Document:

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                                                                   Exhibit 10.12

                                   SYNTApharma

                                                           April 8, 2004

Dr. Mitsunori Ono
[ADDRESS]

Dear Dr. Ono:

     The purpose of this letter agreement is to set forth our mutual
understanding and agreement with respect to your resignation from employment
with Synta Pharmaceuticals Corp. (the "Company"). In consideration of the mutual
covenants set forth herein, the receipt and sufficiency of which you
acknowledge, we have agreed as follows:

1.   RESIGNATION FROM EMPLOYMENT. Your resignation from employment shall be
     effective as of the close of business, Thursday, January 1, 2004 (your
     "resignation date"), and you shall have relinquished as of that date any
     and all positions that you have held with the Company, including but not
     limited to President and Chief Operating Officer of the Company. You shall
     not be considered an employee of the Company for any purpose after that
     date and shall have no authority to act on behalf of the Company.

2.   TERMINAL PAY. You agree that you have received all compensation to which
     you are entitled in connection with your employment through your
     resignation date. You agree to make no claims for further compensation from
     the Company of any type, including bonus payments, commission payments, and
     vacation pay. You acknowledge that, except to the extent provided herein,
     the Company is under no obligation to provide you with the consideration
     described below, including, but not limited to, the severance payment.

3.   SEVERANCE PAYMENT. The Company shall pay you a lump sum amount of
     $200,000.00 (less all applicable federal, state or local tax withholding,
     F.I.C.A., and any other applicable payroll deductions), subject to the
     Company's receipt of this letter agreement and General Release signed by
     you and the expiration of the seven (7) day revocation period contained in
     paragraph 16 hereof. The Company shall also pay you monthly installments
     totaling $250,000.00 commencing in March 2004 for eighteen (18) months
     consisting of seventeen (17) payments in the amount of $13,888.89 and the
     final payment in the amount of $13,888.87. In the event that your death
     shall precede the Company's full payment of any of the amounts set forth in
     this Section 3. then any unpaid amounts under this section shall continue
     to be paid upon the schedule set forth above to your surviving spouse or,
     if there is no surviving spouse, to your estate, unless the Company
     receives a written instrument signed by you designating a beneficiary and
     providing the beneficiary's social security number (if any) and home
     address.

4.   STOCK OPTIONS. You acknowledge that, pursuant to Incentive Stock Option
     Agreement No. 004 dated December 13, 2002 ("ISOA No. 004") and Incentive
     Stock Option

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     Agreement No. 099 dated June 17, 2003 ("ISOA No. 099"), your right to
     exercise ISOA No. 004 for an aggregate of 812,500 shares (the "Original
     Vested Shares") of the Company's common stock, $.0001 par value per share
     (the "Common Stock"), vested as of your resignation date. You agree that as
     of your resignation date and subject to the approval of the Compensation
     Committee of the Company's Board of Directors:

     (i)      ISOA No. 099 shall terminate; and

     (ii)     the terms of ISOA No. 004 shall be amended as follows:

              a.   All references to "1,500,000 shares" on the face of ISOA No.
                   004 will be deleted and replaced with "1,000,000 shares";

              b.   Section 3 of ISOA No. 004 will be deleted in its entirety
                   and replaced with the following:

                        "3. VESTING SCHEDULE. The Option shall be exercisable
                   with respect to 1,000,000 of the Stock Rights Shares
                   immediately."

              c.   Section 4 of the ISOA No. 004 will be deleted in its
                   entirety and replaced with the following:

                        "4. TERM OF OPTION. Subject to earlier termination as
                   provided in this Agreement or the Plan, the Option shall
                   expire on the tenth anniversary of the Grant Date."

     (iii)    upon the effective date of the amendments set forth in paragraph
              4(ii) above, ISOA No. 004 shall cease to be treated as an
              incentive stock option for purposes of the Company's 2001 Stock
              Option Plan and for tax purposes pursuant to Section 422 of the
              Internal Revenue Code of 1986, as amended.

     (iv)     the first aggregate of 187,500 shares you exercise in connection
              with ISOA No. 004, as amended, shall not consist of any Original
              Vested Shares.

              A copy of the First Amendment To Incentive Stock Option Agreement
              No. 004 is attached hereto as Exhibit A.

5.   MEDICAL INSURANCE CONTINUATION. At your option, you may continue to be
     covered under the Company's group medical insurance plan for up to eighteen
     (18) months after your resignation date, subject to the terms and
     conditions provided for in the Consolidated Omnibus Budget Reconciliation
     Act of 1985, as amended ("COBRA"). The Company shall, at your option, pay
     the entire cost of your group medical insurance premiums during the entire
     eighteen (18) months of your COBRA continuation period or until such
     earlier time as you become eligible for alternate medical insurance
     coverage from a new employer provided you have timely and properly elected
     COBRA coverage in accordance with the Company's COBRA election procedures,
     notice of which shall be

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     sent to you under separate cover. You agree promptly to notify the Company
     in writing if and when you become eligible for alternate medical coverage
     during this eighteen (18) month period.

6.   TRANSFER OF RESPONSIBILITIES. You shall cooperate fully with the Company
     and its personnel to provide an orderly transfer of your duties and
     responsibilities. This cooperation includes but is not limited to timely
     compliance with all reasonable requests for information.

