion Shares, other than (i) filings pursuant
to state securities laws (all of which filings have been made by the Company,
other than those which are required to be made after the Closing and which will
be duly made on a timely basis) in connection with the sale of the Preferred
Shares and (ii) with respect to the Registration Rights Agreement, the
registration of the shares covered thereby with the Securities and Exchange
Commission (the "Commission") and filings pursuant to state securities laws.

     SECTION 2.17 Disclosure. Neither this Agreement, nor any Schedule or
                  ----------
Exhibit to this Agreement, contains an untrue statement of a material fact or
omits a material fact necessary to make the statements contained herein or
therein not misleading. None of the statements, documents, certificates or other
items prepared or supplied by the Company with respect to the transactions
contemplated hereby contains an untrue statement of a material fact or omits a
material fact necessary to make the statements contained therein not misleading.

                                      10
<PAGE>

     SECTION 2.18 Offering of the Preferred Shares. Neither the Company nor any
                  --------------------------------
person authorized or employed by the Company as agent, broker, dealer or
otherwise in connection with the offering or sale of the Preferred Shares or any
security of the Company similar to the Preferred Shares has offered the
Preferred Shares or any such similar security for sale to, or solicited any
offer to buy the Preferred Shares or any such similar security from, or
otherwise approached or negotiated with respect thereto with, any person or
persons, and neither the Company nor any person acting on its behalf has taken
or will take any other action (including, without limitation, any offer,
issuance or sale of any security of the Company under circumstances which might
require the integration of such security with Preferred Shares under the
Securities Act of 1933, as amended (the "Securities Act") or the rules and
regulations of the Commission thereunder), in either case so as to subject the
offering, issuance or sale of the Preferred Shares to the registration
provisions of the Securities Act.

     SECTION 2.19 Brokers. The Company has no contract, arrangement or
                  -------
understanding with any broker, finder or similar agent with respect to the
transactions contemplated by this Agreement.

     SECTION 2.20 Officers. Set forth in Schedule II is a list of the names of
                  --------               -----------
the officers of the Company, together with the title or job classification of
each such person and the total cash compensation anticipated to be paid to each
such person by the Company in 2000.

     SECTION 2.21 Transactions With Affiliates. Other than purchases of
                  ----------------------------
Preferred Shares hereunder and under the Series A Agreement, Series B Agreement,
Series C Agreement, Series E Agreement, Series F Agreement and Series G
Agreement (as such terms are defined in Section 6.13 hereof) and under the
Series D Agreement and except as set forth on Schedule III and Schedule IV(B)
                                              ------------     --------------
hereof, no director, officer, employee or stockholder of the Company, or member
of the family of any such person, or any corporation, partnership, trust or
other entity in which any such person, or any member of the family of any such
person, has a substantial interest or is an officer, director, trustee, partner
or holder of more than 5% of the outstanding capital stock thereof, is a party
to any transaction with the Company, including any contract, agreement or other
arrangement providing for the employment of, furnishing of services by, rental
of real or personal property from or otherwise requiring payments to any such
person or firm, other than employment-at-will arrangements in the ordinary
course of business.

     SECTION 2.22 Employees. Each of the officers of the Company, each key
                  ---------
employee and each other employee now employed by the Company who has access to
confidential information of the Company has executed an Employee Nondisclosure
and Developments Agreement substantially in the form of Exhibit C, and such
                                                        ---------
agreements are in full force and effect. No officer or key employee of the
Company has advised the Company (orally or in writing) that he or she intends to
terminate employment with the Company. The Company has complied in all material
respects with all applicable laws relating to the employment of labor, including
provisions relating to wages, hours, equal opportunity, collective bargaining
and the payment of Social Security and other taxes, and with the Employee
Retirement Income Security Act of 1974, as amended ("ERISA").

                                      11
<PAGE>

     SECTION 2.23 Environmental Protection. The Company has not caused or
                  ------------------------
allowed, or contracted with any party for, the generation, use, transportation,
treatment, storage or disposal of any Hazardous Substances (as defined below) in
connection with the operation of its business or otherwise. The Company, the
operation of its business, and to the Company's knowledge, any real property
that the Company owns, leases or otherwise occupies or uses (the "Premises") are
in material compliance with all applicable Environmental Laws (as defined below)
and orders or directives of any governmental authorities having jurisdiction
under such Environmental Laws, including, without limitation, any Environmental
Laws or orders or directives with respect to any cleanup or remediation of any
release or threat of release of Hazardous Substances. The Company has not
received any citation, directive, letter or other communication, written or
oral, or any notice of any proceeding, claim or lawsuit, from any person arising
out of the ownership or occupation of the Premises, or the conduct of its
operations, and the Company is not aware of any basis therefor. The Company has
obtained and is maintaining in full force and effect all necessary permits,
licenses and approvals required by all Environmental Laws applicable to the
Premises and the Company's business operations conducted thereon, and is in
compliance with all such permits, licenses and approvals. The Company has not
caused or allowed a release, or a threat of release, of any Hazardous Substance
unto, at or near the Premises, and, to the best of the Company's knowledge,
neither the Premises nor any property at or near the Premises has ever been
subject to a release, or a threat of release, of any Hazardous Substance. For
the purposes of this Agreement, the term "Environmental Laws" shall mean any
Federal, state or local law or ordinance or regulation pertaining to the
protection of human health or the environment, including, without limitation,
the Comprehensive Environmental Response, Compensation and Liability Act, 42
U.S.C. Sections 9601, et seq., the Emergency Planning and Community
Right-to-Know Act, 42 U.S.C. Sections 11001, et seq., and the Resource
Conservation and Recovery Act, 42 U.S.C. Sections 6901, et seq. For purposes of
this Agreement, the term "Hazardous Substances" shall include oil and petroleum
products, asbestos, polychlorinated biphenyls, urea formaldehyde and any other
materials classified as hazardous or toxic under any Environmental Laws.

     SECTION 2.24 ERISA.
                  -----

     (a) Schedule II lists each employee plan that covers any employee of the
         -----------
Company (each, an "Employee Plan" and collectively, the "Employee Plans"),
copies or descriptions of all of which have previously been made available or
furnished to the Purchasers. With respect to each Employee Plan, the Company has
provided the most recently filed Form 5500 and an accurate summary description
of such plan.

     (b) Schedule II also includes a list of each benefit arrangement of the
         -----------
Company (each, a "Benefit Arrangement" and collectively, the "Benefit
Arrangements"), copies or descriptions of all of which have been made available
or furnished previously to the Purchasers.

     (c) No Employee Plan is a Multiemployer Plan and no Employee Plan is
subject to Title IV of ERISA. The Company and its Affiliates have not incurred
any liability under Title IV of ERISA arising in connection with the termination
of any plan covered or previously covered by Title IV of ERISA.

                                      12
<PAGE>

     (d) None of the Employee Plans or other arrangements listed on Schedule II
                                                                    -----------
covers any non-United States employee or former employee of the Company.

     (e) No "prohibited transaction," as defined in Section 406 of ERISA or
Section 4975 of the Code, has occurred with respect to any Employee Plan.

     (f) Each Employee Plan which is intended to be qualified under Section
401(a) of the Code is so qualified and has been so qualified during the period
from its adoption to date, and each trust forming a part thereof is exempt from
tax pursuant to Section 501(a) of the Code. The Company has furnished to the
Purchasers copies of the most recent Internal Revenue Service determination
letters with respect to each such plan. Each Employee Plan has been maintained
in compliance with its terms and with the requirements prescribed by any and all
statutes, orders, rules and regulations, including but not limited to ERISA and
the Code, which are applicable to such plan.

     (g) Each Employee Plan and each Benefit Arrangement has been maintained in
substantial compliance with its terms and with the requirements prescribed by
any and all statutes, orders, rules and regulations which are applicable to such
Employee Plan and Benefit Arrangement.

