Document:

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                                                                   Exhibit 10.15

                            FOURTH AMENDMENT TO LEASE

      This FOURTH AMENDMENT TO LEASE (this "Fourth Amendment") of September 30,
2004 is executed by and between KS Parcel A, LLC as successor to Kendall Square,
LLC ("Landlord") and Vertex Pharmaceuticals Incorporated ("Tenant").

      Reference is made to that certain lease dated January 18, 2001, as amended
by First Amendment to Lease dated as of May 9, 2002, Confirmation of
Commencement Date and Rentable Square Footage dated January 30, 2003, Second
Amendment to Lease dated as of September 16, 2003, Third Amendment to Lease
dated as of December 22, 2003, a certain letter agreement dated March 24, 2004
acknowledging agreement that, taking account of the Phase I Work Holiday Period
the Phase I Work completion date is July 20, 2004, and a certain letter
agreement dated September 17, 2004 extending the Phase I Work completion date to
September 30, 2004 (as amended, the "Lease"), by and between Landlord and Tenant
with respect to a portion of the Building on the Lot (as such terms are defined
in the Lease) located in Cambridge, Massachusetts.

      Landlord and Tenant desire further to amend the terms of the Lease, to
ratify and confirm the terms of the Lease as amended, and to confirm certain
other matters, all as more particularly set forth below.

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

      1. Each capitalized term which is used but not defined herein shall have
the respective meaning ascribed thereto in the Lease.

      2. The last sentence of the next to last paragraph of Section 3.2.1 of the
Lease, as previously amended by the Second Amendment, is deleted and the
following is substituted in place thereof: "Tenant has completed certain of the
Phase I Work as such Phase I Work is described in Attachment A to the Second
Amendment to Lease (such completed Phase I Work being the "Phase IA Work"
covered by a Certificate of Occupancy issued on July 19, 2004 by the City of
Cambridge and provided to Landlord by Tenant on July 20, 2004) and the parties
desire to acknowledge such completion, make provision for the commencement and
completion of the remaining Phase I Work described below ("Phase IB Work") and
make certain changes with respect to the Phase II Work, more particularly
described below, and its timing. On or before November 5, 2004 Tenant shall
submit Construction Documents for those portions of the Phase IB Work not
previously approved by Landlord as well as for the Phase II Work for Landlord's
approval under other provisions of the Lease, provided, however, that in the
event the subtenant under the Momenta Sublease, as defined below, does not
provide Tenant with construction plans on the date required for submission of
such plans to Tenant under the Momenta Sublease, the date for submittal of
Construction Documents for the Phase IB Work

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shall be extended one day for each day the subtenant submittal is delayed under
the Momenta Sublease, such submittal, however, not to be later than November 19,
2004, shall thereafter diligently obtain applicable permits and commence
construction and, subject to Sections 7.1 (h) and 10.6 hereof, shall (a)
substantially complete the Phase IB Work on or before April 30, 2005, (b)
substantially complete the Phase II Work on or before April 30, 2005, and (c)
commence on or before October 1, 2005 and substantially complete all Tenant's
Work as to the remainder of the Premises ("Phase III Work") within thirty-six
(36) months after the Commencement Date, all such improvements to be performed
in accordance with Construction Documents approved by Landlord as provided for
in the Lease, and to have a cost of approximately $18 million in the case of the
combined Phase IA and IB Work and approximately $17 million in the aggregate for
the Phase II and Phase III Work. The "Phase IB Work" consists of any remaining
unfinished Phase I Work and includes, without limitation, those portions of the
tenant build-out required to be performed by Tenant as sublandlord under a
sublease with Momenta Pharmaceuticals, Inc as subtenant dated September 14, 2004
("Momenta Sublease") consisting of approximately 44,000 rentable square feet on
the fourth (4th) floor and 1,000 rentable square feet on the first (1st) floor
as such areas are described in the Momenta Sublease. The "Phase II Work"
consists of the remainder of the build out under the Momenta Sublease, all of
the sixth (6th) floor of the building and such additional space on the first
floor of the building as shall be necessary to build out seventy-five (75%)
percent of the Premises under the Lease.