7.   CONFIDENTIALITY. You agree, to the extent permitted by law, to keep
     confidential and not to disclose the existence or terms of this letter
     agreement or sums paid under this letter agreement to anyone or to any
     organization, except you may disclose such information to your spouse,
     attorney, and financial advisor, provided you have received in advance
     their promises to maintain this information in strict confidence; provided,
     however, that nothing in this Agreement will prevent you from cooperating
     with or participating in any proceeding before the EEOC, the MCAD or any
     other federal, state or local agency or entity. You also agree that you
     will not, without the Company's prior written consent, reveal or disclose
     to any person or entity outside of the Company or use for your own benefit
     or for the benefit of any other person or entity, any fact or information
     which was disclosed to or developed by you during the course of your
     employment with the Company, and is not generally available to the public,
     including but not limited to information and facts concerning the business
     plans, customers, future customers, suppliers, licensors, licensees,
     partners, investors, affiliates, training methods and materials, financial
     information, sales prospects, client lists, Inventions (as defined in
     paragraph 11), or any other scientific, technical, trade or business secret
     or confidential or proprietary information of the Company ("Confidential
     Information").

8.   RETURN OF PROPERTY. You acknowledge that you have returned to the Company
     all property of the Company that is in your possession or under your
     control, including, without limitation, the laptop computer, printer, other
     computer accessories, pager, corporate credit card, telephone card, Company
     keys, and any and all written or digital-based files, documents,
     communications with the Company's directors, officers, employees and
     consultants, and other information with respect to the Company's
     management, business operations or customers, including all files,
     documents, or other information containing Confidential Information. You
     further acknowledge and agree that payment of your final severance payment
     at the end of the eighteen (18) month severance period will be conditioned
     upon your return of the company-issued automobile to the Company, in good
     condition, normal wear and tear excluded, by July 1, 2005. It is understood
     that you may continue to use the company-issued automobile for the duration
     of the eighteen (18) month lease period.

9.   COOPERATION IN LITIGATION. At the Company's request, you agree to assist,
     consult with, and cooperate with the Company in any litigation or
     administrative proceeding or inquiry that involves the Company, subject to
     reimbursement for your reasonable out of pocket expenses, such as travel,
     meals, or lodging.

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10.  NON-DISPARAGEMENT. You further agree, to the extent permitted by law, that
     you will not, at any time after the date hereof, make any remarks or
     comments, orally or in writing, to the Company's shareholders, officers,
     directors, employees, agents, or others, which or who have, or could
     reasonably be anticipated to have, business dealings with the Company,
     which remarks or comments reasonably could be construed to be derogatory or
     disparaging to the Company or any of its shareholders, officers, directors,
     employees, attorneys or agents, or which reasonably could be anticipated to
     be damaging or injurious to the Company's reputation or good will or to the
     reputation or good will of any person associated with the Company. The
     Company agrees, to the extent permitted by law, that neither the Chief
     Executive Officer or Scientific Founder will, at any time after the date
     hereof, make any remarks or comments, orally or in writing, to third
     parties which remarks or comments reasonably could be construed to be
     derogatory or disparaging to you which reasonably could be anticipated to
     be damaging or injurious to your reputation or good will.

11.  OWNERSHIP OF INVENTIONS AND DEVELOPMENTS. You agree that all ideas,
     discoveries, creations, manuscripts and properties, innovations,
     improvements, know-how, inventions, designs, developments, apparatus,
     techniques, algorithms, software, mask works, methods, and formulae which
     you worked upon, conceived, made, developed or improved, during your
     employment by the Company whether or not reduced to practice and whether or
     not patentable, copyrightable, or otherwise protectable, alone or in
     conjunction with any other party, and whether or not at the request or upon
     the suggestion of the Company (all of the foregoing being hereinafter
     referred to as the "Inventions"), are the sole and exclusive property of
     the Company. The Company is and will be the sole owner of all patents,
     copyrights and other proprietary rights in and with respect to such
     Inventions. To the fullest extent permitted by law, such Inventions will be
     deemed works made for hire. You hereby transfer and assign to the Company
     any proprietary rights which you may have or acquire in any such Inventions
     and you waive any moral rights or other special rights which you may have
     or accrue therein. You agree to execute any documents and take any actions
     that may be required to effect and confirm such transfer and assignment and
     waiver. In the event that the Company is unable for any reason to secure
     your signature to any lawful and necessary document required to perfect its
     rights in and to any Inventions as set forth in this paragraph 12, you
     hereby designate the Company as your agent for, and grant to the Company a
     power of attorney with full power of substitution, which power of attorney
     shall be deemed coupled with an interest, for the purpose of effecting the
     foregoing assignments from you to the Company.

12.  NON-SOLICITATION. You agree that you will not, while receiving severance
     payments from the Company, directly or indirectly, whether as owner,
     partner, shareholder, consultant, agent, employee, co-venturer or
     otherwise, engage or participate in the hiring of any Company employees or
     consultants or engage or participate in the solicitation of or attempt to
     solicit any Company employees or consultants resulting in the termination
     of their employment or relationship with the Company or resulting in their
     working for any other business, person or company. For purposes of this
     Paragraph 12, employees and consultants shall include those who were
     employed or had a relationship with the Company during the twelve month
     period prior to the resignation date.

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13.  BREACH OF AGREEMENT. You understand and agree that any material breach of
     your obligations under this letter agreement will immediately render the
     Company's obligations and agreements hereunder null and void, all payments
     pursuant to paragraphs 3 and 5 shall immediately cease, and, to the extent
     permitted by law, you shall repay to the Company all sums you have been
     paid or sums paid on your behalf pursuant to paragraphs 3 and 5.