     (h) All contributions and payments accrued under each Employee Plan and
Benefit Arrangement, determined in accordance with prior funding and accrual
practices, as adjusted to include proportional accruals for the period ending on
the Closing Date, will be discharged and paid on or prior to the Closing Date
except to the extent reflected on the Balance Sheet. Except as disclosed in
writing to the Purchasers prior to the date hereof, there has been no amendment
to, written interpretation of or announcement (whether or not written) by the
Company or any of its ERISA Affiliates relating to, or change in employee
participation or coverage under, any Employee Plan or Benefit Arrangement that
would increase materially the expense of maintaining such Employee Plan or
Benefit Arrangement above the level of the expense incurred in respect thereof
for the fiscal year ended prior to the date hereof.

     (i) There is no contract, agreement, plan or arrangement covering any
employee or former employee of the Company that, individually or collectively,
could give rise to the payment of any amount that would not be deductible
pursuant to the terms of Section 280G of the Code.

     (j) No tax under Section 4980B of the Code has been incurred in respect of
any Employee Plan that is a group health plan, as defined in Section 5000(b)(1)
of the Code.

     (k) With respect to the employees and former employees of the Company,
there are no employee post-retirement medical or health plans in effect, except
as required by Section 4980B of the Code.

     (l) No employee of the Company will become entitled to any bonus,
retirement, severance or similar benefit or enhanced benefit solely as a result
of the transactions contemplated hereby.

                                      13
<PAGE>

     (m) The Company does not have, nor is it reasonably expected to have, any
liability under Title IV of ERISA.

     SECTION 2.25 Real Property Holding Corporation. The Company is not a real
                  ---------------------------------
property holding corporation within the meaning of Section 897(c)(2) of the Code
and any regulation promulgated thereunder.

                                   ARTICLE III

                REPRESENTATIONS AND WARRANTIES OF THE PURCHASERS

     Except as previously disclosed in writing to the Company, each Purchaser
severally represents and warrants to the Company that:

     (a) it is a "qualified institutional buyer" as defined under Rule 144A
under the Securities Act (which definition is set forth in Schedule V attached
                                                           ----------
hereto) and was not organized for the specific purpose of acquiring the
Preferred Shares;

     (b) it has sufficient knowledge and experience in investing in companies
similar to the Company in terms of the Company's stage of development so as to
be able to evaluate the risks and merits of its investment in the Company and it
is able financially to bear the risks thereof;

     (c) it has had an opportunity to discuss the Company's proposed business,
management and financial affairs with the Company's management;

     (d) the Preferred Shares being purchased by it are being acquired for its
own account for the purpose of investment and not with a view to or for sale in
connection with any distribution thereof;

     (e) it understands that (i) the Preferred Shares and the Conversion Shares
have not been registered under the Securities Act by reason of their issuance in
a transaction exempt from the registration requirements of the Securities Act
pursuant to Section 4(2) thereof or Rule 505 or 506 promulgated under the
Securities Act, (ii) the Preferred Shares and, upon conversion thereof, the
Conversion Shares must be held indefinitely unless a subsequent disposition
thereof is registered under the Securities Act or is exempt from such
registration, (iii) the Preferred Shares and the Conversion Shares will bear a
legend to such effect and (iv) the Company will make a notation on its transfer
books to such effect; and

     (f) if it sells any Conversion Shares pursuant to Rule 144A promulgated
under the Securities Act, it will take all necessary steps in order to perfect
the exemption from registration provided thereby, including (i) obtaining on
behalf of the Company information to enable the Company to establish a
reasonable belief that the purchaser is a qualified

                                      14
<PAGE>

institutional buyer and (ii) advising such purchaser that Rule 144A is being
relied upon with respect to such resale.

                                   ARTICLE IV

                          CONDITIONS TO THE OBLIGATIONS
                                OF THE PURCHASERS

         The obligation of each Purchaser to purchase and pay for the Preferred
Shares being purchased by it on the Closing Date is, at its option, subject to
the satisfaction, on or before the Closing Date, of the following conditions:

     (a) Representations and Warranties to be True and Correct. The
         -----------------------------------------------------
representations and warranties contained in Article II shall be true, complete
and correct in all material respects on and as of the Closing Date with the same
effect as though such representations and warranties had been made on and as of
such date, and the President and Chief Financial Officer of the Company shall
have certified to such effect to the Purchasers in writing.

     (b) Performance. The Company shall have performed and complied in all
         -----------
material respects with all agreements contained herein required to be performed
or complied with by it prior to or at the Closing Date.

     (c) All Proceedings to be Satisfactory. All corporate and other proceedings
         ----------------------------------
to be taken by the Company in connection with the transactions contemplated
hereby and all documents incident thereto shall be satisfactory in form and
substance to the Purchasers and their counsel, and the Purchasers and their
counsel shall have received all such counterpart originals or certified or other
copies of such documents as they reasonably may request.

     (d) Supporting Documents. The Purchasers and their counsel shall have
         --------------------
received copies of the following documents:

          (i) the Charter, certified as of a recent date by the Secretary of
     State of the State of Delaware; and

          (ii) (A) a complete copy of the By-laws of the Company as in effect on
     the Closing Date; and (B) a complete copy of all resolutions adopted by the
     Board of Directors and the stockholders of the Company authorizing the
     execution, delivery and performance of this Agreement and the Registration
     Rights Agreement Amendment, the issuance, sale and delivery of the
     Preferred Shares and the reservation, issuance and delivery of the
     Conversion Shares, and a secretary's certificate to the effect that all
     such resolutions are in full force and effect and are all the resolutions
     adopted in connection with the transactions contemplated by this Agreement
     and the Registration Rights Agreement Amendment.

                                      15
<PAGE>

          (e) Registration Rights Agreement Amendment. The Company shall have
              ---------------------------------------
     executed and delivered the Registration Rights Agreement Amendment.

          (f) Charter. The Charter shall read in its entirety as set forth in
              -------
     Exhibit B.
     ---------

         (g) Opinion of Counsel. Purchasers participating in such Closing shall
             ------------------
     have received an opinion of Dechert Price & Rhoads, dated the date of such
     Closing, satisfactory in form and substance to such Purchasers and
     Purchasers' counsel.

          (h) Fees of Purchasers' Counsel. The Company shall have paid in
              ---------------------------
     accordance with Section 6.01 the fees and disbursements of Purchasers'
     counsel invoiced at the Closing or any Additional Closing.

          (i) Due Diligence. The Purchasers shall have completed their business,
             -------------
     tax, accounting, regulatory, legal and other due diligence review to the
     reasonable satisfaction of the Purchasers.

         (j) Approvals and Consents. The Company shall have received all
             ----------------------
      governmental and regulatory approvals and consents and all other requisite
      third party approvals and consents which are necessary to consummate this
      Agreement and the transactions contemplated thereby.

All such documents shall be satisfactory in form and substance to the Purchasers
and their counsel.

