Document:

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                                                                     EXHIBIT 4.3

                      PREDIX PHARMACEUTICALS HOLDINGS, INC.

                                 FIRST AMENDMENT
                                       TO
                          REGISTRATION RIGHTS AGREEMENT

      This First Amendment (the "Amendment") to the Registration Rights
Agreement (the "Registration Rights Agreement"), dated October 30, 2003, by and
between Predix Pharmaceuticals Holdings, Inc., a Delaware corporation (the
"Company"), and RRD International, LLC ("RRD"), is made as of July 7, 2005, by
and among the Company and RRD. Capitalized terms used herein but not defined
shall have the meanings ascribed to such terms in the Registration Rights
Agreement.

      WHEREAS, the Company is contemplating an initial public offering of shares
of its Common Stock;

      WHEREAS, Section 2(a) of the Registration Rights Agreement provides RRD
incidental registration rights (the "incidental rights");

      WHEREAS, the Registration Rights Agreement may be modified, changed,
discharged or terminated by the agreement of the Company and the holders of more
than fifty percent (50%) in interest of the Registrable Shares held by the
Holders; and

      WHEREAS, the Company and RRD (comprising more than fifty percent (50%) in
interest of the Registrable Shares held by the Holders) believe that it is in
the best interests of the Company and RRD to amend Section 2(a)(i) of the
Registration Rights Agreement such that the incidental rights of Section 2(a) do
not apply to an Initial Public Offering.

      NOW THEREFORE, the Company and RRD, hereby agree to amend the Registration
Rights Agreement as follows:

1. AMENDMENT TO REGISTRATION RIGHTS AGREEMENT.

      a. SECTION 2(a)(i)

      Section 2(a)(i) of the Registration Rights Agreement is hereby amended by
deleting the first sentence of such Section 2(a)(i) in its entirety and
inserting in lieu thereof the following sentence:

      "If at any time subsequent to the expiration of any lock-up agreement that
      the Company's stockholders have entered into with the underwriters in
      connection with an Initial Public Offering the Company proposes to file a
      Registration Statement covering shares of Common Stock (other than a
      Registration Statement covering shares (i) to be sold solely for the
      account of Other Holders, or (ii) that

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      were acquired pursuant to either (a) an acquisition of a company of which
      they were formerly stockholders, (b) a "private placement" under the
      Securities Act or (c) Rule 144A under the Securities Act) at any time and
      from time to time, it will, prior to such filing, give written notice to
      all holders of Regsitrable Shares (a "Holder") of its intention to do so;
      provided, that no such notice need be given if no Registrable Shares are
      to be included therein as a result of a written notice from the managing
      underwriter pursuant to Section 2(a)(ii) below.

      b. NO OTHER AMENDMENT

      Except as otherwise expressly set forth herein, all terms and conditions
of the Registration Rights Agreement shall remain in full force and effect.
Except as expressly set forth herein, nothing herein shall be construed to be an
amendment or a waiver of any requirements of the Registration Rights Agreement.

2. MISCELLANEOUS.

      a. MODIFICATION AND AMENDMENT.

      This Amendment shall not be modified or amended except by an instrument in
writing signed by or on behalf of the parties hereto.

      b. COUNTERPARTS.

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

                            [Signature Pages Follow]

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      IN WITNESS WHEREOF, the parties hereto have executed this Amendment as of
the day and year first above written.

                                PREDIX PHARMACEUTICALS HOLDINGS, INC.

                                By: /s/ MICHAEL KAUFFMAN
                                    --------------------------------------------
                                    Name: Michael Kauffman
                                    Title: President and Chief executive Officer

                                RRD INTERNATIONAL, LLC

                                By: /s/ JOSEPH P. CLANCY
                                    --------------------------------------------
                                    Name: Joseph P. Clancy
                                    Title: President and CEO

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                          REGISTRATION RIGHTS AGREEMENT

      REGISTRATION RIGHTS AGREEMENT (the "Agreement"), dated as of the 30th day
of October 2003, by and between RRD International, LLC, a Delaware limited
liability company ("RRD"), and Predix Pharmaceuticals Holdings, Inc., a Delaware
corporation (the "Company").

                              PRELIMINARY STATEMENT

      WHEREAS, the Company and RRD have entered into that certain Master
Services Agreement, dated as of the date hereof (the "Master Services
Agreement"), and Work Order No. 1 to the Master Services Agreement, dated as of
the date hereof ("Work Order No. 1), pursuant to which RRD has agreed to perform
certain services for the Company, in exchange for, in pertinent part, the
Company's issuance to RRD of warrants to purchase one hundred and eighty
thousand (180,000) shares (the "Shares") of common stock, par value $0.01 per
share (the "Common Stock"), of the Company, pursuant to a warrant agreement of
even date herewith (the "Warrant Agreement"); and

      WHEREAS, the Company and RRD desire to provide for certain arrangements
with respect to the registration of the Shares under the Securities Act of 1933,
as amended.

      NOW THEREFORE, in consideration of these premises, and the respective
promises and covenants contained herein, the parties hereto agree as follows:

            SECTION 1. Certain Definitions.

      As used in this Agreement, the following terms shall have the following
respective meanings:

            "Commission" means the Securities and Exchange Commission, or any
other federal agency at the time administering the Securities Act.

            "Exchange Act" means the Securities Exchange Act of 1934, as
amended, or any successor federal statute, and the rules and regulations of the
Commission issued under such Act, as they each may, from time to time, be in
effect.

            "Holders" means holders of Registrable Shares.

            "Indemnified Party" means a party entitled to indemnification
pursuant to Section 2(d).

            "Indemnifying Party" means a party obligated to provide
indemnification pursuant to Section 2(d).

            "Initial Public Offering" means the initial underwritten public
offering of shares of Common Stock pursuant to an effective Registration
Statement under the Securities Act resulting in gross proceeds to the Company of
at least $25 million (prior to

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the deduction of underwriters' commissions, discounts and expenses) and at a
pre-money valuation of the Company of no less than $100 million.