      3. Clause (h) of Section 7.1 of the Lease is deleted (up to but not
including the parenthesis beginning with the words, "the foregoing events
described in clauses (a) through (h)...") and the following is substituted in
place thereof: "(h) due to any cause other than a Force Majeure Event, (i) if
Tenant fails to make the Construction Documents submission to Landlord for the
Phase IB Work or Phase II by the date set forth in the next to last paragraph of
Section 3.2.1 hereof or shall suspend or otherwise fail to diligently pursue the
construction of any Tenant's Work or if Tenant shall fail to provide the Letters
of Credit in the amounts, at the times and in the form required under a certain
Agreement of Surety Indemnity between Tenant and The Lyme Timber Company, the
surety furnishing the bond with respect to the Phase I, II and III Work, and any
such default shall continue for thirty (30) days after written notice from
Landlord to Tenant in any such case, or (ii) if Tenant fails to substantially
complete the Phase IB Work or the Phase II Work by the respective dates pursuant
to such next to last paragraph of Section 3.2.1 hereof and such completion
failure shall continue for more than two (2) months in each case, or (iii) if
Tenant fails to commence or to substantially complete the Phase III Work by the
respective dates determined pursuant to such next to last paragraph of Section
3.2.1 and any such commencement failure shall continue for nine (9) months or
completion failure shall continue for six (6) months in the case of Phase III
Work, in each case after written notice from Landlord to Tenant,"

      4. As hereby amended, the Lease is ratified, confirmed and continued in
effect.

      5. The parties acknowledge that within ten (10) business days following
the execution of this Amendment, Tenant will increase the letter of credit
furnished to Landlord in accordance with Sections 2.3, 4.1 and 10.11 of the
Lease by $1,535,800 bringing the total thereof to $17,655,800, being the amount
the parties agree is required under the Lease.

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      6. By execution below, the surety which has issued a bond pursuant to a
certain Agreement of Surety Indemnity dated as of September 16, 2003 has
consented to this Fourth Amendment, it being understood and agreed with said
surety that except with respect to the underlying Lease obligations expressly
modified by this Fourth Amendment, all other obligations with respect to said
Agreement of Surety Indemnity, including without limitation the obligations to
provide letters of credit in the amounts and at the times therein provided, are
ratified, confirmed and continued in effect except as follows: wherever the
words "Phase I Work" are used in such Agreement of Surety Indemnity, the same
shall hereafter mean "Phase IA and Phase IB Work".

      Executed as of the date first written above.

LANDLORD:                                 TENANT:

KS PARCEL A, LLC                          VERTEX PHARMACEUTICALS
By:Kendall Square, LLC, its Manager       INCORPORATED
   By: Lyme Properties, LLC, its Manager

          By: /s/ David Clem              By: /s/ Kenneth S. Boger
              -----------------               ----------------------------
              Its Manager                     Name: Kenneth S. Boger
                                              Title: Senior VP & General Counsel

The undersigned surety under an Agreement of Surety Indemnity dated as of
September 16, 2003 hereby consents to the foregoing Fourth Amendment to Lease.

The Lyme Timber Company, a New Hampshire limited partnership

By:   Woodland Management Associates, LLC, a
      New Hampshire limited liability company,
      its general partner

      By: /s/ David M. Roby
          ------------------
      David M. Roby, Manager

                                       3<PAGE>
                                                                   Exhibit 10.16

                            FIFTH AMENDMENT TO LEASE

      This FIFTH AMENDMENT TO LEASE (this "Fifth Amendment") dated as of April
15, 2005 is executed by and between KS Parcel A, LLC as successor to Kendall
Square, LLC ("Landlord") and Vertex Pharmaceuticals Incorporated ("Tenant").

