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                                                                   Exhibit 4.2.1

                           SYNTA PHARMACEUTICALS CORP.

                 AMENDED AND RESTATED INVESTOR RIGHTS AGREEMENT

     THIS AMENDED AND RESTATED INVESTOR RIGHTS AGREEMENT (this "Agreement")
dated as of December 13, 2002, is by and among Synta Pharmaceuticals Corp., a
Delaware corporation (the "Company"), Robert A. Day and Mountain Trail
Investments, LLC, (singly, a "DAY INVESTOR", and collectively, the "DAY
INVESTOR"), Keith R. Gollust, Gollust Trust II, and Wyandanch Partners, L.P.
(singly, a "GOLLUST INVESTOR", and collectively, the "GOLLUST INVESTOR") and
Cxsynta LLC, an affiliate of Caxton Corporation (the "Caxton Investor") (each an
"INVESTOR" and, collectively, the "INVESTORS").

     Reference is hereby made to that certain Investor Rights Agreement dated
May 16, 2002 by and between the Company and Cxsynta LLC, which agreement is
hereby amended and restated in its entirety and shall be without further force
or effect as of the date hereof

                                    RECITALS

     WHEREAS, the Investors have purchased the number of shares (the "SHARES")
of the Common Stock, $.0001 par value, of the Company (the "COMMON STOCK") set
forth opposite their respective names on SCHEDULE A hereto;

     WHEREAS, concurrently with the execution of this Agreement, the Investors
have executed the Amended and Restated Stockholders' Agreement of even date
herewith (the "STOCKHOLDERS' AGREEMENT") with the Company pursuant to which all
parties have agreed to provide for certain board of director representation and
meeting requirements, co-sale rights and rights of first refusal with respect to
the Common Stock; and

     WHEREAS, the Company wishes to provide the Investors with certain
information rights, registration rights and rights of first refusal in
conjunction with the purchase of their respective Shares;

     NOW, THEREFORE, in consideration of the mutual covenants and agreements
contained herein, and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto agree as
follows:

1.   DEFINITIONS. For purposes of this Agreement:

          (a)  The term "AFFILIATE" means any general or limited partner of any
     person that is a partnership, any member or manager of any person that is a
     limited liability company or any person or entity that, directly or
     indirectly, through one or more intermediaries, controls or is controlled
     by, or is under common control with, such person.

          (b)  The term "ACT" means the Securities Act of 1933, as amended, or
     any similar federal statute, and the rules and regulations of the SEC, all
     as shall be in effect at the time.

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          (c)  The term "BUDGET" has the meaning set forth in SECTION 3.3
     hereof.

          (d)  The term "COMPANY INDEMNIFIED PARTIES" has the meaning set forth
     in SECTION 2.6(a) hereof.

          (e)  The term "DEMAND PERIOD" has the meaning set forth in SECTION
     2.2(a) hereof.

          (f)  The term "FORM S-3" means such form under the Act as in effect on
     the date hereof or any registration form under the act subsequently adopted
     by the SEC that permits inclusion or incorporation of substantial
     information by reference to other documents filed by the Company with the
     SEC.

          (g)  The term "GAAP" means generally accepted accounting principles as
     consistently applied by the Company.

          (h)  The term "HOLDER" means any person owning or having the right to
     acquire Registrable Securities, or any assignee thereof, in accordance with
     SECTION 2.8 hereof.

          (i)  The term "INITIAL OFFERING" means the Company's first firm
     commitment underwritten public offering of its Common Stock under the Act.

          (j)  The term "1934 ACT" means the Securities Exchange Act of 1934, as
     amended, or any similar federal statute, and the rules and regulations of
     the SEC, all as shall be in effect at the time.

          (k)  The term "NEW SECURITIES" shall mean any equity securities of the
     Company, whether now authorized or not, and rights, options, or warrants to
     purchase said equity securities, and securities of any type whatsoever that
     are, or may become, convertible into said equity securities; PROVIDED,
     HOWEVER that "New Securities" does not include: (i) securities offered to
     the public pursuant to an Initial Offering; (ii) securities issued pursuant
     to the acquisition of another corporation or entity by the Company by
     merger, purchase of substantially all of the assets, or other
     reorganization whereby the Company acquires a majority of the voting power
     of such corporation or entity; (iii) up to ten million (10,000,000) shares
     of Common Stock issued or issuable to employees, consultants or directors
     of the Company pursuant to the Company's 2001 Stock Plan (including shares
     issued or issuable upon exercise of options previously granted), which
     number of shares may be adjusted upward by the affirmative vote of the
     Company's Board of Directors; (iv) up to three hundred sixty eight thousand
     eight hundred ninety four (368,894) shares of Common Stock issuable to
     employees of the Company pursuant to the Company's 2002 Employee Stock
     Purchase Plan; (v) securities issued to strategic partners of the Company,
     such as biotechnology, pharmaceutical, drug manufacturing or clinical
     research companies; (vi) securities issued to licensors of technology to
     the Company; or (vii) securities issued in connection with any bank lines
     of credit, equipment lease transactions, or real estate transactions; in
     each case as approved by the Company's Board of Directors.

          (1)  The term "NOTICE OF ACCEPTANCE" has the meaning set forth in
     SECTION 4.3

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     hereof.

          (m)  The term "OFFER" has the meaning set forth in SECTION 4.2 hereof.

          (n)  The term "PRO RATA AMOUNT" has the meaning set forth in SECTION
     4.1 hereof.

          (o)  The term "REFUSED SECURITIES" has the meaning set forth in
     SECTION 4.4 hereof.

