Document:

<PAGE>
                                                                   Exhibit 10.19

                            FIRST AMENDMENT TO LEASE

      THIS FIRST AMENDMENT TO LEASE is made by and between David E. Clem and
David M. Roby, as Trustees of Fort Washington Realty Trust under Declaration of
Trust dated June 19, 1995 recorded in the Suffolk County Registry of Deeds in
Book ________, Page ________, and filed for registration as Document No. 9762301
in the Suffolk County Registry District of the Land Court ("Landlord"), and
Vertex Pharmaceuticals Incorporated ("Tenant") as of the ____ day of December,
1995.

      WHEREAS, Fort Washington Limited Partnership, the predecessor in interest
to Landlord, and Tenant entered into that certain Lease dated March 3, 1995 (the
"Lease") with respect to a portion of the property located at 40 Erie Street,
Cambridge, Massachusetts, as more particularly described therein (the "Premises;
and

      WHEREAS, Landlord and Tenant desire to amend and modify the terms of the
Lease as more particularly set forth herein.

      NOW, THEREFORE, in consideration of the mutual promises herein contained,
and other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, Landlord and Tenant hereby agree as follows:

      1. Section 1.1 of the Lease is hereby amended by deleting the definitions
of the following terms in their entirety: "Annual Fixed Rent Rate," "Annual
Fixed Rent Rate for Phase I Space," "Annual Fixed Rent Rate for phase II
Space," "Annual Fixed Rent Rate for Phase III space," "Build-Out Rent Rate,"
"Phase I Space," "Phase II Space," "Phase III Space," "Premises," "Premises
Design Floor Area," "Scheduled Substantial Completion Dates," "Tenant
Allowance," "Tenant's Design Completion Date," and "Tenant's Proportionate
Fraction." Section 1.1 is hereby further amended by inserting the following new
definitions:

      ANNUAL FIXED RENT RATE:      The sum of:

                                   (i) the Annual Fixed Rent Rate for the Phase
                                   I Space and (ii) the Annual Fixed Rent Rate
                                   for the Phase III Space, subject to
                                   adjustment in accordance with Article IV
                                   hereof.

      ANNUAL FIXED RENT RATE
      FOR PHASE I SPACE:           Build-Out Rent Rate multiplied
                                   by 44,874 r.S.f., subject to adjustment in
                                   accordance with Article IV hereof.

<PAGE>

      ANNUAL FIXED RENT RATE
      FOR PHASE II SPACE:          $0.00, the parties hereby acknowledging and
                                   agreeing that the portion of the Premises
                                   referred to as the "Phase II Space" in the
                                   Lease has been combined with, and constitutes
                                   a portion of, the "Phase I Space," as such
                                   term is defined in this First Amendment to
                                   the Lease.

      ANNUAL FIXED RENT RATE
      FOR PHASE III SPACE:         Shell Rent Rate multiplied by 14,448 r.s.f.,
                                   subject to adjustment in accordance with
                                   Article IV hereof.

      BUILDINGS:                   The Building of which the Premises are a part
                                   known and numbered as 40 Erie Street,
                                   Cambridge, MA, containing approximately
                                   100,454 r.s.f.

      BUILDING DESIGN FLOOR
      AREA                         100,454 r.s.f.

      BUILD-OUT RENT RATE:         $39.50 p.r.s.f., subject to adjustment in
                                   accordance with Section 2.3 hereof.

      PHASE I SPACE:               The portion of the Premises consisting of
                                   approximately 44,874 rentable square feet
                                   ("r.s.f.") to be located within the Premises
                                   in accordance with Complete Plans therefor
                                   pursuant to Section 3.1 hereof.

      PHASE II SPACE:              The term "Phase II Space" as used herein
                                   shall have no meaning, the parties hereby
                                   agreeing that the portion of the Premises
                                   referred to as the "Phase II Space" in the
                                   Lease has been combined with, and constitutes
                                   a portion of, the "Phase I Space," as such
                                   term is defined in this First Amendment to
                                   the Lease.

                                      -2-
<PAGE>

      PHASE III SPACES:            The portion of the Premises consisting of
                                   approximately 14,448 r.s.f. to be located
                                   within the Premises in accordance with
                                   Complete Plans therefor pursuant to
                                   Section 3.1 hereof.

      PREMISES:                    Approximately 59,322 r.s.f. of, space in the
                                   Building, as shown on, Exhibit A, consisting
                                   of the Phase I Space and the Phase III Space
                                   (each such Phase being sometimes hereinafter
                                   referred to as a "Phase of the Premises").

