Document:

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                                                                   Exhibit 10.20

February 7, 2006

Martin Williams
[ADDRESS]

Dear Martin:

On behalf of Synta Pharmaceuticals, I am pleased to offer you the position of
Senior Vice President, Business and Commercial Development reporting to Safi
Bahcall for Synta Pharmaceuticals Corp. (hereinafter "Synta Pharmaceuticals" or
the "Company"). In this position, you will have responsibility for corporate
development, commercial development/marketing, corporate communications, and
such other duties as the Company may at time to time reasonable assign to you.

1. EFFECTIVE DATE: The effective date of your employment will be February 27,
2006.

2. COMPENSATION: Your initial base salary will be $260,000 annually; payable at
a semi-monthly rate of $10,833.34, from which all applicable taxes and other
customary employment-related deductions will be taken. You will be eligible to
receive an annual performance based bonus. Cash bonuses for fully meeting and
exceeding expecations undwer 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.

Subject to the approval of the Company's Board of Directors, you will be granted
a stock option to purchase 300,000 shares of the Company's common stock pursuant
to the terms of the Synta Pharmaceuticals Corp. 2001 Stock Plan (the "Plan") and
formal stock option agreement. All stock option grants shall be priced at the
fair market value on the grant date and are subject to a vesting schedule over
four years (25% vest after the first year and the remainder in equal portions
quarterly over the next three years.)

For the first annual performance review following your hire date, all
pay-for-performance compensation (such as merit increases and annual stock
option grants) will be pro-rated to reflect your start date.

In your first paycheck, you will also receive a $15,000.00 lump sum bonus, from
which all applicable taxes and other customary employment-related deductions
will be taken. This one time bonus is payable, in full, back to Synta
Pharmaceuticals should you choose to leave the company within one year of your
start date. This bonus repayment obligation shall not apply if you are
terminated "without cause", as explained in Section 3 below.

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3.   SEVERANCE: In the event the Company terminates your employment without
     cause after the first six months of your employment, the Company will make
     a one-time severance payment to you on the date of termination equal to
     six-months base pay, (b) after three months but before completion of six
     months of employment, the Company will make a one time severance payment
     equal to three months base pay, and you will be eligible to continue your
     health benefits for the period of time covered by the severance payment at
     the same cost you would have paid if you were an active employee.

     For purposes of this letter, termination "without cause" shall include, but
     not be limited to, (a) your termination after a Change in Control, as
     defined in the Company's 2001 Stock Plan (as amended and restated), or (b)
     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 pleasing of guilty or nolo contender 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.

4. BENEFITS: As a full-time 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.

5. 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.

6. 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.

7. 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

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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.

8. ORIENTATION: On your first day of employment, please arrive at 45 Hartwell
Avenue at 8:30am for benefits enrollment with Human Resources.

Martin, 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 12pm, February 13th. After that date, this offer will lapse.

Sincerely,

SYNTA PHARMACEUTICALS CORP.

/s/ STEPHEN M. GANSLER
----------------------
Stephen M. Gansler
Vice President of Human Resources

Agreed to and accepted:

Name:  /s/ MARTIN WILLIAMS          Date: February 19, 2006
       -------------------

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                                                                       EXHIBIT A

                           Synta Pharmaceuticals Corp.
                               45 Hartwell Avenue
                               Lexington, MA 02421

February 27, 2006

Martin Williams
[ADDRESS]

Dear Martin:

     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 that may be purchased in lieu
of purchasing services performed or products produced,

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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.

                (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, 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.

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          (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.

     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.

          (c) SURVIVAL OF ACKNOWLEDGEMENTS AND AGREEMENTS. Except as expressly
set forth hereunder, your acknowledgements and agreements set forth in this
Section 2 shall survive the termination of your employment with the Company.

     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,

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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.

          (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.

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     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.

     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

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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.

     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.