          (p)  The term "REGISTER," "REGISTERED," and "REGISTRATION" refer to a
     registration effected by preparing and filing a registration statement or
     similar document in compliance with the Act, and the declaration or
     ordering of effectiveness of such registration statement or document.

          (q)  The term "REGISTRABLE SECURITIES" means (i) any shares of Common
     Stock held by an Investor (ii) any shares of capital stock of the Company
     acquired by an Investor (or any transferee of an Investor) after the date
     hereof pursuant to the Stockholders' Agreement and (iii) any Common Stock
     issued as (or issuable upon the conversion or exercise of any warrant,
     right or other security) a dividend or other distribution with respect to
     or because of stock splits, stock dividends, reclassifications,
     recapitalizations, or similar events, or in exchange for, or in replacement
     of the shares referenced in (i) and (ii) above, excluding in all cases,
     however, any Registrable Securities sold by a person in a transaction in
     which his, her or its rights under SECTION 2 hereof are not assigned.
     Registrable Securities shall exclude any shares which (A) have been
     registered under the Securities Act pursuant to an effective registration
     statement filed thereunder and disposed of in accordance with the
     registration statement covering them, or (B) may be publicly sold pursuant
     to and in compliance with SEC Rule 144 in any ninety (90) day period,
     provided that such shares shall not be excluded if (x) the number of shares
     proposed to be sold by such Investor is larger than the number of shares
     that may be sold in any single 90-day period pursuant to Rule 144 or (y)
     such Investor believes in good faith that a sale pursuant to Rule 144 will
     be less advantageous to it than a sale pursuant to Section 2.1 or 2.2.

          (r)  The term "REQUESTING HOLDERS" has the meaning set forth in
     SECTION 2.2(a) hereof.

          (s)  The term "SEC" means the Securities and Exchange Commission.

          (t)  The term "SELLING HOLDER" has the meaning set forth in SECTION
     2.1(a) hereof.

          (u)  The term "SELLING HOLDER INDEMNIFIED PARTIES" has the meaning set
     forth in SECTION 2.6(b) hereof.

          (v)  The term "VIOLATION" has the meaning set forth in SECTION 2.6(a)
     hereof.

2.   Registration Rights. The Company covenants and agrees as follows:

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     2.1  PIGGYBACK REGISTRATION.

           (a)  If (but without any obligation to do so) the Company proposes to
register (including for this purpose a registration effected by the Company for
stockholders other than the Holders) any of its stock or other securities under
the Act in connection with the public offering of such securities (other than a
registration statement on Form S-8 or Form S-4, or their successors, or any
registration statement covering only securities proposed to be issued in
exchange for securities or assets of another corporation), the Company shall, at
such time, promptly give each Holder written notice of such registration. Upon
the written request of each Holder (a "SELLING HOLDER") given within thirty (30)
days after mailing of such notice by the Company in accordance with the
provisions hereof, the Company shall, subject to the provisions of SECTION
2.1(c), use all reasonable efforts to cause to be registered under the Act all
of the Registrable Securities that each such Holder has requested to be
registered.

           (b)  The Company shall have the right to terminate or withdraw any
registration initiated by it under this SECTION 2.1 prior to the effectiveness
of such registration whether or not any Holder has elected to include securities
in such registration. The expenses of such withdrawn registration shall be borne
by the Company in accordance with SECTION 2.5 hereof.

           (c)  In connection with any offering involving an underwriting of
shares of the Company's capital stock, the Company shall not be required under
this SECTION 2.1 to include any of the Holders' securities in such underwriting
unless they accept the terms of the underwriting as agreed upon between the
Company and the underwriters selected by it, which terms shall not contravene
any of the terms hereof without the consent of the Selling Holders holding fifty
percent (50%) of the Registrable Securities requested to be included in such
registration statement, and enter into such an underwriting agreement in
customary form with an underwriter or underwriters selected by the Company. In
connection with any such underwriting agreement, no Selling Holder shall be
required to make representations and warranties other than representations and
warranties regarding such Selling Holder's ownership and title to the
Registrable Securities being sold by it and its plan of distribution with
respect to its Registrable Securities. The number of securities which shall be
included in such registration shall be in such quantity as the managing
underwriter determines in its sole discretion will not materially and adversely
affect the offering by the Company. If the total number of securities, including
Registrable Securities, requested by stockholders to be included in such
offering exceeds the number of securities that the managing underwriter
determines in its sole discretion will not materially and adversely affect the
offering, then the Company shall be required to include in the offering only
that number of such securities, including Registrable Securities, that the
managing underwriter determines in writing in its sole discretion will not
materially and adversely affect the offering (the securities so included to be
apportioned pro rata among the Selling Holders according to the total amount of
securities entitled to be included therein owned by each Selling Holder or in
such other proportions as shall mutually be agreed to by such Selling Holders).
Notwithstanding anything to the contrary contained in this Agreement, to the
extent the stockholders of the Company that have requested to have securities
included in such registration include stockholders other than Holders exercising
contractual demand registration rights, then the Company will include in such
registration, to the extent of the number and type which the Company is so
advised can be sold in such offering, (i) FIRST all Registrable Securities
requested for inclusion held by the Holders exercising contractual demand

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registration rights, and

(ii) SECOND such securities requested to be included in such registration
statement by all other stockholders. For purposes of the foregoing parenthetical
concerning apportionment, for any selling stockholder that is a Holder of
Registrable Securities and that is a partnership, limited liability company or
corporation, the partners, members, retired partners, retired members and
stockholders of such Holder, or the estates and family members of any such
partners, members, retired partners and retired members and any trusts for the
benefit of any of the foregoing persons shall be deemed to be a single Selling
Holder, and any pro rata reduction with respect to such Selling Holder shall be
based upon the aggregate amount of Registrable Securities owned by all such
related entities and individuals.