            "Other Holders" means holders of securities of the Company (other
than the Holders) who are entitled, by contract with the Company, to have
securities included in a Registration Statement pursuant to the terms of that
certain Stockholders Agreement, dated August 8, 2003, by and among the Company
and the Stockholders, as defined therein.

            "Prospectus" means the prospectus included in any Registration
Statement, as amended or supplemented by an amendment or prospectus supplement,
including post-effective amendments, and all material incorporated by reference
or deemed to be incorporated by reference in such Prospectus.

            "Registrable Shares" means (a) the Shares, and (b) any other shares
of Common Stock issued in respect of such Shares (because of stock splits, stock
dividends, reclassifications, recapitalizations or similar events); provided,
however, that shares of Common Stock which are Registrable Shares shall cease to
be Registrable Shares (i) upon any sale pursuant to a Registration Statement, or
(ii) at such time, following an Initial Public Offering, as they become eligible
for sale pursuant to Rule 144 under the Securities Act. Wherever reference is
made in this Agreement to a request or consent of holders of a certain
percentage of Registrable Shares, the determination of such percentage shall
include shares of Common Stock issuable upon conversion of the Warrants granted
to Holder under the Warrant Agreement ("Warrant") even if such conversion has
not been effected.

            "Registration Expenses" means all expenses incurred by the Company
in complying with the provisions of Section 2, including, without limitation,
all registration and filing fees, exchange listing fees, printing expenses,
state Blue Sky fees and expenses, and the expense of any special audits incident
to or required by any such registration, but excluding underwriting discounts,
selling commissions and the fees and expenses of Selling Stockholders' own
counsel.

            "Registration Statement" means a registration statement filed by the
Company with the Commission for a public offering and sale of securities of the
Company (other than a registration statement on Form S-8 or Form S-4, or their
successors, or any other form for a similar limited purpose, or any registration
statement covering only securities proposed to be issued in exchange for
securities or assets of another corporation).

            "Securities Act" means the Securities Act of 1933, as amended, or
any successor federal statute, and the rules and regulations of the Commission
issued under such Act, as they each may, from time to time, be in effect.

            "Selling Stockholder" means any Holder owning Registrable Shares
included in a Registration Statement.

            SECTION 2. Registration Rights.

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            (a) Incidental Registration.

                        (i) Whenever the Company proposes to file a Registration
      Statement covering shares of Common Stock (other than a Registration
      Statement covering shares to be sold solely for the account of Other
      Holders, which shares were acquired pursuant to either (i) an acquisition
      of a company of which they were formerly stockholders, (ii) a "private
      placement" under the Securities Act or (iii) Rule 144A under the
      Securities Act) at any time and from time to time, it will, prior to such
      filing, give written notice to all holders of Registrable Shares (a
      "Holder") of its intention to do so; provided, that no such notice need be
      given if no Registrable Shares are to be included therein as a result of a
      written notice from the managing underwriter pursuant to Section 2(a)(ii)
      below. Upon the written request of a Holder given within 20 days after the
      Company provides such notice (which request shall state the intended
      method of disposition of such Registrable Shares), the Company shall use
      its commercially reasonable efforts to cause all Registrable Shares which
      the Company has been requested by such Holder to register to be registered
      under the Securities Act to the extent necessary to permit their sale or
      other disposition in accordance with the intended methods of distribution
      specified in the request of such Holder; provided that the Company shall
      have the right to postpone or withdraw any registration effected pursuant
      to this Section 2(a) without obligation to any Holder.

                        (ii) If the registration for which the Company gives
      notice pursuant to Section 2(a)(i) is a registered public offering
      involving an underwriting, the Company shall so advise the Holders as a
      part of the written notice given pursuant to Section 2(a)(i). In such
      event, (i) the right of any Holders to include its Registrable Shares in
      such registration pursuant to this Section 2(a) shall be conditioned upon
      such Holder's participation in such underwriting on the terms set forth
      herein and (ii) all Holders including Registrable Shares in such
      registration shall enter into an underwriting agreement upon customary
      terms with the underwriter or underwriters selected for the underwriting
      by the Company. If any Holder who has requested inclusion of its
      Registrable Shares in such registration as provided above disapproves of
      the terms of the underwriting, such person may elect, by written notice to
      the Company, to withdraw its shares from such Registration Statement and
      underwriting. If the managing underwriter advises the Company in writing
      that marketing factors require a limitation on the number of shares to be
      underwritten, and subject to the superior rights, if any, of any other
      holder of the Company's capital stock then outstanding, which other
      holders shall by virtue of such superior rights receive preference over
      the Holders (the "Superior Rights Holders") the shares held by holders
      other than Holders and Other Holders shall be excluded from such
      Registration Statement and underwriting to the extent deemed advisable by
      the managing underwriter, and, if a further reduction of the number of
      shares is required, and, subject to the rights of Superior Rights Holders,
      if any, the number of shares that may be included in such Registration
      Statement and underwriting shall be allocated among all Holders and Other
      Holders requesting registration in proportion, as nearly as practicable,
      to the respective number of shares of Common Stock (on an as-converted
      basis) held by

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      them on the date the Company gives the notice specified in Section 2(a)(i)
      above. If any Holder or Other Holder would thus be entitled to include
      more shares than such holder requested to be registered, the excess shall
      be allocated among other requesting Holders and Other Holders pro rata in
      the manner described in the preceding sentence.

            (b) Registration Procedures.

