Document:

<PAGE>

                                  Exhibit 10.10

                                                December 21, 2000

                        CONFIDENTIAL

Richard H. Aldrich
75 Cambridge Parkway, #E702
Cambridge, MA 02139

Dear Rich:

      This will confirm our conversation regarding your separation from Vertex
Pharmaceuticals Incorporated ("Vertex").

      Your employment with Vertex will terminate effective as of June 21, 2002
(the "Termination Date"). You will remain an employee of the Company, subject to
all your obligations as an employee of Vertex, between now and the Termination
Date.

      Effective immediately, Vertex accepts your resignation from office as
Senior Vice President and Chief Business Officer and Clerk of Vertex, Director
of Vertex Pharmaceuticals (Europe) Limited and Clerk of Vertex Securities Corp.,
and from any and all other officerships and directorships of Vertex and its
subsidiaries. In addition, you will cease to serve as Vertex's representative on
the Board of Directors of Altus Biologics Inc.

     Vertex will continue to pay you your usual salary, at the biweekly rate of
$11,311 less all applicable taxes and other withholdings, through the
Termination Date. Your base salary will be subject to review annually by the
Compensation Committee of the Board or by the full Board in accordance with
Vertex's usual practices, but will not be less than the biweekly rate of
$11,311, and you will be eligible to receive such bonuses, stock option grants
and other compensation as may be determined from time to time by the
Compensation Committee or the Board.

     In addition, through the Termination Date you will be entitled to such life
insurance, health insurance and other employee benefits as may be offered or
generally made available by Vertex to its executive employees from time to time.

      In addition to your regular benefits described above, if you accept the
offer below, the following terms and conditions will apply:

      1. Vertex will grant you stock options for the purchase of 75,000 shares
of Vertex Common Stock at an exercise price equal to the Fair Market Value per
share of Vertex Common Stock on the date of grant, as determined under the 1996
Stock and Option Plan. Such options will vest over a period of 18 months, in 6
quarterly installments from the date of grant.

      2. Vertex will pay you a cash bonus for the year 2000 in the amount of
$100,000.

<PAGE>

Richard H. Aldrich
Page 2
Confidential

      3. Neither party will knowingly make any statement, take any action, or
conduct himself or itself in any way that such party has reason to believe may
adversely affect the reputation of, or goodwill toward, the other.

      4. You acknowledge the unique nature of the business of Vertex and the
need of Vertex to maintain its competitive advantage in the industry through the
protection of its trade secrets and proprietary information. Accordingly, you
agree that through the Termination Date and for a period of one (1) year
thereafter, you shall not, directly or indirectly, within the United States of
America or its territories or possessions or within any other country in which
Vertex conducts or plans to conduct its business or distributes any of its
products or renders any services (determined as of the effective date of this
agreement), engage in business with, own an interest in, be employed by, or
consult or advise for, any person or entity (except as a holder of not more than
a two percent (2%) equity interest in a publicly-traded entity) which is in
competition with the business of the Company, provided that the foregoing shall
not preclude your employment by any multi-divisional employer having a division
which engages in activities in competition with the business of Vertex so long
as you are not be involved in the operations or management of such competitive
division. For purposes hereof, an entity shall be deemed to be in competition
with the business of Vertex if it is engaged in the discovery of novel
pharmaceuticals, using structure-based rational drug design as its primary
research methodology. You further agree that through the Termination Date and
for a period of at least one (1) year after Termination Date you will not
solicit any other employee of or consultant or advisor to, or client, supplier,
customer or partner of Vertex to terminate his, her or its relationship with
Vertex.

      5. Upon your acceptance of these terms, you hereby remise, release and
forever discharge Vertex, its predecessors and successors in interest, and its
shareholders, directors, agents, attorneys, employees, assigns and heirs ("the
Releasees") of and from any and all debts, demands, actions, causes of action,
suits, accounts, covenants, contracts, guarantees, bonds, warrantees,
agreements, torts, damages, statutes, and any and all claims and liabilities
whatsoever of every name and nature, both in law and in equity, including
without limitation claims under the Americans with Disabilities Act, the Family
Medical Leave Act, the federal Age Discrimination in Employment Act, and the
Older Workers Benefits Protection Act, and claims of discrimination of any kind,
however arising, which you may have or have had, against the Releasees, or which
are based on facts on which you now have knowledge, and specifically (but
without limiting the generality hereof) on account of or arising out of or in
connection with your employment by Vertex and the cessation thereof; provided
that the foregoing shall not apply to any events occurring after the date of
this agreement.