           (d)  In the event a Selling Holder (i) is unable to include in a
registration by the Company under this SECTION 2.1 all of the Registrable
Securities that such Holder has requested, or (ii) disapproves the terms of the
underwriting as agreed upon between the Company and the underwriters selected by
it, the Selling Holder will have the demand registration rights set forth in
SECTION 2.2.

     2.2  DEMAND REGISTRATION.

           (a)  Commencing at least one hundred eighty (180) days from the
effective date of a registration statement that could have included Registrable
Securities under SECTION 2.1 and expiring two (2) years from such effective date
or until such time as the registration statement delayed pursuant to Section
2.2.(b) has been granted effectiveness (the "DEMAND PERIOD"), any Holders
entitled to demand registration rights under Section 2.1(d) (the "REQUESTING
HOLDERS") shall be entitled to request in writing during the Demand Period that
the Company effect the registration, qualification or compliance of the
Registrable Securities owned by such Requesting Holders; PROVIDED, HOWEVER, that
the expected aggregate price to the public of the Registrable Securities will
equal or exceed five million dollars ($5,000,000). If the Requesting Holders
intend to distribute the Registrable Securities by means of an underwriting,
they shall so advise the Company in their request. The underwriter shall be
reasonably acceptable to the Company.

           (b)  The Company shall file a registration statement covering the
Registrable Securities so requested to be registered as soon as practicable
after receipt of the request or requests of the Requesting Holders, and shall
use its commercially reasonable efforts to effect such registration,
qualification or compliance (including, without limitation, the execution of an
undertaking to file post-effective amendments, appropriate qualification under
applicable blue sky or other state securities laws and appropriate compliance
with applicable regulations issued under the Securities Act and any other
governmental requirements or regulations) as may be so requested and as would
permit or facilitate the sale and distribution of all or such portion of such
Registrable Securities as are specified in such request; PROVIDED, HOWEVER, that
if: (i) in the good faith judgment of the Board of Directors of the Company,
such registration would be seriously detrimental to the Company and the Board of
Directors of the Company concludes, as a result, that it is essential to defer
the filing of such registration statement at such time, and (ii) the Company
shall furnish to such Holders a certificate signed by the Chief Executive
Officer of the Company stating that in the good faith judgment of the Board of
Directors of the Company, it would be seriously detrimental to the Company for
such registration statement to be filed in the near future

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and that it is, therefore, essential to defer the filing for a period of not
more than one hundred twenty (120) days after receipt of the request of the
Requesting Holders; and FURTHER PROVIDED, that the Company shall not defer its
obligation in this manner more than once in any twelve-month period.

           (c) The Company shall not be required to effect more than one (1)
registration pursuant to this SECTION 2.2 during the Demand Period.

     2.3  OBLIGATIONS OF THE COMPANY. Whenever required under this SECTION 2 to
effect the registration of any Registrable Securities, the Company shall, at the
earliest possible date:

           (a)  prepare and file with the SEC a registration statement with
respect to such Registrable Securities and use all reasonable efforts to cause
such registration statement to become effective, and, upon the request of the
Holders of a majority of the Registrable Securities registered thereunder, keep
such registration statement effective for one hundred twenty (120) days from the
effective date or, if earlier, until the distribution contemplated in the
Registration Statement has been completed;

           (b)  as promptly as possible prepare and file with the SEC such
amendments and supplements to such registration statement and the prospectus
used in connection with such registration statement as may be necessary to
comply with the provisions of the Act with respect to the disposition of all
securities covered by such registration statement;

           (c)  as promptly as possible furnish to the Holders such numbers of
copies of the registration statement and amendments thereto, a prospectus,
including a preliminary prospectus, in conformity with the requirements of the
Act, and such other documents as they may reasonably request in order to
facilitate the disposition of Registrable Securities owned by them;

           (d)  as promptly as possible use all reasonable efforts to register
and qualify the securities covered by such registration statement under such
other securities or blue sky or other state securities laws of such
jurisdictions as shall be reasonably requested by the Holders, provided that the
Company shall not be required in connection therewith or as a condition thereto
to qualify to do business or to file a general consent to service of process in
any such states or jurisdictions;

           (e)  as promptly as possible in the event of any underwritten public
offering, enter into and perform its obligations under an underwriting
agreement, in usual and customary form, with the managing underwriter of such
offering;

           (f)  as promptly as possible notify each Holder of Registrable
Securities covered by such registration statement at any time when a prospectus
relating thereto is required to be delivered under the Act of the happening of
any event as a result of which the prospectus included in such registration
statement, as then in effect, includes an untrue statement of a material fact or
omits to state a material fact required to be stated therein or necessary to
make the statements therein not misleading in the light of the circumstances
then existing;

           (g)  as promptly as possible cause all such Registrable Securities
registered pursuant hereunder to be listed on each securities exchange on which
similar securities issued by

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the Company are then listed;

           (h)  as promptly as possible. provide a transfer agent and registrar
for all Registrable Securities registered pursuant hereunder and a CUSIP number
for all such Registrable Securities, in each case not later than the effective
date of such registration; and

           (i)  if such securities are being sold in an underwritten offering,
as promptly as possible furnish at the request of any Holder requesting
registration of Registrable Securities pursuant hereto, on the date that such
Registrable Securities are delivered to the underwriters for sale in connection
with a registration pursuant hereto, (A) an opinion, dated such date, of the
counsel representing the Company for the purposes of such registration, in form
and substance as is customarily given to underwriters in an underwritten public
offering, addressed to the underwriters, and to the Holders requesting
registration of Registrable Securities, and (B) a letter dated such date, from
the independent certified public accountants of the Company, in form and
substance as is customarily given by independent certified public accountants to
underwriters in an underwritten public offering, addressed to the underwriters,
and to the Holders requesting registration of Registrable Securities.