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          (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 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

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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/ STEPHEN M. GANSLER
                                           ----------------------
                                           Stephen M. Gansler
                                           Vice President of Human Resources

Agreed to and accepted:

/s/ MARTIN WILLIAMS
-------------------
Name: Martin Williams

[ADDRESS]

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

Date: February 27, 2006
     ------------------

                                      A-11<Page>

                      SCIENTIFIC ADVISORY BOARD AGREEMENT

     This Agreement, effective as of September 1, 2003 (the "Effective Date"),
is made between Synta Pharmaceuticals Corp. (the "Company"), a Delaware
corporation, and Judah Folkman, MD (the "Consultant").

                                    RECITALS

     WHEREAS, the Consultant is a member of the faculty at HARVARD MEDICAL
SCHOOL ("HMS") and appointed at CHILDREN'S HOSPITAL Boston ("CHILDREN'S
HOSPITAL" or "the Institution"), which permit the Consultant to perform limited
consulting services for companies;

     WHEREAS, the Company desires that the Consultant provide advice and
assistance to the Company in his or her area of expertise; and

     WHEREAS, the Consultant desires to provide such advice and assistance to
the Company under the terms and conditions of this Agreement;

     NOW, THEREFORE, in consideration of the mutual covenants set forth herein,
the Company and the Consultant hereby agree as follows:

     1.   SERVICES.
          --------

          a.   The Consultant shall render to the Company or its designee such
consulting services as the Company may mutually agree from time to time (the
"Services"); provided, however, that the Consultant shall not be required to
devote more than FIVE (5) days per month in the provision of such Services.
Meetings with Consultant shall be scheduled as mutually convenient. Consultant
may, but shall not be required to, participate by conference telephone. The
Company shall provide the Consultant with reasonable prior notice of any
Services the Company requires. The Consultant shall use reasonable efforts not
use any facilities, funds, or equipment owned or administered by the Institution
in the performance of the Services, except with the prior written consent of the
Company and in accordance with all applicable policies of the Institution.

          b.   It is understood that the purpose of the Consulting is to provide
periodic review and advice relevant to certain Company matters, and that neither
Consultant nor Company will benefit if Consultant provides inaccurate advice or
commentary based on insufficient information. To that end, Company shall provide
Consultant, in advance of meetings, with accurate, unbiased and sufficient
information for him to review the subject matter thereof, and shall promptly
provide further information that Consultant reasonably deems relevant to forming
any pertinent conclusions relevant to the matter for discussion. It is expressly
understood that Consultant has no fiduciary obligation to Company, but instead a
contractual one described by the terms of this Agreement; that his role is to
provide independent advice uninfluenced by commercial concerns; and that service
as a Consultant does not require him to be an advocate for Company or its
products in any forum, public or private. Company expressly agrees that under no
circumstances will this role be compromised or inaccurately represented.

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2.       COMPENSATION.

          2.1  CONSULTING FEES. The Company shall pay to the Consultant a
consulting fee in the amount of $50,000.00 per annum for performance of the
Services. Such fees shall payable in four (4) equal calendar quarter
installments of twelve thousand five hundred dollars ($12,500.00) each,
beginning on SEPTEMBER 1, 2003.

          2.2  STOCK OPTIONS. The Company will issue to the Consultant a
non-qualified stock option to purchase 100,000 shares of the common stock of the
Company, $.0001 par value per share, at a purchase price of $2.7108 per share,
such option to vest as follows: 25% vesting on the first anniversary of the
grant date of the option (the grant date being the actual date when the
Company's Board of Directors grants the option to the Institution) and
thereafter 6.25% vesting per quarter over the following three (3) year period,
provided that this Agreement remains in effect on the date in which vesting
occurs. The option will be subject to the terms and conditions of the Company's
2001 Stock Plan (as amended on August 21, 2002), and shall have additional terms
and conditions, as set forth on the stock option agreement certificate to be
provided to the Consultant following the grant. Company understands that it is
Consultant's intention immediately to transfer such options to Children's
Hospital Boston or Children's Medical Center Corporation, pursuant to a transfer
agreement approved by the Company, and hereby consents to such transfer.

