<PAGE>

                                                                 Exhibit 10.20


                            MASTER SERVICES AGREEMENT

This Master Services Agreement (this "Master Agreement") when signed by both
parties will set forth the terms and conditions between Novirio Pharmaceuticals,
Inc., 125 CambridgePark Drive, Cambridge, MA 02140, USA, a corporation organized
and existing under the laws of the Commonwealth of Massachusetts (hereinafter
"Company"), and Quintiles Scotland Ltd, Research Avenue South, Heriot-Watt
Research Park, Riccarton, Edinburgh, EH14 4AP, a corporation organized and
existing under the laws of England (hereinafter "Quintiles").

                                   BACKGROUND:

A.   Company is in the business of developing, manufacturing and distributing
pharmaceutical products on behalf of Novirio Pharmaceuticals Limited, a
corporation organised and existing under the laws of the Cayman Islands.
Quintiles is in the business of providing research and development services for
the pharmaceutical and biotechnology industry.

B.   Company and Quintiles desire to enter into this Master Agreement to provide
the terms and conditions upon which Company may engage Quintiles from
time-to-time to provide contract research and development services for
individual projects by executing individual Work Orders (as defined below)
specifying the details of the services and the related terms and conditions.

                                   AGREEMENT:

1.   SCOPE OF THE AGREEMENT; SERVICES TO BE PROVIDED; WORK ORDERS.

     (a)  SCOPE OF AGREEMENT. As a "master" form of contract, this Master
          Agreement allows the parties to contract for multiple projects through
          the issuance of multiple Work Orders (as discussed in Section 1(c)
          below). This Master Agreement covers the provision of services by
          Quintiles and, accordingly, this Master Agreement represents the
          Agreement by which Company may contract with Quintiles on a global
          basis for research and development services consisting of
          pharmaceutical sciences, clinical trial supplies and related services,
          requested by Company and agreed by Quintiles.

     (b)  NATURE OF SERVICES. The research and development services covered by
          this Master Agreement include expert consultation, pharmaceutical
          science services and related research and development services
          requested by Company and agreed to by Quintiles as set forth in the
          relevant Work Order (collectively "the Services").

     (c)  WORK ORDERS. The specific details of each project under this Master
          Agreement (each "Project") shall be separately negotiated and
          specified in writing on terms and in a form acceptable to the parties
          (each such writing, a "Work Order"). Each Work Order will include, as
          appropriate, the Project protocol, scope of work,

<PAGE>

          timeline, budget and payment schedule. Each Work Order shall be
          subject to all of the terms and conditions of this Master Agreement,
          in addition to the specific details set forth in the Work Order. To
          the extent any terms or provisions of a Work Order conflict with the
          terms and provisions of this Master Agreement, the terms and
          provisions of this Master Agreement shall control, unless the Work
          Order expressly and specifically states an intent to supersede the
          Master Agreement on a specific matter (but then only with respect to
          the particular Work Order and with respect only to the matter so
          specified).

2.   PAYMENT OF FEES AND EXPENSES. Company will pay Quintiles for fees and
     expenses in accordance with each Work Order. Unless otherwise agreed in a
     particular Work Order, the following shall apply:, (a) the total fees for a
     particular Project will not exceed the budget applicable to such Project
     and attached to the Work Order; (b) Quintiles shall be reimbursed by
     Company for all reasonable and necessary expenses incurred in the
     performance of the Services provided that documentation substantiating the
     expenses for which reimbursement is sought is provided to the Company; and
     (c) Quintiles will invoice Company monthly for the fees and expenses
     relating to the Project. Payment of all undisputed amounts shall be made by
     Company within thirty (30) days of receipt of each monthly itemized
     invoice. If any portion of an invoice is disputed, then Company shall pay
     the undisputed amounts and the parties shall use good faith efforts to
     reconcile (and pay) the disputed amount as soon as practicable. Company
     shall pay Quintiles interest in an amount equal to 1% per month (or the
     maximum lesser amount permitted by law) of all undisputed amounts owing
     hereunder and not paid within 45 days of receipt of the corresponding
     invoice. The currency utilized for invoicing and payments shall be set
     forth in each Work Order and, as a general matter, will reflect the local
     exchange applicable to the country in which the most significant amount of
     work is being performed for the relevant Work Order. If the Project spans
     greater than a calendar year, the parties shall agree to a reasonable
     inflation adjustment mechanism, which will be described in the
     corresponding Work Order.

