<PAGE>
                                                                    Exhibit 10.3

NOVIRIO LEASE
AUGUST 21, 2001


                                 LEASE AGREEMENT

1.    PARTIES

This Lease Agreement ("Lease") executed as of this 22 day of August, 2001 by and
between West Cambridge Science Park, LLC, a Massachusetts limited liability
corporation having a mailing address of 38 Pequossette Road, Belmont,
Massachusetts 02478-3459 ("Lessor"), which expression shall include its
successors, and assigns where the context so admits, and Novirio
Pharmaceuticals, Inc., a Massachusetts corporation having a mailing address of
125 CambridgePark Drive, Cambridge, Massachusetts 02140 ("Lessee"), which
expression shall include its successors and assigns where the context so admits.

2.    LEASED PREMISES

Lessor hereby leases to Lessee and Lessee hereby leases from Lessor that portion
of the building (the "Building") located at 763E Concord Avenue, Cambridge,
Massachusetts (the "Property") totaling approximately six thousand seven hundred
seventy-one (6,771) gross square feet ("GSF") shown on Exhibit "A" attached
hereto, together with fourteen (14) parking spaces shown on Exhibit "B",
(together referred to as the "Leased Premises") and the right to use in common
with others the ways to access said Leased Premises. The Property is part of the
development known as West Cambridge Science Park ("West Cambridge Science
Park").

3.    TERM

The term of this Lease (the "Term" or "Lease Term") shall be for three (3) years
commencing on July 1, 2002 (the "Commencement Date") and terminating on June 30,
2005. In the event that Lessor terminates for any reason its existing Lease with
Zycos, Inc., for the Leased Premises (Lessee being Zycos, Inc.'s current
subtenant), it is agreed that Lessor and Lessee shall execute an amendment to
this Lease providing that the Commencement Date of this Lease shall become the
date of the termination of Zycos, Inc.'s said Lease, with the termination date
of this Lease (June 30, 2005) being unaffected.

Lessee shall have the option to terminate this Lease at midnight on June 30,
2004 (the "Early Termination Date") under the following conditions:

      (a)   In order to exercise said option of early termination, Lessee shall
            give Lessor written notice of termination on or before April 30,
            2004.

      (b)   At the time Lessee notifies Lessor that Lessee is exercising its
            option for an early termination of this Lease, Lessee shall pay to
            Lessor by certified check or checks the sum of One Hundred
            Twenty-Five Thousand and 00/100 Dollars ($125,000.00) as
            consideration for such early termination, (the "Early Termination
            Fee"). If Lessee does not duly pay the Early Termination Fee as
            herein provided, then Lessee's attempted exercise of its option to
            terminate this Lease on the Early Termination Date shall be void and
            of no force or effect.
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NOVIRIO LEASE
AUGUST 21, 2001


      (c)   It is stipulated and agreed that the security deposit (as provided
            in Section 7 hereof) may not be used (in whole or in part) to
            satisfy Lessee's obligation to pay the Early Termination Fee.

4.    RENT

Base Rent for the first twelve months of the Lease Term shall be at the rate of
Two Hundred Twenty-Five Thousand Two Hundred Twelve and 25/100 Dollars
($225,212.25) plus Ten Thousand Eighty and 00/100 Dollars ($10,080.00) on
account of the fourteen (14) parking spaces described in Section 2 hereof, which
Base Rent (excluding, however, the $10,080 payable on account of the parking
spaces) shall be subject to annual escalation as hereinafter provided.

Each year on the anniversary of the Commencement Date, the Base Rent shall be
increased by a percentage equal to the greater of (a) 3.5% or (b) the percentage
increase in the Consumer Price Index, All-Items, for All Urban Consumers (CPI-U)
(1982-84 = 100), as published by the Bureau of Labor Statistics of the U.S.
Department of Labor (the "Index"), over the immediately preceding twelve month
period ending with the most recent calendar quarter for which said Index has
been published; provided, however, that if said Index shall no longer be
published, then the index reflecting the rate of inflation in the economy which
is the most comparable thereto shall be used in lieu thereof.

Base Rent for the Lease Term shall be payable in equal monthly installments,
with the first twelve such installments to be in the amount of $19,607.69 each,
with each such installment to be due and payable in advance on the first day of
each month commencing on the Commencement Date; provided, however, that if the
Commencement Date does not fall on the first day of the month, Lessee shall pay,
in advance on such date, the Base Rent pro-rated for the number of days then
remaining in the month.

5.    ADDITIONAL RENT

As "Additional Rent", Lessee shall pay to Lessor its Proportionate Share (as
hereinafter defined) of operating expenses related to the Property incurred by
Lessor, including, but not limited to, real estate taxes, insurance, building
and parking area maintenance and repair, water and sewer, back-flow prevention
testing, grounds maintenance and landscaping, snowplowing, sanding and ice
removal and the shuttle bus or van service to be provided pursuant to the
Parking and Transportation Demand Management Plan for West Cambridge Science
Park (see also Section 30 hereof).

Lessee's said Proportionate Share shall be as follows:

      (a)   For real estate taxes, insurance, building and parking maintenance
            and repair, grounds maintenance and landscaping, common areas
            operational and maintenance expenses, Lessee's Proportionate Share
            shall be that percentage which is equal to the ratio of the Leased
            Premises GSF to the total GSF of the Building, which is thirty-three
            and 9/100's percent (33.9%).


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NOVIRIO LEASE
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            Lessee shall pay one-twelfth (1/12) of the estimated annual amounts
            of such operating costs on the first day of each month. The amounts
            paid shall be adjusted annually when the actual amounts are known.

            The current Lessee's Proportionate Share charges, as currently
            estimated for calendar year 2002,* are as follows:

<TABLE>
<S>                                                      <C>
            Real Estate Taxes (Building & Parking)       $    22,437.72
            Insurance                                    $    6,155.00
            Common Electric-Parking                      $    914.87
            Grounds Maintenance                          $    1,963.90
                                                         -------------
            TOTAL                                        $    31,471.49 per year
                                                         $    2,622.62 per month
</TABLE>

      (b)   For water and sewer which shall be separately metered, Lessee's
            Proportionate Share shall be one hundred percent (100%) of the
            expenses reflected on a separate meter.

      (c)   For gas and electric consumption (which shall be separately metered
            to service the Leased Premises) and back-flow prevention maintenance
            and testing, Lessee's Proportionate Share shall be one hundred
            percent (100%).