     2.4  INFORMATION FROM HOLDER. It shall be a condition precedent to the
obligations of the Company to take any action pursuant to this SECTION 2 with
respect to the Registrable Securities of any Selling Holder that such Holder
shall furnish to the Company such information regarding itself, the Registrable
Securities held by it, and the intended method of disposition of such securities
as shall be required to effect the registration of such Holder's Registrable
Securities.

     2.5  EXPENSES OF REGISTRATION. All expenses (other than underwriting
discounts and commissions and the fees and expenses of counsel to the Selling
Holders) incurred in connection with registrations, filings or qualifications
pursuant to SECTIONS 2.1 and 2.2 including without limitation all registration,
filing and qualification fees, printers' and accounting fees, fees and
disbursements of counsel for the Company shall be borne by the Company.

     2.6  INDEMNIFICATION. In the event any Registrable Securities are included
in a registration statement under this SECTION 2:

           (a)  The Company will indemnify and hold harmless each Holder, the
partners, members, managers, officers, directors and stockholders of each
Holder, any underwriter (as defined in the Act) for such Holder and each person,
if any, who controls such Holder or underwriter within the meaning of the Act or
the 1934 Act (collectively, the "COMPANY INDEMNIFIED PARTIES"), against any
losses, claims, damages or liabilities (joint or several) to which they may
become subject under the Act, the 1934 Act or any state securities laws, insofar
as such losses, claims, damages, or liabilities (or actions in respect thereof)
arise out of or are based upon any of the following statements, omissions or
violations (collectively, a "VIOLATION"): (i) any untrue statement or alleged
untrue statement of a material fact contained in such registration statement,
including any preliminary prospectus or final prospectus contained, therein or
any amendments or supplements thereto, (ii) the omission or alleged omission to
state therein a material fact required to be stated therein, or necessary to
make the statements therein not misleading, or (iii) any violation or alleged
violation by the Company of the Act, the 1934 Act, any blue sky or other state
securities laws or any rule or regulation promulgated under the Act, the

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1934 Act or any blue sky or other state securities laws; and the Company will
pay to each such Company Indemnified Party any legal or other expenses
reasonably incurred by them in connection with investigating or defending any
such loss, claim, damage, liability or action; PROVIDED, HOWEVER, that the
indemnity agreement contained in this SECTION 2.6(a) shall not apply to amounts
paid in settlement of any such loss, claim, damage, liability or action if such
settlement is effected without the consent of the Company (which consent shall
not be unreasonably withheld), nor shall the Company be liable in any such case
for any such loss, claim, damage, liability or action to the extent that it
arises out of or is based upon a Violation that occurs in reliance upon and in
conformity with written information furnished expressly for use in connection
with such registration by any such Company Indemnified Party, and FURTHER
PROVIDED in the case of a demand registration pursuant to SECTION 2.2, that the
foregoing indemnity agreement with respect to any preliminary prospectus shall
not inure to the benefit of any Company Indemnified Party, from whom the person
asserting any such losses, claims, damages or liabilities purchased shares in
the offering, if a copy of the prospectus (as then amended or supplemented if
the Company shall have furnished any amendments or supplements thereto) was not
sent or given by or on behalf of such Company Indemnified Party to such person,
if required by law so to have been delivered, at or prior to the written
confirmation of the sale of the shares to such person, and if the prospectus (as
so amended or supplemented) would have cured the defect giving rise to such
loss, claim, damage or liability.

           (b)  Each Selling Holder will indemnify and hold harmless the
Company, each of its directors, each of its officers who has signed the
registration statement, each person, if any, who controls the Company within the
meaning of the Act, any underwriter, any other Holder selling securities in such
registration statement and any controlling person of any such underwriter or
other Holder (collectively, the "SELLING HOLDER INDEMNIFIED PARTIES"), against
any losses, claims, damages or liabilities (joint or several) to which any of
the foregoing Selling Holder Indemnified Parties may become subject, under the
Act, the 1934 Act or any state securities laws, insofar as such losses, claims,
damages or liabilities (or actions in respect thereto) arise out of or are based
upon any Violation, in each case to the extent (and only to the extent) that
such Violation occurs in reliance upon and in conformity with written
information furnished by such Holder expressly for. use in connection with such
registration; and each such Holder will reimburse a Selling Holder Indemnified
Party for any legal or other expenses reasonably incurred by such person in
connection with investigating or defending any such loss, claim, damage,
liability or action; PROVIDED, HOWEVER, that the indemnity agreement contained
in this SECTION 2.6(b) shall not apply to amounts paid in settlement of any such
loss, claim, damage, liability or action if such settlement is effected without
the consent of the Selling Holder (which consent shall not be unreasonably
withheld); and FURTHER PROVIDED, that in no event shall any indemnity under this
SECTION 2.6(b) exceed the net proceeds (after underwriting discounts and
commissions) from the offering received by such Selling Holder; and FURTHER
PROVIDED, that the foregoing indemnity agreement with respect to any preliminary
prospectus shall not inure to the benefit of a Selling Holder Indemnified Party
from whom the person asserting any such losses, claims, damages or liabilities
purchased shares in the offering, if a copy of the prospectus (as then amended
or supplemented if the Company shall have furnished any amendments or
supplements thereto) was not sent or given by or on behalf of such Selling
Holder Indemnified Party to such person, if required by law so to have been
delivered, at or prior to the written confirmation of the sale of the shares to
such person, and if the prospectus (as so amended or supplemented) would have
cured the defect giving rise to such loss, claim, damage or liability.