14.  MUTUAL GENERAL RELEASE. You, for yourself and your heirs, legal
     representatives, beneficiaries, assigns and successors in interest, hereby
     knowingly and voluntarily release, remise and forever discharge the Company
     and its successors, assigns, former or current shareholders, officers,
     directors, employees, agents, attorneys and representatives ("Company
     Released Parties") whether in their individual or official capacities, from
     any and all actions or causes of action, suits, debts, claims, complaints,
     contracts, controversies, agreements, promises, damages, claims for
     attorneys' fees, costs, interest, punitive damages or reinstatement,
     judgments and demands whatsoever, in law or equity, you now have, may have
     or ever had, known or unknown from the beginning of the world to this date,
     including, without limitation, any claims under the Age Discrimination in
     Employment Act, 29 U.S.C. Section 621 ET SEQ.; Title VII of the Civil
     Rights Act of 1964, 42 U.S.C. Section 2000e ET SEQ.; the Employee
     Retirement Income Security Act of 1974, as amended, 29 U.S.C. Section 1000
     ET SEQ.; Massachusetts General Laws, Chapters 149, 151B, 214; the
     Massachusetts Civil Rights Act; the Massachusetts Equal Rights Act; the
     Americans with Disabilities Act, 42 U.S.C. Section 12101 et seq.; claims
     for breach of contract or based on tort; and any other statutory,
     regulatory or common law causes of action ("the Released Claims"). The
     Company hereby knowingly and voluntarily releases, remises and forever
     discharges you from any and all actions or causes of action, suits, debts,
     claims, complaints, contracts, controversies, agreements, promises,
     damages, claims for attorneys' fees, costs, interest or punitive damages,
     judgments and demands whatsoever, in law or equity, the Company now has,
     may have or ever had, known or unknown from the beginning of the world to
     this date, including, without limitation, claims for breach of contract or
     based on tort, and any other statutory, regulatory or common law causes of
     action. THE PARTIES HEREBY ACKNOWLEDGE AND UNDERSTAND THAT THIS IS A
     GENERAL RELEASE.

15.  MUTUAL COVENANT NOT TO SUE. To the extent permitted by law, you
     specifically agree not to commence any legal action against any of the
     Company Released Parties arising out of or in connection with the Released
     Claims. To the extent permitted by law, you expressly agree that if you
     commence such an action in violation of this Agreement, you shall indemnify
     the Company Released Parties for the full and complete costs of defending
     such an action and enforcing this Agreement, including reasonable
     attorneys' fees (whether incurred in a third party action or in an action
     to enforce this Agreement), court costs, and other related expenses. You
     further agree that, to the extent permitted by law, if you commence such an
     action despite the provisions of this Agreement, you shall be obligated to
     return to the Company the full amount of all sums paid to you, or on your
     behalf, pursuant to paragraphs 3 and 5. To the extent permitted by law, the
     Company specifically agrees not to commence any legal action against you
     arising out of or in connection with the Released Claims. To the extent
     permitted by law, the Company

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     expressly agrees that if the Company commences such an action in violation
     of this Agreement, the Company shall indemnify you for the full and
     complete costs of defending such an action and enforcing this Agreement,
     including reasonable attorneys' fees (whether incurred in a third party
     action or in an action to enforce this Agreement), court costs, and other
     related expenses.

16.  ACKNOWLEDGMENT. You acknowledge and agree that you understand the meaning
     of this letter agreement and that you freely and voluntarily enter into it
     and the General Release contained herein. You agree that no fact, evidence,
     event, or transaction, whether known or unknown, shall affect in any manner
     the final and unconditional nature of the agreements and releases set forth
     herein. You acknowledge that you have been advised that you have twenty-one
     (21) days to consider this General Release and to consult with an attorney
     prior to executing it. For a period of seven (7) days after executing this
     General Release, you may revoke this General Release by providing written
     notice of said revocation to Dr. Safi R. Bahcall at the address of the
     Company set forth above and this General Release shall not become effective
     or enforceable until said seven-day period has expired.

17.  MISCELLANEOUS. This letter agreement shall be construed in accordance with
     the laws of the Commonwealth of Massachusetts without regard to choice or
     conflict of law principles. A waiver of any breach of or failure to comply
     fully with any provision of this letter agreement by either party shall not
     operate or be construed as a waiver of any subsequent breach thereof or
     failure so to comply. If any portion or provision of this letter agreement
     shall to any extent be deemed invalid or unenforceable, the remainder of
     this letter agreement shall not be affected thereby and each portion and
     provision of this letter agreement shall be valid and enforceable to the
     fullest extent permitted. To avoid any possible misunderstanding, the
     Company intends this letter agreement to be a comprehensive statement of
     the terms of your resignation. This letter agreement supersedes any prior
     understanding or statement made to you by the Company regarding your
     positions with the Company or your arrangements with the Company for the
     period after your resignation. For the same reason, any modifications of
     the terms set forth in this letter agreement must be in writing and signed
     by you and by me on behalf of the Company.

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     Please indicate your agreement to the terms of this letter agreement by
signing and dating the last page of the enclosed copy of this letter agreement,
and return it to me not later than the close of business on May 12, 2004, which
you acknowledge to be twenty-one (21) days from the date of your receipt of this
letter agreement.

                                          Sincerely,

                                          /s/ DR. SAFI R. BAHCALL
                                          ---------------------------
                                          Dr. Safi R. Bahcall
                                          Chief Executive Officer

AGREED TO AND EXECUTED UNDER SEAL THIS 21 day of April, 2004.