      (d)   Lessee shall be responsible for all of Lessee's operational
            expenses, including Lessee's equipment repair and maintenance and
            Lessee's own utilities such as gas and electric as provided in
            Paragraphs 8 and 9 hereof.

      (e)   Lessee shall pay its Proportionate Share of thirty-three and 9/100's
            percent (33.9%) of snowplowing and sanding expenses of areas of the
            parking areas dedicated to the Building, including accesses to the
            Building's loading dock and dumpster locations within ten days of
            the submission of invoices related thereto.

      (f)   For shuttle bus or van service to be provided pursuant to the
            Parking and Transportation Demand Management Plan for West Cambridge
            Science Park, Lessee's Proportionate Share shall be the lesser of
            twelve per cent (12%) or $100.00 per month during the first two
            years of the Lease Term and the lesser of twelve percent (12%) or
            $150.00 per month during the remainder of the Lease Term.

      (g)   Lessor shall not charge Lessee for management fees or capital
            replacement.

      (h)   Notwithstanding the provisions of Section 5(a) to the contrary, any
            increase in Additional Rent solely on account of grounds maintenance
            and landscaping, building and parking area maintenance and repair,
            snowplowing, sanding and ice removal in any given year shall be
            limited to no more than five percent (5%).


----------

*These charges are estimates based on currently available information and, as
such, are subject to adjustment once the actual charges are known.


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NOVIRIO LEASE
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6.    RENT DUE DATE

Rent for the Term of the Lease shall be due and payable monthly in advance on
the first day of each month. All rent shall be paid without offset or reduction.
In the event Lessee fails to make payments of rent on the due date, then Lessee,
without prior notice from Lessor, shall be charged and pay interest on said
unpaid rent at the rate of the Wall Street Journal Prime Rate plus four (4)
percent during the time the arrearage continues. In the event Lessee fails to
make such payment within three (3) days of the due date, then Lessee, without
prior notice from Lessor, shall be charged and pay a late charge fee equal to
one and one-half percent (1.5%) of the amount due for each month or portion
thereof.

7.    SECURITY DEPOSIT

Upon the execution of this Lease, Lessee shall pay to the Lessor the amount of
Fifty Thousand and 00/100 Dollars ($50,000.00) which shall be added to the Five
Thousand and 00/100 Dollars ($5,000.00) already being held by Lessor so as to
total Fifty-Five Thousand and 00/100 Dollars ($55,000.00). This amount, together
with interest thereon at the rate of two percent (2%) per annum which shall be
credited by Lessor to Lessee, shall be held as a security deposit hereunder. In
lieu of said cash security deposit, Lessee, at Lessee's option, may deliver to
Lessor at the time this Lease is executed an unconditional, irrevocable Letter
of Credit in the amount of Seventy-Five Thousand and 00/100 Dollars (U.S.
$75,000.00) naming Lessor as beneficiary, which Letter of Credit shall be issued
by a United States bank approved by Lessor and shall otherwise be in form
acceptable to Lessor. Said Letter of Credit shall have a term at least equal in
length to the Lease Term or if for a term shorter than the Lease Term, shall
provide that if the Letter of Credit is not extended or renewed at least
forty-five (45) days prior to its then expiration date, Lessor shall have the
unconditional right to draw upon it in full. Said Letter of Credit shall
constitute the security deposit and may be drawn upon either by Lessor's
certification that Lessor is entitled to the security deposit under the terms of
this Section 7 or in the event of a failure to extend or renew it as specified
in the immediately preceding sentence.

The security deposit shall be held as a security for the due performance of each
and every obligation of Lessee as in this Lease provided. Lessor may commingle
the Security Deposit in one or more bank accounts with other funds of Lessor.
Lessor may use the Security Deposit to cure any default by Lessee under the
terms of this Lease, and Lessee shall immediately pay to Lessor on demand, as
Additional Rent, the amount so expended and any additional amount as is required
to cause the security deposit at all times to equal the amount set forth above.
Lessor shall assign the security deposit to any successor owner of the Building,
and thereafter Lessor shall have no further responsibility therefor or liability
on account thereof. Upon the expiration (or any earlier termination) of the
Lease Term, Lessor shall inspect the Premises, make such deductions from the
security deposit as Lessor reasonably estimates may be required to cure any
default by Lessee under the terms of this Lease, and pay the balance of the
security deposit, if any, to Lessee within thirty (30) days of such expiration
or termination. The security deposit may not be used by Lessee in lieu of rent
payment(s) including, but not limited to, the last months' Base Rent payments or
the payment of any Additional Rent.

8.    OPERATING COSTS


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NOVIRIO LEASE
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Except as otherwise provided in Paragraph 9 hereof, Lessee shall pay all costs
and expenses of the operation and management of the Leased Premises and for
repairs and replacement, cleaning and maintenance of the Leased Premises
including, without limitation, the following: (i) the cost of electrical energy
for the operation of any machine, appliance or device used in the operation of
the Leased Premises; (ii) the cost of all utilities, whether they are used for
furnishing heat, air conditioning or other purposes; (iii) all bills for fuel
furnished to the Leased Premises; (iv) the maintenance and repair of the
neutralization tank, including removal, disposal and replacement of its chips,
subject to Paragraph 29 hereof; (v) trash disposal and lawful and proper
disposal of any chemicals or refuse: (vi) window cleaning and janitorial
services, including janitorial equipment and supplies; (vii) the material and
labor for cleaning the Leased Premises and sidewalk areas adjacent to the Leased
Premises, including removing ice, snow and debris from such areas. The cost of
utilities shall be as reflected on separate meters for the Leased Premises.