                                    ARTICLE V

                            COVENANTS OF THE COMPANY

         The Company covenants and agrees with each of the Purchasers that:

     SECTION 5.01 Financial Statements, Reports, Etc.. The Company shall furnish
                  -----------------------------------
to a Purchaser (provided that such Purchaser signs a customary confidentiality
agreement with the Company):

     (a) within ninety (90) days after the end of each fiscal year of the
Company a consolidated balance sheet of the Company as of the end of such fiscal
year and the related consolidated statements of income, stockholders' equity and
cash flows for the fiscal year then ended, prepared in accordance with generally
accepted accounting principles and certified by a firm of independent public
accountants of recognized national standing selected by the Board of Directors
of the Company;

     (b) within thirty (30) days after the end of each fiscal quarter of the
Company a consolidated balance sheet of the Company as of the end of such fiscal
quarter and the related consolidated statements of income, stockholders' equity
and cash flows for the fiscal

                                      16
<PAGE>

quarter then ended, unaudited but prepared in accordance with generally accepted
accounting principles;

     (c) so long such Purchaser holds not less than 100,000 Preferred Shares or
Conversion Shares (or a combination of both), within thirty (30) days after the
end of each month in each fiscal year (other than the last month in each fiscal
year) a consolidated balance sheet of the Company and the related consolidated
statements of income, stockholders' equity and cash flows, unaudited but
prepared in accordance with generally accepted accounting principles and
certified by the Chief Financial Officer of the Company, such consolidated
balance sheet to be as of the end of such month and such consolidated statements
of income, stockholders' equity and cash flows to be for such month and for the
period from the beginning of the fiscal year to the end of such month, in each
case with comparative statements for the prior fiscal year, provided that the
Company's obligations under this Section 5.01(c) shall terminate upon the
completion of a firm commitment underwritten public offering of the Company's
securities;

     (d) at the time of delivery of each monthly statement pursuant to Section
5.01(c), a management narrative report explaining all significant variances from
forecasts and all significant current developments in staffing, marketing, sales
and operations;

     (e) no later than thirty (30) days prior to the start of each fiscal year,
consolidated capital and operating expense budgets, cash flow projections and
income and loss projections for the Company in respect of such fiscal year, all
itemized in reasonable detail and prepared on a monthly basis, and, promptly
after preparation, any revisions to any of the foregoing;

     (f) promptly following receipt by the Company, each audit response letter,
accountant's management letter and other written report submitted to the Company
by its independent public accountants in connection with an annual or interim
audit of the books of the Company;

     (g) promptly after the commencement thereof, notice of all actions, suits,
claims, proceedings, investigations and inquiries of the type described in
Section 2.07 that could reasonably be expected to materially adversely affect
the Company;

     (h) promptly upon sending, making available or filing the same, all press
releases, reports and financial statements that the Company sends or makes
available to its stockholders or directors or files with the Commission; and

     (i) promptly, from time to time, such other information regarding the
business, prospects, financial condition, operations, property or affairs of the
Company as such Purchaser reasonably may request.

     SECTION 5.02 Right of First Refusal. So long as the Company has not
                  ----------------------
consummated an Initial Public Offering (as hereafter defined), the Company
shall, prior to any issuance by the Company of any of its securities (other than
debt securities with no equity feature), offer to each

                                      17
<PAGE>

Purchaser by written notice the right, for a period of twenty (20) days, to
purchase all of such securities for cash at an amount equal to the price or
other consideration for which such securities are to be issued; provided,
however, that the first refusal rights of the Purchasers pursuant to this
Section 5.02 shall not apply to securities issued (A) upon conversion of any of
the Preferred Shares, (B) as a stock dividend or upon any subdivision of shares
of Common Stock, provided that the securities issued pursuant to such stock
dividend or subdivision are limited to additional shares of Common Stock, (C)
pursuant to subscriptions, warrants, options, convertible securities, or other
rights which are listed in Schedule III as being outstanding on the date of this
                           ------------
Agreement, but not including those described in (F) below, (D) solely in
consideration for the acquisition (whether by merger or otherwise) by the
Company of all or substantially all of the stock or assets of any other entity,
which such acquisition has been approved by the Board of Directors of the
Company, (E) pursuant to a firm commitment underwritten public offering, (F)
pursuant to the exercise of options to purchase Common Stock granted to
directors, officers, employees or consultants of the Company in connection with
their service to the Company or pursuant to the exercise of options to purchase
Common Stock granted to or Common Stock issued to licensors or transferors of
technology to the Company, not to exceed in the aggregate 17,750,000 shares
(appropriately adjusted to reflect stock splits, stock dividends, combinations
of shares and the like with respect to the Common Stock) (the shares exempted by
this clause (F) being hereinafter referred to as the "Reserved Employee and
Technology Shares"), and (G) upon the exercise of any right which was not itself
in violation of the terms of this Section 5.02. The Company's written notice to
the Purchasers shall describe the securities proposed to be issued by the
Company and specify the number, price and payment terms. Each Purchaser may
accept the Company's offer as to the full number of securities offered to it or
any lesser number, by written notice thereof given by it to the Company prior to
the expiration of the aforesaid twenty (20) day period, in which event the
Company shall promptly sell and such Purchaser shall buy, upon the terms
specified, the number of securities agreed to be purchased by such Purchaser.
Notwithstanding the foregoing, if the Purchasers agree, in the aggregate, to
purchase more than the full number of securities offered by the Company, then
each Purchaser accepting the Company's offer shall first be allocated the lesser
of (i) the number of securities which such Purchaser agreed to purchase and (ii)
the number of securities as is equal to the full number of securities offered by
the Company multiplied by a fraction, the numerator of which shall be the number
of shares of Common Stock held by such Purchaser as of the date of the Company's
notice of offer (treating such Purchaser, for the purpose of such calculation,
as the holder of the number of shares of Common Stock which would be issuable to
such Purchaser upon conversion, exercise or exchange of all securities
(including but not limited to the Preferred Shares) held by such Purchaser on
the date such offer is made, that are convertible, exercisable or exchangeable
into or for (whether directly or indirectly) shares of Common Stock) and the
denominator of which shall be the aggregate number of shares of Common Stock
(calculated as aforesaid) held on such date by all Purchasers who accepted the
Company's offer, and the balance of the securities (if any) offered by the
Company shall be allocated among the Purchasers accepting the Company's offer in
proportion to their relative equity ownership interests in the Company
(calculated as aforesaid), provided that no Purchaser shall be allocated more
than the number of securities which such Purchaser agreed to purchase and
provided further that in cases covered by this sentence all Purchasers shall be
allocated among them the full number of securities offered by the Company. The
Company shall be free at any time prior to ninety (90) days after the date of
its notice of offer to

                                      18
<PAGE>

the Purchasers, to offer and sell to any third party or parties the number of
such securities not agreed by the Purchasers to be purchased by them, at a price
and on payment terms no less favorable to the Company than those specified in
such notice of offer to the Purchasers. However, if such third party sale or
sales are not consummated within such ninety (90) day period, the Company shall
not sell such securities as shall not have been purchased within such period
without again complying with this Section 5.02. For purposes of this Section
5.02, (x) the term "Purchasers" shall include the Series A Purchasers, the
Series B Purchasers, the Series C Purchasers, the Series E Purchasers, the
Series F Purchasers, the Series G Purchasers (as such terms are defined in
Section 6.13 hereof) and Purchasers of Series H Preferred Stock holding at least
100,000 shares of Series H Preferred Stock and (y) the term "Preferred Shares"
shall include the shares of Series A Convertible Preferred Stock, the shares of
Series B Convertible Preferred Stock, the shares of Series C Convertible
Preferred Stock, the shares of Series E Convertible Preferred Stock, the shares
of Series F Convertible Preferred Stock and the shares of Series G Convertible
Preferred Stock purchased pursuant to the Series A Agreement, the Series B
Agreement, the Series C Agreement, the Series E Agreement, the Series F
Agreement and the Series G Agreement, respectively (as such terms are defined in
Section 6.13). "Initial Public Offering" means a underwritten public offering
pursuant to an effective registration statement under the Securities Act in
respect of the offer and sale of shares of Common Stock for the account of the
Company resulting in aggregate net proceeds to the Company and any stockholder
selling shares of Common Stock in such offering of not less than $25,000,000 and
a public offering price per share of not less than $2.85 per share (as adjusted
for any combination, division, subdivision, stock split, reverse stock split or
similar event relating to the Common Stock).