                        (i) If and whenever the Company is required by the
      provisions of this Agreement to use its commercially reasonable efforts to
      affect the registration of any Registrable Shares under the Securities
      Act, the Company shall:

                        (1)   file with the Commission a Registration Statement
                              with respect to such Registrable Shares and use
                              its commercially reasonable efforts to cause that
                              Registration Statement to become effective as soon
                              as possible;

                        (2)   as expeditiously as possible prepare and file with
                              the Commission any amendments and supplements to
                              the Registration Statement and the prospectus
                              included in the Registration Statement as may be
                              necessary to comply with the provisions of the
                              Securities Act (including the anti-fraud
                              provisions thereof) and to keep the Registration
                              Statement, effective for 12 months from the
                              effective date or such lesser period until all
                              such Registrable Shares are sold;

                        (3)   as expeditiously as possible furnish to each
                              Selling Stockholder such reasonable numbers of
                              copies of the Prospectus, including any
                              preliminary Prospectus, in conformity with the
                              requirements of the Securities Act, and such other
                              documents as such Selling Stockholder may
                              reasonably request in order to facilitate the
                              public sale or other disposition of the
                              Registrable Shares owned by such Selling
                              Stockholder;

                        (4)   as expeditiously as possible use its commercially
                              reasonable efforts to register or qualify the
                              Registrable Shares covered by the Registration
                              Statement under the securities or Blue Sky laws of
                              such states as the Selling Stockholders shall
                              reasonably request, and do any and all other acts
                              and things that may be necessary or desirable to
                              enable

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                              the Selling Stockholders to consummate the public
                              sale or other disposition in such states of the
                              Registrable Shares owned by the Selling
                              Stockholders; provided, however, that the Company
                              shall not be required in connection with this
                              paragraph (4) to qualify as a foreign corporation
                              or execute a general consent to service of process
                              in any jurisdiction;

                        (5)   as expeditiously as possible, cause all such
                              Registrable Shares to be listed on each securities
                              exchange or automated quotation system on which
                              similar securities issued by the Company are then
                              listed;

                        (6)   promptly provide a transfer agent and registrar
                              for all such Registrable Shares not later than the
                              effective date of such Registration Statement;

                        (7)   promptly make available for inspection by the
                              Selling Stockholders, any managing underwriter
                              participating in any disposition pursuant to such
                              Registration Statement, and any attorney or
                              accountant or other agent retained by any such
                              underwriter or selected by the Selling
                              Stockholders, all financial and other records,
                              pertinent corporate documents and properties of
                              the Company and cause the Company's officers,
                              directors, employees and independent accountants
                              to supply all information reasonably requested by
                              any such seller, underwriter, attorney, accountant
                              or agent in connection with such Registration
                              Statement;

                        (8)   as expeditiously as possible, notify each Selling
                              Stockholder, promptly after it shall receive
                              notice thereof, of the time when such Registration
                              Statement has become effective or a supplement to
                              any Prospectus forming a part of such Registration
                              Statement has been filed; and

                        (9)   as expeditiously as possible following the
                              effectiveness of such Registration Statement,
                              notify each seller of such Registrable Shares of
                              any request by the Commission for the amending or
                              supplementing of such Registration Statement or
                              Prospectus.

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                        (ii) If the Company has delivered a Prospectus to the
      Selling Stockholders and after having done so the Prospectus is amended to
      comply with the requirements of the Securities Act, the Company shall
      promptly notify the Selling Stockholders and, if requested, the Selling
      Stockholders shall immediately cease making offers of Registrable Shares
      and return all Prospectuses to the Company. The Company shall promptly
      provide the Selling Stockholders with revised Prospectuses and, following
      receipt of the revised Prospectuses, the Selling Stockholders shall be
      free to resume making offers of the Registrable Shares.

                        (iii) In the event that, in the judgment of the Company,
      it is advisable to suspend use of a Prospectus included in a Registration
      Statement due to pending material developments or other events that have
      not yet been publicly disclosed and as to which the Company believes
      public disclosure would be detrimental to the Company, the Company shall
      notify all Selling Stockholders to such effect, and, upon receipt of such
      notice, each such Selling Stockholder shall immediately discontinue any
      sales of Registrable Shares pursuant to such Registration Statement until
      such Selling Stockholder has received copies of a supplemented or amended
      Prospectus or until such Selling Stockholder is advised in writing by the
      Company that the then current Prospectus may be used and has received
      copies of any additional or supplemental filings that are incorporated or
      deemed incorporated by reference in such Prospectus. Notwithstanding
      anything to the contrary herein, the Company shall not exercise its rights
      under this Section 2(b)(iii) to suspend sales of Registrable Shares for a
      period in excess of 30 days consecutively or 60 days in any 365-day
      period.

            (c) Allocation of Expenses. The Company will pay all Registration
Expenses for all registrations under this Agreement.

            (d) Indemnification and Contribution.

                        (i) In the event of any registration of any of the
      Registrable Shares under the Securities Act pursuant to this Agreement,
      the Company will indemnify and hold harmless each Selling Stockholder,
      each underwriter of such Registrable Shares, and each other person, if
      any, who controls such Selling Stockholder or underwriter within the
      meaning of the Securities Act or the Exchange Act against any losses,
      claims, damages or liabilities, joint or several, to which such Selling
      Stockholder, underwriter or controlling person may become subject under
      the Securities Act, the Exchange Act, state securities or Blue Sky laws or
      otherwise, insofar as such losses, claims, damages or liabilities (or
      actions in respect thereof) arise out of or are based upon (i) any untrue
      statement or alleged untrue statement of any material fact contained in
      any Registration Statement under which such Registrable Shares were
      registered under the Securities Act, any preliminary prospectus or final
      prospectus contained in the Registration Statement, or any amendment or
      supplement to such Registration Statement, (ii) the omission or alleged
      omission to state a material fact required to be stated therein or
      necessary to make the statements therein not

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      misleading, or (iii) any violation or alleged violation by the Company of
      the Securities Act, the Exchange Act, any state securities law or any rule
      or regulation promulgated under the Securities Act, the Exchange Act or
      any state securities law in connection with the Registration Statement or
      the offering contemplated thereby; and the Company will reimburse such
      Selling Stockholder, underwriter and each such controlling person for any
      legal or any other expenses reasonably incurred by such Selling
      Stockholder, underwriter or controlling person in connection with
      investigating or defending any such loss, claim, damage, liability or
      action; provided, however, that the Company will not be liable in any such
      case to the extent that any such loss, claim, damage or liability arises
      out of or is based upon any untrue statement or omission made in such
      Registration Statement, preliminary prospectus or prospectus, or any such
      amendment or supplement, in reliance upon and in conformity with
      information furnished to the Company, in writing, by or on behalf of such
      Selling Stockholder, underwriter or controlling person specifically for
      use in the preparation thereof.