                                          /s/ DR. MITSUNORI ONO
                                          ---------------------------
                                          Dr. Mitsunori Ono

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                                    EXHIBIT A

           FIRST AMENDMENT TO INCENTIVE STOCK OPTION AGREEMENT NO. 004

This First Amendment to Incentive Stock Option Agreement No. 004 (the
"Amendment") is dated as of April 8. 2004 and is entered into by and between
SYNTA PHARMACEUTICALS CORP., a Delaware corporation (the "Company"), and
MITSUNORI ONO, an individual (the "Grantee").

Reference is made to that certain Incentive Stock Option Agreement No. 004 dated
December 13, 2002, by and between the Company and the Grantee (the "Option") as
issued pursuant to the 2001 Stock Plan adopted by the Company (the "Stock
Plan"). Reference also is made to that certain Separation Agreement dated as of
April 8, 2004 and by and between the Company and the Grantee (collectively, the
"Separation Agreement"). Capitalized terms used, but not defined, herein shall
have the meanings as set forth in the Option.

WHEREAS, Grantee's employment with the Company terminated upon January 1, 2004
(the "Resignation Date");

WHEREAS, pursuant to the terms of the Option, Grantee's right to exercise the
Option for an aggregate of 812,500 shares (the "Original Vested Shares") vested
upon the Resignation Date; and

WHEREAS, the Company and the Grantee wish to amend the Option pursuant to the
terms of the Separation Agreement.

NOW, THEREFORE, for good and valuable consideration, the receipt and sufficiency
of which is hereby acknowledged by the parties, the parties hereby agree as
follows:

1.   The Option is hereby amended as follows:

     a.   All references to "1,500,000 shares" on the face of the Option are
hereby deleted and replaced with "1,000,000 shares".

     b.   Section 3 of the Option is deleted in its entirety and hereby replaced
with the following:

          "3. VESTING SCHEDULE. The Option shall be exercisable with respect to
     1,000,000 of the Stock Rights Shares immediately, provided that the first
     aggregate of 187,500 Stock Rights Shares that the Participant exercises
     shall not be Original Vested Shares."

     c.   Section 4 of the Option is deleted in its entirety and hereby replaced
with the following:

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          "4. TERM OF OPTION. Subject to earlier termination as provided in this
     Agreement or the Plan, the Option shall expire on the tenth anniversary of
     the Grant Date."

2.   All other terms and conditions of the Option remain in full force and
effect, and are not modified in any way except as expressly set forth herein.
However, in the event of a conflict between the terms and conditions of the
Option and this Amendment, the terms of this Amendment shall govern.

3.   The Grantee hereby acknowledges that the Option as amended by this
Amendment will not be an incentive stock option that complies with Section 422
of the Internal Revenue Code of 1986, as amended.

4.   The Grantee further acknowledges that the exercise of the amended Option by
the Grantee could have negative tax consequences for the Grantee, and that the
Grantee has had sufficient opportunity to discuss the tax consequences of the
foregoing amendments to the Option with his tax advisor(s) prior to entering
into this Amendment.

5.   This Amendment shall be governed by and construed in accordance with the
laws of the State of Delaware, without regard to conflict of laws principles.

IN WITNESS WHEREOF, the parties hereto have executed this Amendment as of the
date first set forth above.

GRANTEE                                   COMPANY

                                          SYNTA PHARMACEUTICALS CORP.

 /s/ MITSUNORI ONO                        /s/ SAFI R. BAHCALL
 ---------------------                    ------------------------
Mitsunori Ono                             By:  Safi R. Bahcall
                                          Its: Chief Executive Officer<Page>

                                                                   Exhibit 10.13

April 30, 2004

John A. McCarthy, Jr.
[ADDRESS]

Dear John:

On behalf of Synta Pharmaceuticals, I am pleased to offer you the position of
Senior Vice President Corporate Development and Chief Financial Officer
reporting to Safi Bahcall, the President and Chief Executive Officer of Synta
Pharmaceuticals Corp. (hereinafter "Synta Pharmaceuticals" or the "Company").

1.   START DATE: Your first day of employment will be May 17, 2004.

2.   BASE COMPENSATION: Your initial base salary will be $240,000 annually
payable on a semimonthly basis, from which all applicable taxes and other
customary employment-related deductions will be taken.

3.   BONUSES: You will be eligible to receive annual performance based bonuses.
Cash bonuses for fully meeting and exceeding expectations under the Company's
proposed bonus program are expected to be in the 10-20% range, with a full
target level of 20%. Such bonus, if any, will be granted at the discretion of
the Company's Board of Directors.

4.   STOCK OPTION: Subject to the approval of the Company's Board of Directors,
you will be granted an incentive stock option to purchase a total of 350,000
shares of the Company's common stock. The shares will vest pursuant to the terms
of the Synta Pharmaceuticals Corp. 2001 Stock Plan (the "Plan") and a formal
stock option agreement that you will receive after the grant is approved. All
stock option grants shall be priced at the fair market value on the grant date,
which will be your first day of employment. Provided that you are still employed
by the Company, the Option shall become exercisable in cumulative installments
of 25% of the Stock Right Shares on the one-year anniversary of your grant date,
and thereafter 6.25% of the Stock Rights Shares upon the end of each following
calendar quarter.

5.   BENEFITS: As an employee, you will be eligible to participate in certain
Company-sponsored benefit plans to the same extent as, and subject to the same
terms, conditions and limitations applicable to other employees of the Company
of similar rank and tenure. All benefits may be changed or modified from time to
time at the Company's sole discretion.