     SECTION 5.03 Reserve for Conversion Shares. The Company shall at all times
                  -----------------------------
reserve and keep available out of its authorized but unissued shares of Common
Stock, for the purpose of effecting the conversion of the Preferred Shares and
otherwise complying with the terms of this Agreement, such number of its duly
authorized shares of Common Stock as shall be sufficient to effect the
conversion of the Preferred Shares from time to time outstanding or otherwise to
comply with the terms of this Agreement (the "Conversion Shares"). If at any
time the number of authorized but unissued shares of Common Stock shall not be
sufficient to effect the conversion of the Preferred Shares or otherwise to
comply with the terms of this Agreement, the Company will forthwith take such
corporate action as may be necessary to increase its authorized but unissued
shares of Common Stock to such number of shares as shall be sufficient for such
purposes. The Company will obtain any authorization, consent, approval or other
action by or make any filing with any court or administrative body that may be
required under applicable state securities laws in connection with the issuance
of shares of Common Stock upon conversion of the Preferred Shares.

     SECTION 5.04 Corporate Existence. The Company shall maintain its corporate
                  -------------------
existence, rights and franchises in full force and effect.

     SECTION 5.05 Properties, Business, Insurance. The Company shall maintain
                  -------------------------------
its properties and business with insurance from financially sound and reputable
insurers against such casualties and contingencies and of such types and in such
amounts as is customary for companies similarly situated, which insurance shall
be deemed by the Company to be sufficient.

                                      19
<PAGE>

The Company shall not cause or permit any assignment or change in beneficiary
and shall not borrow against any such policy. If requested by Purchasers holding
at least sixty percent (60%) of the outstanding Preferred Shares, the Company
shall add one designee of such Purchasers as a notice party for each such policy
and shall request that the issuer of each policy provide such designee with ten
(10) days' notice before such policy is terminated (for failure to pay premiums
or otherwise) or assigned or before any change is made in the beneficiary
thereof.

     SECTION 5.06 Inspection, Consultation and Advice. The Company shall permit
                  -----------------------------------
each Purchaser holding, in aggregate with its partners, shareholders or other
affiliates, in excess of 10% of the Series H Convertible Preferred Stock and
such persons as it may designate, at such Purchaser's expense, and subject to
the execution of an appropriate confidentiality agreement, to visit and inspect
any of the properties of the Company, examine their books and take copies and
extracts therefrom, discuss the affairs, finances and accounts of the Company
with its officers, employees and public accountants (and the Company hereby
authorizes said accountants to discuss with such Purchaser and such designees
such affairs, finances and accounts), and consult with and advise the management
of the Company as to its affairs, finances and accounts, all at reasonable times
and upon reasonable notice.

     SECTION 5.07 Restrictive Agreements Prohibited. The Company shall not
                  ---------------------------------
become a party to any agreement which by its terms restricts the Company's
performance of this Agreement, the Registration Rights Agreement or the Charter.

     SECTION 5.08 Transactions with Affiliates. Except for transactions
                  ----------------------------
contemplated by this Agreement or as otherwise approved by the Board of
Directors, the Company shall not enter into any material transaction with any
director, officer, employee or holder of more than 5% of the outstanding capital
stock of any class or series of capital stock of the Company, member of the
family of any such person, or any corporation, partnership, trust or other
entity in which any such person, or member of the family of any such person, is
a director, officer, trustee, partner or holder of more than 5% of the
outstanding capital stock thereof, except for transactions on customary terms
related to such person's employment.

     SECTION 5.09 Use of Proceeds. The Company shall use the cash proceeds from
                  ---------------
the sale of the Preferred Shares for the continued development of existing
product candidates, research and development of additional product candidates,
manufacturing, commercialization and marketing expenditures, expansion of the
Company's facilities, working capital and potential acquisitions of products,
technologies or businesses and other general corporate purposes.

     SECTION 5.10 By-laws. The Company shall at all times cause its By-laws to
                  -------
provide that, (a) unless otherwise required by the laws of the State of
Delaware, (i) any two directors and (ii) any holder or holders of at least 25%
of the outstanding shares of Series H Convertible Preferred Stock, shall have
the right to call a meeting of the Board of Directors or stockholders and (b)
the number of directors fixed in accordance therewith shall in no event conflict
with any of the terms or provisions of the Series H Convertible Preferred Stock
as set forth in the Charter. The Company shall at all times maintain provisions
in its By-laws and/or Charter indemnifying all directors against liability and
absolving all directors from liability to the Company and its stockholders to
the maximum extent permitted under the laws of the State of Delaware.

                                      20
<PAGE>

     SECTION 5.11 Performance of Contracts. The Company shall not materially
                  ------------------------
amend, modify, terminate, waive or otherwise alter, in whole or in part, any of
the Employee Nondisclosure and Developments Agreements without the consent of
the Company's Board of Directors.

     SECTION 5.12 Vesting of Reserved Employee Shares. The Company shall not
                  -----------------------------------
grant to any of its employees options to purchase Reserved Employee Shares which
will become exercisable at a rate in excess of 25% per annum from the date of
such grant without the approval of the Company's Board of Directors.

     SECTION 5.13 Employee Nondisclosure and Developments Agreements. The
                  --------------------------------------------------
Company shall use its best efforts to obtain an Employee Nondisclosure and
Developments Agreement in substantially the form of Exhibit C, or in such other
                                                    ---------
form as is approved by the Board of Directors, from all future officers, key
employees and other employees who will have access to confidential information
of the Company, upon their employment by the Company.

     SECTION 5.14 Compliance with Laws. The Company shall comply with all
                  --------------------
applicable laws, rules, regulations and orders, noncompliance with which could
reasonably be expected to materially adversely affect its business or condition,
financial or otherwise.

     SECTION 5.15 Keeping of Records and Books of Account. The Company shall
                  ---------------------------------------
keep adequate records and books of account, in which complete entries will be
made in accordance with generally accepted accounting principles consistently
applied, reflecting all financial transactions of the Company, and in which, for
each fiscal year, all proper reserves for depreciation, depletion, obsolescence,
amortization, taxes, bad debts and other purposes in connection with its
business shall be made.

     SECTION 5.16 Rule 144A Information. The Company shall, at all times during
                  ---------------------
which it is neither subject to the reporting requirements of Section 13 or 15(d)
of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), nor
exempt from reporting pursuant to Rule 12g3-2(b) under the Exchange Act, provide
in writing, upon the written request of any Purchaser or a prospective buyer of
Preferred Shares or Conversion Shares from any Purchaser, all information
required by Rule 144A(d)(4)(i) (or any successor provision thereto) of the
General Regulations promulgated by the Commission under the Securities Act
("Rule 144A Information"). The Company also shall, upon the written request of
any Purchaser, cooperate with and assist such Purchaser or any member of the
National Association of Securities Dealers, Inc. PORTAL system in applying to
designate and thereafter maintain the eligibility of the Preferred Shares or
Conversion Shares, as the case may be, for trading through PORTAL. The Company's
obligations under this Section 5.16 shall at all times be contingent upon the
relevant Purchaser's obtaining from the prospective buyer of Preferred Shares or
Conversion Shares a written agreement to take all reasonable precautions to
safeguard the Rule 144A Information from disclosure to anyone other than a
person who will assist such buyer in evaluating the purchase of any Preferred
Shares or Conversion Shares.

                                      21
<PAGE>

     SECTION 5.17 Future Subsidiaries. In the event the Company shall acquire or
                  -------------------
create a subsidiary or subsidiaries, the Company agrees that the covenants
contained in this Article V shall apply to the Company and such subsidiaries on
a consolidated basis.