      Upon your acceptance of these terms, Vertex hereby remises, releases and
forever discharges you, your assigns and heirs of and from any and all debts,
demands, actions, causes of action, suits, accounts, covenants, contracts,
guarantees, bonds, warrantees, agreements, torts, damages, statutes, and any and
all claims and liabilities whatsoever of every name and nature, both in law and
in equity, however arising, which you may have or have had, against you, your
assigns and heirs, or which are based on facts on which Vertex now has
knowledge, and specifically (but without limiting the generality hereof) on
account of or arising out of or in connection with your employment by Vertex and
the cessation thereof; provided that the foregoing shall not apply to any events
occurring after the date of this agreement.

<PAGE>

Richard H. Aldrich
Page 3
Confidential

      6. You acknowledge that you have carefully read and understand all the
provisions of this agreement and you are voluntarily entering into this
agreement. You have not relied upon any representation or any statement not set
forth herein with regard to the subject matter, basis or effect of this
agreement or otherwise.

      7. If you accept these terms, this agreement sets forth the entire
agreement between you and Vertex, and fully supersedes any and all prior
agreements or understandings between the parties hereto pertaining to the
subject matter hereof; provided, however, that your Employee Non-Disclosure and
Inventions Agreement will remain in full force and effect in accordance with its
terms, and your Vertex stock option agreements will remain in full force and
effect in accordance with their respective terms and the terms of the plans
under which they were granted.

      8. This agreement shall be binding upon the parties hereto and their
heirs, representatives, successors and assigns.

      9. If any provision of this agreement should be determined to be
unenforceable by any court of law, the remaining provisions shall be severable
and enforceable in accordance with their terms.

      10. This agreement will be governed by, and construed in accordance with,
the laws of the Commonwealth of Massachusetts.

      I urge you to consult with an attorney of your choice regarding this
offer. You will have 21 days to consider this offer and, if you accept, you will
have 7 days after you accept the offer within which you may rescind your
acceptance. This Agreement will not become effective until the 7-day rescission
period has passed. If you wish to accept the above offer, please sign below and
return one copy of this letter to me not later than 5:00 p.m. on December 28,
2000, or this offer will expire automatically.

                                       VERTEX PHARMACEUTICALS INCORPORATED

                                       By: /s/ Joshua S. Boger
                                           -------------------------------------
                                           Joshua S. Boger
                                           Chief Executive Officer

ACCEPTED AND AGREED:

/s/ Richard H. Aldrich
----------------------------------
Richard H. Aldrich

21 December 2000
----------------------------------
Date<PAGE>

                                  EXHIBIT 10.14

      The Company has omitted from this Exhibit portions of the Lease for which
the Company has requested confidential treatment from the Securities and
Exchange Commission. The portions of the Lease for which confidential treatment
has been requested have been deleted and marked with asterisks surrounded by
brackets ([****]) and have been filed separately with the Securities and
Exchange Commission.

                            THIRD AMENDMENT TO LEASE

      This THIRD 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 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 at40 Erie
Street, Cambridge, Massachusetts (the "Building"), as more particularly
described in the lease, as amended by a First Amendment to Lease (the "First
Amendment") and a Second Amendment to Lease (the "Second Amendment"). The lease,
Fist Amendment to Lease and Second Amendment to Lease are herein collectively
referred to as the "Lease".

      WHEREAS, the tenant has requested, and the Landlord has agreed, to further
amend the Lease upon the terms and conditions set forth in this Third Amendment
to Lease.

      WHEREAS, Landlord and Tenant desire to amend and modify the terms of the
Lease and to ratify and confirm the terms of the Lease as amended by the First
Amendment and the Second 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 the 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. Section 1 of the Second Amendment (other than the last sentence
thereof) is hereby deleted in its entirety.

      3. Effective as of October 1, 1998 (the "Effective Date"), 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 to the
Second Amendment for the layout of the Additional Space.