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           (c)  Promptly after receipt by an indemnified party under this
SECTION 2.6 of notice of the commencement of any action (including any
governmental action), such indemnified party will, if a claim in respect thereof
is to be made against any indemnifying party under this SECTION 2.6, deliver to
the indemnifying party a written notice of the commencement thereof and the
indemnifying party shall have the right to participate in, and, to the extent
the indemnifying party so desires, jointly with any other indemnifying party
similarly noticed, to assume the defense thereof with counsel mutually
satisfactory .to the parties; provided, however, that an indemnified party
(together with all other indemnified parties that may be represented without
conflict by one counsel) shall have the right to retain one separate counsel,
with the fees and expenses to be paid by the indemnifying party, if
representation of such indemnified party by the counsel retained by the
indemnifying party would be inappropriate due to actual conflict of interests
between such indemnified party and any other party represented by such counsel
in such proceeding. The failure to deliver written notice to the indemnifying
party within a reasonable time of the commencement of any such action, if
prejudicial to its ability to defend such action, shall relieve such
indemnifying party of any liability to the indemnified party under this SECTION
2.6, but the omission so to deliver written notice to the indemnifying party
will not relieve it of any liability that it may have to any indemnified party
otherwise than under this SECTION 2.6.

           (d)  If the indemnification provided for in this SECTION 2.6 is held
by a court of competent jurisdiction to be unavailable to an indemnified party
with respect to any loss, liability, claim, damage or expense referred to
herein, then the indemnifying party, in lieu of indemnifying such indemnified
party hereunder, shall contribute to the amount paid or payable by such
indemnified party as a result of such loss, liability, claim, damage or expense
in such proportion as is appropriate to reflect the relative fault of the
indemnifying party on the one hand and of the indemnified party on the other in
connection with the statements or omissions that resulted in such loss,
liability, claim, damage or expense, as well as any other relevant equitable
considerations. The relative fault of the indemnifying party and of the
indemnified party shall be determined by reference to, among other things,
whether the untrue or alleged untrue statement of a material fact or the
omission to state a material fact relates to information supplied by the
indemnifying party or by the indemnified party, and the parties' relative
intent, knowledge, access to information, and opportunity to correct or prevent
such statement or omission; PROVIDED HOWEVER, that, in any such case (A) no such
Holder will be required to contribute any amount in excess of the net proceeds
(after underwriting discounts and commissions) from the offering received by
such Holder, and (B) no person or entity guilty of fraudulent misrepresentation
(within the meaning of Section 11(f) of the Act) will be entitled to
contribution from any person or entity who was not guilty of such fraudulent
misrepresentation.

           (e)  Notwithstanding anything to the contrary in the foregoing, to
the extent that the provisions on indemnification and contribution contained in
the underwriting agreement entered into in connection with the underwritten
public offering are in conflict with the foregoing provisions, the provisions in
this Agreement shall control, unless otherwise consented to in writing by the
Holders of at least two-thirds of the Registrable Securities.

           (f)  The obligations of the Company and Holders under this SECTION
2.6 shall survive the completion of any offering of Registrable Securities in a
registration statement under this SECTION 2, and otherwise.

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           2.7  REPORTS UNDER SECURITIES EXCHANGE ACT OF 1934. With a view to
making available to the Holders the benefits of SEC Rule 144 and any other rule
or regulation of the SEC that may at any time permit a Holder to sell securities
of the Company to the public without registration or pursuant to a registration
on Form S-3, the Company agrees to:

           (a)  make and keep public information available, as those terms are
understood and defined in SEC Rule 144, at all times after ninety (90) days
after the effective date of the Initial Offering;

           (b)  file with the SEC in a timely manner all reports and other
documents required of the Company under the Act and the 1934 Act; and

           (c)  furnish to any Holder, so long as the Holder owns any
Registrable Securities, promptly upon request (i) a written statement by the
Company that it has complied with the reporting requirements of SEC Rule 144 (at
any time after ninety (90) days after the effective date of the Initial
Offering), the Act and the 1934 Act (at any time after it has become subject to
such reporting requirements), or that it qualifies as a registrant whose
securities may be resold pursuant to Form S-3 (at any time after it so
qualifies), (ii) a copy of the most recent annual or quarterly report of the
Company and such other reports and documents so filed by the Company, and (iii)
such other information as may be reasonably requested in availing any Holder of
any rule or regulation of the SEC that permits the selling of any such
securities without registration or pursuant to such form.

     2.8  ASSIGNMENT OF REGISTRATION RIGHTS. The rights to cause the Company to
register Registrable Securities pursuant to this SECTION 2 may be assigned (but
only with all related obligations) by a Holder to a transferee or assignee of
such securities that (i) is an Affiliate of the Holder or (ii) after such
assignment or transfer, holds at least one percent (1%) of the issued and
outstanding shares of the Company's Common Stock, provided that: (a) the Company
is, within a reasonable time after such transfer, furnished with written notice
of the name and address of such transferee or assignee and the securities with
respect to which such registration rights are being assigned; and (b) such
transferee or assignee agrees in writing to be bound by and subject to the terms
and conditions of this Agreement, including without limitation the provisions of
SECTION 2.9 below.