                        (ii) In the event of any registration of any of the
      Registrable Shares under the Securities Act pursuant to this Agreement,
      each Selling Stockholder, severally and not jointly, will indemnify and
      hold harmless the Company, each of its directors and officers and each
      underwriter (if any) and each person, if any, who controls the Company or
      any such underwriter within the meaning of the Securities Act or the
      Exchange Act, against any losses, claims, damages or liabilities, joint or
      several, to which the Company, such directors and officers, underwriter or
      controlling person may become subject under the Securities Act, Exchange
      Act, state securities or Blue Sky laws or otherwise, insofar as such
      losses, claims, damages or liabilities (or actions in respect thereof)
      arise out of or are based upon (i) any untrue statement or alleged untrue
      statement of a material fact contained in any Registration Statement under
      which such Registrable Shares were registered under the Securities Act,
      any preliminary prospectus or final prospectus contained in the
      Registration Statement, or any amendment or supplement to the Registration
      Statement, or (ii) any omission or alleged omission to state a material
      fact required to be stated therein or necessary to make the statements
      therein not misleading, if and to the extent (and only to the extent) that
      the statement or omission was made in reliance upon and in conformity with
      information relating to such Selling Stockholder furnished in writing to
      the Company by such Selling Stockholder specifically for use in connection
      with the preparation of such Registration Statement, prospectus, amendment
      or supplement; provided, however, that the obligations of a Selling
      Stockholder hereunder shall be limited to an amount equal to the gross
      proceeds to such Selling Stockholder of Registrable Shares sold in
      connection with such registration.

                        (iii) Each Indemnified Party shall give notice to the
      Indemnifying Party promptly after such Indemnified Party has actual
      knowledge of any claim as to which indemnity may be sought, and shall
      permit the Indemnifying Party to assume the defense of any such claim or
      any litigation resulting therefrom; provided, that counsel for the
      Indemnifying Party, who shall

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      conduct the defense of such claim or litigation, shall be approved by the
      Indemnified Party (whose approval shall not be unreasonably withheld,
      conditioned or delayed); and, provided, further, that the failure of any
      Indemnified Party to give notice as provided herein shall not relieve the
      Indemnifying Party of its obligations under this Section 2(d) except to
      the extent that the Indemnifying Party is adversely affected by such
      failure. The Indemnified Party may participate in such defense and retain
      its own counsel at such party's expense; provided, however, that the
      Indemnifying Party shall pay such reasonable fees and expenses if, in the
      reasonable opinion of counsel to the Indemnified Party, either (x) one or
      more defenses are available to the Indemnified Party that are not
      available to the Indemnifying Party, or (y) a conflict or potential
      conflict exists between the Indemnified Party, on the one hand, and the
      Indemnifying Party, on the other hand, that would make separate
      representation advisable; and provided, further, that in no event shall
      the Indemnifying Party be required to pay the expenses of more than one
      law firm as counsel for the Indemnified Party. The Indemnifying Party also
      shall be responsible for the expenses of such defense if the Indemnifying
      Party does not elect to assume such defense. No Indemnifying Party, in the
      defense of any such claim or litigation shall, except with the consent of
      each Indemnified Party, consent to entry of any judgment or enter into any
      settlement which does not include as an unconditional term thereof the
      giving by the claimant or plaintiff to such Indemnified Party of a release
      from all liability in respect of such claim or litigation, and no
      Indemnified Party shall consent to entry of any judgment or settle such
      claim or litigation without the prior written consent of the Indemnifying
      Party, which consent shall not be unreasonably withheld, conditioned or
      delayed.

                        (iv) In order to provide for just and equitable
      contribution in circumstances in which the indemnification provided for in
      this Section 2(d) is due in accordance with its terms but for any reason
      is held to be unavailable to an Indemnified Party in respect to any
      losses, claims, damages and liabilities referred to herein, then the
      Indemnifying Party shall, in lieu of indemnifying such Indemnified Party,
      contribute to the amount paid or payable by such Indemnified Party as a
      result of such losses, claims, damages or liabilities to which such party
      may be subject in such proportion as is appropriate to reflect the
      relative fault of the Company on the one hand and the Selling Stockholders
      on the other in connection with the statements or omissions which resulted
      in such losses, claims, damages or liabilities, as well as any other
      relevant equitable considerations. The relative fault of the Company and
      the Selling Stockholders shall be determined by reference to, among other
      things, whether the untrue or alleged untrue statement of material fact
      related to information supplied by the Company or the Selling Stockholders
      and the parties' relative intent, knowledge, access to information and
      opportunity to correct or prevent such statement or omission. The Company
      and the Selling Stockholders agree that it would not be just and equitable
      if contribution pursuant to this Section 2(d)(iv) were determined by pro
      rata allocation or by any other method of allocation which does not take
      account of the equitable considerations referred to above. Notwithstanding
      the provisions of this Section 2(d)(iv), (i) in no case shall any one
      Selling Stockholder

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      be liable or responsible for any amount in excess of the gross proceeds
      received by such Selling Stockholder from the offering of Registrable
      Shares and (ii) the Company shall be liable and responsible for any amount
      in excess of such proceeds; provided, however, that no person guilty of
      fraudulent misrepresentation (within the meaning of Section 11(f) of the
      Securities Act) shall be entitled to contribution from any person who was
      not guilty of such fraudulent misrepresentation. Any party entitled to
      contribution will, promptly after receipt of notice of commencement of any
      action, suit or proceeding against such party in respect of which a claim
      for contribution may be made against another party or parties under this
      Section 2(d)(iv), notify such party or parties from whom contribution may
      be sought, but the omission so to notify such party or parties from whom
      contribution may be sought shall not relieve such party from any other
      obligation it or they may have thereunder or otherwise under this Section
      2(d)(iv). No party shall be liable for contribution with respect to any
      action, suit, proceeding or claim settled without its prior written
      consent, which consent shall not be unreasonably withheld, conditioned or
      delayed.