6.   EMPLOYMENT PERIOD: Your employment with the Company will be at-will,
meaning that you will not be obligated to remain employed by the Company for any
specified period of time; likewise, the Company will not be obligated to
continue your employment for any specific period and may terminate your
employment at any time, with or without cause. No provision of this letter shall
be construed to create an express or implied employment contract.

                                       A-1
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7.   SEVERANCE: In the event the Company terminates your employment without
cause, the Company will make a one-time severance payment to you one week after
the date of termination equal to: 3 months if your employment period has been
less than 6 months, 6 months of base salary if your employment period has been
between 6 and 12 months, or 12 months of base salary if your employment period
has been 12 or more months. In addition to the one-time severance payment, the
Company will provide for a continuation of health care coverage for a 12 month
period.

     For purposes of this letter, termination "without cause" shall include, but
not be limited to, your resignation following a significant and material
diminution in your title, salary, duties or responsibilities by the Company or a
requirement that you relocate to an office more than 50 miles from Lexington,
MA. The preceding sentence notwithstanding, "cause" shall include (but is not
limited to): (i) any substantial malfeasance or non-feasance of duty, (ii) any
material breach by you of any of the terms of the Confidential Information
Agreement and Non-Competition Agreement between you and the Company, (iii) any
attempt by you to secure any improper personal profit in connection with the
business of the Company or any of its affiliates, (iv) your conviction, or the
entry of a pleading of guilty or nolo contendre by you to, any crime involving
moral turpitude or any felony, or (v) any conduct substantially injurious or
prejudicial to the business of the Company or its affiliates.

     Concurrently with the receipt of your severance payment, and as a condition
to such receipt, you shall execute and deliver to the Company your written
release of the Company from any and all claims and causes of action against the
Company arising in connection with your employment with the Company.

8.   CONTINGENCIES: Our employment offer to you is contingent upon (1) your
execution of the standard form of Non-Competition, Confidentiality and
Inventions Agreement (a copy of which is attached hereto as EXHIBIT A); (2) your
ability, as required under federal law, to establish your employment eligibility
as a U.S. citizen, a lawful permanent resident of the U.S. or an individual
specifically authorized for employment by the Immigration and Naturalization
Service; and (3) completion of a satisfactory background check. If any of the
foregoing conditions are not met, this employment offer shall be null and void.

9.   JURISDICTION AND WAIVER: In the case of any dispute, this offer of
employment shall be interpreted under the laws of the Commonwealth of
Massachusetts. By accepting this offer of employment, you agree that any action,
demand, claim or counterclaim in connection with any aspect of your employment
with the Company, or any separation of employment (whether voluntary or
involuntary) from the Company, shall be resolved in a court of competent
jurisdiction in Massachusetts by a judge alone, and you knowingly waive and
forever renounce your right to a trial before a civil jury.

10.  ORIENTATION: On your first day of employment, please see Human Resources
for benefits orientation and enrollment at 9:30am.

                                       A-2
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We are very enthusiastic about the prospect of your joining us as a Synta
Pharmaceuticals employee. Please indicate your acceptance of the foregoing by
signing one enclosed copy of this letter and returning it to Human Resources by
May 10, 2004. After that date, this offer will lapse.

Sincerely,

/s/ SAFI BAHCALL
-----------------------
Safi Bahcall
President and CEO
SYNTA PHARMACEUTICALS CORP.

Agreed to and accepted:

Name: /s/ JOHN A. MCCARTHY                Date:  5/7/04
      ----------------------                   -------------

                                       A-3
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                                                                       EXHIBIT A

                           Synta Pharmaceuticals Corp.
                               45 Hartwell Avenue
                               Lexington, MA 02421

April 30, 2004

John A. McCarthy, Jr.
[ADDRESS]

Dear John:

     This letter is to confirm our understanding with respect to (i) your
agreement not to compete with Synta Pharmaceuticals Corp. or its subsidiaries or
affiliates (collectively, the "Company") and (ii) your agreement to protect and
preserve information and property which is confidential and proprietary to the
Company (the terms and conditions agreed to in this letter shall hereinafter be
referred to as the "Agreement"). You hereby acknowledge and agree that you are
an "at-will" employee and that no provision of this Agreement shall be construed
to create an express or implied employment contract, or a promise of employment
for a specific period of time, and the Company expressly reserves the right to
end your employment at any time, with or without notice or cause.

     In consideration of your employment by the Company, the mutual promises and
covenants contained in this Agreement, and for other good and valuable
consideration, the receipt and sufficiency of which are hereby mutually
acknowledged, we have agreed as follows:

     1.   PROHIBITED COMPETITION AND SOLICITATION.

          (a)  CERTAIN ACKNOWLEDGMENTS AND AGREEMENTS.

               (i)     We have discussed, and you recognize and acknowledge the
competitive and proprietary aspects of the business of the Company.

               (ii)    You will devote your full time and efforts to the
business of the Company and, during the period of your employment with the
Company (the "Term") and for a period of twelve (12) months following
termination of your employment (whether such termination is voluntary or
involuntary), shall not participate, directly or indirectly, in any capacity, in
any business which is competitive with the Company without the prior written
consent of the Company. You acknowledge and agree that a business will be deemed
competitive with the Company if it conducts research, performs any of the
services or manufactures or sells any of the products provided or offered by the
Company or if it performs any other services and/or engages in the production,
manufacture, distribution or sale of any product similar to services performed
or products produced, manufactured, distributed or sold by the Company within
the Field of Interest (as defined below) at any time during the period of your
employment with the Company.