          2.3  REIMBURSEMENT OF EXPENSES. The Company shall reimburse the
Consultant for reasonable travel and other out-of-pocket expenses incurred by
the Consultant in the performance of the Services, provided that the Consultant
shall have submitted to the Company written expense statements and other
supporting documentation in a form that is reasonably satisfactory to the
Company. Company will accommodate Consultant's request to arrange, at Company's
expense, for all of his travel and accommodations in connection with such
meetings if they occur outside the Boston metropolitan area. If Consultant makes
such arrangements, the Company shall provide the Consultant with a check for any
amounts due under this Section within thirty (30) days after the Company
receives satisfactory documentation. Notwithstanding the foregoing, the
Consultant shall not incur total expenses in excess of $1,000 per occurrence
without the prior written approval of the Company.

          2.4  NO BENEFITS. The Consultant acknowledges and agrees that the
Company will not provide the Consultant with any employee benefits, including
without limitation any employee stock purchase plan, social security,
unemployment, medical, or pension payments. The Consultant is an independent
contractor and not an employee of the Company. Notwithstanding the foregoing,
Company shall indemnify, defend and hold harmless Consultant, and CHILDREN'S
HOSPITAL, its corporate affiliates, current or future directors, trustees,
officers, faculty, medical and professional staff, employees, students and
agents and their respective successors, heirs and assigns (the "Indemnitees"),
against any claim, liability, cost, damage, deficiency, loss, expense or
obligation of any kind or nature (including without limitation reasonable
attorneys' fees and other costs and expenses of litigation) incurred by or
imposed upon the Indemnitees or any one of them in connection with any claims,
suits, actions, demands or judgments arising out of this Agreement (including,
but not limited to, actions in the form of tort, warranty, or strict liability),
except to the extent caused by the Consultant's misconduct or negligence.

                                       2
<Page>

3.   TERM AND TERMINATION.

     3.1  TERM. This Agreement shall commence on the Effective Date and shall
remain in effect for a period of one (1) year, unless earlier terminated as
provided in this Article 3; provided, however, that the term of this Agreement
shall automatically extend for additional one-year periods until the Consultant
gives the Company written notice that the Agreement will not continue, which
notice must be received by the Company at least sixty (60) days prior to the
expiration of the term.

     3.2  TERMINATION WITHOUT CAUSE. Either party may terminate this Agreement
for any reason upon sixty (60) days prior written notice to the Consultant.

     3.3  TERMINATION WITH CAUSE. In the event that a party commits a material
breach of its obligations under this Agreement, the other party may terminate
this Agreement upon sixty (60) days prior written notice to the party in breach,
unless the breach is cured within such sixty-day notice period. Notwithstanding
the foregoing, either party may terminate this Agreement immediately upon
written notice if either party breaches or threatens to breach any provision of
Article 4 or Sections 6.5 or 6.6..

     3.4  SURVIVAL. The following provisions shall survive the expiration or
termination of this Agreement: Articles 4 and 5; Sections 6.5., 6.6.,
6.10., and 6.12.

4.   CONFIDENTIAL INFORMATION AND PROPRIETARY MATERIALS.

     4.1  CONFIDENTIAL INFORMATION.

          4.1.1 DEFINITION OF CONFIDENTIAL INFORMATION. Confidential Information
shall mean, subject to the exceptions below, any technical or business
information furnished by the Company to the Consultant in connection with this
Agreement or developed by the Consultant within the scope and in the course of
performing the Services. Such Confidential Information may include, without
limitation, trade secrets, know-how, inventions, technical data or
specifications, testing methods, business or financial information, research and
development activities, product and marketing plans, and customer and supplier
information.

          4.1.2 OBLIGATIONS. The Consultant shall

     (a)  maintain all Confidential Information in strict confidence;

     (b)  use all Confidential Information solely for the purpose of providing
the Services as requested by the Company; and

     (c)  reproduce the Confidential Information only to the extent necessary
for providing the Services as requested by the Company, with all such
reproductions being considered Confidential Information.