                        (v) The rights and obligations of the Company and the
      Selling Stockholders under this Section 2(d) shall survive the termination
      of this Agreement.

            (e) Information by Holder. Each Holder of Registrable Shares
included in any registration shall furnish to the Company such information
regarding such Holder and the distribution proposed by such Holder as the
Company may reasonably request in writing and as shall be required in connection
with any registration, qualification or compliance referred to in this
Agreement.

            (f) "Lock-Up" Agreement; Confidentiality of Notices. Each Holder and
any transferee thereof, if requested by the Company and the managing underwriter
of an underwritten offering, shall not sell or otherwise transfer or dispose of
any Shares held by such Holder or such transferee for a period of 180 days
following the effective date of the Registration Statement for such offering;
provided, that all stockholders of the Company then holding at least 5% of the
outstanding Common Stock (on an as-converted basis) and all executive officers
and directors of the Company enter into similar agreements.

The Company may impose stop-transfer instructions with respect to the Shares
subject to the foregoing restriction until the end of such 180-day period.

Any Holder and any transferee thereof receiving any written notice from the
Company regarding the Company's plans to file a Registration Statement shall
treat such notice confidentially and shall not disclose such information to any
person other than as necessary to exercise its rights under this Agreement.

            (g) Rule 144 Requirements. After the earliest of (i) the closing of
the sale of securities of the Company pursuant to a Registration Statement, (ii)
the registration by the Company of a class of securities under Section 12 of the
Exchange Act, or (iii) the

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issuance by the Company of an offering circular pursuant to Regulation A under
the Securities Act, the Company agrees to:

                        (i) make and keep current public information about the
      Company available, as those terms are understood and defined in Rule 144;

                        (ii) use its commercially reasonable efforts to file
      with the Commission in a timely manner all reports and other documents
      required of the Company under the Securities Act and the Exchange Act (at
      any time after it has become subject to such reporting requirements); and

                        (iii) furnish to any holder of Registrable Shares upon
      request (i) a written statement by the Company as to its compliance with
      the reporting requirements of Rule 144 and of the Securities Act and the
      Exchange Act (at any time after it has become subject to such reporting
      requirements), (ii) a copy of the most recent annual or quarterly report
      of the Company, and (iii) such other reports and documents of the Company
      as such holder may reasonably request to avail itself of any similar rule
      or regulation of the Commission allowing it to sell any such securities
      without registration.

            (h) Termination. All of the Company's obligations to register
Registrable Shares under Section 2(a) shall terminate upon the earlier of (i)
three years after the closing of the Initial Public Offering or (ii) the date on
which no Holder holds any Registrable Shares.

            SECTION 3. Governing Law. This Agreement shall be governed by, and
construed in accordance with, the laws of the State of Delaware.

            SECTION 4. Benefits of Agreement. This Agreement shall be binding
upon and inure to the benefit of the parties hereto and all Permitted
Transferees, as such term is defined in Warrant Agreement.

            SECTION 5. Notices. Any notice, demand or request required or
permitted to be given under the provisions of this Agreement: (a) shall be in
writing; (b) shall be delivered personally, including by means of telecopy or
courier, or mailed by registered or certified mail, postage prepaid and return
receipt requested; (c) shall be deemed given on the date of personal delivery or
on the date set forth on the return receipt; and (d) shall be delivered or
mailed as follows or to such other address as any party may from time to time
direct the Company in writing:

                        (i) if to the Company, at Predix Pharmaceuticals
      Holdings, Inc., 10 K Gill Street, Woburn, Massachusetts 01801, Attention:
      Chief Financial Officer; and

                        if to any Holder, at 11 North Washington Street, Suite
      310, Rockville, Maryland 20850, Attention: Joseph P. Clancy, or at such
      other address as may be furnished in writing by the Holder to the Company,
      with a copy to

                                       10
<PAGE>

      Curtin, Law, Roberson, Dunigan & Salans, 900 M Street, NW, Washington, DC
      20036, Attn: Thomas Dunigan, Esq. or

                        (ii) at its respective address set forth on the
      signature page hereto.

            SECTION 6. Modification. Except as otherwise provided herein,
neither this Agreement nor any provision hereof may be modified, changed,
discharged or terminated except: (a) by an instrument in writing signed by the
party against whom the enforcement of any modification, change, discharge or
termination is sought; or (b) by the agreement of holders of more than fifty
percent (50%) in interest of the Registrable Shares held by the Holders;
provided, however, such modification, change, discharge or termination so
approved as set forth in clause (b) affects all Registrable Shares equally; and,
provided, further, all Holders shall be bound by such modification, change,
discharge or termination with the same force and effect as if all such Holders
agreed thereto.

            SECTION 7. Severability. In the event that any one or more of the
provisions contained in this Agreement or in any other instrument referred to
herein shall, for any reason, be held to be invalid, illegal or unenforceable,
such illegality, invalidity or unenforceability shall not affect any other
provisions of this Agreement.

            SECTION 8. Counterparts. This Agreement may be executed in one or
more counterparts, each of which shall be deemed to be an original, but all of
which taken together shall constitute one and the same instrument.

            SECTION 9. Pronouns. Whenever the context may require, any pronouns
used in this Agreement shall include the corresponding masculine, feminine or
neuter forms, and the singular form of nouns and pronouns shall include the
plural, and vice versa.

            SECTION 10. Entire Agreement. This Agreement constitutes the entire
agreement of the parties with respect to the subject matter hereof, and
supercedes and replaces all previous agreements relating to the subject matter
hereof. In the event of any conflict between this Agreement and any other
agreement or instrument with respect to the subject matter hereof, the
provisions of this Agreement shall control.