9.    MAINTENANCE

Lessee covenants throughout the Term of this Lease, at its sole cost and
expense, to keep and maintain the Leased Premises and all fixtures, equipment
and systems therein, including all plumbing, backflow preventers, neutralization
tank(s) (subject to Paragraph 29 hereof), sprinkler, heating, air conditioning,
electrical and gas fixtures, systems and equipment, all ceiling, doors and door
frames, window glass and window frames, carpeting and flooring, inside walls of
the Leased Premises, in good repair and condition, subject to ordinary wear and
tear and damage by casualty (subject to Paragraph 19 hereof), making all repairs
thereto as may be required. Lessor will deliver the Leased Premises in good
order on the Commencement Date, subject, however, to ordinary wear and tear by
the current tenant Zycos, Inc. and by Lessee, as Zycos, Inc.'s subtenant. With
respect to the heating, ventilating and air conditioning ("HVAC") systems and
equipment, Lessee shall obtain and maintain in force during the Term of this
Lease a maintenance contract with a reputable HVAC service contractor so as to
insure that the HVAC systems and equipment are maintained on a regular,
preventive basis, including, but not limited to, regular changes of filters and
belts, and regular preventive maintenance checks of refrigerant and pumps.
Lessee shall not permit the Leased Premises to be overloaded, damaged,
punctured, stained, stripped or defaced, nor suffer any waste. Lessee shall
obtain Lessor's written consent before erecting any sign on the Leased Premises,
except that Lessor agrees that Lessor will provide to Lessee signage space on
the existing signage board(s) located outside the Building and shall reimburse
Lessee for Lessee's reasonable one-time cost of installing Lessee's sign on the
front of the Building. All of Lessee's signage shall conform to the existing
signage styles. Notwithstanding the above, Lessee and Lessor shall share equally
(50/50) the entirety of any capital expenditure related to plumbing, Building
backflow preventers, sprinkler, heating, air conditioning, electrical and gas
fixtures, systems and equipment (excluding (a) regular maintenance and repair,
(b) replacement items which cost less than $500.00, and (c) items under
maintenance contracts) which cost in the aggregate up to ten thousand and 00/100
dollars ($10,000.00) per year, with Lessor paying any excess over ten thousand
dollars and 00/100 ($10,000.00) per year.

Lessor agrees, at Lessor's sole cost and expense, to maintain and repair the
roof and structure of the Building in the same condition as it is at the
commencement of the Term or as it may be put

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NOVIRIO LEASE
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in during the Term of this Lease, reasonable tear and wear, excepted, and damage
by fire and other casualty excepted subject to Paragraph 19 hereof.

10.   USE

Lessee shall use the Leased Premises only for the purpose of research and
development and business office use. Lessee shall secure all necessary local,
state and federal permits and licenses for the conduct of its business.

11.   COMPLIANCE WITH LAWS

Lessee acknowledges that the Leased Premises, the Building and the Property
shall not be possessed, occupied or used for any purpose or in any manner which
will be improper, noisy or offensive, or contrary to any applicable Federal,
state and local law, by-law, ordinance, code, rule or regulation or any guidance
or interpretation of any governmental authority relating thereto (hereinafter
collectively referred to as the "Legal Requirements"). Without limiting the
generality of the foregoing, except in accordance with all other applicable
provisions of this Lease and all Legal Requirements (and having given Lessor
copies of all required permits, licenses and approvals), Lessee shall not bring
or permit to be brought, stored, used or generated in or on the Leased Premises,
the Building or elsewhere on the Property any "Hazardous Materials" (as
hereinafter defined). "Hazardous Materials" means and includes any substance,
waste material, effluent or emission, whether in a solid, gas, liquid or other
state, which is radioactive or deemed hazardous, toxic, a pollutant or a
contaminant under any Federal, state or local law, by-law, ordinance, code,
rule, regulation, executive order or other administrative order, judgment,
decree, injunction or other judicial order of or by any governmental authority,
now or hereafter in effect, relating to pollution or to the protection of
health, safety or the environment. By way of illustration and not limitation,
"Hazardous Materials" includes "oil", "hazardous materials", "hazardous wastes",
and "hazardous substances" as defined in the Comprehensive Environmental
Response, Compensation and Liability Act, 42 U.S.C. Section 9601 et seq., as
amended, and the Resource Conservation and Recovery Act of 1976, 42 U.S.C.
Section 6901 et seq., as amended, and the regulations promulgated thereunder,
and also Massachusetts General Laws, Chapter 21C and Chapter 21E and the
regulations promulgated thereunder. If and to the extent that any Hazardous
Materials are brought, stored, used, or generated in or on the Leased Premises,
Lessee agrees to indemnify, defend and hold harmless, Lessor, all other tenants
and occupants of the Building, the Property and West Cambridge Science Park and
all of their respective employees, agents, consultants, contractors, guests and
invitees from and against all loss, liability, damage and expense (including
consultants' and attorneys' fees and expenses) ever caused by, resulting from or
associated with such Hazardous Materials, including, without limitation, all
costs of assessment, containment, removal, remediation or other response, any
injury or death to any person or any loss of or damage to any property
(including natural resource damages), regardless of whether Lessee or Lessee's
employees, agents, consultants and contractors have been negligent or otherwise
at fault with respect to said Hazardous Materials. For purposes of this
indemnification Lessor shall be deemed strictly liable on account of all such
Hazardous Materials, regardless of fault. If, at any time during the Lease Term,
any lender or governmental authority requires any investigation or assessment,
including, without limitation, sampling and analytical testing, to determine
whether

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NOVIRIO LEASE
AUGUST 21, 2001


there has been any release or threatened release of Hazardous Materials by
Lessee or anyone claiming by, through or under Lessee, and if there is
reasonable basis to conclude that such a release or threatened release has
occurred, then Lessee shall reimburse Lessor upon demand, as Additional Rent,
for the costs thereof. Lessee shall execute affidavits, certifications and the
like, as may be reasonably requested by Lessor, from time to time with respect
to Lessee's best knowledge and belief concerning the presence, procedures for
management and any releases or threatened releases of Hazardous Materials in or
on the Leased Premises, the Building or the Property. Lessor reserves the right
to enter the Premises at reasonable times (provided twenty-four (24) hours'
notice is given to Lessee, except in the case of emergency) to inspect the same
for Hazardous Materials. If at any time during the Lease Term, Lessee or anyone
claiming by, through or under Lessee uses or stores radioactive materials on the
Premises, Lessee's obligations under this Section 11 shall include compliance
with all so-called "close-out" procedures of the Nuclear Regulatory Commission
or other Federal, state or local governmental authorities having jurisdiction
over radioactive materials.

12.   FIRE INSURANCE

Lessee shall not permit any use of the Leased Premises which will make voidable
any insurance on the Property, or on the contents of said Property, or which
shall be contrary to any law or regulation from time to time established by the
New England Fire Insurance Rating Association, or any similar body succeeding to
its powers. Lessee shall on demand reimburse Lessor, and all other tenants at
the Property all extra insurance premiums caused by Lessee's use of the Leased
Premises or the Property.