     2.9  "MARKET STAND-OFF' AGREEMENT. Each Holder hereby agrees that it will
not, without the prior written consent of the managing underwriter, during the
period commencing on the date of the final prospectus relating to the Initial
Offering and ending on the date specified by the Company and the managing
underwriter (such period not to exceed one hundred eighty (180) days): (i) lend,
offer, pledge, sell, contract to sell, sell any option or contract to purchase,
purchase any option or contract to sell, grant any option, right or warrant to
purchase, or otherwise transfer or dispose of, directly or indirectly, any
shares of Common Stock or any securities convertible into or exercisable or
exchangeable for Common Stock (whether such shares or any such securities are
then owned by the Holder or are thereafter acquired), or (ii) enter into any
swap or other arrangement that transfers to another, in whole or in part, any of
the economic consequences of ownership of the Common Stock, whether any such
transaction described in clause (i) or (ii) above is to be settled by delivery
of Common Stock or such other securities, in cash or otherwise;

                                       10
<Page>

PROVIDED, HOWEVER, that all executive officers and directors of the Company and
all other holders of at least one percent (1%) of Common Stock enter into
similar agreements. The underwriters in connection with the Initial Offering are
intended third-party beneficiaries of this SECTION 2.9 and shall have the right,
power and authority to enforce the provisions hereof as though they were a party
hereto.

     In order to enforce the foregoing covenant, the Company may impose
stop-transfer instructions with respect to the Registrable Securities of each
Holder (and the shares or securities of every other person subject to the
foregoing restriction) until the end of such period.

     2.10 TERMINATION OF REGISTRATION RIGHTS. No Holder shall be entitled to
exercise any right provided for in this SECTION 2 after the earlier to occur
of(a) the date that is five (5) years following the consummation of the Initial
Offering, and (b) the date on which all of the Investors' Registrable Shares may
be sold within a ninety (90) day period pursuant to SEC Rule 144.

3.   INFORMATION RIGHTS. Subject to the limitations set forth in SECTION 3.4:

     3.1 ACCESS TO RECORDS. The Company agrees to afford to each Investor, upon
such Investor's reasonable prior request, free and full access, during normal
business hours, to all books, records and properties of the Company and to all
officers and employees of the Company having responsibility for financial or
accounting matters generally, for any reasonable purpose whatsoever.

     3.2  FINANCIAL REPORTS. The Company agrees to furnish each Investor with
the following:

          (a)  QUARTERLY REPORTS. Within thirty (30) days after the end of each
     fiscal quarter, an unaudited financial statements of the Company, which
     shall be prepared in accordance with GAAP (except that the financial report
     may (i) be subject to normal year-end audit adjustments and (ii) not
     contain all notes thereto which may be required in accordance with GAAP).

          (b)  ANNUAL REPORTS. Within one hundred twenty (120) days after the
     end of each fiscal year of the Company, audited financial statements of the
     Company, prepared in accordance with GAAP.

          (c)  OTHER REPORTS AND INFORMATION. Within a reasonable period of
     time, such other reports and financial information as may be reasonably
     requested by an Investor.

     3.3  BUDGET. At least twenty (20) days prior to the beginning of each
fiscal year of the Company, the Company shall prepare and submit a budget for
such fiscal year (the "BUDGET") to the Board of Directors of the Company and the
Investors. The Budget shall be accepted as the Budget for such fiscal year when
it has been approved by the Board of Directors of the Company.

     3.4  LIMITATIONS ON RIGHTS OF THE INVESTORS UNDER SECTION 3. The Company
shall provide the access rights and information required by SECTION 3 to an
Investor so long as such Investor

                                       11
<Page>

shall own at least five percent (5%) of the issued and outstanding Common Stock
of the Company (the "THRESHOLD AMOUNT"), PROVIDED that, in the case of a Day
Investor, the Threshold Amount shall be determined based upon the aggregate
amount of the Company's issued and outstanding Common Stock held by the Day
Investors, and FURTHER PROVIDED that, in the case of a Gollust Investor, the
Threshold Amount shall be determined based upon the aggregate amount of the
Company's issued and outstanding Common Stock held by the Gollust Investors. The
Company's obligations to the Investors pursuant to SECTION 3 shall terminate
upon the closing of an Initial Offering.

     4. RIGHT OF FIRST REFUSAL.

     4.1  The Company hereby grants to each Investor, so long as such Investor
shall own at least the Threshold Amount, the right of first refusal to purchase
a pro rata portion of any New Securities that the Company may, from time to
time, propose to sell or issue (the "PRO RATA AMOUNT"), PROVIDED that, in the
case of a Day Investor, the Threshold Amount shall be determined based upon the
aggregate amount of the Company's issued and outstanding Common Stock held by
the Day Investors, and FURTHER PROVIDED that, in the case of a Gollust Investor,
the Threshold Amount shall be determined based upon the aggregate amount of the
Company's issued and outstanding Common Stock held by the Gollust Investors.
Each Investor's Pro Rata Amount, for purposes of this right of first refusal, is
the ratio of (i) the number of shares of Common Stock then held of record by
such Investor, assuming the full conversion into Common Stock of any convertible
shares of the capital stock of the Company held by such Investor, to (ii) the
total number of shares of Common Stock outstanding immediately prior to the
issuance of New Securities, assuming the conversion into Common Stock of any
convertible shares of the Company's capital stock then outstanding.