      PREMISES DESIGN FLOOR
      AREA:                        59,322 r.s.f.

      SCHEDULED SUBSTANTIAL
      COMPLETION DATES:            Phase I Space: January 1, 1996

                                   Phase III Space: January 1, 2001

      TENANT ALLOWANCE:            Phase I Space: $4,206,650.00

                                   Phase III Space: $0.00

                                   Phase I Structural Modifications: $166,900.00

      TENANT'S DESIGN
      COMPLETION DATE              Phase I Space:  April 1, 1995

                                   Phase III Space:  June 1, 2000

      TENANT'S PROPORTIONATE
      FRACTION:                    59.05% subject to adjustments as provided in
                                   Section 2.3 hereof."

      2. The Lease is hereby amended by deleting Exhibit A thereto and inserting
in place thereof Exhibit A attached hereto.

      3. Any and all references contained in the Lease to the "Phase I Space"
or the "Phase II Space" shall mean and refer to the portion of the Premises
defined in this First Amendment as the "Phase I Space." The Parties hereby
acknowledge and agree that the portion of the Premises referred to as the
"Phase II Space" in the original

                                       -3-
<PAGE>

Lease has been combined with, and constitutes a portion of, the "Phase I Space,"
as such term is defined in this First Amendment to the Lease.

      4. Section 2.3 of the Lease is hereby amended by inserting the following
at the end thereof:

            "In the event that the actual floor area of the Phase I Space after
            completion thereof is different from the floor area for the Phase I
            Space set forth in Article I of this Lease, then the Build-Out Rent
            Rate shall be adjusted in accordance with the following formula:

            Adjusted
            Build-Out
            Rent Rate     =   $18.00

                                 +

                           TI Allowance p.s.f. x (36.75 - $18.00)
                           ------------------
                               $85 p.s.f

Where "TI Allowance p.s.f." is equal to  $4,373,550
                                         -----------------
                                         Actual Floor Area
                                         of Phase I Space"

      5. Section 4.1 (a) of the Lease is hereby amended by deleting the second
grammatical paragraph thereof and inserting in place thereof the following:

      "Landlord and Tenant hereby agree that the Annual Fixed Rent Rate shall
      consist of the sum of (i) the Annual Fixed Rent Rate for the Phase I Space
      and (ii) the Annual Fixed Rent Rate for the Phase III Space, as the same
      may be adjusted in accordance with the provisions of this Article IV."

      6. Section 4.1(c) is hereby deleted in its entirety and the following new
Section 4.1(c) is hereby inserted in place thereof:

      "(c) Adjustment for Unused Tenant Allowance. In the event that the cost
      of constructing the Leasehold Improvements for the Phase I Space and the
      Phase I Structural Modifications is less than the sum of the Tenant
      Allowance for the Leasehold Improvements Phase I Space and the Tenant
      Allowance for the Phase I Structural Modifications, the Annual Fixed Rent
      Rate for the Phase I Space set forth in Section 1.1 shall be adjusted in
      accordance with the following formula:

                                       -4-
<PAGE>

Adjusted Annual
Fixed Rent Rate
for Phase I Space          =       Shell Rent Rate

                                         +

                                   Actual TI     x   [Build Out Rent Rate -
                                   ---------          Shell Rent Rate]
                                   Tenant Allowance

      Where "Actual TI" equals the actual portion of the sum of (i) the Tenant
      Allowance for the Leasehold Improvements for the Phase I Space and (ii)
      the Tenant Allowance for the Phase I Structural Modifications expended by
      Landlord in connection with the Leasehold Improvements for the Phase I
      Space and the Phase I Structural Modifications.

      By way of example, if the actual cost of completing the Leasehold
      Improvements for the Phase I Space is $4,100,000 and the actual cost of
      completing the Phase I Structural Modifications is $150,000, the Annual
      Fixed Rent for the Phase I Space shall be adjusted as follows:

      Adjusted Annual
      Fixed Rent for
      Phase I Space                =         $18.00 p.r.s.f

                                                   +

              ($4,100,000 + $150,000)  x    ($39.50 - $18.00)
              ----------------------
              $4,373,550

              = $18.00 - $20.89

              = $38.89

      7. Section 4.1(d) is hereby amended by deleting the words "Phase II Space"
each time they appear in said subsection and inserting in place thereof the
words "Phase I Structural Modifications".