      4. The first paragraph of Section 2 of the Second Amendment is hereby
deleted in its entirety and the following is inserted in place thereof:

      The term "Additional Space Term" as used in the Lease shall mean the
      period commencing on the Effective Date and terminating on March 18, 2009.
<PAGE>
                                      -2-

      5. Section 5 of the Second Amendment is hereby deleted in its entirety and
the following is inserted in place thereof:

      5. Commencing on the Effective Date and continuing throughout the
      Additional Space Term, Tenant shall pay to Landlord Annual Fixed Rent for
      the Additional Space in the amount of [**CONFIDENTIAL**], subject to
      adjustment as set forth in the Lease, including Sections 2 and 6 of the
      Second Amendment. The Annual Fixed Rent for the Additional Space shall be
      payable in equal monthly installments in advance on the first day of each
      month and pro-rated for any portion of a calendar month during the
      Additional Space Term. In accordance with the foregoing, Tenant shall pay
      [****] to Landlord on October 1, 1998 and on the first day of each
      month thereafter until adjustment of Annual Fixed Rent for the Additional
      Space as set forth in the Lease.

      6. Commencing on the Effective Date and continuing to and including March
18, 1999 (the "Parking Term"), Landlord hereby leases to Tenant and Tenant
hereby leases from Landlord one hundred twenty (120) parking spaces (Tenant's
Additional Parking Spaces") on the Lot upon the terms and conditions hereinafter
set forth. Tenant shall pay to Landlord as Additional Rent, together with the
payments of Annual Fixed Rent for the Additional Space [**CONFIDENTIAL**] per
month for each of Tenant's Additional Parking Spaces, which amount shall be
prorated for the month of March, 1999. Tenant shall have the right to sublease
any of Tenant's Additional Parking Spaces to Millennium without the consent of
Landlord, but Tenant shall remain liable for the monthly rental fee for Tenant's
Additional Parking Spaces. Landlord reserves the right from time to time to
alter or relocate any or all of Tenant's Additional Parking Spaces on or off the
Lot, provided that the total number of parking spaces available to Tenant on or
off the Lot is not decreased and any of Tenant's Additional Parking Spaces
relocated off the Lot are located within one thousand (1000) feet of the
Building, provided, however, that upon completion of construction by Landlord of
the garage at 47 Erie Street, Cambridge, Massachusetts (the "Garage"), the
completion of which is expected by March, 1999, any of Tenant's Additional
Parking Spaces which have been relocated shall be located in the Garage for the
remainder, if any, of the Parking Term.

      7. Landlord hereby represents and warrants that Landlord and Millennium
have terminated Millennium's lease of the Additional Space and Tenant's
Additional Parking Spaces effective as of the Effective Date.

      8. Anything in the Lease to the contrary notwithstanding (i) Landlord
shall have no obligation to evict Millennium from the Additional Space or
Tenant's Additional Parking Spaces or to dispossess Millennium thereof, (ii)
Tenant may sublet all or such portion of the Additional Space to Millennium for
the period commencing on the Effective Date and terminating on such date within
the Term as Tenant and Millennium may agree and on such other terms and
conditions as are consistent with and not in violation of the Lease and as
Tenant otherwise may deem acceptable, (iii) Tenant may sublet all or such number
of
<PAGE>
                                      -3-

Tenant's Additional Parking Spaces to Millennium for the period commencing on
the Effective Date and terminating on or before March 18, 1999 and on such other
terms and conditions as are consistent with and not in violation of the Lease,
and (iv) any parking spaces subleased by Tenant to Millennium for any period
after March 18, 1999 shall consist of parking spaces leased to Tenant pursuant
to the Lease and shall not consist of any of Tenant's Additional Parking Spaces.

      9. Landlord shall have no liability or obligation to Tenant or Millennium
with respect to matters set forth in a letter dated February 5, 1998 from Tenant
to Mr. Dan Winny and a letter dated February 20, 1998 from Landlord to
Millennium, copies of which are attached hereto as Exhibits A and B
respectively, and incorporated herein by reference thereto, including without
limitation, any work performed by or on behalf of Millennium with respect to the
Additional Space, the restoration thereof and the replacement and/or storage of
fixtures removed from the Additional Space by Millennium, and Tenant hereby
releases Landlord from all such liabilities and obligations.