     SECTION 5.18 Observation Rights. So long as MPM Capital (or any successor
                  ------------------
thereto) ("MPM") (or entities affiliated with MPM) owns shares of Series H
Convertible Preferred Stock, MPM shall have the right to designate one observer
to attend meetings of the Board of Directors (such designee, an "Observer"). The
Observer shall not have the right to vote on any matter presented to the Board
of Directors. The Company shall give the Observer written notice of each meeting
of the Board of Directors at the same time and in the same manner as the members
of the Board of Directors receive notice of such meetings, and the Company shall
permit the Observer to attend as an observer all meetings of its Board of
Directors. The Observer shall be entitled to receive all written materials and
other information given to the directors in connection with such meetings at the
same time such materials and information are given to the directors, and the
Observer shall keep such materials and information confidential. If the Company
proposes to take any action by written consent in lieu of a meeting of its Board
of Directors, the Company shall give written notice thereof to the Observer
prior to the effective date of such consent. The Company shall provide the
Observer all written materials and other information given to the directors in
connection with such action by written consent at the same time such materials
and information are given to the directors, and the Observer shall keep such
materials and information confidential. Notwithstanding the foregoing, the
Company reserves the right not to provide information and to exclude the
Observer from any meeting or portion thereof if the Company believes, as
determined in good faith by its Board of Directors, that delivery of such
information or attendance at such meeting by the Observer (a) would adversely
affect the attorney-client privilege between the Company and its counsel, (b)
would violate any fiduciary obligations of the Board of Directors, or (c) would
involve a conflict of interest with regard to the Observer, or if the Observer
is a direct competitor of the Company. The rights of MPM pursuant to this
Section 5.18 shall terminate on the earlier to occur of (i) twelve months after
the consummation of an Initial Public Offering, or (ii) the acquisition (whether
by merger or otherwise) of substantially all of the stock or assets of the
Company by any entity unaffiliated with the Company.

                                   ARTICLE VI

                                  MISCELLANEOUS

     SECTION 6.01 Expenses. Each party hereto will pay its own expenses in
                  --------
connection with the transactions contemplated hereby, whether or not such
transactions shall be consummated, provided, however, that in the event the
Closing occurs, the Company shall pay the reasonable invoiced, out of pocket
fees and expenses, not to exceed an aggregate amount of $40,000, incurred by the
Purchasers' attorneys in connection with legal, corporate and patent due
diligence.

     SECTION 6.02 Survival of Agreements. All covenants, agreements,
                  ----------------------
representations and warranties made herein or in the Registration Rights
Agreement Amendment, or any certificate

                                      22
<PAGE>

or instrument delivered to the Purchasers pursuant to or in connection with this
Agreement or the Registration Rights Agreement Amendment, shall survive the
execution and delivery of this Agreement and the Registration Rights Agreement
Amendment, the issuance, sale and delivery of the Preferred Shares, and the
issuance and delivery of the Conversion Shares, and all statements contained in
any certificate or other instrument delivered by the Company hereunder or
thereunder or in connection herewith or therewith shall be deemed to constitute
representations and warranties made by the Company; provided that all such
representations and warranties shall terminate two years from the date they are
made.

     SECTION 6.03 Brokerage. Each party hereto will indemnify and hold harmless
                  ---------
the others against and in respect of any claim for brokerage or other
commissions relative to this Agreement or to the transactions contemplated
hereby, based in any way on agreements, arrangements or understandings made or
claimed to have been made by such party with any third party.

     SECTION 6.04 Parties in Interest. All representations, covenants and
                  -------------------
agreements contained in this Agreement by or on behalf of any of the parties
hereto shall bind and inure to the benefit of the respective successors and
assigns of the parties hereto whether so expressed or not. Without limiting the
generality of the foregoing, all representations, covenants and agreements
benefiting the Purchasers shall inure to the benefit of any and all subsequent
holders from time to time of Preferred Shares or Conversion Shares.

     SECTION 6.05 Notices. All notices, requests, consents and other
                  -------
communications hereunder shall be in writing and shall be delivered in person,
mailed by certified or registered mail, return receipt requested, or sent by
telecopier or telex, addressed as follows:

     (a) if to the Company, at 371 Phoenixville Pike, Malvern, PA 19355,
Attention: President, with a copy to James A. Lebovitz, Esq., Dechert Price &
Rhoads, 4000 Bell Atlantic Tower, 1717 Arch Street, Philadelphia, PA 19103; and

     (b) if to any Purchaser, at the address of such Purchaser set forth in
Schedule I;
----------

or, in any such case, at such other address or addresses as shall have been
furnished in writing by such party to the others.

     SECTION 6.06 Governing Law. This Agreement shall be governed by and
                  -------------
construed in accordance with the laws of the State of Delaware.

     SECTION 6.07 Entire Agreement. This Agreement, including the Schedules and
                  ----------------
Exhibits hereto, constitutes the sole and entire agreement of the parties with
respect to the subject matter hereof. All Schedules and Exhibits hereto are
hereby incorporated herein by reference.

     SECTION 6.08 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.

                                      23
<PAGE>

     SECTION 6.09 Amendments. This Agreement may be amended or modified, and
                  ----------
provisions hereof may be waived, with the written consent of the Company and the
holders of at least two-thirds (2/3) of the outstanding shares of Common Stock
issued or issuable upon conversion of the Preferred Shares. Otherwise this
Agreement may not be amended or waived or any provision hereof waived.

     SECTION 6.10 Severability. If any provision of this Agreement shall be
                  ------------
declared void or unenforceable by any judicial or administrative authority, the
validity of any other provision and of the entire Agreement shall not be
affected thereby.

     SECTION 6.11 Titles and Subtitles. The titles and subtitles used in this
                  --------------------
Agreement are for convenience only and are not to be considered in construing or
interpreting any term or provision of this Agreement.

     SECTION 6.12 Certain Defined Term. As used in this Agreement, the following
                  --------------------
term shall have the following meaning (such meaning to be equally applicable to
both the singular and plural forms of the term defined):

     "person" shall mean an individual, corporation, trust, partnership, joint
venture, unincorporated organization, government agency or any agency or
political subdivision thereof, or other entity.

     SECTION 6.13 Prior Agreements. By their signature below, each of the
                  ----------------
Purchasers who are also parties to that certain Series A Convertible Preferred
Stock Purchase Agreement (the "Series A Agreement") between the Company and the
purchasers named therein dated as of November 7, 1994, (each, a "Series A
Purchaser") and that certain Series B Convertible Preferred Stock Purchase
Agreement (the "Series B Agreement") between the Company and the purchasers
named therein dated as of March 1, 1996, (each, a "Series B Purchaser"), that
certain Series C Convertible Preferred Stock Purchase Agreement (the "Series C
Agreement") between the Company and the purchasers named therein dated as of May
1, 1997 (each, a "Series C Purchaser"), that certain Series E Convertible
Preferred Stock Purchase Agreement (the "Series E Agreement") between the
Company and the purchasers named therein dated as of December 8, 1998 (each, a
"Series E Purchaser"), that certain Series F Convertible Preferred Stock
Purchase Agreement (the "Series F Agreement") between the Company and the
purchaser named therein dated as of July 22, 1999, (the "Series F Purchaser")
and that certain Series G Convertible Preferred Stock Purchase Agreement (the
"Series G Agreement") between the Company and the Purchasers named therein dated
as of January 10, 2000, (each a "Series G Purchaser") hereby (i) waives, except
to the extent set forth on Schedule I hereto, the right to purchase shares of
                           ----------
Series H Convertible Preferred Stock sold pursuant to this Agreement. The
signature of each Series A Purchaser, Series B Purchaser, Series C Purchaser,
Series E Purchaser, Series F Purchaser and Series G Purchaser below shall also
constitute such party's agreement to the right of first refusal granted in
Section 5.02 hereof and the termination of Sections 5.02 of the Series A
Agreement, the Series B Agreement, the Series C Agreement, the Series E
Agreement , the Series F Agreement and the Series G Agreement.

                      [Signature pages follow immediately.]