     4.2  The Company shall not issue, sell or exchange, agree to issue, sell or
exchange, or reserve or set aside for issuance, sale or exchange any New
Securities unless the Company shall deliver to the Investors a written notice of
any proposed or intended issuance, sale or exchange of New Securities (the
"OFFER"), which Offer shall (i) identify and describe the New Securities, (ii)
describe the price and other terms upon which they are to be issued, sold or
exchanged, and the number or amount of the New Securities to be issued, sold or
exchanged, (iii) identify the persons or entities, if known, to which or with
which the New Securities are to be offered, issued, sold or exchanged and (iv)
offer to issue and sell to or exchange with each of the Investors their
respective Pro Rata Amount. Each Investor shall have the right, for a period of
thirty (30) days following delivery of the Offer, to purchase or acquire, at a
price and upon the other terms specified in the Offer, the number or amount of
New Securities described above. The Offer by its terms shall remain open and
irrevocable for such 30-day period.

     4.3  To accept an Offer, in whole or in part, an Investor must deliver a
written notice to the Company prior to the end of the 30-day period of the
Offer, setting forth the portion of the Pro Rata Amount that such Investor
elects to purchase (the "NOTICE OF ACCEPTANCE").

     4.4  The Company shall have ninety (90) days from the expiration of the
period set forth in SECTION 4.2 above to issue, sell or exchange all or any part
of such New Securities as to which a Notice of Acceptance has not been given by
an Investor (the "REFUSED SECURITIES"), but only to the offerees or purchasers
(if identified) and only upon terms and conditions (including,

                                       12
<Page>

without limitation, unit prices and interest rates) which are described in the
Offer.

     4.5  In the event the Company shall propose to sell less than all the
Refused Securities (any such sale to be in the manner and on the terms specified
in SECTION 4.4 above), then an Investor may, at its sole option and in its sole
discretion, reduce the number or amount of the New Securities specified in its
Notice of Acceptance to an amount that shall be not less than the number or
amount of the New Securities that such Investor elected to purchase pursuant to
SECTION 4.3 above multiplied by a fraction, (i) the numerator of which shall be
the number or amount of New Securities the Company actually proposes to issue,
sell or exchange (including New Securities to be issued or sold to Investor
pursuant to SECTION 4.3 above prior to such reduction) and (ii) the denominator
of which shall be the number or amount of all New Securities that the Company
initially proposed to offer, sell or exchange as described in the Offer. In the
event that an Investor so elects to reduce the number or amount of New
Securities specified in its Notice of Acceptance, the Company may not issue,
sell or exchange more than the reduced number or amount of the New Securities
unless and until such securities have again been offered to the Investors in
accordance with SECTION 4.2 above.

     4.6  Upon the closing of the issuance, sale or exchange of all or less than
all the Refused Securities and the payment in full therefor by the Investor(s)
to the Company in immediately available funds, the Investor(s) shall acquire
from the Company, and the Company shall issue to the Investor(s), the number of
New Securities specified in the Notices of Acceptance, as reduced pursuant to
SECTION 4.5 above if the Investor(s) has so elected, upon the terms and
conditions specified in the Offer. The purchase by the Investor(s) of any New
Securities is subject in all cases to the preparation, execution and delivery by
the Company and each Investor returning a Notice of Acceptance of a purchase
agreement relating to such New Securities reasonably satisfactory in form and
substance to such Investor(s) and its or their counsel.

5.   MISCELLANEOUS.

     5.1  SUCCESSORS AND ASSIGNS. Except as otherwise provided herein, the terms
and conditions of this Agreement shall inure to the benefit of and be binding
upon the respective successors and assigns of the parties. Nothing in this
Agreement, express or implied, is intended to confer upon any party other than
the parties hereto or their respective successors and assigns any rights,
remedies, obligations, or liabilities under or by reason of this Agreement,
except as expressly provided in this Agreement.

     5.2  GOVERNING LAW. This Agreement shall be governed by and construed under
the laws of the State of Delaware, other than the laws relating to conflict or
choice of laws.

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

     5.4  TITLES AND SUBTITLES AND CONSTRUCTION. The titles and subtitles used
in this Agreement are used for convenience only and are not to be considered in
construing or interpreting this Agreement. Unless the context of this Agreement
clearly requires otherwise, (i) references to the plural include the singular,
and the singular the plural, (ii) references to one

                                       13
<Page>

gender include all genders, (iii) "or" has the inclusive meaning frequently
identified with the phrase "and/or," (iv) "including" has the inclusive meaning
frequently identified with the phrase "but not limited to" and (v) "hereunder"
or "herein" refer to the entire Agreement. The section and other headings
contained in this Agreement are for reference purposes only and shall not
control or affect the construction of this Agreement or the interpretation
thereof in any respect. Section and subsection references are to this Agreement
unless otherwise specified.

     5.5  NOTICES. All notices, requests, consents, and other communications
under this Agreement shall be in writing, and shall be delivered by hand or sent
by reputable overnight courier service or electronic facsimile transmission
(with a copy sent by first class mail, postage prepaid), or mailed by United
States first class certified or registered mail, return receipt requested,
postage prepaid:

     If to the Company, at Synta Pharmaceuticals Corp., 45 Hartwell Avenue,
Lexington, Massachusetts 02421, attention: Dr. Safi R. Bahcall, Chief Executive
Officer, facsimile number (530) 323-7045, with a copy to Nixon Peabody LLP, 101
Federal Street, Boston, MA 02110 attention: Michael K. Barron, Esq., facsimile
number (866) 947-1784), or at such other address as may be furnished in writing
by the Company to the Investors;

     If to Cxsynta LLC, at Caxton Corporation, Princeton Plaza, Building 2, 731
Alexander Road, Princeton, New Jersey 08540, attention Scott B. Bernstein, Esq.,
facsimile number 609- 419-0470, or at such other address as may be furnished in
writing by the Investor to the Company.