3.   CHANGE ORDERS; WORK ORDER AMENDMENTS.

     (a)  Any change in the details of a Work Order (including, but not limited
          to, changes in the Project's scope and/or any one or number of the
          duties, responsibilities and tasks undertaken by Quintiles) shall
          require a written Change Order (herein so called) in a form acceptable
          to both parties. Either party may request or initiate a Change Order
          provided that Quintiles shall not commence any work under a Change
          Order without Company's prior written authorisation and Quintiles
          shall accept all Change Orders requested by Company, subject to: (i)
          Quintiles ability to do the work requested; (ii) Quintiles having
          adequate capacity available to carry out the work requested pursuant
          to the Change Order; and (iii) equitable adjustment(s) in the fees and
          expenses payable to Quintiles and/or to the time allotted to complete
          the allotted Work Order as amended pursuant to the Change Order.
          Notwithstanding the foregoing, Quintiles' right to request or initiate
          a Change Order shall be limited to events or circumstances not within
          Quintiles control or not determinable by Quintiles at the time of
          entering into the Work Order or the Change Order sought to be
          modified. Each Change Order shall


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          detail the specific changes to a task, responsibility, duty or
          applicable budget, timeline or other details, requested by the
          initiating party. If the Change Order is initiated by Company,
          Quintiles shall furnish Company with an estimate of the necessary
          changes, if any, upon the applicable budget (whether an increase or
          decrease) and/or timeline associated with implementing the Change
          Order. The Change Order will become effective upon the execution of
          the Change Order by both parties. Quintiles will be given a reasonable
          period of time within which to implement changes in a Change Order.
          Both parties agree to act in good faith and promptly when considering
          a Change Order requested by the other party and will not unreasonably
          withhold approval of a requested Change Order. Without limiting the
          foregoing, (a) Quintiles shall act in good faith when evaluating any
          changes to the budget or timeline associated with a requested Change
          Order, and (b) Company shall act in good faith and shall not
          unreasonably withhold approval of reasonable changes requested by
          Quintiles in fees, costs, expenses or the timeline resulting from,
          among other appropriate reasons, forces outside the reasonable control
          of Quintiles and changes in the assumptions on which the initial
          budget was based resulting from information or the occurrence of
          events not determinable at the time the initial or subsequently agreed
          to budget was prepared. Quintiles reserves the right to postpone
          effecting material changes in the Project's scope until such time as
          the parties agree to and execute the corresponding Change Order.

     (b)  In lieu of a Change Order, a change in a Work Order may be evidenced
          by an amendment to the relevant Work Order duly executed by Company
          and Quintiles in form acceptable to both parties.

4.   CONFIDENTIALITY. WHEN USED IN THIS MASTER AGREEMENT THE TERM
     "REPRESENTATIVE" SHALL MEAN THE RESPECTIVE EMPLOYEES, OFFICERS, DIRECTORS,
     REPRESENTATIVES OR AGENTS OF QUINTILES OR THE COMPANY AS THE CASE MAY BE.
     It is understood that during the course of this Master Agreement, Quintiles
     and its Representatives may be exposed to trade secrets, know how, data and
     information which is confidential and proprietary to Company. All such
     trade secrets, know how, data and information (hereinafter "Company
     Confidential Information") written, verbal, magnetic, optical or in other
     form tangible or intangible, made available, disclosed, or otherwise made
     known to Quintiles and its Representatives or derived from such information
     by Quintiles or its Representatives as a result of services under this
     Master Agreement shall be considered confidential and shall be considered
     the sole property of Company. All information regarding Quintiles'
     operations, including but not limited to Quintiles' Property (as defined in
     Section 5.0 below), disclosed by Quintiles to Company in connection with
     this Master Agreement is proprietary, confidential information belonging to
     Quintiles (the "Quintiles Confidential Information", and together with the
     Company Confidential Information, the "Confidential Information"). The
     Confidential Information shall be used by the receiving party and its
     Representatives only for purposes of performing the receiving party's
     obligations hereunder. Each party agrees that it will not reveal, publish
     or otherwise disclose the Confidential Information of the other party to
     any third party including its Representatives, unless such persons have
     entered into appropriate confidentiality