13.   ALTERATIONS

Lessee shall not make structural alterations or additions or any alterations or
additions which may adversely impact the structure of the Building or the Leased
Premises (hereinafter referred to as "structural alterations or additions").
Should Lessee wish to make non-structural alterations or additions to the
interior of the Leased Premises, including, but not limited to, alterations or
additions to, or removal of, non-bearing walls, partitions, ceilings, flooring
or floor coverings (including carpeting), doors or elements of the heating,
ventilating, air conditioning, plumbing or electrical systems, Lessee shall in
each such case obtain Lessor's consent thereto, which consent shall not be
unreasonably withheld provided that the requirements of this Section 13 and any
other applicable requirements of this Lease are duly satisfied. Prior to
Lessor's reviewing Lessee's request for consent to any such non-structural
alterations or additions, Lessee shall provide to Lessor design plans and
specifications describing in detail the proposed alterations or additions and
including any applicable color schemes or choices for surface treatments and/or
floor coverings (including carpeting).

Prior to undertaking any such non-structural alterations or additions which
require a Building Permit from the City of Cambridge or any other
construction/installation permit from a governmental authority, Lessee shall, at
Lessee's sole expense, (unless otherwise mutually agreed upon by Lessor and
Lessee in writing as to any given project):


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NOVIRIO LEASE
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      (a)   engage the services of a qualified architect, engineer and/or
            contractor to determine the need for and the design of the
            contemplated alterations or additions; and

      (b)   provide to Lessor the architect's, engineer's and/or contractor's
            report, together with the design plans and specifications, for the
            alterations or additions when requesting Lessor's consent to
            implement them.

Should Lessor's consent be granted for the making of alterations or additions to
the interior of the Leased Premises, Lessee shall duly obtain all necessary
permits, licenses and approvals prior to the commencement of the work and shall
perform the work (if applicable) in accordance with the architect's, engineer's
and/or contractor's plans and specifications therefor. Should the contemplated
alterations or additions be substantial, including, but not limited to,
alterations or additions to, or removal of, non-bearing walls, partitions,
ceilings, flooring or floor coverings (including carpeting), doors or elements
of the heating, ventilating, air conditioning, plumbing or electrical systems,
then, prior to performing any work, Lessee shall deposit, at Lessor's option,
with Lessor a sum of money equal to the cost of restoring the Leased Premises to
their original condition. Should Lessee make any alterations or additions to the
Leased Premises, and said alterations or additions result in a violation of
applicable zoning, building, health or safety laws, by-laws, ordinances, codes,
rules or regulations or any other Legal Requirements as to any part of the
Leased Premises, even if such part is not being altered or changed (including
those parts of the Leased Premises which then have non-conforming status as to
any such Legal Requirements), then Lessee shall be fully responsible for curing
all such violations at Lessee's sole expense and shall reimburse Lessor on
demand, as Additional Rent, for any expenses (including consultants' and
attorneys' fees) that Lessor may incur in connection with such violation(s).

All such allowed alterations or additions shall be made at Lessee's sole expense
and shall be of a quality at least equal to that of the present construction.
Lessee shall not permit any mechanics' liens, or similar liens, to attach to the
Leased Premises for labor or materials furnished to Lessee, or claimed to have
been furnished to Lessee, in connection with work of any character performed or
claimed to have been performed at the direction of Lessee, and shall cause any
such liens to be released of record or bonded over forthwith without cost to
Lessor. Any alterations or additions made by Lessee shall become the property of
Lessor upon the expiration or any earlier termination of the Lease Term, unless
Lessor specifies at the time of Lessor's granting its consent to the alterations
or additions that Lessee remove some or all of such alterations or additions, in
which case Lessee shall do so and shall fully repair prior to the expiration or
earlier termination of the Lease Term any damage, injury or disruption to the
Building and the Leased Premises caused by or resulting from such removal. In
the event that Lessee fails to so remove its alterations and additions at the
expiration or any earlier termination of the Lease Term as required by the
preceding sentence and/or to repair any damage, injury or disruption caused
thereby or resulting therefrom, Lessor shall have the right to use all or part
of any monies deposited by Lessee pursuant to this Section, (as well as the
security deposit, should such monies prove insufficient) for purposes of such
removal and/or repairs. Any monies deposited by Lessee pursuant to this Section
which are not so used and applied by Lessor shall be promptly refunded to Lessee
once the said removal and/or repairs have been duly completed. The right to use
said

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NOVIRIO LEASE
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monies deposited by Lessee pursuant to this Section, as well as the security
deposit, shall be in addition to, and not by way of limitation of, all other
rights and remedies which Lessor shall have on account of Lessee's failure to
perform Lessee's obligations under the terms of this Lease.

14.   SUBLEASING AND ASSIGNING

Lessee shall not assign or sublet the whole or any part of the Leased Premises
without Lessor's prior written consent, which consent shall not be unreasonably
withheld or delayed, provided that (a) Lessee shall not be in default under the
terms of this Lease; (b) Lessee remains a business entity in good standing and
financially capable of continuing to perform its obligations under this Lease;
and (c) Lessor determines, in the exercise of its good faith judgment, that the
proposed assignee or sublessee has sufficient financial resources, good
reputation and character of proposed occupancy to fulfill Lessee's obligations
under this Lease. For purposes of the preceding sentence, Lessor agrees to
approve or reject a proposed assignment or subletting within ten (10) business
days after receiving from Lessee copies of the proposed instrument of assignment
or sublease, together with the financial and other relevant information
regarding the assignee or subtenant, as the case may be.

If Lessor consents to an assignment or sublease: (a) Lessee shall promptly
deliver to Lessor a fully executed copy of said assignment or sublease, which
shall be in all material respects the form previously submitted to Lessor for
review; and (b) following any such assignment or sublease, Lessee shall remain
primarily liable to Lessor under the terms of this Lease (which liability shall
be joint and several with the assignee or sublessee).

Regardless of whether Lessor grants such consent to an assignment or sublease,
Lessee shall reimburse Lessor on demand, as Additional Rent, for all reasonable
costs and expenses (including, without limitation, reasonable attorneys' fees)
in excess of $1,500.00 incurred by Lessor in responding to a request for such
consent.

Any assignment or sublease shall be subject to all of the terms and provisions
of this Lease, including without limitation this Section 14. If Lessee enters
into any such assignment or sublease, Lessor may, at any time and from time to
time after the occurrence of a default hereunder, collect rent from such
assignee or sublessee, and apply the net amount collected against Lessee's
obligations hereunder, but no such assignment or sublease or collection of
rental shall be deemed an acceptance by Lessor of such assignee or sublessee as
a lessee hereunder or as a release of the original named Lessee hereunder.