                                       A-4
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               (iii)   You further acknowledge and agree that, during the course
of your employment with the Company, the Company will furnish, disclose or make
available to you confidential and proprietary information related to the
Company's business and that the Company may provide you with unique and
specialized training. You also acknowledge that such confidential information
and such training have been developed and will be developed by the Company
through the expenditure by the Company of substantial time, effort and money and
that all such confidential information and training could be used by you to
compete with the Company.

          (b)  NON-SOLICITATION. During the Term and for a period of twelve (12)
months following termination of your employment, whether such termination is
voluntary or involuntary, you shall not, without the prior written consent of
the Company:

               (i)     either individually or on behalf of or through any third
party, solicit, divert or appropriate or attempt to solicit, divert or
appropriate, any customer of the Company with which you had any contact at any
time during the Term, located within the Restricted Territory with the effect or
intention of reducing or limiting the amount of business the customer does with
the Company; or

               (ii)    either individually or on behalf of or through any third
party, directly or indirectly, solicit, entice or persuade or attempt to
solicit, entice or persuade any employees of or consultants to the Company
(other than your spouse), who have been employees or consultants of the Company
at any time during the Term, or who are employees at the time of the
solicitation, to leave the services of the Company.

          (c)  FIELD OF INTEREST. As used herein, the term "Field of Interest"
means the research of, and/or the development, manufacture and sale of, any
therapeutic or diagnostic product that is developed, manufactured or sold by the
Company at any time during the Term, as documented in the bi-weekly scientific
project reports or other scientific planning documents of the company (the
"Scientific Reports") prepared by the Company during the Term. You hereby
acknowledge and agree that the Field of Interest shall be assessed for purposes
of this Agreement as of the date on which your employment with the Company
terminates, which assessment shall include, without limitation, a review of the
applicable Scientific Reports.

          (d)  REASONABLENESS OF RESTRICTIONS. You further acknowledge and agree
that (i) the activities which are prohibited by this Section 1 are narrow and
reasonable in relation to the skills which represent your principal salable
asset both to the Company and to your other prospective employers, and (ii)
given the global nature of the Company's business, including its need to market
its services and sell its products in a large geographic area in order to have a
sufficient customer base to make the Company's business profitable, the
geographic, length of time and substantive scope of the provisions of this
Section 1 are reasonable, legitimate and fair to you.

          (e)  SURVIVAL OF ACKNOWLEDGMENTS AND AGREEMENTS. Except as expressly
set forth hereunder, your acknowledgments and agreements set forth in this
Section 1 shall survive the termination of your employment with the Company for
the periods set forth above.

                                       A-5
<Page>

     2.   PROTECTED INFORMATION.

          (a)  CONFIDENTIALITY OBLIGATIONS. You shall at all times, both during
the Term and thereafter, maintain in confidence and shall not, without the prior
written consent of the Company, use, except in the course of performance of your
duties for the Company, disclose or give to others any Confidential Information
of the Company. As used herein, the term "Confidential Information" shall mean
any information which is disclosed to or developed by you during the course of
performing services for, or receiving training from, the Company, and is not
generally available to the public, including but not limited to confidential
information concerning business plans, customers, future customers, suppliers,
licensors, licensees, partners, investors, affiliates or others, training
methods and materials, financial information, sales prospects, client lists,
Company Inventions (as defined in Section 3), or any other scientific,
technical, trade or business secret or confidential or proprietary information
of the Company or of any third party provided to you during the Term. In the
event anyone not employed or otherwise engaged by the Company seeks information
from you in regard to any such Confidential Information or any other secret or
confidential work of the Company, or concerning any fact or circumstance
relating thereto, you will promptly notify the chief executive officer of the
Company.

          (b)  LIMITED EXCEPTIONS. The restrictions in Section 2(a) hereof shall
not apply to information that, as can be established by competent written
records: (i) was publicly known at the time of the Company's communication
thereof to you; (ii) becomes publicly known through no fault of yours subsequent
to the time of the Company's communication thereof to you; (iii) was in your
possession free of any obligation of confidence at the time of the Company's
communication thereof to you; or (iv) is developed by you independently of and
without reference to or use of any of the Company's Confidential Information. In
the event that you are required by law, regulation or court order to disclose
any of the Company's Confidential Information, you shall (i) first notify the
Company of such disclosure requirement and (ii) furnish only that portion of the
Confidential Information that is legally required and will exercise all
reasonable efforts to obtain reliable assurances that confidential treatment
will be accorded the Confidential Information.

     3.   OWNERSHIP OF INTELLECTUAL PROPERTY IDEAS.

          (a)  PROPERTY OF THE COMPANY. As used in this Agreement, the term
"Inventions" shall mean all ideas, discoveries, creations, manuscripts and
properties, innovations, improvements, know-how, inventions, designs,
developments, apparatus, techniques, methods, biological processes, cell lines,
laboratory notebooks and formulae, whether patentable, copyrightable or not,
including all rights to obtain, register, perfect and enforce any of the
foregoing. You hereby agree that any Inventions which you may conceive, reduce
to practice or develop during the Term in connection with the business
activities of the Company or otherwise within the Field of Interest, alone or in
conjunction with any other party, whether during or out of regular business
hours, and whether at the request or upon the suggestion of the Company, or
otherwise (collectively, the "Company Inventions"), shall be the sole and
exclusive property of the Company. You hereby assign to the Company all of your
right, title and interest in and to all such Company Inventions and hereby agree
that you shall not publish any of the Company Inventions without the prior
written consent of the Company.