                                      24
<PAGE>

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

                                      ADOLOR CORPORATION

                                      By:
                                         -----------------------------
                                      Name:
                                      Title:
[Corporate Seal]

Attest:

---------------------------
Assistant Secretary

                                  PURCHASERS:

                                  MPM BIOVENTURES II, L.P.

                                  By: MPM Asset Management II, L.P.,
                                  its General Partner

                                  By: MPM Asset Management II LLC,
                                  its General Partner

                                  By:
                                      --------------------------------
                                       Name:
                                       Title:

                                  MPM BIOVENTURES II-QP, L.P.

                                  By: MPM Asset Management II, L.P.,
                                  its General Partner

                                  By: MPM Asset Management II LLC,
                                  its General Partner

                                  By:
                                      -------------------------------
                                       Name:
                                       Title:

                                      25
<PAGE>

                                  MPM BIOVENTURES GMBH & CO. PARALLEL-
                                  BETEILIGUNGS KG

                                  By: MPM Asset Management II, L.P.,
                                  its General Partner

                                  By: MPM Asset Management II LLC,
                                  its General Partner

                                  By:
                                      ----------------------------------------
                                       Name:
                                       Title:

                                  ZESIGER CAPITAL GROUP LLC, As Agent and
                                  Attorney-in-fact for City of Milford Pension
                                  & Retirement Fund

                                  By:
                                      ----------------------------------------
                                       Name:
                                       Title:

                                  ZESIGER CAPITAL GROUP LLC, As Agent and
                                  Attorney-in-fact for Norwalk Employees'
                                  Pension Plan

                                  By:
                                      ----------------------------------------
                                       Name:
                                       Title:

                                  ZESIGER CAPITAL GROUP LLC, As Agent and
                                  Attorney-in-fact for Public Employee
                                  Retirement System of Idaho

                                  By:
                                      ----------------------------------------
                                       Name:
                                       Title:

                                  ZESIGER CAPITAL GROUP LLC, As Agent and
                                  Attorney-in-fact for City of Stamford
                                  Firemen's Pension Fund

                                  By:
                                      ----------------------------------------
                                       Name:
                                       Title:

                                      26
<PAGE>

                                  SPECIAL SITUATIONS FUND III LP

                                  By:
                                      ----------------------------------------
                                       Name: David Greenhouse
                                       Title: Managing Director

                                  SPECIAL SITUATIONS CAYMAN FUND L.P.

                                  By:
                                      ----------------------------------------
                                       Name: David Greenhouse
                                       Title: Managing Director

                                  SPECIAL SITUATIONS PRIVATE EQUITY FUND L.P.

                                  By:
                                      ----------------------------------------
                                       Name: David Greenhouse
                                       Title: Managing Director

                                  LOMBARD ODIER & CIE

                                  By:
                                      ----------------------------------------
                                       Name:
                                       Title:

                                  S.R. ONE LIMITED

                                  By:
                                      ----------------------------------------
                                       Name: Brenda D. Gavin
                                       Title: President

                                  WPG ENTERPRISE FUND II, L.L.C.

                                  By: WPG Venture Partners III, L.P., Fund
                                      Investment Advisory Member

                                  By:
                                      ----------------------------------------
                                      Name: Gill Cogan
                                      Title: General Partner

                                      27
<PAGE>

                                  WEISS, PECK & GREER VENTURE ASSOCIATES III,
                                  L.L.C.

                                  By: WPG Venture Partners III, L.P.,
                                  Fund Investment Advisory Member

                                  By:
                                      ----------------------------------------
                                      Name: Gill Cogan
                                      Title: General Partner

                                  ALTA CALIFORNIA PARTNERS, L.P.

                                  By: Alta California Management Partners, LP

                                  By:
                                      ----------------------------------------
                                       Name:
                                       Title: General Partner

                                  ALTA EMBARCADERO PARTNERS, LLC

                                  By:
                                      ----------------------------------------
                                       Name:
                                       Title: Member

                                  By:

                                  By:
                                      ----------------------------------------
                                       Name:
                                       Title:

                                  ARCH VENTURE FUND, III, L.P.,
                                  a Delaware limited partnership

                                  By:  ARCH VENTURE PARTNERS, L.L.C.,
                                  its General Partner

                                  By:
                                      ----------------------------------------
                                       Name:
                                       Title:

                                      28
<PAGE>

                              DLJ CAPITAL CORPORATION

                              By:
                                  ----------------------------------------
                                   Name:
                                   Title:

                                      29<PAGE>

                                                                    Exhibit 4.16

                                 AMENDMENT NO. 6

                                       TO

                          REGISTRATION RIGHTS AGREEMENT

     AMENDMENT NO. 6, dated as of June 29, 2000 by and among Adolor Corporation,
a Delaware corporation (the "Company"), certain holders of the Company's
outstanding securities (collectively, the "Existing Investors") and those
purchasers listed in Schedule I to the Series H Convertible Preferred Stock
Purchase Agreement (the "Purchase Agreement") dated the date hereof
(collectively, including such purchasers who participate in any Additional
Closing (as defined in the Purchase Agreement) and who execute a counterpart to
this Agreement, the "Purchasers").

     WHEREAS, the Company and the Existing Investors are parties to that
Registration Rights Agreement (the "Registration Rights Agreement") by and among
the Company and the parties named therein dated as of the 7th day of November
1994, as amended by Amendment No. 1 to the Registration Rights Agreement dated
as of February 27, 1996, Amendment No. 2 to the Registration Rights Agreement
dated as of May 1, 1997, Amendment No. 3 to the Registration Rights Agreement
dated as of December 8, 1998, Amendment No. 4 to the Registration Rights
Agreement dated as of July 22, 1999, and Amendment No. 5 to the Registration
Rights Agreement dated as of January 10, 2000; and

     WHEREAS, the Purchasers are purchasing from the Company and the Company is
issuing and selling to the Purchasers up to 23,921,569 shares (the "Series H
Shares") of Series H Convertible Preferred Stock, par value $.01 per share,
("Series H Stock") of the Company at the aggregate purchase price of up to
$36,600,000 pursuant to the Purchase Agreement; and

     WHEREAS, it is a condition to the sale of the Series H Shares that the
Registration Rights Agreement be amended to grant the Purchasers certain rights
thereunder, and the parties hereto desire to amend the Registration Rights
Agreement as set forth below.

     NOW, THEREFORE in consideration of the foregoing and the promises and
covenant contained herein, the parties hereby agree as follows:

1.   That Section 1 of the Registration Rights Agreement, as amended to date, be
     and hereby is further amended to delete the definition of "Preferred
     Shares" therein and replace it with the following:

            "Preferred Shares" shall mean shares of the Company's Series A
            Convertible Preferred Stock, par value $.01 per share, Series B
            Convertible Preferred Stock, par value $.01 per share, Series C
            Convertible Preferred Stock, par value $.01 per share, Series E
            Convertible Preferred Stock, par value $.01 per share, Series F
            Convertible Preferred Stock, par value $.01 per share, Series G
            Convertible Preferred Stock, par value $.01 per share and Series H
            Convertible Preferred Stock, par value $.01 per share.
<PAGE>

2.   That Section 4 of the Registration Rights Agreement, as amended to date, be
     and hereby is further amended by inserting the following paragraph
     immediately after the first paragraph designated as 4(a) of said Section 4:

               After the closing of a Series H Qualified Public Offering (as
               defined in the Amended and Restated Certificate of Incorporation
               of Adolor Corporation), if holders of more than 49% of the
               outstanding shares of Series H Convertible Preferred Stock,
               including Common Stock issued on conversion of shares of Series H
               Convertible Preferred Stock, request that the Company register
               all or a portion of the shares of Series H Convertible Preferred
               Stock held by such requesting holder or holders under the
               Securities Act, the Company will use its best efforts to cause
               such shares to be registered if the shares of Series H
               Convertible Preferred Stock for which registration has been
               requested shall have an aggregate offering price to the public of
               not less than $5,000,000.