      Reference is made to that certain lease dated January 18, 2001, as amended
by First Amendment to Lease dated as of May 9, 2002, Confirmation of
Commencement Date and Rentable Square Footage dated January 30, 2003, Second
Amendment to Lease dated as of September 16, 2003, Third Amendment to Lease
dated as of December 22, 2003, a certain letter agreement dated March 24, 2004
acknowledging agreement that, taking account of the Phase I Work Holiday Period,
the Phase I Work completion date is July 20, 2004, and a certain letter
agreement dated September 17, 2004 extending the Phase I Work completion date to
September 30, 2004 and Fourth Amendment to Lease dated September 30, 2004 (as
amended, the "Lease"), by and between Landlord and Tenant with respect to a
portion of the Building on the Lot (as such terms are defined in the Lease)
located in Cambridge, Massachusetts.

      Landlord and Tenant desire further to amend the terms of the Lease, to
ratify and confirm the terms of the Lease as amended, and to confirm certain
other matters, all as more particularly set forth below.

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

      1. Each capitalized term which is used but not defined herein shall have
the respective meaning ascribed thereto in the Lease.

      2. The third to last sentence of the next to last paragraph of Section
3.2.1 of the Lease, as previously amended by the Fourth Amendment, is modified
to delete the dates "April 30, 2005" in the two places they appear and
substitute the dates "May 30, 2005". As so changed the phrases read in part:
"...shall (a) substantially complete the Phase IB Work on or before May 30,
2005, (b) substantially complete the Phase 11 Work on or before May 30, 2005,
...."

      3. Upon the execution hereafter of counterpart copies of this Fifth
Amendment by Landlord and surety of the Paragraph Three Consent set forth below
in their sole discretion, the May 30, 2005 dates in Paragraph 3 above shall be
deemed amended to "September 1, 2005".

      4. As hereby amended, the Lease is ratified, confirmed and continued in
effect.

      5. By execution below, the surety which has issued a bond pursuant to a
certain Agreement of Surety indemnity dated as of September 16, 2003 has
consented to this Fifth Amendment, it being understood and agreed with said
surety that except with respect to the underlying Lease obligations expressly
modified by this Fifth Amendment, all other obligations

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with respect to said Agreement of Surety Indemnity, including without limitation
the obligations to provide letters of credit in the amounts and at the times
therein provided, are ratified, confirmed and continued in effect.

      Executed as of the date first written above.

LANDLORD:                                       TENANT:

KS PARCEL A, LLC                                VERTEX PHARMACEUTICALS
By: Kendall Square, LLC, its Manager            INCORPORATED
      By: Lyme Properties, LLC, its Manager

          By: /s/ David M. Roby                 By: /s/ Kenneth S. Boger
              ---------------------                 ----------------------------
              Its Manager                           Name: Kenneth S. Boger
                                                    Title: Senior VP and General
                                                           Counsel

The undersigned surety under an Agreement of Surety Indemnity dated as of
September 16, 2003 hereby consents to the foregoing Fifth Amendment to Lease.

The Lyme Timber Company, a New
Hampshire limited partnership

By: Woodland Management Associates, LLC, a
    New Hampshire limited liability company,
    its general partner

    By: /s/ David M. Roby
        -----------------------------
    David M. Roby, Manager

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                               Paragraph 3 Consent

By their signatures, the undersigned hereby give their consent to the date
change effected by Paragraph 3 above as of this_____day of____________, 2005.

The Lyme Timber Company, a New Hampshire limited partnership

By: Woodland Management Associates, LLC, a New Hampshire limited liability
    company, its general partner

    By: /s/ David M. Roby
        --------------------------
    David M. Roby, Manager

KS PARCEL A, LLC
By: Kendall Square, LLC, its Manager
    By: Lyme Properties, LLC, its Manager

        By: /s/ David M. Roby
            ---------------------------
            Its Manager

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