                                       A-6
<Page>

          (b)  COOPERATION. During the Term, you agree that, without further
compensation, you will disclose promptly to the Company in writing, all Company
Inventions you conceive, reduce to practice or develop during the Term (or, if
based on or related to any Confidential Information of the Company obtained by
you during the Term, within one (1) year after the termination of your
employment). You further agree that you will fully cooperate with the Company,
its attorneys and agents in the preparation and filing of all papers and other
documents as may be reasonably required to perfect the Company's rights in and
to any of such Company Inventions, including, but not limited to, joining in any
proceeding to obtain patents, copyrights, trademarks or other legal rights of
the United States and of any and all other countries on such Company Inventions;
PROVIDED, THAT, the Company will bear the expense of such proceedings (including
all of your reasonable expenses). You further agree that any patent or other
legal right covering any Company Invention so issued to you, personally, shall
be assigned by you to the Company without charge by you. You further acknowledge
that all original works of authorship made by you, whether alone or jointly with
others within the scope of your employment and which are protectable by
copyright are "works made for hire" within the meaning of the United States
Copyright Act, 17 U.S.C. Section 101, as amended, the copyright of which shall
be owned solely, completely and exclusively by the Company. If any Company
Invention is considered to be work not included in the categories of work
covered by the United States Copyright Act, 17 U.S.C. Section 101, as amended,
such work shall be owned solely by, or hereby assigned or transferred completely
and exclusively to, the Company. If the Company is unable because of your mental
or physical incapacity or for any other reason, after reasonable effort, to
secure your signature on any document or documents needed to obtain or enforce
any patent, copyright, trademarks or any other rights covering Inventions or
original works of authorship assigned by you to the Company as required above,
you hereby irrevocably designate and appoint the Company and its duly authorized
officers and agents as your agent and attorney-in-fact, to act for and in your
behalf and stead to execute and file any application or assignment and to do all
other lawfully permitted acts to further the prosecution and issuance to the
Company of patents, copyright registrations, trademark registrations or similar
protections covering the Inventions with the same legal force and effect as if
executed by you.

     4.   PROVISIONS NECESSARY AND REASONABLE/BREACH/ATTORNEYS' FEES. You agree
that (i) the provisions of Sections 1, 2 and 3 of this Agreement are necessary
and reasonable to protect the Company's Confidential Information, Company
Inventions, and goodwill and (ii) in the event of any breach of any of the
covenants set forth herein, the Company would suffer substantial irreparable
harm and would not have an adequate remedy at law for such breach. In
recognition of the foregoing, you agree that in the event of a breach or
threatened breach of any of these covenants, in addition to such other remedies
as the Company may have at law, without posting any bond or security, the
Company shall be entitled to seek and obtain equitable relief, in the form of
specific performance, and/or temporary, preliminary or permanent injunctive
relief, or any other equitable remedy which then may be available. The seeking
of such injunction or order shall not affect the Company's right to seek and
obtain damages or other equitable relief on account of any such actual or
threatened breach. In the event the Company takes any court action with respect
to your breach or threatened breach of this Agreement, and prevails in such
action, you shall be obligated to reimburse the Company for its reasonable
attorneys' fees and costs incurred in such action.

                                       A-7
<Page>

     5.   DISCLOSURE TO FUTURE EMPLOYERS. You agree that you will provide, and
that the Company may similarly provide in its discretion, a copy of the
covenants contained in Sections 1, 2 and 3 of this Agreement to any business or
enterprise which you may directly, or indirectly, own, manage, operate, finance,
join, control or in which you participate in the ownership, management,
operation, financing, or control, or with which you may be connected as an
officer, director, employee, partner, principal, agent, representative,
consultant or otherwise.

     6.   REPRESENTATIONS REGARDING PRIOR WORK AND LEGAL OBLIGATIONS.

          (a)  You represent that you have no agreement or other legal
obligation with any prior employer or any other person or entity that restricts
your ability to engage in employment discussions with, employment with, or to
perform any function for, the Company.

          (b)  You represent that you have been advised by the Company that at
no time should you divulge to or use for the benefit of the Company, any trade
secret or confidential or proprietary information of any previous employer. You
acknowledge that you have not divulged or used any such information for the
benefit of the Company.

          (c)  You acknowledge that the Company is basing important business
decisions on these representations, and affirm that all of the statements
included herein are true.

     7.   RECORDS. Upon termination of your employment relationship with the
Company, you shall deliver to the Company any property of the Company which may
be in your possession including products, materials, memoranda, notes, records,
reports, or other documents or photocopies of the same.

     8.   NO CONFLICTING AGREEMENTS. You hereby represent and warrant that you
have no commitments or obligations inconsistent with this Agreement and you
hereby agree to indemnify and hold the Company harmless against loss, damage,
liability or expense arising from any claim based upon circumstances alleged to
be inconsistent with such representation and warranty.

     9.   GENERAL.

          (a)  NOTICES. All notices, requests, consents and other communications
hereunder shall be in writing, shall be addressed to the receiving party's
address set forth below or to such other address as a party may designate by
notice hereunder, and shall be either (i) delivered by hand, (ii) made by telex,
telecopy or facsimile transmission with confirmed receipt thereof (and with a
copy of such telex, telecopy or facsimile, together with a copy of the
confirmation sent to the recipient by regular U.S. mail on the next business
day), (iii) sent by overnight courier, or (iv) sent by registered mail, return
receipt requested, postage prepaid.

If to the Company:     Synta Pharmaceuticals Corp.
                       45 Hartwell Avenue
                       Lexington, MA 02421
                       Attn:  Chief Executive Officer

If to you:             To the address set forth on the signature page of this
                       Agreement.