Notwithstanding anything herein to the contrary, Company agrees that it shall
not disclose to Consultant any information which is Company Confidential
Information: (i) except to the extent necessary for Consultant to fulfill his
obligations to Company under this Agreement; or (ii)

                                       3
<Page>

unless Consultant has agreed in writing to accept such disclosure. All other
information and communications between Company and Consultant shall be deemed to
be provided to Consultant by Company on a non-confidential basis. Company agrees
that Consultant shall not be liable to Company or to any third party claiming by
or through Company for any unauthorized disclosure or use of Company
Confidential Information which occurs despite Consultant's compliance with his
obligations under this Agreement.

          4.1.3 EXCEPTIONS. The obligations of the Consultant under
Section 4.1.2. above shall not apply to the extent that such information

     (a)  was in the public domain prior to the time of its disclosure under
this Agreement;

     (b)  entered the public domain after the time of its disclosure under this
Agreement through means other than an unauthorized disclosure resulting from an
act or omission by the Consultant;

     (c)  was independently developed or discovered by the Consultant or
others ;

     (d)  is or was disclosed to the Consultant at any time, whether prior to or
after the time of its disclosure under this Agreement, by a third party having
no fiduciary relationship with the Company and having no obligation of
confidentiality with respect to such Confidential Information;

     (e)  is required to be disclosed to comply with applicable laws or
regulations, or with a court or administrative order. To the extent practicable
the consultant shall provide Company with prior notice of such disclosure and
cooperate with Company in taking reasonable steps to obtain confidential
treatment for such disclosure and, if possible, to minimize the extent of such
disclosure;

     (f)  in the case of information prepared by Consultant, is encompassed
within and derived from Consultant's academic and professional commitments to
CHILDREN'S HOSPITAL, HMS, and/or any other consulting or research engagement,
provided that Confidential Information described in this clause (f) which
constitutes Inventions shall be subject to the intellectual property provisions
of Section 5 of this Agreement

     4.2  PROPRIETARY MATERIALS.

          4.2.1 DEFINITION OF PROPRIETARY MATERIALS. "Proprietary Materials"
shall mean any tangible chemical, biological, or physical research materials
furnished by the Company to the Consultant in connection with this Agreement and
any such materials developed by the Consultant solely in the course of
performing the Services. (If such materials developed by Consultant are also
subject to CHILDREN'S HOSPITAL policies, they shall not be considered
Proprietary Materials for purposes of this Section 4, but will instead be
governed by the principles stated in Section 5, and Company shall have an option
to their use as stated therein.) In the case of biological materials,
Proprietary Materials shall also include other materials ordinarily engendered
by the original materials, including without limitation any progeny derived from
a cell line (including naturally occurring mutants), monoclonal antibodies

                                       4
<Page>

produced by hybridoma cells, DNA or RNA replicated from isolated DNA or RNA,
recombinant proteins produced by a cell line, recombinant proteins produced
through use of isolated DNA or RNA, and substances routinely purified from any
source material included in the original materials.

          4.2.2 LIMITED USE. The Consultant shall use Proprietary Materials
solely for the purpose of providing the Services as requested by the Company.
The Consultant shall use the Proprietary Materials only in compliance with all
applicable governmental laws and regulations, and not for any IN VIVO
experiments on human subjects.

          4.2.3 LIMITED DISPOSITION. The Consultant shall not transfer or
distribute any Proprietary Materials to any third party without the prior
written consent of the Company.

     4.3  RETURN OF CONFIDENTIAL INFORMATION AND PROPRIETARY MATERIALS. Upon the
termination of this Agreement, or earlier at the request of the Company, the
Consultant shall return to the Company all originals, copies, and summaries of
documents, materials, and other tangible manifestations of Confidential
Information in the possession or control of the Consultant, except that
Consultant may retain one copy, on a completely confidential basis, for archival
purposes. Upon the termination of this Agreement, or earlier at the request of
the Company, the Consultant shall at the instruction of the Company either
destroy or return any unused Proprietary Materials that remain in his or her
possession.