15.   SUBORDINATION

This Lease shall be subject and subordinate to any and all mortgages, deeds of
trust and other instruments in the nature of a mortgage, now or at any time
hereafter, a lien or liens on the Property. This Section shall be
self-operative, but notwithstanding the self-operative effect, Lessee shall,
when requested, promptly execute and deliver such written instruments as shall
be requested to evidence the subordination of this Lease to said mortgages,
deeds of trust or other such instruments in the nature of a mortgage.


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Lessor represents that, at the present time, there are two mortgages encumbering
the Property. Should in the future the Property be encumbered by additional
mortgages from lenders, Lessor shall provide Lessee with a non-disturbance
agreement from said lenders having mortgages on the Property recognizing the
terms of this Lease. With respect to expenses associated with providing Lessee
with said non-disturbance agreement from lenders having mortgages on the
Property, Lessor shall pay all of the legal expenses related to the initial
preparation of such agreements and shall pay up to $250.00 of the legal expenses
of lender's counsel related to the negotiation of any such agreement, and Lessee
shall pay any amount in excess of $250.00

16. LESSOR'S ACCESS

Upon twenty-four (24) hours notice from Lessor to Lessee, Lessee shall permit
Lessor, its agents employees and contractors, during Lessee's business hours, to
enter all parts of the Leased Premises to inspect the same and to enforce and
carry any provision of this Lease, provided, however, that in case of an
emergency situation, such access shall be provided at any time upon Lessor's or
Lessor's manager's oral request. Within six (6) months of the expiration of this
Lease, the Lessor may show the Leased Premises to others, with reasonable prior
notice and so as not to unreasonably interfere with the normal conduct of
Lessee's business.

17.   INDEMNIFICATION

Lessee shall save Lessor harmless from all loss, damage, liability and expense,
including reasonable attorneys' fees and expenses, occasioned by anything
occurring on the Leased Premises unless caused by the negligence of the Lessor,
and from all loss, liability, damage and expense, including reasonable
attorneys' fees and expenses, whenever occurring at the Leased Premises or the
Property occasioned by any omission, fault, neglect or other misconduct of the
Lessee, its employees, agents, guests and invitees.

18.   LESSEE'S INSURANCE

Lessee shall maintain with respect to the Leased Premises and the Property a
comprehensive public liability insurance in the amount of $2,000,000 with
property damage insurance in limits of $1,000,000 and an umbrella policy of at
least $5,000,000 in responsible companies qualified to do business in
Massachusetts and in good standing therein insuring Lessor as well as Lessee
against injury to persons or damage to property as provided therein. Lessee
shall deposit with Lessor certificates for such insurance upon the execution of
this Lease, naming Lessor as additional insured under the policies, and
thereafter at least thirty (30) days prior to the expiration of any such
policies. All such insurance certificates shall provide that such policies shall
not be canceled without at least ten (10) days prior written notice to each
insured named therein. Lessee shall also maintain adequate coverage for its
personal property at the Leased Premises.

19.   FIRE, CASUALTY AND EMINENT DOMAIN

Should seventy percent (70%) of the Leased Premises or sixty percent (60%) of
the Property be damaged by fire or other casualty, or be taken by eminent
domain, Lessor may elect to terminate

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NOVIRIO LEASE
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this Lease by notice given within thirty (30) days of the event. When such fire,
casualty or taking renders the Leased Premises substantially unsuitable for
their intended use, Lessee may elect to terminate this Lease if:

      (a)   Lessor fails to give written notice within thirty (30) days of
            intention to restore Leased Premises, or

      (b)   Lessor fails to restore the Leased Premises to a condition
            substantially suitable for their intended use within ninety (90)
            days of said fire, casualty or taking.

Lessor reserves, and Lessee grants to Lessor, all rights which Lessee may have
for damages or injuries to the Leased Premises for any taking by eminent domain,
except for damage to Lessee's fixtures, property or equipment.

20.   DEFAULT AND BANKRUPTCY

In the event that:

      (a)   Lessee shall default in the payment of any installment of Rent or
            Additional Rent herein specified and such default shall continue for
            ten (10) days after written notice thereof; provided, however, that
            if a rental payment default shall occur three times within any
            twelve month period, then Lessor shall not thereafter be required to
            give such notice of default to Lessee and to accord Lessee said
            right to cure; or

      (b)   Lessee shall default in observance or performance of any other of
            Lessee's covenants, agreements, or obligations hereunder and such
            default has not been corrected within thirty (30) days after written
            notice thereof or such additional time as is reasonably required to
            correct such default provided Lessee is diligently prosecuting the
            correction, or immediately if the default has been corrected and
            Lessee is again in default in the same manner; or

      (c)   Lessee or any guarantor makes any assignment for the benefit of
            creditors, commits any act of bankruptcy or files a petition under
            any bankruptcy or insolvency law; or if such a petition is filed
            against Lessee or any guarantor and is not dismissed within thirty
            (30) days; or if a receiver or similar officer becomes entitled to
            Lessee's leasehold hereunder and it is not returned to Lessee within
            thirty (30) days, or if such is taken on execution or other process
            of law in any action against Lessee; or

      (d)   any representation or warranty made herein or in any certificate,
            financial statement or other instrument furnished in connection with
            this Lease shall prove to be materially false;

      (e)   any attempted assignment of this Lease or any attempted subletting
            of all or any part of the Leased Premises without the prior written
            consent of Lessor;


                                       11
<PAGE>
NOVIRIO LEASE
AUGUST 21, 2001


      (f)   if, after any permitted assignment or sublease in accordance with
            the provisions of Section 14 hereof, Lessee shall be subject to any
            of the conditions or events specified in Subsection 20 (c) hereof,
            then