                                       A-8
<Page>

     All notices, requests, consents and other communications hereunder shall be
deemed to have been given either (i) if by hand, at the time of the delivery
thereof to the receiving party at the address of such party set forth above,
(ii) if made by telex, telecopy or facsimile transmission, at the time that
receipt thereof has been acknowledged by electronic confirmation or otherwise,
(iii) if sent by overnight courier, on the next business day following the day
such notice is delivered to the courier service, or (iv) if sent by registered
mail, on the fifth business day following the day such mailing is made.

          (b)  ENTIRE AGREEMENT. This Agreement embodies the entire agreement
and understanding between the parties hereto with respect to the subject matter
hereof and supersedes all prior oral or written agreements and understandings
relating to the subject matter hereof. No statement, representation, warranty,
covenant or agreement of any kind not expressly set forth in this Agreement
shall affect, or be used to interpret, change or restrict, the express terms and
provisions of this Agreement.

          (c)  MODIFICATIONS AND AMENDMENTS. The terms and provisions of this
Agreement may be modified or amended only by written agreement executed by the
parties hereto.

          (d)  WAIVERS AND CONSENTS. The terms and provisions of this Agreement
may be waived, or consent for the departure therefrom granted, only by written
document executed by the party entitled to the benefits of such terms or
provisions. No such waiver or consent shall be deemed to be or shall constitute
a waiver or consent with respect to any other terms or provisions of this
Agreement, whether or not similar. Each such waiver or consent shall be
effective only in the specific instance and for the purpose for which it was
given, and shall not constitute a continuing waiver or consent.

          (e)  ASSIGNMENT. The Company may assign its rights and obligations
hereunder to any person or entity that succeeds to all or substantially all of
the Company's business or that aspect of the Company's business in which you are
principally involved. Your rights and obligations under this Agreement may not
be assigned by you without the prior written consent of the Company.

          (f)  BENEFIT. All statements, representations, warranties, covenants
and agreements in this Agreement shall be binding on the parties hereto and
shall inure to the benefit of the respective successors and permitted assigns of
each party hereto. Nothing in this Agreement shall be construed to create any
rights or obligations except among the parties hereto, and no person or entity
shall be regarded as a third-party beneficiary of this Agreement.

          (g)  GOVERNING LAW. This Agreement and the rights and obligations of
the parties hereunder shall be construed in accordance with and governed by the
laws of the Commonwealth of Massachusetts, without giving effect to the conflict
of laws principles thereof.

          (h)  JURISDICTION. Any legal action or proceeding with respect to this
Agreement may be brought in the courts of the Commonwealth of Massachusetts or
of the United States of America. By execution and delivery of this Agreement,
each of the parties

                                       A-9
<Page>

hereto accepts for itself and in respect of its property, generally and
unconditionally, the jurisdiction of the aforesaid courts.

          (i)  SEVERABILITY. The parties intend this Agreement to be enforced as
written. However, (i) if any portion or provision of this Agreement shall to any
extent be declared illegal or unenforceable by a duly authorized court having
jurisdiction, then the remainder of this Agreement, or the application of such
portion or provision in circumstances other than those as to which it is so
declared illegal or unenforceable, shall not be affected thereby, and each
portion and provision of this Agreement shall be valid and enforceable to the
fullest extent permitted by law and (ii) if any provision, or part thereof, is
held to be unenforceable because of the duration of such provision or the
geographic area covered thereby, the Company and you agree that the court making
such determination shall have the power to reduce the duration and/or geographic
area of such provision, and/or to delete specific words and phrases
("blue-penciling"), and in its reduced or blue-penciled form such provision
shall then be enforceable and shall be enforced.

          (j)  HEADINGS AND CAPTIONS. The headings and captions of the various
subdivisions of this Agreement are for convenience of reference only and shall
in no way modify, or affect the meaning or construction of any of the terms or
provisions hereof.

          (k)  NO WAIVER OF RIGHTS, POWERS AND REMEDIES. No failure or delay by
a party hereto in exercising any right, power or remedy under this Agreement,
and no course of dealing between the parties hereto, shall operate as a waiver
of any such right, power or remedy of the party. No single or partial exercise
of any right, power or remedy under this Agreement by a party hereto, nor any
abandonment or discontinuance of steps to enforce any, such right, power or
remedy, shall preclude such party from any other or further exercise thereof or
the exercise of any other right, power or remedy hereunder. The election of any
remedy by a party hereto shall not constitute a waiver of the right of such
party to pursue other available remedies. No notice to or demand on a party not
expressly required under this Agreement shall entitle the party receiving such
notice or demand to any other or further notice or demand in similar or other
circumstances or constitute a waiver of the rights of the party giving such
notice or demand to any other or further action in any circumstances without
such notice or demand.

          (l)  COUNTERPARTS. This Agreement may be executed in one or more
counterparts, and by different parties hereto on separate counterparts, each of
which shall be deemed an original, but all of which together shall constitute
one and the same instrument.

     If the foregoing accurately sets forth our agreement, please so indicate by
signing and returning to us the enclosed copy of this letter.

                                          Very truly yours,

                                          SYNTA PHARMACEUTICALS CORP.

                                          By: /s/ SAFI BAHCALL
                                              -----------------------------
                                              Safi Bahcall
                                              Chief Executive Officer

                                      A-10
<Page>

Agreed to and accepted:

/s/ JOHN A. MCCARTHY, Jr.
---------------------------
Name: John A. McCarthy, Jr.

---------------------------

---------------------------
Address:

Date:  5/7/04
     ----------

                                      A-11

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