      10. The Lease, as amended hereby, is hereby ratified and confirmed in all
respects.
<PAGE>
                                      -4-

Executed as of the 1st day of October, 1998:

                              LANDLORD:

                              FORT WASHINGTON REALTY TRUST

                              /s/ David E. Clem, Trustee
                              ------------------------------------------
                              David E. Clem, Trustee
                              And not individually

                              FORT WASHINGTON REALTY TRUST

                              /s/ David M. Roby
                              ------------------------------------------
                              David M. Roby, Trustee
                              And not individually

                              TENANT:

                              VERTEX PHARMACEUTICALS
                              INCORPORATED

                              By:   /s/ Richard H. Aldrich
                                    ------------------------------------
                              Name:
                              Title:
<PAGE>

                                    EXHIBIT A

[VERTEX LOGO]  Vertex Pharmaceuticals Incorporated
               130 Waverly Street, Cambridge, MA  02139-4242
               Tel. 617-577-6000, Fax 617-577-6680
               http://www.vpharm.com

February 5, 1998

Mr. Dan Winny
C/o Lyme Properties/FWLP
P.O. Box 266
On the Common
Lyme, NH  03768

Dear Dan:

I am writing as a follow-up to various conversations and meetings regarding the
proposed renovations to FWRC/40 Erie St. by Millennium Pharmaceuticals. By
virtue of this letter, I am granting approval to proceed with the work as noted
on the official contract drawings submitted. For reference, those drawings are
as follows:

      Architectural A0, D01, D02, A01, A02, A03    Issued for Approval 1/23/98
      Plumbing P1.0, P1.1, P1.2                    Issued for Approval 1/28/98
      HVAC H1.1, H1.2                              Issued for Approval 1/28/98
      Electrical E1, E2, E3, E4                    Issued for Approval 1/28/98

Regarding the conditions for approval, I would include the following:

      1) All areas are to be 100% restored to the condition and function that
      was existing prior to this work being done. Restoration is to be the same
      specification as was existing.

      2) All fixtures that are removed will be inventoried and kept in secure
      off-site storage by Millennium. Vertex will approve the condition of the
      fixtures prior to going into storage and will also approve the condition
      when they return for installation to ensure that the fixtures have
      remained in the same condition while in storage. Millennium will be
      required to bear the cost of replacing any fixtures that are damaged while
      in storage.

      3) Any change orders to these plans must be approved by Vertex. We will
      provide approval within 24 hours of receipt of such changes.

      4) Millennium is to provide as-built drawings for this work within 30 days
      of completion of construction.

      5) Any interruption in building systems (electrical, fire alarms, plumbing
      etc.) that requires shutdowns that will affect Vertex must be requested
      for approval by Vertex no less than 48 hours in advance with either myself
      or Koji Kubota.

Please accept this letter as authorization to proceed from Vertex. If you need
any further information, please let me know. I appreciate being kept up to date
as the project progresses.

Sincerely,

/s/ Alfred Vaz, Jr.

Alfred Vaz, Jr.
Vice President Facilities & Operations
<PAGE>

                                    EXHIBIT B

                          Fort Washington Realty Trust
                               Post Office Box 266
                            Lyme, New Hampshire 03768
                                Tel: 603-795-2129
                                Fax: 603-795-4789

February 20, 1998

Arthur Brunelle
Millennium Pharmaceuticals, Inc.
640 Memorial Drive
Cambridge, MA  02139-4815

Re:   Millennium Build-Out Changes at 40 Erie Street

Dear Arthur,

As required by the terms of your lease, the architectural and engineering
drawings by D.T.S. Shaw and Associates dated 1/28/98 showing miscellaneous
build-out changes have been reviewed by Vertex and by ourselves.

This letter constitutes approval to proceed with the work, subject to
conditions. An approval letter from Vertex, dated 2/5/98 from Al Vaz, is
attached. The conditions for approval are those listed 1 through 5 in the Vertex
letter. You should pay particular attention to Item #3, relating to review of
changes. Changes will also be reviewed by us, and as-build record drawings
provided to us.

On a related matter, we have reviewed the submittal from D.T.S. Shaw dated
2/10/98 showing the revised location of the generator which is part of the
previously reviewed computer room package (plan attached). This location is
approved subject to details showing how the trailer is to be supported on the
planting area, and to the same conditions relating to building penetrations and
treatment of exterior exposed conduits which applied to the previous location.
Again, complete restoration is required at the termination of your lease. All
work must comply with all applicable codes, ordinances and regulations.