      8. Notwithstanding the provisions of Section 10.11 of the Lease, Landlord
and Tenant hereby agree that Tenant shall not be required to provide a
replacement for the existing letter of credit to cover the Security Deposit
Amount effective as a result of this Amendment until such time, as either: (i)
the Annual Fixed Rent Rate shall be adjusted in accordance with Section 4.1(b),
Section 4.1(c), Section 4.1(d), Section 10.12, Section 10.13 or Section 10.14 of
the Lease, (ii) Landlord applies all or any portion of the existing letter of
credit to cure any default by Tenant or (iii) the existing letter of credit
shall expire under its own terms.

                                       -5-
<PAGE>

      9. (a) Landlord and Tenant hereby acknowledge that certain structural
modifications to the Phase I Space (the "Phase I Structural Modifications")
would be necessary to support the Second Floor Space in the event that Tenant
hereafter elects to exercise its Option to Expand. Landlord hereby agrees to
perform such Phase I Structural Modifications on or before the Scheduled
Substantial Completion Date for the Phase I Space. The Structural Modifications
shall be performed in accordance with that certain "GMP Approval" form accepted
by Tenant on July 27, 1995 (the "GMP Approval"), modifying the work of that
certain Construction Contract for the Leasehold Improvements dated May 9, 1995
by and between Fort Washington Limited Partnership and Siena Construction
Corporation. The Tenant Allowance for the Phase I Structural Modifications is
set forth in Section 1.1 of the Lease.

         (b) The terms and conditions of Article 3 of the Lease shall apply
to the Phase I Structural Modifications as though such work constituted a
portion of the Leasehold Improvements for the Phase I Space; provided,
however, that, without limitation of the provisions of Paragraph 4.1(c) hereof:
(i) Tenant shall be separately billed for Landlord's actual cost of the
Leasehold Improvements for the Phase I Space and for the Phase I Structural
Modifications, (ii) the unused portion, if any, of the Tenant Allowance for the
Leasehold Improvements for the Phase I Space shall not be applied to reduce or
offset any TIR due with respect to the Phase I Structural Modifications, and
(iii) the unused portion, if any, of the Tenant Allowance for the Phase I
Structural Modifications shall not be applied to reduce or offset any TIR due
with respect to the Leasehold Improvements for the Phase I Space.

      10. The Lease is hereby amended by deleting the table and drawings
attached to Exhibit F of the Lease immediately following (but not including)
"Figure 2: Typical Building Core Spaces" thereof in their entirety and inserting
the table and drawings attached hereto as "Exhibit B" in place thereof.

      11. The parties hereby acknowledge and agree that the measurements and
calculations set forth in this Amendment of the Premises Design Floor Area,
the Building Design Floor Area, the Phase I Space and Phase III Space and such
other items set forth herein, are the most current measurements and
calculations thereof and supercede all prior measurements and calculations
thereof.

      12. Initially capitalized terms used and not otherwise defined in this
Amendment shall have the meanings given to such terms in the Lease.

      13. Except as modified or amended by this Amendment, the Lease is hereby
ratified, confirmed and approved by Landlord and Tenant and shall remain in full
force and effect.

                                       -6-
<PAGE>

      EXECUTED as a sealed instrument as of the date first above written.

                                        LANDLORD:

                                        /s/ David E. Clem
                                        ----------------------------------------
                                        David E. Clem, as Trustee of Fort
                                        Washington Realty Trust, and not
                                        individually

                                        /s/ David M. Roby
                                        ----------------------------------------
                                        David M. Roby, as Trustee of Fort
                                        Washington Realty Trust, and not
                                        individually

                                        TENANT:

                                        VERTEX PHARMACEUTICALS INCORPORATED

                                        By: /s/ Richard H. Aldrich
                                            ------------------------------------
                                            Name: Richard H. Aldrich
                                            Title: Senior Vice President

                                        By: /s/ [ILLEGIBLE]
                                            ------------------------------------
                                        Name: [ILLEGIBLE]
                                        Title: Senior Director of Financial and
                                               Treasurer

                                      -7-<PAGE>
                                                                   Exhibit 10.20

                            SECOND AMENDMENT TO LEASE

      This SECOND AMENDMENT TO LEASE is made by and between David E. Clem and
David M. Roby, Trustees of Fort Washington Realty Trust under Declaration of
Trust dated June 19, 1995 and recorded with the Middlesex County (South
District) Registry of Deeds in Book 25422, Page 360 (the "Landlord") and Vertex
Pharmaceuticals Incorporated (the "Tenant").