                                      -3-
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     agreements enforceable by the disclosing party or the prior written consent
     of the disclosing party has been obtained. The foregoing obligations shall
     not apply to Confidential Information which:

     (a)  is or becomes generally available to the public other than as a result
          of a disclosure by the receiving party;

     (b)  becomes available to the receiving party on a non-confidential basis
          from a source which is not prohibited from disclosing such information
          by a legal, contractual or fiduciary obligation to the disclosing
          party;

     (c)  the receiving party develops independently of any disclosure by the
          disclosing party;

     (d)  was in the receiving party's possession or known to the receiving
          party prior to its receipt from the disclosing party as shown by
          contemporaneous written evidence; or

     (e)  is required by law or judicial order to be disclosed or is required to
          be disclosed in connection with the defence or prosecution of
          litigation, provided, that the receiving party has given to the
          disclosing party prior written notice of such disclosure and the
          receiving party takes all reasonable actions to avoid or minimize the
          disclosure of Confidential Information.

     This obligation of confidentiality and non-disclosure shall remain in
     effect for a period of ten (10) years after the termination of this Master
     Agreement.

5.   OWNERSHIP AND INVENTIONS. All data and information necessary for Quintiles
     to conduct project assignments will be forwarded by Company to Quintiles.
     All data and information generated or derived by Quintiles as the result of
     services performed by Quintiles under this Master Agreement shall be and
     remain the exclusive property of Company. Any inventions that may evolve
     from the data and information described above or as the result of services
     performed by Quintiles under this Master Agreement shall belong to Company
     and Quintiles agrees to assign its rights in all such inventions and/or
     related patents to Company. Quintiles agrees to use ALL REASONABLE EFFORTS
     to cause its Representatives, at COMPANY'S cost and expense, to cooperate
     with COMPANY and deliver any documents, instruments or agreements useful or
     necessary to evidence the assignment of and to facilitate the prosecution
     of related patents. Notwithstanding the foregoing, Company acknowledges
     that Quintiles possesses certain inventions, processes, know-how, trade
     secrets, improvements, other intellectual properties and other assets,
     including but not limited to analytical methods, procedures and techniques,
     computer technical expertise and software, which have been independently
     developed by Quintiles (collectively "Quintiles' Property"). Company and
     Quintiles agree that any Quintiles' Property or improvements thereto which
     are used, improved, modified or developed by Quintiles under or during the
     term of this Master Agreement are the sole and exclusive property of
     Quintiles.


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6.   INDEPENDENT CONTRACTOR RELATIONSHIP. For the purposes of this Master
     Agreement, the parties hereto are independent contractors and nothing
     contained in this Master Agreement shall be construed to place them in the
     relationship of partners, principal and agent, employer/employee or joint
     venturers. Quintiles agrees that it shall have no power or right to bind or
     obligate Company, nor shall Quintiles hold itself out as having such
     authority.

7.   REGULATORY COMPLIANCE. In carrying out its responsibilities under this
     Master Agreement and each Work Order, Quintiles agrees its services will be
     conducted in compliance with all applicable laws, rules and regulations,
     including the U.S. Food, Drug and Cosmetic Act and the regulations
     promulgated pursuant thereto or any equivalent laws pertaining to
     jurisdictions in which the Services are provided, and with the standards of
     care customary in the contract research organization industry. Company
     warrants that neither any assignment or task requested by Company nor the
     conduct thereof as provided in this Master Agreement or in any Work Order
     shall violate any applicable law or regulation of which Company has or
     should have knowledge. Company shall notify Quintiles promptly in writing
     of any governmental or regulatory inspection or inquiry concerning any
     services being rendered by Quintiles or any study or Project to which such
     services relate.