Sincerely,

/s/ David Clem

David Clem, Trustee
DEC:fp

Cc:  Al Vaz
     Janet Bush
     Chip Crane
     Gary Shaw
<PAGE>

[VERTEX LOGO]  Vertex Pharmaceuticals Incorporated
               130 Waverly Street, Cambridge, MA  02139-4242
               Tel. 617-577-6000, Fax 617-577-6680
               http://www.vpharm.com

February 5, 1998

Mr. Dan Winny
C/o Lyme Properties/FWLP
P.O. Box 266
On the Common
Lyme, NH  03768

Dear Dan:

I am writing as a follow-up to various conversations and meetings regarding the
proposed renovations to FWRC/40 Erie St. by Millennium Pharmaceuticals. By
virtue of this letter, I am granting approval to proceed with the work as noted
on the official contract drawings submitted. For reference, those drawings are
as follows:

      Architectural A0, D01, D02, A01, A02, A03    Issued for Approval 1/23/98
      Plumbing P1.0, P1.1, P1.2                    Issued for Approval 1/28/98
      HVAC H1.1, H1.2                              Issued for Approval 1/28/98
      Electrical E1, E2, E3, E4                    Issued for Approval 1/28/98

Regarding the conditions for approval, I would include the following:

      1) All areas are to be 100% restored to the condition and function that
      was existing prior to this work being done. Restoration is to be the same
      specification as was existing.

      2) All fixtures that are removed will be inventoried and kept in secure
      off-site storage by Millennium. Vertex will approve the condition of the
      fixtures prior to going into storage and will also approve the condition
      when they return for installation to ensure that the fixtures have
      remained in the same condition while in storage. Millennium will be
      required to bear the cost of replacing any fixtures that are damaged while
      in storage.

      3) Any change orders to these plans must be approved by Vertex. We will
      provide approval within 24 hours of receipt of such changes.

      4) Millennium is to provide as-built drawings for this work within 30 days
      of completion of construction.

      5) Any interruption in building systems (electrical, fire alarms, plumbing
      etc.) that requires shutdowns that will affect Vertex must be requested
      for approval by Vertex no less than 48 hours in advance with either myself
      or Koji Kubota.

Please accept this letter as authorization to proceed from Vertex. If you need
any further information, please let me know. I appreciate being kept up to date
as the project progresses.

Sincerely,

/s/ Alfred Vaz, Jr.

Alfred Vaz, Jr.
Vice President Facilities & Operations
<PAGE>

                 [GRAPHIC OF GENERATOR LOCATION PLAN, SCHEME 4]
<PAGE>

                            FOURTH AMENDMENT TO LEASE

      This FOURTH AMENDMENT TO LEASE (this "Fourth Amendment") is executed 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 (the "Registry") in Book
25422, Page 360 and filed with the Middlesex South Registry District of Land
Court (the "Registry District") as Document No. 976230 (the "Landlord") and
Vertex Pharmaceuticals Incorporated (the "Tenant").

      Reference is hereby made to that certain 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"), a Second Amendment to Lease (the "Second Amendment") and a Third
Amendment to Lease (the "Third Amendment"). The lease, First Amendment, Second
Amendment and Third Amendment are herein collectively referred to as the
"Lease".

      WHEREAS, Landlord and Tenant desire to amend and modify the terms of the
Lease and to ratify and confirm the terms of the Lease as amended by the First
Amendment, the Second Amendment and the Third 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 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 definition of "Lot" as set forth in Section 1.1 of the Lease is
hereby deleted and the following definition is hereby inserted in place thereof:

            The Land shown on Exhibit A-1 to this Fourth Amendment as the "Fort
            Washington Premises" and more particularly described on Exhibit A-2
            to this Fourth Amendment, both of which are attached hereto and
            incorporated herein by reference thereto.

      3. Anything in the Lease to the contrary notwithstanding, references in
the Lease to "Exhibit A-1" or "Exhibit A-2", including Section 1.2 of the Lease,
shall mean Exhibit A-1 attached hereto and Exhibit A-2 attached hereto,
respectively.

      4. Simultaneously with the execution hereof, Landlord and Tenant shall
execute, acknowledge and deliver an Amended Notice of Lease in the form of
Exhibit B
<PAGE>

attached hereto, which Landlord agrees to record with the Registry and file with
the Registry District.