            SECTION 11. Arbitration. Any controversy or claim arising out of or
in conjunction with this Agreement (other than an action for injunctive relief)
shall be settled by arbitration in accordance with the rules of the American
Arbitration Association then in effect in the State of Delaware and judgment
upon such award rendered by the arbitrator shall be final and binding upon the
parties and may be entered and enforced in any court having jurisdiction
thereof. The arbitration shall be held in the State of Delaware. The arbitration
award shall include attorneys' fees and costs to the prevailing party.

                            [Signature Page Follows]

                                       11
<PAGE>

      IN WITNESS WHEREOF, the parties hereto have executed this Registration
Rights Agreement as of the day and year first above written.

                    PREDIX PHARMACEUTICALS HOLDINGS, INC.

                        By: /s/ NIK HAYES
                            -----------------------------------------
                            Nik Hayes
                            Chief Operating Officer and Chief Financial Officer

                    RRD INTERNATIONAL, LLC

                        By: /s/ JOSEPH P. CLANCY
                            --------------------------------------------

                            Name: Joseph P. Clancy

                            Title: Chief Executive Officer

                            Address: 11 North Washington Street, Ste 310
                                Rockville, MD 20850

                                       12<PAGE>

                                                                     EXHIBIT 4.4

                     PREDIX PHARMACEUTICALS HOLDINGS, INC.

                           FIRST AMENDMENT TO WARRANT

      This First Amendment (the "Amendment") to the Warrant (the "Warrant"),
dated October 30, 2003, by and between Predix Pharmaceuticals Holdings, Inc., a
Delaware corporation (the "Company"), and RRD International, LLC ("RRD"), is
made as of July 7, 2005, by and among the Company and RRD. Capitalized terms
used herein but not defined shall have the meanings ascribed to such terms in
the Warrant.

      WHEREAS, the Company and RRD entered into the Warrant on October 30, 2003;

      WHEREAS, the Company and RRD desire to amend the Warrant to reflect
certain changes in the terms of the Warrant as agreed by them; and

      WHEREAS, pursuant to Section 13(h) of the Warrant, the Warrant may be
amended, waived or terminated only by an instrument in writing signed by the
Company and RRD.

      NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the Company and RRD hereby agree
as follows:

1. AMENDMENT TO WARRANT. The Warrant is hereby amended by deleting Section 2 in
its entirety and inserting in lieu thereof the following:

      " 2. Exercise; Payment.

            (a) Cash Exercise. The purchase rights represented by this Warrant
      may be exercised by the Holder, in whole or in part, at any time, or from
      time to time, by the surrender of this Warrant (together with a duly
      executed notice of exercise (the "Notice of Exercise") in the form
      attached hereto as Exhibit A) at the address of the Company's principal
      corporate offices set forth in Section 6 hereof, and by payment to the
      Company of an amount equal to the Exercise Price multiplied by the number
      of the Shares being purchased, which amount may be paid by the Holder by
      wire transfer or check payable to the order of the Company. The Person in
      whose name any certificate representing the Shares issuable upon any
      exercise of this Warrant shall be deemed to have become the holder of
      record of, and shall be treated for all purposes as the record holder of
      the Shares represented thereby (and such Shares shall be deemed to have
      been issued) immediately prior to the close of business on the date or
      dates upon which such surrender and payment are made. As used herein, the
      term "Person" means any individual or any corporation, partnership, trust,
      limited liability company or other entity or organization of any kind.

            (b) Cashless Exercise. If the Fair Market Value (as defined below)
      of the Shares exceeds the Exercise Price, in lieu of paying the Exercise
      Price in cash as

<PAGE>

      set forth in Section 2(a) hereof, the Holder may exercise this Warrant
      partially or in full (to the extent not previously exercised) pursuant to
      a Cashless Exercise (as defined below). For purposes of this Warrant,
      "Cashless Exercise" shall mean an event whereby the Company shall issue to
      the Holder the number of Shares determined as follows:

                  N      =       B-A
                                 ---
                                  Y

                  where:

                  N   = the number of Shares to be issued to Holder

                  Y   = the Fair Market Value of one Share

                  A = the aggregate Exercise Price (the number of Shares for
      which this Warrant is exercised x Exercise Price)

                  B = the aggregate Fair Market Value (i.e., Fair Market Value x
      the number of Shares for which this Warrant is exercised)

            (c) Stock Certificates. In the event of any exercise of the rights
      represented by this Warrant, as promptly as practicable on or after the
      date of exercise and in any event within thirty days thereafter, the
      Company at its expense shall issue and deliver to the Person or Persons
      entitled to receive the same a certificate or certificates representing
      the number of Shares issued upon such exercise. In the event that this
      Warrant is exercised in part, as promptly as practicable on or after the
      date of exercise and in any event within thirty days thereafter, the
      Company at its sole expense will execute and deliver a new Warrant of like
      tenor exercisable for the number of Shares for which this Warrant may then
      be exercised.

            (d) Taxes. The issuance of the Shares upon the exercise of this
      Warrant, and the delivery of certificates or other instruments
      representing such Shares, shall be made without charge to the Holder for
      any tax or other charge of whatever nature in respect of such issuance and
      the Company shall bear any such taxes in respect of such issuance."

2. MISCELLANEOUS.

      (a) Modification and Amendment. This Amendment and any provision hereof
may be amended, waived or terminated only by an instrument in writing signed by
the Company and the Holder.

      (b) Counterparts. This Amendment may be executed in one or more
counterparts each of which will be deemed an original, but all of which together
shall constitute one and the same instrument.

<PAGE>

      (c) No Other Amendment. Except as otherwise expressly set forth herein,
all terms and conditions of the Warrant shall remain in full force and effect.
Except as expressly set forth herein, nothing herein shall be construed to be an
amendment or a waiver of any requirements of the Warrant.

                            [Signature Pages Follow]

<PAGE>

      IN WITNESS WHEREOF, the parties hereto have executed this Amendment as of
the day and year first above written.

