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                                                                   EXHIBIT 10.50

                     FIRST AMENDMENT TO EMPLOYMENT AGREEMENT

         This AMENDMENT TO EMPLOYMENT AGREEMENT (the "First Amendment") made as
of July 1, 2001 between ARIAD Pharmaceuticals, Inc., a Delaware corporation (the
"Company"), and Timothy P. Clackson, Ph.D. (the "Employee").

         The Company and the Employee have entered into an Employment Agreement
dated as of June 8, 2000 (the "Agreement"), and the parties hereto desire to
further amend certain provisions of the Agreement.

         NOW, THEREFORE, in consideration of the premises set forth herein and
for other good and valuable consideration, the receipt of which is hereby
acknowledged, the parties hereto agree to further amend the Agreement as
follows:

         I. Term of Employment. The first sentence of Section 2 is hereby
amended to read as follows:

                  "The term of the Employee's employment under the Agreement is
         hereby extended to December 31, 2003 (the "Term"), unless sooner
         terminated pursuant to Section 4 or 5 of this Agreement; provided,
         however, that this Agreement shall automatically be renewed for
         successive one-year terms (the Term and, if the period of employment is
         so renewed, such additional period(s) of employment are collectively
         referred to herein as the "Term") unless terminated by written notice
         given by either party to the other at least 90 days prior to the end of
         the applicable Term."

         II. Compensation. Section 3.1 is hereby replaced and amended in its
entirety as follows:

                  "3.1. As full compensation for all services to be rendered
         pursuant to this Agreement, the Company agrees to pay the Employee a
         salary at the fixed rate of $185,000 per annum as of the date hereof
         and increased thereafter during the Term, by amounts, if any, to be
         determined by the Board of Directors of the Company (the "Board") in
         its sole discretion, payable in equal semi-monthly installments, less
         such deductions or amounts to be withheld as shall be required by
         applicable law and regulations."

         III. Definitions. The definition of the Company's "Field of Interest"
in Section 14 (b) of the Agreement is hereby amended to read as follows:

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                  The "Company's Field of Interest" is the discovery,
         development and commercialization of (i) pharmaceutical products based
         on (a) intervention in signal transduction pathways and (b) gene and
         cell therapy, and (ii) technologies to regulate genes and proteins. The
         Company's Field of Interest may be changed at any time at the sole
         discretion of the Company.

         IV. This Amendment shall be governed by and construed and enforced in
accordance with the laws of the Commonwealth of Massachusetts applicable to
agreements made and to be performed entirely in Massachusetts.

         V. Except as modified by this First Amendment, the Agreement remains in
full force and effect and unchanged.

                  IN WITNESS WHEREOF, the parties have executed this First
Amendment as of the date first written above.

                                ARIAD PHARMACEUTICALS, INC.

                                By:      /s/ Harvey J. Berger
                                    -----------------------------------
                                    Harvey J. Berger, M.D.
                                    Chairman and Chief Executive Officer

                                EMPLOYEE

                                   /s/ Timothy P. Clackson
                                --------------------------------
                                Timothy P. Clackson, Ph.D.

                                       2<PAGE>

                                                                   EXHIBIT 10.51

                      SECOND AMENDMENT EMPLOYMENT AGREEMENT

         This AMENDMENT TO EMPLOYMENT AGREEMENT (the "Second Amendment") made
as of June 12, 2002, between ARIAD Pharmaceuticals, Inc., a Delaware corporation
(the "Company"), and Timothy P. Clackson, Ph.D. (the "Employee").

         The Company and the Employee have entered into an Employment Agreement
dated as of June 8, 2000 (the "Agreement"), as previously amended, and the
parties hereto desire to further amend certain provisions of the Agreement.

         NOW, THEREFORE, in consideration of the premises set forth herein and
for other good and valuable consideration, the receipt of which is hereby
acknowledged, the parties hereto agree to further amend the Agreement as
follows:

         I. Employment, Duties and Acceptance. The second sentence of Section
1.1 is hereby amended to read as follows:

                  "The Employee's title shall be designated by the Chief
         Executive Officer and shall be Senior Vice President, Science and
         Technology.

         II. Term of Employment. The first sentence of Section 2 is hereby
amended to read as follows:

                  "The term of the Employee's employment under the Agreement is
         hereby extended to December 31, 2005 (the "Term"), unless sooner
         terminated pursuant to Section 4 or 5 of this Agreement; provided,
         however, that this Agreement shall automatically be renewed for
         successive one-year terms (the Term and, if the period of employment is
         so renewed, such additional period(s) of employment are collectively
         referred to herein as the "Term") unless terminated by written notice
         given by either party to the other at least 90 days prior to the end of
         the applicable Term."