     If any governmental or regulatory authority conducts or gives notice to
     Quintiles of its intent to conduct an inspection at any investigational
     site or to take any other regulatory action with respect to any study or
     Project or services provided under this Master Agreement, Quintiles will
     promptly give Company notice thereof, including all information pertinent
     thereto. Quintiles shall permit a representative of COMPANY to review any
     materials and other information submitted or made available to any
     investigating authority and TO USE ALL REASONABLE EFFORTS TO ALLOW SUCH
     REPRESENTATIVE to be present at an investigational site inspection. Company
     acknowledges that Company may not direct the manner in which Quintiles
     fulfills its obligations to permit inspection by governmental entities. It
     shall not be a breach of this Master Agreement for Quintiles to comply with
     the demands and requests of any governmental entity in accordance with
     Quintiles' reasonable judgement or to fail to inform and consult with
     Company before complying with any such demand or request if notification is
     not practicable.

8.   RELATIONSHIP WITH INVESTIGATORS. If a particular Work Order obligates
     Quintiles to contract with investigators or investigative sites
     (collectively, "Investigators") or facilitate Company's contracting with
     Investigators (or other independent contractors such as central
     laboratories), then any such contract shall be on a form mutually
     acceptable to Quintiles and Company, which contract may include, without
     limitation, provisions addressing the specific duties and standards of the
     parties, confidentiality, indemnification, ownership of property and patent
     rights, and insurance coverage. Company shall be responsible to promptly
     review, comment on and/or approve such form contracts. Company acknowledges
     that (a) an Investigator engaged for a particular Project shall be solely
     responsible for his or her (or its) own independent medical judgment and
     his or her (or its) acts and omissions in performing the clinical
     investigation and related services, and (b) Quintiles shall have no
     responsibility


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     whatsoever for the acts and omissions of any such Investigator; rather,
     Quintiles' sole responsibility with respect to any such Investigator shall
     be those responsibilities specifically set forth in the applicable Work
     Order.

9.   CONFLICTING AGREEMENTS. Quintiles represents and warrants to Company that
     it is not a party to any agreement which would prevent it from fulfilling
     its obligations under this Master Agreement and that during the term of
     this Master Agreement, Quintiles will not enter into an agreement to
     provide services which would in any way prevent it from providing the
     services contemplated under this Master Agreement.

10.  PUBLICATION. From time to time it may be in the mutual interest of
     Quintiles and Company to publish articles relating to services performed as
     a part of this Master Agreement. Publication of project assignment results
     in whole or in part, shall be within the sole and absolute discretion of
     Company. Results may not be presented or submitted for publication or
     referred to in any publication in whole or in part, without the prior
     express written consent of Company. Company reserves the unqualified right
     to reject any article utilizing any data generated from Quintiles' services
     under this Master Agreement before such article is presented or submitted
     for publication. Company will not use Quintiles' name in connection with
     any publication or promotion without Quintiles' prior written consent.

11.  RETURN OR STORAGE OF COMPANY'S MATERIALS. At the completion of services by
     Quintiles or the earlier termination of this Master Agreement all materials
     and other data and all copies and reproductions thereof in whole or in part
     provided by the Company for the conduct of services, produced as a result
     of the services provided under this Master Agreement or otherwise owned by
     Company, regardless of the method of storage or retrieval, shall at the
     direction of the Company be (a) delivered to Company in such form as is
     then currently in the possession of Quintiles, subject to the payment
     obligations set forth in Sections 2 and 13 herein, (b) retained by
     Quintiles for Company for a period of two years, or (c) disposed of unless
     such materials are otherwise required to be stored or maintained by
     Quintiles as a matter of law or regulation. The costs associated with
     storage, delivery OR DISPOSAL of materials or other data at the Company's
     direction shall be paid by Company.

12.  INDEMNIFICATION AND LIABILITY LIMITS

     (a)  Company shall indemnify, defend and hold harmless Quintiles and its
          corporate affiliates and its and their directors, officers and
          employees (each a "Quintiles Indemnified Party") from and against any
          and all actual losses, damages, liabilities, reasonable legal fees and
          expenses, (collectively, "Quintiles Losses"), resulting or arising
          from third party claims, actions, proceedings `or litigation arising
          from or in connection with any such Quintiles Indemnified Party's
          performance of this Master Agreement, any Work Order or the Services
          contemplated herein (including without limitation any product
          liability claim) save to the extent such losses are determined to have
          resulted solely from the negligence or intentional misconduct of the
          Quintiles Indemnified Party seeking indemnity hereunder.