                           PREDIX PHARMACEUTICALS HOLDINGS, INC.

                                By: /s/ Michael Kauffman
                                    --------------------------------------------
                                    Name: Michael Kauffman
                                    Title: President and Chief executive Officer

                           RRD INTERNATIONAL, LLC

                                By: /s/ Joseph P. Clancy
                                    --------------------------------------------
                                    Name:  Joseph P. Clancy
                                    Title: President and CEO
<PAGE>

      THIS WARRANT AND THE SHARES OBTAINABLE UPON ITS EXERCISE HAVE NOT BEEN
      REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR ANY STATE
      SECURITIES LAWS (COLLECTIVELY, THE "ACTS"), AND MAY NOT BE OFFERED, SOLD
      OR OTHERWISE TRANSFERRED, PLEDGED OR HYPOTHECATED UNLESS AND UNTIL
      REGISTERED UNDER ALL APPLICABLE ACTS OR UNLESS AN OPINION OF COUNSEL IS
      DELIVERED TO THE ISSUER IN FORM AND SUBSTANCE SATISFACTORY TO THE ISSUER
      TO THE EFFECT THAT AN EXEMPTION FROM REGISTRATION IS AVAILABLE UNDER ALL
      APPLICABLE ACTS.

No. CS-1                                                        October 30, 2003

                      PREDIX PHARMACEUTICALS HOLDINGS, INC.

                          WARRANT FOR NEW COMMON STOCK

      THIS CERTIFIES THAT, for value received, RRD International, LLC, or its
registered assigns (the "Holder"), is entitled, subject to the vesting
provisions set forth herein, to subscribe for and purchase from Predix
Pharmaceuticals Holdings, Inc., a Delaware corporation (the "Company"), at any
time prior to or upon October 30, 2008, or, in the event such date is a
Saturday, Sunday or United States holiday, the next business day (the
"Expiration Date"), up to one hundred eighty thousand (180,000) shares (the
"Shares") of the Company's New Common Stock, $0.01 par value (the "Common
Stock"), at an exercise price of $1.00 per share (the "Exercise Price").

      This Warrant is subject to the following terms and conditions:

      1. Vesting.

      (a) Upon the terms and subject to the conditions set forth herein, this
Warrant shall vest and Warrants shall be exercisable in five equal installments
upon the execution of this Agreement and the achievement of certain milestones
set forth below in accordance with the following vesting schedule:

            (i) 20% of this Warrant (representing 36,000 Shares) shall vest upon
execution of this Agreement by both parties hereto.

            (ii) 20% of this Warrant (representing 36,000 Shares) shall vest
immediately upon completion of the pre-IND meetings for the Company's PRX-00023
product with the FDA or similar meetings with a foreign equivalent agency;
provided, however, that if such pre-IND meetings or foreign equivalent meetings
are not required, the portion of the Warrant subject to vesting under this
clause (ii) shall vest upon completion of the milestone set forth in clause
(iii) below.

                                       1
<PAGE>

            (iii) 20% of this Warrant (representing 36,000 Shares) shall vest
immediately upon filing of the IND for the Company's PRX-00023 product with the
FDA or upon an equivalent filing with a foreign agency.

            (iv) 20% of this Warrant (representing 36,000 Shares) shall vest
immediately upon enrollment of the first subject/patient in clinical trials for
the Company's PRX-00023 product.

            (v) 20% of this Warrant (representing 36,000 Shares) shall vest on
July 31, 2004.

Notwithstanding anything herein to the contrary, the milestones set forth above
and the vesting provisions of this Warrant Agreement may be accelerated pursuant
to the provisions of the Master Services Agreement and any Work Orders thereto
entered into between the Company and the Holder of this Warrant as of October
30, 2003.

      2. Exercise; Payment.

      (a) Cash Exercise. The purchase rights represented by this Warrant may be
exercised by the Holder, in whole or in part, at any time, or from time to time,
by the surrender of this Warrant (together with a duly executed notice of
exercise (the "Notice of Exercise") in the form attached hereto as Exhibit A) at
the address of the Company's principal corporate offices set forth in Section 6
hereof, and by payment to the Company of an amount equal to the Exercise Price
multiplied by the number of the Shares being purchased, which amount may be paid
by the Holder by wire transfer or check payable to the order of the Company. The
Person in whose name any certificate representing the Shares issuable upon any
exercise of this Warrant shall be deemed to have become the holder of record of,
and shall be treated for all purposes as the record holder of the Shares
represented thereby (and such Shares shall be deemed to have been issued)
immediately prior to the close of business on the date or dates upon which such
surrender and payment are made. As used herein, the term "Person" means any
individual or any corporation, partnership, trust, limited liability company or
other entity or organization of any kind.

      (b) Stock Certificates. In the event of any exercise of the rights
represented by this Warrant, as promptly as practicable on or after the date of
exercise and in any event within thirty days thereafter, the Company at its
expense shall issue and deliver to the Person or Persons entitled to receive the
same a certificate or certificates representing the number of Shares issued upon
such exercise. In the event that this Warrant is exercised in part, as promptly
as practicable on or after the date of exercise and in any event within thirty
days thereafter, the Company at its sole expense will execute and deliver a new
Warrant of like tenor exercisable for the number of Shares for which this
Warrant may then be exercised.

      (c) Taxes. The issuance of the Shares upon the exercise of this Warrant,
and the delivery of certificates or other instruments representing such Shares,
shall be made

                                       2
<PAGE>

without charge to the Holder for any tax or other charge of whatever nature in
respect of such issuance and the Company shall bear any such taxes in respect of
such issuance.