Lessor shall have the right thereafter, while such default continues, to
re-enter and take the complete possession of the Leased Premises, to declare the
term of this Lease ended (forcibly if necessary and to the extent permitted by
law) and to remove Lessee's effects, without prejudice to any remedies which
might be otherwise used for arrears of rent or other default and without being
deemed guilty of any manner of trespass. Lessee shall be liable for the Base
Rent, Additional Rent and any other charges due from Lessee as current damages
for the residue of the Term and shall indemnify Lessor against all loss of rent
and other payments which Lessor may incur by reason of such termination during
the residue of the Term. Lessor agrees that it shall use commercially reasonable
efforts to re-let the Leased Premises, provided, however, that this obligation
shall be deemed satisfied if Lessor lists the Leased Premises for lease with a
real estate broker or rental agent having experience with leasing commercial
real estate in the City of Cambridge, Massachusetts. At any time within 120 days
from such termination, whether or not Lessor shall have collected any such
current charges payable under any of the foregoing provisions of this Section,
Lessor may elect to recover, and Lessee shall forthwith pay, as liquidated final
damages in lieu of such current damages beyond the date of such election, an
amount equal to the excess, if any, of the Base Rent, Additional Rent and any
other charges due from Lessee for the residue of the Term minus the fair market
value rent for the Leased Premises for the same period of time. Lessee agrees
that Lessor may re-let the Leased Premises, or any portions thereof, for a
period which may, at Lessor's option, be less than or exceed the balance of the
Term of this Lease, and may grant concessions or free rent. Lessor may make such
alterations, repairs, replacements and decorations to the Leased Premises as
Lessor, in Lessor's reasonable judgment, considers advisable or necessary for
the purpose of re-letting the Leased Premises. If Lessee shall default in the
observance of any conditions or covenants on Lessee's part to be observed or
performed under or by virtue of any provisions in any provisions of this Lease
(including, but not limited to, the timely payment of Base Rent or Additional
Rent), Lessor, without any obligation to do so and without thereby waiving such
default, may, upon ten (10) days notice to Lessee (except in the event of an
emergency), remedy such default for the account and at the expense of Lessee. If
Lessor makes any expenditures or incurs any obligations for the payment of money
in connection therewith including, but not limited to, reasonable attorney's
fees in instituting, prosecuting or defending any action or proceeding, such
sums paid or obligations incurred shall be paid with interest at the rate of the
Wall Street Journal Prime Rate plus four (4) percent per annum and costs to
Lessor by Lessee as Additional Rent.

21.   LIABILITY OF OWNER

Lessor shall not be liable under the terms of this Lease or otherwise for damage
or injury caused by, resulting from or associated with the leakage or release of
water, whether attributable to roof leaks, pipes or plumbing fixtures bursting,
leaking or overflowing, or other sources unless such leakage or release of water
is directly attributable to the gross negligence of Lessor. IN ADDITION AND
NOTWITHSTANDING ANY OTHER PROVISION OF THIS LEASE TO THE CONTRARY, UNDER NO
CONDITION OR CIRCUMSTANCE SHALL LESSOR BE LIABLE TO LESSEE OR TO ANY OTHER PARTY
FOR ANY SPECIAL, REMOTE OR CONSEQUENTIAL DAMAGES.


                                       12
<PAGE>
NOVIRIO LEASE
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No owner of the Property shall be liable hereunder except for breaches of
Lessor's obligations occurring during the period of ownership. The obligations
of Lessor shall be binding upon Lessor's interest in said Property, but not upon
other assets of Lessor, and no individual partner, agent, trustee, stockholder,
officer, director, employee or beneficiary of Lessor shall be personally liable
for the performance of Lessor's obligations hereunder.

22.   REMEDIES CUMULATIVE

No reference to any specific right or remedy shall preclude Lessor from
exercising any other right or from having any other remedy or from maintaining
any action to which it may otherwise be entitled at law or in equity. No failure
by Lessor to insist upon the strict performance of any agreement, term, covenant
or condition hereof, and no acceptance of full or partial rent during the
continuance of such breach, shall constitute a waiver of any breach by Lessee
under this Lease. No breach by any other Lessee under any other lease of any
portion of the Property shall affect or alter this Lease in any way whatsoever.

23.   NOTICE

Any notice from Lessor to Lessee relating to this Lease, the Leased Premises or
the Property or to the use, possession and occupancy thereof, shall be deemed
duly given or served, if hand delivered or if sent to Lessee by registered or
certified mail, return receipt requested, or by a recognized overnight courier
service, postage and other charges prepaid, addressed to Lessee at 125
CambridgePark Drive, Cambridge, MA 02140, Attn: Andrea J. Corcoran, Senior Vice
President - Legal and Administration. Any notice from Lessee to Lessor relating
to this Lease, the Leased Premises or the Property or to the use, possession and
occupancy thereof, shall be deemed duly given or served, if mailed to Lessor by
registered or certified mail, return receipt requested, or by a recognized
overnight courier service, postage and other charges prepaid, addressed to
Lessor at the following address or to any other address as Lessor may from time
to time advise Lessee of in writing:

To Lessor:                                  With a copy to:

WEST CAMBRIDGE SCIENCE PARK, LLC            MICHAEL P. LAST, ESQUIRE
ATTN. JACQUES SULTAN, MANAGER               RACKEMANN, SAWYER & BREWSTER
38 PEQUOSSETTE ROAD                         ONE FINANCIAL CENTER
BELMONT, MA 02478-3459                      BOSTON, MA 02111

Such notice shall be deemed given either when received or three business days
after being mailed in the foregoing manner, whichever shall first occur.

24.   SURRENDER

At the expiration of the Term, whether by termination or any other manner of
expiration or cessation of Lessee's right of possession hereunder, Lessee shall
vacate the Leased Premises and remove all Lessee's goods and effects from the
Leased Premises (including, without limiting the generality of the foregoing,
all furniture, Hazardous Materials, waste of any nature, any and all


                                       13
<PAGE>
NOVIRIO LEASE
AUGUST 21, 2001


contaminated material [including the neutralization chips] and all signs,
stickers and lettering affixed or painted by Lessee, either inside or outside of
the Leased Premises, including on floors, doors, walls or cabinetry).

Lessee shall deliver to Lessor the Leased Premises (including, without limiting
the generality of the foregoing, its plumbing, HVAC, electrical, chemical hoods,
benches and neutralization tank systems) and all keys, locks thereto, and other
fixtures connected therewith and all alterations and additions made to or upon
the Leased Premises (provided Lessor does not elect otherwise as provided
herein), in good condition, damage by fire or other casualty and reasonable wear
and tear only excepted. Lessee shall, at its sole cost and expense, repair any
and all damage resulting from the removal of any fixtures. In the event of the
Lessee's failure to remove any of Lessee's property from the Leased Premises, or
deliver the Leased Premises in good condition as provided herein, Lessor is
hereby authorized, without liability to Lessee for loss or damage thereto, and
at the sole risk of Lessee, to remove and store any of the property at Lessee's
expense, or to retain the same under Lessor's control or to sell the same at
public or private sale, and to apply the net proceeds of such sale to the
payment of any sum due hereunder, or to destroy such property. Lessor may make
all necessary repairs, maintenance and work which are Lessee's responsibility as
provided herein, and charge Lessee as Additional Rent any expenses and
obligations which Lessor has incurred in connection herewith (including, without
limitation, reasonable attorneys' fees incurred by Lessor in connection
herewith).