     If to Robert A. Day, at Mountain Trail Investments, LLC, 865 South Figueroa
St., Suite 700, Los Angeles, California 90017, facsimile number 213-452-2822,
Attn: Jonathan D. Jaffrey, or at such other address as may be furnished in
writing by the Investor to the Company.

     If to Mountain Trail Investments, LLC, at Mountain Trail Investments, LLC,
865 South Figueroa St., Suite 700, Los Angeles, California 90017, facsimile
number 213-452-2822, Attn: Jonathan D. Jaffrey, or at such other address as may
be furnished in writing by the Investor to the Company, with a copy to Richard
N. Foster at 21 East 79th Street, New York, New York 10021.

     If to Keith R. Gollust, at Keith R. Gollust c/o Gollust Management, Inc.,
500 Park Avenue, Suite 510, New York, New York 10022, facsimile number
212-319-8779, or at such other address as may be furnished in writing by the
Investor to the Company.

     If to Gollust Trust II, at Keith R. Gollust, c/o Gollust Management, Inc.,
500 Park Avenue, Suite 510, New York, New York 10022 facsimile number
212-319-8779, or at such other address as may be furnished in writing by the
Investor to the Company.

     If to Wyandanch Partners, L.P., at Keith R. Gollust, c/o Gollust
Management, Inc., 500 Park Avenue, Suite 510, New York, New York 10022 facsimile
number 212-319-8779, or at such other address as may be furnished in writing by
the Investor to the Company.

     Notices provided in accordance with this SECTION 5.5 shall be deemed
delivered (i) if personally delivered or sent by electronic facsimile
transmission with written confirmation, when

                                       14
<Page>

received, or (ii) if sent by a nationally recognized overnight courier service,
twenty four (24) hours after deposit with such courier service, or (iii) if sent
by United States certified or registered mail, return receipt requested, forty
eight (48) hours after deposit in the mail.

     5.6  EXPENSES. If any action at law or in equity is necessary to enforce or
interpret the terms of this Agreement, the prevailing party shall be entitled to
reasonable attorneys' fees, costs and necessary disbursements in addition to any
other relief to which such party may be entitled.

     5.7  ENTIRE AGREEMENT; AMENDMENTS AND WAIVERS. This Agreement constitutes
the full and entire understanding and agreement among the parties with regard to
the subject matter hereof. To the extent any of the terms of this Agreement are
inconsistent with the terms of any subscription agreement for the Shares
executed by an Investor or the Stockholders' Agreement, the applicable terms of
this Agreement shall control with respect to the subject matter hereof. Any term
of' this Agreement may be amended and the observance of any term of this
Agreement may be waived (either generally or in a particular instance and either
retroactively or prospectively), only with the written consent of the Company
and each of the Investors. Any amendment or waiver effected in accordance with
this paragraph shall be binding upon each of the Investors, its successors and
assigns, and the Company.

     5.8  SPECIFIC PERFORMANCE. The parties recognize that various of the rights
of the Investors under this Agreement are unique and, accordingly, the Investors
(and their respective successors and assigns) shall, in addition to such other
remedies as may be available to each of them at law or in equity, have the right
to enforce their respective rights hereunder by actions for injunctive relief
and specific performance to the extent permitted by law.

     5.9  SEVERABILITY. If one or more provisions of this Agreement are held to
be unenforceable under applicable law, such provision shall be excluded from
this Agreement and the balance of the Agreement shall be interpreted as if such
provision were so excluded and shall be enforceable in accordance with its
terms.


      [REMAINDER OF PAGE LEFT BLANK INTENTIONALLY. SIGNATURE PAGE FOLLOWS.]

                                       15
<Page>

     IN WITNESS WHEREOF, the parties have executed this Agreement, under seal,
as of the date first above written.


                                    SYNTA PHARMACEUTICAL CORP.

                                    By:   /s/ SAFI R. BAHCALL
                                       -------------------------------
                                    Print Name:   Safi R. Bahcall
                                               -----------------------
                                    Title: Chief Executive Officer
                                           ---------------------------


                                    CxSYNTA LLC

                                    By:   /s/ SCOTT B. BERNSTEIN
                                       -------------------------------
                                    Print Name:   Scott B. Bernstein
                                               -----------------------
                                    Title:  Secretary
                                          ----------------------------


                                    MOUNTAIN TRAIL INVESTMENTS, LLC

                                    By:   /s/ RICHARD N. FOSTER
                                       -------------------------------
                                    Print Name:   Richard N. Foster
                                               -----------------------
                                    Title:  Attorney-in-Fact
                                          ----------------------------

                                    /s/ ROBERT A. DAY
                                    ----------------------------------
                                    Robert A. Day

                                    /s/ KEITH R. GOLLUST
                                    ----------------------------------
                                    Keith R. Gollust


                                    GOLLUST TRUST II

                                    By:   /s/ KENNETH S. DAVIDSON
                                       -------------------------------
                                    Print Name:   Kenneth S. Davidson
                                               -----------------------
                                    Title:  Trustee
                                          ----------------------------


                                    WYANDANCH PARTNERS, L.P.

                                    By:   /s/ KEITH R. GOLLUST
                                       -------------------------------
                                    Print Name:   Keith R. Gollust
                                               -----------------------
                                    Title:  G.P.
                                          ----------------------------


       [SIGNATURE PAGE CO AMENDED AND RESTATED INVESTOR RIGHTS AGREEMENT]

                                       16