         III. Compensation. Section 3.1 is hereby replaced and amended in its
entirety as follows:

                  "3.1. As full compensation for all services to be rendered
         pursuant to this Agreement, the Company agrees to pay the Employee,
         during the Term, a salary at the fixed rate of $213,000 per annum
         during the first year of the Term and increased each year thereafter,
         by amounts, if any, to be determined by the Board of Directors of the
         Company (the "Board") in its sole discretion, payable in equal
         semi-monthly installments, less such deductions or amounts to be
         withheld as shall be required by applicable law and regulations."

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         IV. Termination by the Employee. Section 5 is hereby replaced and
amended in its entirety as follows:

                  "5.1. The Employee may terminate this Agreement, if any one or
         more of the following shall occur: (a) a material breach of the terms
         of this Agreement by the Company and such breach continues for 30 days
         after the Employee gives the Company written notice of such breach;

                           (b) the Company shall make a general assignment for
         benefit of creditors; or any proceeding shall be instituted by the
         Company seeking to adjudicate it as bankrupt or insolvent, or seeking
         liquidation, winding up, reorganization, arrangement, adjustment,
         protection, relief, or composition of it or its debts under law
         relating to bankruptcy, insolvency or reorganization or relief of
         debtors, or seeking entry of an order for relief of the appointment of
         a receiver, trustee, or other similar official for it or for any
         substantial part of its property or the Company shall take any
         corporate action to authorize any of the actions set forth above in
         this subsection 5.1(b);

                           (c) an involuntary petition shall be filed or an
         action or proceeding otherwise commenced against the Company seeking
         reorganization, arrangement or readjustment of the Company's debts or
         for any other relief under the Federal Bankruptcy Code, as amended, or
         under any other bankruptcy or insolvency act or law, state or federal,
         now or hereafter existing and remain undismissed or unstayed for a
         period of 30 days;

                           (d) a receiver, assignee, liquidator, trustee or
         similar officer for the Company or for all or any part of its property
         shall be appointed involuntarily, or

                           (e) a Change in Control as defined in Section 14."

                  V. Severance. Section 6 is hereby replaced and amended in its
entirety as follows:

                  "6. If (i) the Company terminates this Agreement without Cause
         or (ii) the Employee terminates this Agreement pursuant to Section
         5.1(a), then: (1) except in the case of death or disability, the
         Company shall continue to pay Employee his current salary for the
         remaining period of the applicable Term; (2) all options granted
         pursuant to this Agreement that would have vested during the Term shall
         vest immediately prior to such termination; (3) the Company shall
         continue to provide all benefits subject to COBRA at its expense for up
         to one year.

                  In the event of a consummation of a Change in Control of the
         Company, and if the Employee gives notice of termination within 90 days
         after such occurrence, then (i) all stock, stock options, stock awards
         and similar equity rights granted to the Employee shall immediately
         vest and remain fully exercisable through their original term with all
         rights; and (ii) the Company shall

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         continue to pay Employee his current salary for the shorter of (a) six
         months, or (b) the remaining period of the applicable Term."

         VI. Definitions. The definition of the Company's "Field of Interest" in
Section 14 (b) of the Agreement is hereby amended to read as follows:

                  The "Company's Field of Interest" is the discovery,
                  development and commercialization of pharmaceutical products
                  based on (a) intervention in cell signaling and (b) gene and
                  cell therapy. The Company's Field of Interest may be changed
                  at any time at the sole discretion of the Company and upon
                  written notice to Employee.

                  The definition of "Change in Control" shall be added as
         Section 14 (e) of the Agreement as follows:

                  " 'Change in Control' means the occurrence of any of the
         following events (without the consent of the Employee):

                  (i) Any corporation, person or other entity makes a tender or
         exchange offer for shares of the Company's Common Stock pursuant to
         which such corporation, person or other entity acquires more than 50%
         of the issued and outstanding shares of the Company's Common Stock;

                  (ii) The stockholders of the Company approve a definitive
         agreement to merge or consolidate the Company with or into another
         corporation or to sell or otherwise dispose of all or substantially all
         of the Company's assets; or

                  (iii) Any person within the meaning of Section 3 (a) (9) or
         Section 13 (d) of the Securities Exchange Act of 1934 acquires more
         than 50% of the combined voting power of Company's issued and
         outstanding voting securities entitled to vote in the election of the
         Board."

         VII. This Amendment shall be governed by and construed and enforced in
accordance with the laws of the Commonwealth of Massachusetts applicable to
agreements made and to be performed entirely in Massachusetts.

         VIII. Except as modified by this Second Amendment, the Agreement
remains in full force and effect and unchanged.

                       THIS SPACE INTENTIONALLY LEFT BLANK

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         IN WITNESS WHEREOF, the parties have executed this Second Amendment as
of the date first written above.

                                    ARIAD PHARMACEUTICALS, INC.

                                    By:      /s/ Harvey J. Berger
                                        ----------------------------------
                                        Harvey J. Berger, M.D.
                                        Chairman and Chief Executive Officer

                                    EMPLOYEE

                                          /s/ Timothy P. Clackson
                                    -------------------------------------
                                    Timothy P. Clackson, Ph.D.

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