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.



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.



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
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Date