      Reference is hereby made to that certain lease (the "Lease") dated March
3, 1995, by and between Landlord's predecessor, Fort Washington Limited
Partnership and Tenant with respect to a portion of the property (the
"Premises") located at 40 Erie Street, Cambridge, Massachusetts, (the
"Building") as more particularly described in the Lease as amended by a First
Amendment to Lease (the "First Amendment").

      WHEREAS, the Tenant has requested, and the Landlord has agreed, to further
amend the Lease to add additional space to the Premises upon the terms and
conditions set forth in this Second Amendment to Lease.

      WHEREAS, Landlord and Tenant desire to amend and modify the terms of the
Lease to incorporate the additional space and to ratify and confirm the terms of
the Lease as amended by the First Amendment as more particularly set forth
below.

      NOW, THEREFORE, in consideration of the mutual promises herein contained,
and other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, Landlord and Tenant agree as follows:

      1. Upon occupancy by Tenant, the definition of the Premises set forth in
the Lease shall be amended to include the addition of 41,132 r.s.f. of space
(the "Additional Space") in the Building currently leased to Millennium
Pharmaceuticals, Inc. ("Millennium"). See Exhibit A annexed hereto for the
layout of the Additional Space.

      2. Tenant shall take occupancy of the Additional Space beginning on the
later of (i) the date upon which Millennium vacates the Additional Space, or
(ii) March 19, 1999, and continuing for a period of ten (10) years from the date
upon which Tenant occupies the Additional Space (the "Additional Space Term").
On or after March 19, 1999, if necessary, Landlord shall use best efforts to
expedite Millennium's departure from the Additional Space, including filing an
eviction proceeding. Landlord warrants and represents that according to the
terms of its lease with Millennium that the lease expires on March 18, 1999 as
to the Premises and the associated parking spaces. As to the Premises and the
associated parking spaces, Landlord hereby agrees that Landlord will not extend
or renew the term of Millennium's lease or waive any failure by Millennium to
vacate. Landlord shall not be held liable for any loss or damage incurred by
Tenant as a

<PAGE>

                                      -2-

result of a hold-over by Millennium. Landlord represents that in addition to
other sums for holding over, Millennium must pay a holdover premium equal to the
greater of (a) twice the then fair market rent as reasonably determined by
Landlord, or (b) the total of the Fixed Rent, Additional Rent (as those terms
are defined in the Millennium lease) and all other payments then payable under
the Millennium lease. Landlord agrees that it shall not waive the payment to
Landlord of any such holdover premium by Millennium.

      Provided that Tenant has exercised in each instance its options to extend
the Lease Term for the original Premises: (a) Tenant shall have two (2) options
to extend the Additional Space Term (the "Additional Space Options") for
successive periods of five (5) years each (the "Additional Space Extension
Periods"), subject to and on the terms set forth herein. Tenant may only
exercise the Additional Space Options with respect to the entire Additional
Space. If Tenant shall desire to exercise any Additional Space Option, it shall
give Landlord a notice (the "Additional Space Inquiry Notice") of such desire
not later than fifteen (15) months prior to the expiration of the Additional
Space Term of this Lease or the preceding Additional Space Extension Period, as
the case may be. Thereafter, the Fair Market Rent (as defined in Subsection (c)
below) for the applicable Additional Space Extension Period shall be
determined in accordance with Subsection (d) below. After the applicable Fair
Market Rent has been so determined, Tenant shall exercise each Additional Space
Option by giving Landlord notice (the "Additional Space Exercise Notice") of
its election to do so not later than twelve (12) months prior to the expiration
of the Additional Space Term of this Lease, or the preceding Additional Space
Extension Period, as the case may be. If Tenant fails to timely give either the
Additional Space Inquiry Notice or the Additional Space Exercise Notice to
Landlord with respect to any Additional Space Option, Tenant shall be
conclusively deemed to have waived such Additional Space Option hereunder.

      (b) Notwithstanding any contrary provision of this Lease, each Additional
Space Option and any exercise by Tenant thereof shall be void and of no force or
effect unless on the dates. Tenant gives Landlord its Additional Space Inquiry
Notice and Additional Space Exercise Notice for each Additional Space Option
and on the date of commencement of each Additional Space Extension Period (i)
this Lease is in full force and effect, (ii) there is no Event of Default of
Tenant under this Lease, and (iii) Tenant has not assigned or subleased (or
agreed to assign or sublease) more than fifty percent (50%) of the rentable
floor area then comprising the Additional Space.