                                      -6-

<PAGE>

     (b)  Quintiles shall indemnify, defend and hold harmless Company and its
          corporate affiliates and its and their directors, officers and
          employees (each a "Company Indemnified Party") from and against any
          and all actual losses, damages, liabilities, reasonable legal fees and
          expenses (collectively, "Company Losses") resulting or arising from
          any action, proceedings, or litigation to the extent that such Company
          Losses are determined to have resulted solely from the negligence or
          intentional misconduct of a Quintiles Indemnified Party.

     (c)  The party seeking indemnification hereunder (the "Indemnified Parry")
          shall (a) promptly notify the party obligated to indemnify (the
          "Indemnifying Party") of any claim, action, proceeding for which the
          Indemnified Party seeks indemnification; (b) cooperate fully with the
          Indemnifying Party and its legal representatives in the investigation
          of any claim, action or proceedings; and (c) shall not unreasonably
          withhold its approval of the settlement of any claim, action or
          proceeding by Indemnifying Party covered by this indemnification
          provision. The Indemnifying Party's failure to comply with its
          obligations under this provision shall not constitute a breach of this
          Master Agreement nor relieve the Indemnifying Party of its
          indemnification obligations hereunder, except to the extent, if any,
          that the Indemnifying Party's defense or settlement of the affected
          claim, action or proceeding was actually and materially impaired
          thereby.

     (d)  Neither party, nor its affiliates, nor any of its or their respective
          Representatives shall have any liability for any special, incidental,
          or consequential damages, including, but not limited to the loss of
          opportunity, loss of the use of any data or information supplied
          hereunder, or loss of revenue or profit, in connection with or arising
          out of this Master Agreement, any Work Order, the Services performed
          by Quintiles hereunder or the existence, furnishing, functioning, or
          Company's use of any information, documentation or Services provided
          pursuant to this Master Agreement or any Work Order, even if the other
          party shall have been advised of the possibility of such damages. In
          addition, except in instances of gross negligence or intentional
          misconduct, the collective, aggregate liability of Quintiles and its
          affiliates and its and their respective directors, officers, employees
          and agents under this Master Agreement or any Work Order shall not
          exceed the aggregate amount of compensation specified in the Work
          Order affected by the incident in question.

13.  TERM AND TERMINATION. This Master Agreement shall commence on the date of
     execution and shall continue until terminated as hereinafter provided by
     either party. This Master Agreement may be terminated without cause by
     Company or by Quintiles at any time during the term of the Master Agreement
     on ninety (90) days prior written notice to Quintiles or Company, as
     appropriate. All Work Orders in effect shall terminate on the effective
     date of the termination of this Master Agreement. An individual Work Order
     under this Master Agreement may be terminated without cause by Company or
     by Quintiles at any time during the term of the Master Agreement on sixty
     (60) days prior written notice to Quintiles or Company, as appropriate.


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     In the event this Master Agreement or Work Order is terminated, Company's
     sole obligation to Quintiles with respect to the impacted Project(s) shall
     be (a) to pay to Quintiles any fees for Services rendered then due and
     owing to Quintiles because of any completed performance of Quintiles'
     obligations, and (b) to pay all actual costs to complete activities
     associated with the termination and close out of Projects, and (c) to pay
     all additional costs incurred that are required to fulfill regulatory
     requirements, provided that Company shall not pay any such costs incurred
     after ninety (90) days from the date of written notice of termination,
     unless approved in writing by Company. Should Quintiles have a wish to
     terminate, any work already agreed to should continue to be carried out
     during the 60 day or 90 day notice period, with all due effort and at the
     expected rate of progress, unless Company prefer to waive part or all of
     this. Upon the termination of this Master Agreement or any Work Order,
     Quintiles shall deliver to Company all data and materials provided by
     Company to Quintiles for the conduct of Services under the impacted
     Project(s). All statistical data, all statistical reports, all data entries
     and any other documentation produced as the result of Services performed by
     Quintiles under the impacted Project(s) shall be delivered to Company at
     such time as payment has been made to Quintiles for all services performed.