      5. The Lease, as amended hereby, is hereby ratified and confirmed in all
respects.

      Executed under seal as of the 2nd day of February, 2000.

                            LANDLORD:

                            FORT WASHINGTON REALTY TRUST

                            /s/ David E. Clem, Trustee
                            ------------------------------------------------
                            David E. Clem, Trustee
                            And not individually

                            FORT WASHINGTON REALTY TRUST

                            /s/ David M. Roby
                            ------------------------------------------------
                            David M. Roby, Trustee
                            And not individually

                            TENANT:

                            VERTEX PHARMACEUTICALS
                            INCORPORATED

                            By:    /s/ Richard H. Aldrich
                                   -----------------------------------------
                            Name:  Richard H. Aldrich
                            Title: Senior Vice-President, Chief Business Officer

                                      -2-
<PAGE>

                                   EXHIBIT A-1

                      Plan Showing Fort Washington Premises
                                   (attached)
<PAGE>

                                     [PLAN]

<PAGE>

                            FIFTH AMENDMENT TO LEASE

      This FIFTH AMENDMENT TO LEASE (this "Fifth Amendment") is executed 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 (the "Registry") in Book
25422, Page 360 and filed with the Middlesex South Registry District of the Land
Court (the "Registry District") as Document No. 976230 (the "Landlord") and
Vertex Pharmaceuticals Incorporated (the "Tenant").

       Reference is hereby made to that certain 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"), a Second Amendment to Lease (the "Second
Amendment"), a Third Amendment to Lease (the "Third Amendment") and a Fourth
Amendment to Lease (the "Fourth Amendment"). The lease, First Amendment,
Second Amendment, Third Amendment and Fourth Amendment are herein
collectively referred to as the "Lease".

      WHEREAS, Landlord and Tenant desire to amend and modify the terms of the
Lease and to ratify and confirm the terms of the Lease, 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 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. Effective as of May 1, 1999 (the "Amendment Effective Date"), the
definition of Premises set forth in the Lease shall be amended to include the
addition of 400 r.s.f. of space on the second floor of the Building (the "Infill
Space"), which is more particularly shown as "AREA OF INFILL" on Exhibit A
attached hereto and incorporated herein by reference thereto.

      3. From and after the Amendment Effective Date, (A) the term "Additional
Space," as used in the Lease shall mean (i) the Infill Space and (ii) the
Additional Space as defined and described in the Third Amendment, and (B) the
terms and conditions of Section 2 of the Second Amendment, as amended by the
Third Amendment, shall apply to the Infill Space as if originally included
therein as part of the Additional Space.

      4. Effective as of the Amendment Effective Date, Section 5 of the Second
Amendment, as amended by the Third Amendment, is hereby deleted in its entirety
and the following is inserted in place thereof:
<PAGE>

            Commencing on the Amendment Effective Date and continuing throughout
            the Additional Space Term (as such term is defined in the Third
            Amendment), Tenant shall pay to Landlord Annual Fixed Rent for the
            Additional Space in the amount of [**CONFIDENTIAL**], subject to
            adjustment as set forth in the Lease, including Section 2 and 6 of
            the Second Amendment. The Annual Fixed Rent for the Additional Space
            shall be payable in equal monthly installments in advance on the
            first day of each month and prorated for any portion of a calendar
            month during the Additional Space Term. In accordance with the
            foregoing, Tenant shall pay [**CONFIDENTIAL**] to Landlord on the
            Amendment Effective Date and on the first day of each month
            thereafter until adjustment of Annual Fixed Rent for the Additional
            Space as set forth in the Lease.

      5. The Lease, as amended hereby, is hereby ratified and confirmed in all
respects.

      Executed under seal as of the 1st day of May, 1999.

                              LANDLORD:

                              FORT WASHINGTON REALTY TRUST

                              /s/ David E. Clem, Trustee
                              ------------------------------------------
                              David E. Clem, Trustee
                              And not individually

                              FORT WASHINGTON REALTY TRUST

                              /s/ David M. Roby
                              ------------------------------------------
                              David M. Roby, Trustee
                              And not individually

                              TENANT:

                              VERTEX PHARMACEUTICALS
                              INCORPORATED

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

                                      -2-
<PAGE>

                                    EXHIBIT A

                            Plan Showing Infill Space
                                   (attached)
<PAGE>

                                     [PLAN]

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