      (c) All of the terms, provisions, covenants, and conditions of this Lease
shall continue to apply during each Additional Space Extension Period, except
that the Additional Space Annual Fixed Rent Rate during each Additional Space
Extension Period (the "Extension Rent") shall be equal to the fair market rent
for the Additional Space determined as of the date twelve (12) months prior to
expiration of the Additional Space Term or the preceding Additional Space
Extension Period, as the case may be, in accordance with the procedure set forth
in Subsection (d) below (the "Fair Market Rent").

<PAGE>

                                       -3-

      (d) The Fair Market Rent for each Additional Space Extension Period shall
be determined as follows: Within five (5) days after Tenant gives Landlord its
Additional Space Inquiry Notice with respect to any Additional Space Option,
Landlord shall give Tenant notice of Landlord's determination of the Fair Market
Rent for the applicable Additional Space Extension Period. Within ten (10) days
after Tenant receives such notice, Tenant shall notify Landlord of its agreement
with or objection to Landlord's determination of the Fair Market Rent, whereupon
the Fair Market Rent shall be determined by arbitration conducted in the manner
set forth below. If Tenant does not notify Landlord within such ten (10) day
period of Tenant's agreement with or objection to Landlord's determination of
the Fair Market Rent, then the Fair Market Rent for the applicable Additional
Space Extension Period shall be deemed to be Landlord's determination of the
Fair Market Rent as set forth in the notice from Landlord described in this
subsection.

      (e) If Tenant notifies Landlord of Tenant's objection to Landlord's
determination of Fair Market Rent under the preceding subsection, such notice
shall also set forth a request for arbitration and Tenant's appointment of a
commercial real estate broker having at least ten (10) years experience in the
commercial leasing market in the City of Cambridge, Massachusetts (an
"Arbitrator"). Within five (5) days thereafter, Landlord shall by notice to
Tenant appoint a second Arbitrator. Each Arbitrator shall be advised to
determine the Fair Market Rent for the applicable Additional Space Extension
Period within thirty (30) days after Landlord's appointment of the second
Arbitrator. On or before the expiration of such thirty (30) day period, the two
Arbitrators shall confer to compare their respective determinations of the Fair
Market Rent. If the difference between the amounts so determined by the two
Arbitrators is less than or equal to ten percent (10%) of the lower of said
amounts then the final determination of the Fair Market Rent shall be equal to
the average of said amounts. If such difference between said amounts is greater
than ten percent (10%), then the two arbitrators within ten (10) days thereafter
shall appoint a third Arbitrator (the "Third Arbitrator"), who shall be
instructed to determine the Fair Market Rent for the applicable Additional Space
Extension Period within ten (10) days after his appointment by selecting one of
the amounts determined by the other two Arbitrators. Each party shall bear the
cost of the Arbitrator selected by such party. The cost for the Third
Arbitrator, if any, shall be shared equally by Landlord and Tenant.

      3. Tenant shall accept the Additional Space in "as is" condition. Tenant
acknowledges that Landlord has made, in anticipation of Tenant's future
occupancy, for the benefit of Tenant at Landlord's sole cost and expense,
certain improvements to the Additional Space as outlined in Exhibit B. Landlord
agrees to consult with Tenant prior to agreeing to any changes requested by
Millennium to the Additional Space.

<PAGE>

                                       -4-

      4. Upon execution of this Second Amendment to Lease, section 4.1(d) of the
Lease will be stricken in its entirety and be null and void and of no further
force and effect.

      5. Upon occupancy by Tenant of the Additional Space, Tenant shall pay to
Landlord Annual Fixed Rent for the Additional Space in the amount of
$1,460,186.00 (the "Additional Space Annual Fixed Rent Rate"), payable in equal
monthly installments of $121,682.17 in advance on the first day of each calendar
month; and for any portion of a calendar month at the beginning or end of the
Term, at that rate payable in advance for such portion.