     4.4  SURVIVAL OF OBLIGATIONS. The obligations set forth in this Article 4
shall remain in effect for a period of ten (10) years after termination of this
Agreement, except that the obligations of the Consultant to return Confidential
Information and to return or destroy Proprietary Materials shall survive until
fulfilled.

5.   INTELLECTUAL PROPERTY.

     5.1  Consultant and CHILDREN'S HOSPITAL understand and acknowledge that
Company will be providing access to proprietary and valuable information that
Consultant might otherwise not receive. In addition, those parties also
understand that should Consultant, in the course of his advice, invent or
participate in inventing modifications or improvements to Company technology,
Company reasonably seeks to secure such improvements for its own use and
practice. At the same time, Company understands and acknowledges that Consultant
has pre-existing and on-going obligations to HMS, CHILDREN'S HOSPITAL, and the
sponsors of research at CHILDREN'S HOSPITAL (including obligations under grants,
contracts and collaborative agreements, generally). These obligations include a
duty on the part of Consultant to disclose and assign to CHILDREN'S HOSPITAL any
inventions or other proprietary rights arising during the course of such
employment or medical staff membership and any overlapping consulting
arrangements (including this Agreement), and an obligation to ensure that any
consulting agreement he enters into is not in conflict with the CHILDREN'S
HOSPITAL Policy on Inventions and Intellectual Property or in conflict with
other HMS or CHILDREN'S HOSPITAL commitments, such as Consultant's obligation to
publish research results.

     5.2  In order to enter into this Agreement with Consultant, Company
therefore further acknowledges and agrees that in the event that any conflict
should arise between the duties set forth in this Agreement and Consultant's
obligations to HMS, CHILDREN'S HOSPITAL or sponsors of research at CHILDREN'S
HOSPITAL, Consultant shall necessarily notify

                                       5
<Page>

CHILDREN'S HOSPITAL immediately, and that Consultant's obligations to CHILDREN'S
HOSPITAL and sponsors of research at CHILDREN'S HOSPITAL shall take precedence
over the terms of this Agreement.

     5.3  However, the parties agree that it is mutually beneficial that
Consultant be able to participate fully in his role as an adviser, as stated
herein, without being obligated to constrain his comments or contributions based
upon the complexities of applying these conflicting obligations to intellectual
property ownership. Therefore, in order to reconcile these obligations, and
promote Consultant's participation, during the term of this Agreement Consultant
shall promptly report and simultaneously disclose to CHILDREN'S HOSPITAL and to
the President of Company, or his or her designee, all inventions, improvements,
modifications, discoveries, methods and developments, whether patentable or not,
made or conceived by Consultant, or by employees or agents of Company under
Consultant's direction, during the performance of this Agreement that result
directly from Confidential Information provided by Company pursuant to this
Agreement and either embody Company technology or are reduced to practice as a
modification or improvement to Company technology (hereby designated
"Inventions"). Ownership of such Inventions, and any patent rights related
thereto, shall reside with CHILDREN'S HOSPITAL, if the Consultant is the sole
inventor and the Invention is covered by applicable CHILDREN'S HOSPITAL
policies, jointly between CHILDREN'S HOSPITAL and the Company if the Consultant
is not the sole inventor, but the Invention is covered by applicable CHILDREN'S
HOSPITAL policies, or otherwise with Company. If ownership lies solely or
jointly with CHILDREN'S HOSPITAL, then, provided such Inventions are not subject
to prior conflicting obligations to sponsors of research at CHILDREN'S HOSPITAL,
Company shall have an exclusive option, for 120 days following notice of
Consultant's disclosure, to negotiate an exclusive world-wide license, on
reasonable terms customary for CHILDREN'S HOSPITAL, to use, practice, license
and sublicense rights under patents claiming such Inventions within a mutually
agreed field of use. (While the parties believe that conflicting obligations to
research sponsors are unlikely, it is conceivable that in the course of such
sponsored research Inventions useful to Company may emerge; rather than forego
disclosing such fortuitous inventions to Company, to the extent permitted by
such sponsorship and related agreements Consultant and CHILDREN'S HOSPITAL will
endeavor to disclose and license such Inventions pursuant to this Agreement.)

