<Page>

                                                                   Exhibit 10.15

                                                                 October 1, 2002

Dr. Keizo Koya
[ADDRESS]

Dear Keizo:

     We are pleased to make the following employment offer to you at Synta
Pharmaceuticals Corp. (hereinafter "Synta Pharmaceuticals" or the "Company"). We
are looking forward to working with you. The terms of your employment are
detailed below.

Position:                     Your position will be Vice President of Drug
                              Development, and you will work at our office
                              located at 45 Hartwell Avenue, Lexington,
                              Massachusetts. If you continue with the Company,
                              your position and assignments will be subject to
                              change. Synta Pharmaceuticals is a dynamic
                              organization with ever-changing needs, and we will
                              place you where we believe your talents and
                              abilities can be best utilized. As a Synta
                              Pharmaceuticals employee, we expect that you will
                              perform any and all duties and responsibilities
                              normally associated with your position in a
                              satisfactory manner and to the best of your
                              abilities at all times. You will be expected to
                              devote all of your working time to the performance
                              of your duties at the Company throughout your
                              employment. Your employment at all times remains
                              at-will, as discussed below.

Salary:                       Your initial base pay shall be $150,000 annually,
                              payable at a biweekly rate of $5,769.23, from
                              which all applicable taxes and other customary
                              employment-related deductions will be taken.
                              Additionally, you will qualify to receive annual
                              performance-based bonuses. Bonuses for fully
                              meeting and exceeding expectations will be granted
                              in the 10% -20% range. Bonuses over 20% will only
                              be granted for the demonstration of exceptional
                              performance. Any such bonus will be granted at the
                              discretion of the Company's Board of Directors.

Stock Option:                 Subject to the approval of the Company's Board of
                              Directors, you will be granted an incentive stock
                              option to purchase 500,000 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. Vesting shall occur as follows:
                              150,000 at the signing of this offer letter with
                              the remainder to follow the standard Company
                              vesting schedule. Please refer to the Plan and to

<Page>

                              your stock option agreement for further
                              information concerning this aspect of your
                              compensation.

Benefit Plans:                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. The Company will provide you with
                              detailed information on its current benefits,
                              holiday schedule, and vacation and sick leave
                              policies on your start date. All benefits, of
                              course, may be changed or modified from time to
                              time and the provision of any benefits to you in
                              no way changes or impacts your status as an
                              at-will employee.

Starting Date:                Your first day of employment under this offer of
                              employment will be October 1, 2002.

Nature of Relationship:       No provision of this letter shall be construed to
                              create an express or implied employment contract,
                              or a promise of employment for any specific period
                              of time. Your employment with Synta
                              Pharmaceuticals is at-will employment which may be
                              terminated by you or the Company at any time for
                              any reason with or without advance notice.

     Our employment offer to you is contingent upon (i) your execution of the
standard form of Non-Competition, Confidentiality and Inventions Agreement (a
copy of which is attached hereto as Exhibit A), and (ii) 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.

     This letter constitutes our entire offer regarding the terms and conditions
of your prospective employment with Synta Pharmaceuticals. It supersedes any
prior agreements, or other promises or statements (whether oral or written)
regarding the offered terms of employment. The terms of your employment shall be
governed by 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.


                  [Remainder of page intentionally left blank]

                                        2
<Page>

     If you accept the above-described offer, please sign and return the
original copy of this letter; the second copy is for your records. We look
forward to you joining Synta Pharmaceuticals. If you have any questions, please
feel free to call me.


                                          Sincerely,


                                          SYNTA PHARMACEUTICALS CORP.

                                             /s/ SAFI BAHCALL
                                          ---------------------------
                                          Safi Bahcall, Ph.D.
                                          Chief Executive Officer

Agreed to a d accepted

Name: /s/ KEIZO KOYA                      Date: October 12, 2002
      --------------------                     --------------------

                                        3
<Page>

                                                                       EXHIBIT A

                           Synta Pharmaceuticals Corp.
                               45 Hartwell Avenue
                               Lexington, MA 02421

                                                                 October 1, 2002

Dr. Keizo Koya
[ADDRESS]

Dear Keizo:

     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 acknowledge and agree that a business will be deemed
competitive with the Company (a "Competitive Business") 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 Term 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

                                        4
<Page>

that all such confidential information and training could be used by you to
compete with the Company.

          (b)  COVENANTS NOT TO COMPETE. During the period in which you are
employed by the Company (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,
for yourself or on behalf of any other person or entity, directly or indirectly,
either as principal, agent, stockholder, employee, consultant, representative or
in any other capacity, own, manage, operate or control, or be concerned,
connected or employed by, or otherwise associate in any manner with, engage in
or have a financial interest in any Competitive Business (as defined above)
anywhere in the world (the "Restricted Territory"), except that nothing
contained herein shall preclude you from purchasing or owning securities of any
such business if such securities are publicly traded, and provided that your
holdings do not exceed three (3%) percent of the issued and outstanding
securities of any class of securities of such business.

          (c)  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.

          (d)  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.

          (e)  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

                                        5
<Page>

time and substantive scope of the provisions of this Section 1 are reasonable,
legitimate and fair to you.

          (f)  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.

     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,

                                        6
<Page>

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.

     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,

                                        7
<Page>

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 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.

                                        8
<Page>

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.

          (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.

                                        9
<Page>

          (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 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.

                                       10
<Page>

                                          Very truly yours,

                                          SYNTA PHARMACEUTICALS CORP.

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


Agreed to and accepted:

  /s/ KEIZO KOYA
--------------------------
Name:


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


--------------------------
Address

Date: October 12, 2002
     ---------------------

                                       11