If Lessee fails to vacate the Leased Premises in a timely manner upon the
expiration of the Lease Term or any other termination of this Lease, Lessee
shall be a tenant-at-sufferance only subject to all the terms and conditions of
this Lease and, as consideration therefor, shall pay to Lessor commencing on the
date of expiration or any earlier termination of the Lease Term at the rate
equal to one hundred fifty percent (150%) of the Base Rent and Additional Rent
payable during the last month of the Lease Term. Such payments shall not in any
way constitute, or entitle Lessee to, a renewal or extension of this Lease or
grant Lessee any right to continue in possession of the Leased Premises, nor
shall they limit or be in lieu of any other rights or remedies which Lessor may
have against Lessee under the terms of this Lease or under applicable law.

25.   ESTOPPEL CERTIFICATE

Lessor and Lessee agree from time to time, upon not less than fifteen (15) days
prior written request by the requesting party, to execute, acknowledge and
deliver to the requesting party a statement in writing in form satisfactory to
the requesting party certifying that this Lease is unmodified and in full force
and effect; that the responding party has no defenses, offsets or counterclaims
against its obligations to perform under its obligations and covenants under
this Lease; that there are no uncured defaults of the requesting party under
this Lease (or, if there have been any modifications, that this Lease is in full
force and effect as modified and stating the modifications and, if there are any
defenses, offsets, counterclaims, or defaults, setting them forth in reasonable
detail); and the dates to which the Base Rent, Additional Rent and other charges
have been paid. Any such statement delivered pursuant to this Section may be
relied upon by any prospective purchaser or mortgagee of the Property or any
prospective assignee of

                                       14
<PAGE>
NOVIRIO LEASE
AUGUST 21, 2001


any such mortgagee, or any prospective purchaser of Lessee's business or
assignee or subtenant of Lessee.

26.   WAIVER OF SUBROGATION

Any insurance carried by either Lessor or Lessee with respect to the Property or
Leased Premises or personal property therein or occurrences thereon shall, if it
can be so written without additional premium or with an additional premium which
the other party agrees to pay, include a clause or endorsement denying to the
insurer rights of subrogation against the other party to the extent rights have
been waived by the insured prior to occurrence of injury or loss. Each party,
notwithstanding any provisions of this Lease to the contrary, hereby waives any
rights of recovery against the other for injury or loss due to hazards covered
by such insurance to the extent of the indemnification received thereunder.

27.   BROKERAGE

The Brokers named herein, Joe Sultan and Kevin Sullivan of The Conrad Group,
each represent and warrant that he is duly licensed as such by the Commonwealth
of Massachusetts, and joins in this Lease and becomes a party hereto, insofar as
any provisions of this Lease expressly apply to him, and any amendments or
modifications of such provisions to which he agrees in writing. The fees and
expenses of each such broker shall be paid exclusively by Lessor. Lessor and
Lessee each acknowledge and represent to the other that no other brokers,
finders or like parties are entitled to the payment of any commission or
finder's fee with respect to this Lease and the transactions contemplated by it,
and each agrees to indemnify, defend and hold harmless the other with respect to
any such payment.

28.   CONDITION OF THE LEASED PREMISES

Lessor shall deliver the Leased Premises to Lessee solely in an AS-IS CONDITION,
provided that Lessor agrees that the Building structure and roof and all
Building equipment, including HVAC, electric fixtures and plumbing, will be in
good working order, as of the Commencement Date, subject, however, to normal
wear and tear by the current tenant Zycos, Inc. and by Lessee, as Zycos, Inc.'s
subtenant.

Lessor shall have no obligation to make any improvements or changes to the
Building associated with (a) Lessee's occupancy or (b) any work performed by
Lessee, whether or not stated in this Lease.

29.   SPECIAL PROVISIONS

      (a)   LOADING DOCK:

            The loading dock which is identified on the attached floor plan
            shall be Lessee's sole loading dock.

      (b)   USE OF "DARK ROOM" AND "COLD ROOM":


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<PAGE>
NOVIRIO LEASE
AUGUST 21, 2001


            Lessee, at Lessee's sole option, may permit other tenants at the
            Property to use the "Dark Room" and "Cold Room" in common with
            Lessee during normal business hours, with the control of these rooms
            residing with Lessee.

      (c)   NEUTRALIZATION TANK:

            Lessee shall retain the right to use the plumbing dedicated to the
            waste disposal from the Property and the existing central
            neutralization tank. Lessee shall monitor the waste from the Leased
            Premises in accordance with applicable laws which may include the
            installation of additional monitoring and sampling ports (at
            Lessee's expense) after a tenant occupies the adjoining space
            adjacent to the Leased Premises. Lessor shall monitor or cause to be
            monitored the waste from any other tenant in the building which
            enters the common plumbing. If the neutralization chips need to be
            removed and changed as a result of contamination, the cost of such
            work shall be paid by the party causing the contamination, or on a
            pro rata basis if more than one party has caused the contamination,
            which shall be determined on the basis of the monitoring. Lessee
            shall maintain the existing central neutralization tank; provided,
            however, that during such time as the said space adjoining the
            Leased Premises is occupied by a tenant and such tenant shares in
            the use of the central neutralization tank, Lessor shall reimburse
            Lessee for thirty percent (30%) of the cost of maintaining said
            tank.

            Notwithstanding the foregoing to the contrary, during such time as a
            tenant occupies the space adjoining the Leased Premises and either
            (i) said tenant's operations are not compatible with Lessee's
            operations, or (ii) a central neutralization tank is not permitted
            under existing or new law or regulation, then Lessor shall provide
            Lessee with up to six individual under-counter neutralization tanks.
            All under-counter neutralization tanks shall become part of the
            Leased Premises and shall be duly delivered to Lessor upon any
            expiration of the Term of this Lease as provided in Section 24
            hereof.