     5.4  THIRD-PARTY INTELLECTUAL PROPERTY. The Consultant acknowledges that
the Company does not desire to acquire any trade secrets, know-how, confidential
information, or other intellectual property that the Consultant may have
acquired from or developed for any third party, including the Institution
("Third-Party IP"). The Company agrees that in the course of providing the
Services, the Consultant shall not be required to use or disclose any
Third-Party IP, including without limitation any intellectual property of (i)
any former or current employer, (ii) any person for whom the Consultant has
performed or currently performs consulting services, or (iii) any other person
to whom the Consultant has a legal obligation regarding the use or disclosure of
such intellectual property.

     5.5  Notwithstanding any other provision of this Agreement, Company
understands that Consultant has primary professional, academic and ethical
obligations arising in connection with his positions at HMS and CHILDREN'S
HOSPITAL, and that Consultant is subject to policies of those institutions which
protect academic freedom and preserve ownership of

                                       6
<Page>

intellectual property rights. Company agrees that Consultant shall be free to
publish within the scope of his professional and academic duties with respect to
his participation as a Consultant, provided that he does not reveal Confidential
Information. Company therefore agrees that in the course of his professional and
academic duties, Consultant may discuss such participation at conferences, with
colleagues, and with students, residents and fellows as he deems appropriate. In
either context, as well as in the scope of his duties under this Agreement,
Consultant shall be free to conduct himself without restraint or improper
influence, in accordance with HMS and Institutional academic, ethical and
publication standards. Solely in order to permit Company an opportunity to
determine if Confidential Information or Inventions are therein improperly
disclosed, Consultant agrees to use reasonable efforts to (i) provide to Company
at least thirty days in advance of submission to a journal any substantially
complete manuscript that includes such Confidential Information; (ii) provide
notice to Company no later than five working days before submission for
publication or to a conference of any substantially final abstract referring to
such Confidential Information; and (iii) notify Company thirty days in advance
of any conference at which such Confidential Information can foreseeably be
revealed. If within that thirty-day period Company requests a delay in
publication so that a patent may be filed on Inventions disclosed in the
manuscript, Consultant will delay publication for up to an additional sixty days
(not to exceed a total of ninety days from the initial submission of a
manuscript to Company). Company agrees to hold all such submissions and
information in confidence pending publication. Company agrees to notify
Consultant promptly if any action is necessary to delete Confidential
Information. Company has no other right to request alteration or deletion of any
portion of the manuscript or abstract.

6.   MISCELLANEOUS.

     6.1  COUNTERPARTS. This Agreement may be executed in one or more
counterparts, each of which shall be deemed an original, and all of which
together shall be deemed to be one and the same instrument.

     6.2  HEADINGS. All headings in this Agreement are for convenience only and
shall not affect the meaning of any provision hereof.

     6.3  BINDING EFFECT. This Agreement shall inure to the benefit of and be
binding upon the parties and their respective lawful successors, assigns, heirs,
and personal representatives.

     6.4  ASSIGNMENT. This Agreement may not be assigned by either party without
the prior written consent of the other party.

     6.5  NO CONFLICT OF INTEREST. The Consultant and Company mutually represent
that to the best of their knowledge neither currently has any agreement with, or
any other obligation to, any third party that conflicts with the terms of this
Agreement. The parties agree that they shall not intentionally enter into any
such agreement.

     6.6  COMPLIANCE WITH INSTITUTIONAL POLICIES. The Company recognizes that as
a faculty member of the Institution, the Consultant is responsible for ensuring
that any consulting agreement the Consultant enters into with a for-profit
entity is not in conflict with the intellectual property, consulting,
conflict-of-interest, and other policies of the Institution. The Consultant

                                       7
<Page>

represents that to the best of his knowledge this Agreement complies with all
such policies in effect on the Effective Date. The Consultant further represents
that to the best of his knowledge he or she has made all required disclosures to
the Institution and has obtained all necessary approvals of this Agreement from
the appropriate authorities at the Institution.