      6. Article 4.1(b) shall be renumbered as 4.1(b)(1) and the following shall
be added to the Lease as Article 4.1(b)(2):

            (b) (2) Adjustment for CPI - Additional Space. (a) On December 31,
      2000 (the "First Adjustment Date"), the Additional Space Annual Fixed Rent
      Rate shall be increased by multiplying said rate by the lesser of (i) a
      fraction, the numerator of which shall be the Price Index (as hereinafter
      defined) most recently established prior to the First Adjustment Date, and
      the denominator of which shall be the Base Price Index (as hereinafter
      defined), or (ii) one hundred four percent (104%) per year, compounded
      annually over the period of time beginning April 1, 1997 through the First
      Adjustment Date. (b) On December 31, 2005 (the "Second Adjustment Date"),
      the Additional Space Annual Fixed Rent Rate (as adjusted) shall be
      increased by multiplying said rate by the lesser of (i) a fraction, the
      numerator of which shall be the Price Index (as hereinafter defined) most
      recently established prior to the Second Adjustment Date, and the
      denominator of which shall be the Base Price Index (as hereinafter
      defined), or (ii) one hundred four percent (104%) per year; compounded
      annually over the five (5) years of the Additional Space Term of this
      Lease. As used herein, the term "Price Index" shall mean and refer to the
      "Consumer Price Index for Urban Wage Earners and Clerical Workers, for
      the Boston, Massachusetts area, All Items (1982-84-100)" published by the
      Bureau of Labor Statistics of the United States Department of Labor or
      successor or substitute index appropriately adjusted, and the term "Base
      Price Index" shall mean and refer to the Price Index most recently
      established prior to the Commencement Date. In the event the Price Index
      (or a successor or substitute index) shall not be published for the City
      of Boston, Massachusetts area or for the months indicated above, the
      corresponding index for the United States City Average (and if this is not
      available, a reliable governmental or other nonpartisan publication
      evaluating similar or equivalent information as used in the Price Index)
      shall be used. In the event the Price Index ceases to use the 1982-84
      average of

<PAGE>

                                       -5-

      100 as the basis of calculation, or if a substantial change is made in the
      terms or numbers of items contained in the Price Index, then the Price
      Index shall be adjusted to the figure that would have been arrived at had
      the manner of computing the Price Index in effect at the date of this
      Lease not been changed.

      7. Upon commencement of the Additional Space Term, the Tenant's
Proportionate Fraction as set forth in the Lease will be amended to 100%.

      8. The provisions of Paragraph 10.11 of the Lease shall include reference
to the Additional Space Annual Fixed Rent Rate in determining the "Security
Deposit Amount" as the same may be adjusted. Upon commencement of the Additional
Space Term, the Tenant shall increase the Security Deposit Amount by an amount
equal to one (1) year Additional Space Annual Fixed Rent plus additional
amounts, if any, as set forth in paragraph 10.11 as amended. The Security
Deposit Amount shall be adjusted as provided in Section 10.11 by including the
Additional Space Annual Fixed Rent Rate and other rental amounts due with
respect to the Additional Space, as the same may be adjusted in accordance with
Section 4.1(b), Section 4.1(c), Section 10.12, Section 10.13 and Section 10.14
of the Lease as amended. The additional Security Deposit Amount may be in the
form of a Letter of Credit in the form of Exhibit L to the Lease and must be
delivered on the commencement of the Additional Space Term.

      9. Upon Tenant's reasonable request and subject to availability on the lot
upon which the Building is situated, Landlord shall provide additional surface
parking spaces to Tenant on a tenancy-at-will basis for an additional charge of
$75.00 per space per month.

      10. Landlord acknowledges that Tenant presently intends to reconfigure the
Additional Space upon taking occupancy. The process for such reconfiguration of
the Additional Space shall be in accordance with paragraph 3.3 of the Lease.
However, Tenant shall be under no obligation to reconfigure the Additional
Space.

      11. Exhibit I of the Lease is hereby replaced with the Exhibit "I"
attached hereto.

      All capitalized terms used herein shall have the same meaning as set forth
in the Lease.

      Except as otherwise expressly set forth herein, all other terms of the
Lease shall apply to the Additional Space, are hereby ratified and confirmed and
shall remain unchanged and in full force and effect.

<PAGE>

                                      - 6 -

      Executed this 13th day of June, 1997.

                                        LANDLORD:

                                        By: /s/ David E. Clem, Trustee
                                            ------------------------------------
                                            David E. Clem, Trustee as aforesaid
                                            and not individually

                                        By: /s/ David M. Roby
                                            ------------------------------------
                                            David M. Roby, Trustee as aforesaid
                                            and not individually

                                        TENANT:
                                        VERTEX PHARMACEUTICALS INCORPORATED

                                        By: /s/ Richard H. Aldrich
                                            ------------------------------------
                                        Name: Richard H. Aldrich
                                        Title: Senior Vice president

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