      (d)   HVAC:

            Lessee acknowledges that HVAC system shall be considered adequate
            for the term of the Lease and that Lessor shall not be required to
            supplement or make any changes to the HVAC (even if required by
            applicable code).

30.   COMPLIANCE WITH PTDM REQUIREMENTS

In order to comply with the requirements of the City of Cambridge Parking and
Transportation Demand Management Planning Ordinance, Lessor has filed with, and
had approved by, the City of Cambridge Traffic, Parking and Transportation
Department, a Parking and Transportation Demand Management Plan ("PTDM Plan").
Lessee acknowledges having received and reviewed a copy of the PTDM Plan and
hereby agrees duly to comply with (a) all requirements of the PTDM Plan which
are applicable to Lessee and Lessee's employees, agents, guests and invitees, as
said requirements may hereafter be supplemented, amended or otherwise modified
to satisfy

                                       16
<PAGE>
NOVIRIO LEASE
AUGUST 21, 2001


the City of Cambridge and (b) all guidelines, rules and regulations issued from
time to time by Lessor in order duly to implement the requirements of the PTDM
Plan. Consistent with the requirements of the PTDM Plan, Lessee is to discourage
the use of single occupancy vehicle commuting and to promote the use of
alternative modes of transportation.

Lessee agrees promptly to provide Lessor with such information, including,
without limitation, information regarding the number of full-time equivalent
employees using the Leased Premises, as shall be necessary for Lessor to comply
with the monitoring and reporting requirements of the PTDM Plan. In addition,
Lessee shall provide Lessor with periodic written status reports detailing the
measures being implemented by Lessee to comply with the PTDM Plan. Said status
reports shall be provided at least annually or at such more frequent intervals
as may be reasonably requested by Lessor in order to satisfy the requirements of
the City of Cambridge.

31.   LESSEE'S REPRESENTATIONS

If Lessee is a business entity, then the person or persons executing this Lease
on behalf of Lessee jointly and severally warrant and represent in their
individual capacities that:

      (a)   Lessee is duly organized, validly existing and in good standing
            under the laws of the jurisdiction in which such entity was
            organized;

      (b)   Lessee has the authority to own its property and carry on its
            business as contemplated under this Lease;

      (c)   Lessee is in compliance with all the laws and orders of public
            authorities applicable to Lessee;

      (d)   Lessee has duly executed and delivered this Lease;

      (e)   the execution, delivery and performance by Lessee of this Lease (i)
            are within the powers of the Lessee, (ii) will not violate any
            provisions of law or any order of any court or agency of government,
            or any agreement or other instrument to which the Lessee is a party
            or by which it or any of its property is bound, or (iii) will not
            result in the imposition of any lien or charge on any of Lessee's
            property except by the provisions of this Lease; and

      (f)   the Lease is a valid and binding obligation of Lessee in accordance
            with its terms.

Lessee, if a business entity, agrees that breach of the foregoing warranty and
representation shall, at Lessor's election, be a default under this Lease. This
warranty and representation shall survive the termination of the Lease Term.

32.   LESSOR'S REPRESENTATIONS

If Lessor is a business entity, then the person or persons executing this Lease
on behalf of Lessor jointly and severally warrant and represent in their
individual capacities that:


                                       17
<PAGE>
NOVIRIO LEASE
AUGUST 21, 2001


      (a)   Lessor is duly organized, validly existing and in good standing
            under the laws of the jurisdiction in which such entity was
            organized;

      (b)   Lessor has the authority to own its property and carry on its
            business as contemplated under this Lease;

      (c)   Lessor is in compliance with all the laws and orders of public
            authorities applicable to Lessor;

      (d)   Lessor has duly executed and delivered this Lease;

      (e)   the execution, delivery and performance by Lessor of this Lease (i)
            are within the powers of the Lessor, (ii) will not violate any
            provisions of law or any order of any court or agency of government,
            or any agreement or other instrument to which the Lessor is a party
            or by which it or any of its property is bound; and

      (f)   the Lease is a valid and binding obligation of Lessor in accordance
            with its terms.

This warranty and representation shall survive the termination of the Lease
Term.

33.   MISCELLANEOUS

      (a)   SEVERABILITY:

            If any provisions of this Lease or any application thereof shall be
            deemed by a court to be invalid or unenforceable under a given set
            of circumstances, said provisions(s) shall be interpreted so as to
            permit their enforceability to the maximum extent possible, failing
            which, this Lease shall be interpreted as if such provision(s) were
            not a part hereof, and the remainder of this Lease shall not be
            affected thereby.

      (b)   GOVERNING LAW:

            This Lease shall be governed by and construed in accordance with the
            laws of the Commonwealth of Massachusetts.

      (c)   RELATIONSHIP OF PARTIES:

            Lessor and Lessee shall not be considered to joint venturers or
            partners, and neither shall have the power to bind or obligate the
            other except as set forth herein.

      (d)   MODIFICATION:

            No changes, additions, or interlineations made to this Lease shall
            be binding unless initialed or signed by each of the parties.


                                       18
<PAGE>
NOVIRIO LEASE
AUGUST 21, 2001


      (e)   NO OTHER REPRESENTATIONS OR WARRANTIES:

            The Lessee acknowledges that Lessee has not been influenced to enter
            into this Lease nor has it relied upon any warranties or
            representations not set forth or incorporated in this Lease.

      (f)   NO OTHER AGREEMENT:

            This Lease contains all of the agreements of the parties with
            respect to the subject matter thereof and supersedes all prior oral
            and written negotiations and dealings between them with respect to
            such subject matter. The agreement of the parties contained in this
            Lease shall not be modified or amended unless such modification or
            amendment is in writing and signed by the parties.

IN WITNESS WHEREOF, the said parties hereunto set their hands and seals as of
this _____day of August, 2001

LESSOR:                                   LESSEE:
West Cambridge Science Park, LLC          Novirio Pharmaceuticals, Inc.



By: /s/ Jacques Sultan                    By:  /s/ Jean-Pierre Sommadossi, Ph.D.
    ----------------------------------        ----------------------------------
    Jacques Sultan, Manager               Its: Chief Executive Officer
    Being hereunto duly authorized            ----------------------------------
                                               Being hereunto duly authorized


                                    BROKERS:

--------------------------------------        ----------------------------------
Joe Sultan                                    Kevin Sullivan, The Conrad Group


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