     In the event this Master Agreement or any Work Order is terminated,
     Quintiles reserves the right to retain solely for regulatory purpose and
     purposes of evidencing performance of obligations arising under this Master
     Agreement one copy of all materials provided to Company as the result of
     Services performed by Quintiles under the impacted Project(s) for a period
     of TEN (10) years which will remain sealed unless required to be disclosed
     for regulatory purposes or in connection with a dispute regarding the
     Services performed by Quintiles hereunder.

14.  RELATIONSHIP WITH AFFILIATES.

     (a)  Company agrees that Quintiles may utilize the Services of its
          corporate affiliates to fulfill Quintiles' obligations under this
          Master Agreement and any Work Order provided that Quintiles shall
          remain responsible and obligated under this Master Agreement and any
          such Work Order as if Quintiles was directly performing such services.
          Any affiliate so utilized shall be (i) subject to all of the terms and
          conditions applicable to Quintiles under this Master Agreement or any
          Work Order, including, but not limited to, provisions establishing the
          standards for performance, and (ii) entitled to all rights and
          protections afforded Quintiles under this Master Agreement and any
          Work Order, including, but not limited to, the indemnity and
          limitation of liability protections set forth herein. Any such
          affiliate of Quintiles may execute a Work Order directly and, with
          respect to the corresponding Project, the rights and obligations of
          the parties shall be governed by all of the terms and conditions of
          this Master Agreement, to the same extent as if such affiliate was a
          party to this Master Agreement.

     (b)  Company and Quintiles acknowledge that certain affiliates of Company
          may utilize the services of Quintiles (and its affiliates) under this
          Master Agreement and under any Work Order. In such event, (i) Company
          shall cause such affiliate to acknowledge and be bound by all the
          terms and conditions of this Master


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          Agreement and any Work Order, and (ii) Company shall remain
          responsible and obligated under this Master Agreement and any Work
          Order, as if Company was directly receiving the services provided to
          such affiliate. Any such affiliate of Company may execute a Work Order
          directly and, with respect to the corresponding Project, the rights
          and obligations of the parties shall be governed by all of the terms
          and conditions of this Master Agreement, to the same extent as if such
          affiliate was a party to this Master Agreement.

     (c)  When used in this Agreement, the term `"affiliate" shall mean all
          entities controlling, controlled by or under common control with
          Company or Quintiles, as the case may be. When used herein, the term
          "control" shall mean the ability to vote fifty percent (50%) or more
          of the voting securities of any entity.

15.  COOPERATION. All data and information in Company's possession or control
     necessary for Quintiles to conduct Project assignments will be forwarded by
     Company to Quintiles. Quintiles shall not be liable to Company nor be
     deemed to have breached this Master Agreement or any Work Order for errors,
     delays or other consequences arising from Company's failure to provide
     documents, materials or information or to otherwise cooperate with
     Quintiles in order for Quintiles to timely and properly perform its
     obligations.

16.  FORCE MAJEURE. In the event either Party shall be delayed or hindered in or
     prevented from the performance of any act required, hereunder by reasons of
     strike, lockouts, labor troubles, inability to procure materials, failure
     of power or restrictive government or judicial orders, or decrees, riots,
     insurrection, war, Acts of God, inclement weather or other similar reason
     or cause beyond its reasonable control, then performance of such act shall
     be excused for the period of such delay.

17.  REVIEW OF WORK. During the term of this Master Agreement, Quintiles will
     permit Company's representative(s) (unless such representatives are
     competitors of Quintiles) to examine the work performed hereunder and the
     facilities at which the work is conducted at reasonable times and in a
     reasonable manner to determine that the project assignment is being
     conducted in accordance with the agreed task and that the facilities are
     adequate.