3.   That Section 6 of the Registration Rights Agreement, as amended to date, be
     and hereby is further amended by deleting Section 6 in its entirety and
     replacing Section 6 with the following paragraph:

               Registration on Form S-3. If at any time (i) a holder or holders
               ------------------------
               of Preferred Shares or Restricted Stock request that the Company
               file a registration statement on Form S-3 or any successor
               thereto for a public offering of all or any portion of the shares
               of Restricted Stock held by such requesting holder or holders,
               the reasonably anticipated aggregate price to the public of which
               would exceed $1,000,000, and (ii) the Company is a registrant
               entitled to use Form S-3 or any successor thereto to register
               such shares, then the Company shall use its best efforts to
               register under the Securities Act on Form S-3 or any successor
               thereto, for public sale in accordance with the method of
               disposition specified in such notice, the number of shares of
               Restricted Stock specified in such notice. Whenever the Company
               is required by this Section 6 to use its best efforts to effect
               the registration of Restricted Stock, each of the procedures and
               requirements of Section 4 (including but not limited to the
               requirement that the Company notify all holders of Restricted
               Stock from whom notice has not been received and the Founder and
               provide them with the opportunity to participate in the offering)
               shall apply to such registration, provided, however, that there
                                                 --------  -------
               shall be no limitation on the number of registrations on Form S-3
               which may be requested and obtained under this Section 6 by the
               holders of the Preferred Shares or Restricted Stock, except that
               the holders of shares of Series H Convertible Preferred Stock or
               Common Stock issued on conversion of shares of Series H
               Convertible Preferred Stock may request no more than two (2)
<PAGE>

               such registrations, and provided, further, however, that the
                                       --------  -------  -------
               requirements contained in the first sentence of Section 4(a)
               shall not apply to any registration on Form S-3 which may be
               requested and obtained under this Section 6.

4.   That Section 13(a) of the Registration Rights Agreement, as amended to
     date, be and hereby is further amended by deleting in its entirety
     paragraph 13(a) of said Section and replacing paragraph 13(a) with the
     following paragraph:

               All covenants and agreements contained in this Agreement by or on
               behalf of any of the parties hereto shall bind and inure to the
               benefit of the respective successors and assigns of the parties
               hereto (including without limitation transferees of any Preferred
               Shares, restricted Stock or Founder Stock), whether so expressed
               or not, provided, however, that registration rights conferred
                       --------  -------
               herein on the holders of Preferred Shares, Restricted Stock, or
               Founder Stock shall only inure to the benefit of a transferee of
               Preferred Shares, Restricted Stock or Founder Stock if:

               (i) there is transferred to such transferee at least 20% of the
               total shares of Restricted Stock or Founder Stock, as the case
               may be, originally issued pursuant to the Purchase Agreement in
               the case of Restricted Stock or originally issued to the Founder
               with respect to the Founder Stock, to the direct or indirect
               transferor of such transferee;

               (ii) such transferee is partner, or retired partner, shareholder,
               member or affiliate of a party hereto (or with respect to the
               Founder Stock, such transferee is an affiliate of the Founder as
               defined in a certain Stock Restriction Agreement between the
               Company and the Founder as of the date hereof);

               (iii) such transferee is a family member or trust for the benefit
               of any individual holder; or

               (iv) such transferee acquires at least 50,000 shares of Series H
               Convertible Preferred Stock or Common Stock issued on conversion
               of shares of Series H Convertible Preferred Stock, provided that
               the Company is given written notice thereof.

5.   That Section 13(d) of the Registration Rights Agreement, as amended to
     date, be and hereby is further amended by inserting the following clause
     after the final word of said Section 13(d):

               provided, further, however, that any of the rights of the holders
               --------  -------  -------
               of the Series H Convertible Preferred Stock hereunder may not be
               amended without the prior written consent of at least fifty
               percent (50%) of the holders of Series H Convertible Preferred
               Stock.
<PAGE>

6.   That Section 13(g) of the Registration Rights Agreement, as amended to
     date, be and hereby is further amended by inserting the following clause
     after the final phrase in said Section 13(g):

               provided, further, however, that the holders of the Series H
               --------  -------  -------
               Convertible Preferred Stock are so limited by the terms of this
               Section 13(g) only pursuant to a request by the underwriters in
               connection with the initial public offering of the securities of
               the Company.

7.   That any Purchasers who have not previously been made parties to the
     Registration Rights Agreement, as amended, shall become parties to the
     Registration Rights Agreement, as amended; and that any Existing Investors
     who have not previously been made parties to the Registration Rights
     Agreement, as amended, shall become parties to the Registration Rights
     Agreement, as amended.

8.   Capitalized terms used but not otherwise defined herein shall have the
     meanings ascribed to them in the Registration Rights Agreement.

9.   In all other respects, the Registration Rights Agreement is hereby
     ratified, confirmed and approved, and all terms thereof shall remain in
     full force and effect.

10.  This Amendment No. 6 may be executed in counterparts, each of which shall
     constitute an original, but all of which, when taken together, shall
     constitute but one agreement.

                      [Signature Pages Follow Immediately]
<PAGE>

         IN WITNESS WHEREOF, the Company, the Existing Investors and the
Purchasers have executed this Amendment No. 6 as of the day and year first
above.

                                       COMPANY:

                                       ADOLOR CORPORATION

                                       By:
                                          ------------------------------------
                                            Peter J. Schied
                                            Chief Financial Officer

                                       EXISTING INVESTORS/PURCHASERS:

                                       ARCH VENTURE FUND III, L.P.

                                            By:   ARCH VENTURE PARTNERS,
                                                  LLC, its General Partner

                                                  By:
                                                      --------------------------
                                                      Robert Nelsen
                                                      Managing Director

                                       ARCH VENTURE FUND II, L.P.

                                            By:   ARCH MANAGEMENT
                                                  PARTNERS II, L.P.,
                                                  its General Partner

                                            By:   ARCH VENTURE
                                                  PARTNERS, L.P.,
                                                  its General Partner

                                            By:   ARCH VENTURE
                                                  CORPORATION
                                                  its General Partner

                                                  By:
                                                     ---------------------------
                                                      Name:
                                                      Title:
<PAGE>

                                        WEISS, PECK & GREER VENTURE ASSOCIATES
                                        III, L.L.C.

                                            By:  WPG Venture Partners III, L.P.,
                                                 Fund Investment Advisory Member

                                            By:
                                                 -------------------------------
                                                 Name:
                                                 Title: General Partner

                                        WPG ENTERPRISE FUND II, L.L.C.

                                            By:  WPG Venture Partners III, L.P.,
                                            Fund Investment Advisory Member

                                            By:
                                                 -------------------------------
                                                 Name:
                                                 Title: General Partner

                                        WPG LIFE SCIENCE ENTREPRENEUR FUND

                                            By:
                                                 -------------------------------
                                                 Name:
                                                 Title:

                                        FALCON TECHNOLOGY PARTNERS, L.P.

                                            By:
                                                 -------------------------------
                                                 James Rathman
                                                 General Partner
<PAGE>

                                      ONE LIBERTY FUND III

                                          By:  Its General Partner, One Liberty
                                               Partners III, L.P.

                                               By:      A General Partner,
                                                        Edwin M. Kania, Jr.

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

                                      GILDE INTERNATIONAL B.V.

                                          By:  Its Attorney-in-fact, One Liberty
                                               Partners III, L.P.

                                          By:  A General Partner,
                                               Edwin M. Kania, Jr.

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

                                      TECHNOLOGY LEADERS II L.P.