     6.7  NOTICES. All notices, requests, demands and other communications
required or permitted to be given pursuant to this Agreement shall be in writing
and shall be deemed to have been duly given upon the date of receipt if
delivered by hand, recognized national overnight courier, or confirmed facsimile
transmission, or upon the date sent if mailed by registered or certified mail,
return receipt requested, postage prepaid, to the following addresses or
facsimile numbers:

If to Consultant:                              If to Company:

Judah Folkman, M.D.                            Synta Pharmaceuticals Corp.
Children's Hospital Boston                     45 Hartwell Ave.
Department of Surgery, Hunnewell - 1           Lexington, MA 02421
300 Longwood Avenue                            Attn.: Chief Executive Officer
Boston, MA 02115

Either party may change its designated address and facsimile number by notice to
the other party in the manner provided in this Section.

     6.8  AMENDMENT AND WAIVER. This Agreement may be modified, amended, or
supplemented only by means of a written instrument signed by both parties. Any
waiver of any rights or failure to act in a specific instance shall relate only
to such instance and shall not be construed as an agreement to waive any rights
or fail to act in any other instance, whether or not similar.

     6.9  GOVERNING LAW. This Agreement shall be governed by and construed in
accordance with the laws of the Commonwealth of Massachusetts irrespective of
any conflict of laws principles.

     6.10 SEVERABILITY. In the event that any provision of this Agreement shall,
for any reason, be held to be invalid or unenforceable in any respect, such
invalidity or unenforceability shall not affect any other provision hereof, and
this Agreement shall be construed as if such invalid or unenforceable provision
had not been included herein. To the extent this Agreement may be construed in
accordance with the laws of any state that limits the assignability to the
Company of certain inventions, this Agreement shall be interpreted not to apply
to any such invention that a court rules or the Company agrees is subject to
such state limitation.

     6.11 EQUITABLE RELIEF. The parties acknowledge that the restrictions
contained in this Agreement are necessary and reasonable, and that breach
thereof may cause irreparable harm to the other party. Therefore, in addition to
any other remedies that may be available, the aggrieved party may apply for and
obtain immediate injunctive relief in any court of competent jurisdiction to
restrain the breach or threatened breach of, or otherwise to specifically
enforce, any obligations under this Agreement.

                                       8
<Page>

     6.12 ENTIRE AGREEMENT. This Agreement constitutes the entire agreement
between the parties with respect to the subject matter hereof and supersedes any
and all prior or contemporaneous oral and prior written agreements and
understandings.

     6.13 Company shall not use Consultant's name or depiction, or the name,
logos, trademarks, or depictions of CHILDREN'S HOSPITAL, HMS, or any officer,
director, employee, appointee, medical staff member of employee of either, or
any adaptation thereof, in any promotional, advertising or marketing literature,
or in any other way without the prior written consent of CHILDREN'S HOSPITAL,
the individual, or HMS, as appropriate, provided however that in neutral
circumstances that do not imply endorsement or advocacy, or otherwise
misrepresent the terms of this Agreement or Consultant's role, Company may
accurately state that Consultant is a Scientific Advisory Board member and
consultant to Company, and list his professional degrees and titles.

     IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
executed as a sealed instrument effective as of the date first written above.

<Page>

SYNTA PHARMACEUTICALS CORP.

By: /s/ SAFI BAHCALL
   ---------------------------------
Name:  Safi Bahcall, Ph.D.
Title:  Chief Executive Officer

JUDAH FOLKMAN, MD

   /s/ JUDAH FOLKMAN
   ---------------------------------

Accepted and Agreed:

CHILDREN'S HOSPITAL BOSTON

By: /s/ DONALD P. LOMBARDI
   ---------------------------------
     Donald P. Lombardi
     Chief Intellectual Property Officer

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