18.  NOTICES. Any notice required or permitted to be given hereunder by either
     party hereunder shall be in writing and shall be sufficiently given if
     delivered personally or sent by registered or certified mail, return
     receipt requested, postage prepaid or by an internationally recognized
     overnight delivery service to the following addresses:

     If to Quintiles:          Gregory Porter
                               Quintiles Transnational Legal Department
                               P.O. Box 13979
                               Research Triangle Park, North Carolina 27709-3979
                               Fax: (919) 941-2090
                               Tel: (919) 998 2000


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<PAGE>

     If to Company:            Martin L. Bryant, M.D., Ph.D.
                               Executive Vice President, Research & Development
                               Novirio Pharmaceuticals, Inc.
                               125 CambridgePark Drive
                               Cambridge, MA  02140
                               Tel: (617) 250-3100
                               Fax: (6170 250-3101

19.  ASSIGNMENT. Except as stated above in Section 14, neither party may assign
     any of its rights or delegate its obligations under this Master Agreement
     to any party without the express, written consent of the other party.

20.  GOVERNING LAW. This Master Agreement shall be construed, governed,
     interpreted, and applied in accordance with the laws of the country with
     the most significant relationship with the act or omission giving rise to
     the dispute. If any one or more provisions of this Master Agreement shall
     be found to be illegal or unenforceable in any respect, the validity,
     legality and enforceability of the remaining provisions shall not in any
     way be affected or impaired thereby.

21.  SURVIVAL. The rights and obligations of Company and Quintiles, which by
     intent or meaning have validity beyond such termination (including, but not
     limited to, rights with respect to product-related inventions,
     confidentiality, discoveries and improvements, mutual indemnification and
     liability limitations) shall survive the termination of this Master
     Agreement or any Work Order.

22.  ENTIRE AGREEMENT AND MODIFICATION. This Master Agreement contains the
     entire understandings of the parties with respect to the subject matter
     herein, and supersedes all previous agreements (oral and written),
     negotiations and discussions. Notwithstanding the forgoing, the execution
     and delivery of this Master Agreement shall have no effect on the General
     Consulting Agreement between the COMPANY and Quintiles, Inc., a North
     Carolina corporation, dated March 1, 1999, which agreement shall remain
     valid and binding in all respects The parties, from time to time during the
     term of this Master Agreement, may modify any of the provisions hereof only
     by an instrument in writing duly executed by the parties.

23.  INJUNCTIVE RELIEF; SPECIFIC PERFORMANCE. In addition to all other remedies
     available to the parties hereto at law or in equity, each party shall be
     entitled to equitable relief, including injunction and specific performance
     in the event of any breach or threatened breach of any provisions of this
     Master Agreement.

IN WITNESS WHEREOF, this Master Agreement has been executed by the parties
hereto through their duly authorized officers on the date(s) set forth below.

QUINTILES:                                      Company
By: /s/ P. Woodhead                             By: /s/ Martin L. Bryant
   ______________________________                  _____________________________

Name: Dr. Philip J. Woodhead                    Name: Martin L. Bryant
      ___________________________                     __________________________

Title: Director                                 Title: Exec. VP R & D
        ________________________                       _________________________

Date: 27 May 99                                  Date: 27 May 99
      ___________________________                    ___________________________



<PAGE>

                                  ATTACHMENT 1

                                   WORK ORDER
                             QUINTILES STUDY NUMBER:

                          BETWEEN QUINTILES AND COMPANY

TO CONDUCT THE FOLLOWING:
BRIEF DESCRIPTION, according to the agreed study protocol.

PRICE:

         QUINTILES TOTAL PRICE                        (POUND) PRICE

PAYMENT SCHEDULE:

         On initiation                                (pound)
         Monthly installments                         (pound)
         On issue of draft final report               (pound)


SPECIAL CONDITIONS

Any meeting, shipping or travel costs incurred outwith the confines of
Quintiles' facilities in connection with the performance of this project will be
invoiced in addition to the above.

Work will be carried out in accordance with the Terms and Conditions as
specified in the Master Services Agreement (MSA00*01) between Quintiles and
Company, originally executed on [DATE] unless otherwise agreed in writing.

SIGNED ON BEHALF OF                            SIGNED ON BEHALF OF
QUINTILES SCOTLAND LIMITED                     NOVIRIO PHARMACEUTICALS, INC



---------------------------------              --------------------------------
DR. PHILIP J. WOODHEAD
DIRECTOR

DATE:                                          DATE:
QUINTILES
EDINBURGH, SCOTLAND