                                          By:  Technology Leaders II Management
                                          L.P., the General Partner

                                          By:  Technology Leaders Management,
                                          Inc., a General Partner

                                          By:
                                               ---------------------------------
                                               Managing Director

                                      TECHNOLOGY LEADERS II OFFSHORE C.V.

                                          By:  Technology Leaders II Management
                                          L.P., the General Partner

                                               By:   Technology Leaders
                                               Management, Inc., a General
                                               Partner

                                          By:
                                               ---------------------------------
                                               Managing Director
<PAGE>

                                       SONZ PARTNERS, L.P.

                                            By:
                                                     ---------------------------
                                                     Name:
                                                     Title

                                       SONZ/ADOLOR FUND, L.P.

                                            By:
                                                     ---------------------------
                                                     Name:
                                                     Title

                                       ALTA CALIFORNIA PARTNERS, L.P.

                                                By:  Alta California Management
                                                Partners, L.P.

                                                By:
                                                     ---------------------------
                                                     General Partner

                                       ALTA EMBARCADERO PARTNERS, LLC

                                                By:
                                                     ---------------------------
                                                     Member

                                       ------------------------------------
                                       Theodore Stanley

                                       ------------------------------------
                                       Mary Ann Stanley

                                       THE STANLEY RESEARCH FOUNDATION

                                                By:
                                                     ---------------------------
                                                     Name:
                                                     Title
<PAGE>

                                       S.R. ONE, LIMITED

                                            By:
                                                -------------------------------
                                                Name:
                                                Title:

                                       ------------------------------------
                                       Alan L. Maycock

                                       ------------------------------------
                                       Susan M. Maycock

                                       ------------------------------------
                                       John J. Farrar

                                       ------------------------------------
                                       Janet F. Farrar

                                       TGI FUND II, LC

                                            By:  Tredegar Investments, Inc.,
                                                 its Manager

                                            By:
                                                 -------------------------------
                                                 Charles A. Blanchard
                                                 Vice President

                                       VENGOTT, LC

                                            By:
                                                 -------------------------------
                                                 John D. Gottwald
                                                 Vice President

                                       ------------------------------------
                                       Charles A. Blanchard

                                       ------------------------------------
                                       Henry N. Nassau
<PAGE>

                                       ------------------------------------
                                       Allen Bloom

                                       ONE LIBERTY FUND II

                                          By: Its General Partner

                                              By:   A General Partner, Edward M.
                                              Kania, Jr.

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

                                       DOUGLAS A. ALEXANDER (FAMILY PARTNERSHIP)

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

                                       ---------------------------------------
                                       Kenneth A. Fox

                                       ---------------------------------------
                                       Robert Pollan

                                       DECHERT PRICE & RHOADS

                                          By:
                                              ---------------------------------
                                              Name:
                                              Title:

                                       ---------------------------------------
                                       Roger Richards

                                       ---------------------------------------
                                       John Klopp
<PAGE>

                                       ----------------------------------------
                                       Justin Klein

                                       ----------------------------------------
                                       Samuel W. Morris, Jr.

                                       ----------------------------------------
                                       James A. Lebovitz

                                       EATON VANCE WORLDWIDE HEALTH
                                       SCIENCES FUND

                                            By:
                                               --------------------------------
                                                 Samuel Isaly

                                       HARE & CO. (F/A/O FINSBURY WORLDWIDE
                                       PHARMACEUTICAL TRUST)

                                            By:
                                               --------------------------------
                                                 Samuel Isaly

                                       ----------------------------------------
                                       Walter Buckley

                                       ----------------------------------------
                                       Barton J. Winokur
<PAGE>

                                       PURCHASERS:

                                       MPM BIOVENTURES II, L.P.

                                       By: MPM Asset Management II, L.P.,
                                           its General Partner

                                       By: MPM Asset Management II LLC,
                                           its General Partner

                                       By:
                                           -------------------------------------
                                            Name:
                                            Title:

                                       MPM BIOVENTURES II-QP, L.P.

                                       By: MPM Asset Management II, L.P.,
                                           its General Partner

                                       By: MPM Asset Management II LLC,
                                           its General Partner

                                       By:
                                           -------------------------------------
                                            Name:
                                            Title:

                                       MPM BIOVENTURES GMBH & CO. PARALLEL-
                                       BETEILIGUNGS KG

                                       By: MPM Asset Management II, L.P.,
                                           its General Partner

                                       By: MPM Asset Management II LLC,
                                           its General Partner

                                       By:
                                           -------------------------------------
                                            Name:
                                            Title:

                                       ZESIGER CAPITAL GROUP LLC, As Agent and
                                       Attorney-in-fact for City of Milford
                                       Pension & Retirement Fund

                                       By:
                                           -------------------------------------
                                            Name:
                                            Title:
<PAGE>

                                       ZESIGER CAPITAL GROUP LLC, As Agent and
                                       Attorney-in-fact for Norwalk Employees'
                                       Pension Plan

                                       By:
                                           -------------------------------------
                                            Name:
                                            Title:

                                       ZESIGER CAPITAL GROUP LLC, As Agent and
                                       Attorney-in-fact for Public Employee
                                       Retirement System of Idaho

                                       By:
                                           -------------------------------------
                                            Name:
                                            Title:

                                       ZESIGER CAPITAL GROUP LLC, As Agent and
                                       Attorney-in-fact for City of Stamford
                                       Firemen's Pension Fund

                                       By:
                                           -------------------------------------
                                            Name:
                                            Title:

                                       SPECIAL SITUATIONS FUND III LP

                                       By:
                                           -------------------------------------
                                            Name: Austin Marxe
                                            Title: Managing Director

                                       SPECIAL SITUATIONS CAYMAN FUND L.P.

                                       By:
                                           -------------------------------------
                                            Name: Austin Marxe
                                            Title: Managing Director

                                       SPECIAL SITUATIONS PRIVATE EQUITY FUND
                                       L.P.

                                       By:
                                           -------------------------------------
                                            Name: Austin Marxe
                                            Title: Managing Director
<PAGE>

                                    LOMBARD ODIER & CIE

                                    By:
                                        --------------------------------
                                         Name:
                                         Title:

                                    S.R. ONE LIMITED

                                    By:
                                        ------------------------------------
                                         Name: Brenda D. Gavin
                                         Title: President

                                    WPG ENTERPRISE FUND II, L.L.C.

                                    By: WPG Venture Partners III, L.P.,
                                    Fund Investment Advisory Member

                                    By:
                                        ------------------------------------
                                         Name: Gill Cogan
                                         Title: General Partner

                                    WEISS, PECK & GREER VENTURE
                                    ASSOCIATES III, L.L.C.

                                    By: WPG Venture Partners III, L.P.,
                                    Fund Investment Advisory Member

                                    By:
                                        ------------------------------------
                                         Name: Gill Cogan
                                         Title: General Partner

                                    ALTA CALIFORNIA PARTNERS, L.P.

                                    By: Alta California Management Partners, LP

                                    By:
                                        ------------------------------------
                                         Name:
                                         Title: General Partner
<PAGE>

                                    ALTA EMBARCADERO PARTNERS, LLC

                                    By:
                                        ----------------------------------------
                                         Name:
                                         Title: Member

                                    TGI FUND II, LC

                                    By: Tredegar Investments, Inc., its Manager

                                    By:
                                        ----------------------------------------
                                         Name: Charles A. Blanchard
                                         Title: Vice President

                                    ARCH VENTURE FUND, III, L.P.,
                                    a Delaware limited partnership

                                    By:  ARCH VENTURE PARTNERS, L.L.C.,
                                    its General Partner

                                    By:
                                        ----------------------------------------
                                         Name:
                                         Title:

                                    DLJ CAPITAL CORPORATION

                                    By:
                                        ----------------------------------------
                                         Name:
                                         Title:

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