      3. Stock Fully Paid; Reservation of Shares. All of the Shares issuable
upon the exercise of the rights represented by this Warrant will, upon issuance
and receipt of the Exercise Price therefor, be fully paid and nonassessable, and
free from all preemptive rights, rights of first refusal or first offer, taxes,
liens and charges of whatever nature, with respect to the issuance thereof.
During the period within which the rights represented by this Warrant may be
exercised, the Company shall use its best efforts, including soliciting its
stockholders for requisite approval, to at all times have authorized and
reserved for issuance a sufficient number of shares of its Common Stock to
provide for the full exercise of the rights represented by this Warrant. In
connection with any amendment to the Company's Certificate of Incorporation (the
"Certificate of Incorporation") to effect the foregoing, the Holder will vote
all shares of the Company's capital stock held by such Holder in favor of such
amendment. The Company hereby agrees that the issuance of this Warrant shall
constitute full authority to its officers who are charged with the duty of
executing stock certificates to execute and issue the proper certificates for
Shares upon the full or each partial exercise of this Warrant and shares of
Common Stock issuable upon any conversion of all or any portion of such Shares.

      4. Adjustment of Exercise Price and Number of Shares. The number and class
or series of Shares purchasable upon the exercise of this Warrant and the
Exercise Price therefor shall be subject to adjustment from time to time upon
the occurrence of certain events, as follows:

      (a) Adjustment for Reclassification, Consolidation or Merger. If while
this Warrant, or any portion hereof, remains outstanding and unexpired there
shall be (i) a reorganization or recapitalization (other than a combination,
reclassification, exchange or subdivision of shares otherwise provided for
herein), (ii) a merger or consolidation of the Company with or into another
Person in which the Company is not the surviving entity, or a reverse merger in
which the Company is the surviving entity but the shares of the Company's
capital stock outstanding immediately prior to the merger are converted by
virtue of the merger into other property, whether in the form of securities,
cash, or otherwise, or (iii) a sale or transfer of the Company's properties and
assets as, or substantially as, an entirety to any other Person in one
transaction or a series of related transactions, then, as a part of such
reorganization, recapitalization, merger, consolidation, sale or transfer,
unless otherwise directed by the Holder, all necessary or appropriate lawful
provisions shall be made so that the Holder shall thereafter be entitled to
receive upon exercise of this Warrant, during the period specified herein and
upon payment of the Exercise Price then in effect, the greatest number of shares
of stock or other securities or property that a holder of the Shares deliverable
upon exercise of this Warrant would have been entitled to receive in such
reorganization, recapitalization, consolidation, merger, sale or transfer if
this Warrant had been exercised immediately prior to such reorganization,
recapitalization, merger, consolidation, sale or transfer, all subject to
further adjustment as provided in this Section 4. If the per Share consideration
payable to the Holder for Shares in connection with any such transaction is in a
form other than cash or marketable securities, then the value of such
consideration shall be

                                       3
<PAGE>

determined in good faith by the Company's Board of Directors. The foregoing
provisions of this paragraph shall similarly apply to successive
reorganizations, recapitalizations, consolidations, mergers, sales and transfers
and to the stock or securities of any other corporation that are at the time
receivable upon the exercise of this Warrant. In all events, appropriate
adjustment shall be made in the application of the provisions of this Warrant
(including adjustment of the Exercise Price and number of Shares purchasable
pursuant to the terms and conditions of this Warrant) with respect to the rights
and interests of the Holder after the transaction, to the end that the
provisions of this Warrant shall be applicable after that event, as near as
reasonably may be, in relation to any shares or other property deliverable or
issuable after such reorganization, recapitalization, merger, consolidation,
sale or transfer upon exercise of this Warrant.

      (b) Adjustments for Split, Subdivision or Combination of Shares. If the
Company at any time while this Warrant, or any portion hereof, remains
outstanding and unexpired shall split or subdivide any class of securities as to
which purchase rights under this Warrant exist, into a different number of
securities of the same class, the number of shares of such class issuable upon
exercise of this Warrant immediately prior to such split or subdivision shall be
proportionately adjusted and the Exercise Price for such class of securities
shall be proportionately adjusted. If the Company at any time while this
Warrant, or any portion hereof, remains outstanding and unexpired shall combine
any class of securities as to which purchase rights under this Warrant exist,
into a different number of securities of the same class, the number of shares of
such class issuable upon exercise of this Warrant immediately prior to such
combination shall be proportionately adjusted and the Exercise Price for such
class of securities shall be proportionately adjusted.

      (c) Adjustments for Dividends in Stock or Other Securities or Property. If
while this Warrant, or any portion hereof, remains outstanding and unexpired,
the holders of any class of securities as to which purchase rights under this
Warrant exist at the time shall have received, or, on or after the record date
fixed for the determination of eligible stockholders, shall have become entitled
to receive, without payment therefor, other or additional stock or other
securities or property (other than cash) of the Company by way of dividend, then
and in each case, this Warrant shall represent the right to acquire, in addition
to the number of shares of such class of security receivable upon exercise of
this Warrant, and without payment of any additional consideration therefor, the
amount of such other or additional stock or other securities or property (other
than cash) of the Company that such holder would hold on the date of such
exercise had it been the holder of record of the class of security receivable
upon exercise of this Warrant on the date hereof and had thereafter, during the
period from the date hereof to and including the date of such exercise, retained
such shares and/or all other additional stock available by it as aforesaid
during said period, giving effect to all adjustments called for during such
period by the provisions of this Section 4.

      (d) Further Adjustments. In case at any time or from time to time the
Company shall take any action that affects the Shares, other than an action
described herein, then, unless such action will not have a materially adverse
effect upon the rights

                                       4
<PAGE>

of the Holder, the number of Shares into which this Warrant is exercisable shall
be adjusted in such a manner and at such time as shall be equitable in the
circumstances.

      (e) Notice of Adjustments. Upon any adjustment of the Exercise Price and
any increase or decrease in the number of Shares purchasable upon the exercise
of this Warrant, then, and in each such case, the Company, within 30 days
thereafter, shall give written notice thereof to the Holder at the address of
such Holder as shown on the books of the Company which notice shall state the
Exercise Price as adjusted and, if applicable, the increased or decreased number
of Shares purchasable upon the exercise of this Warrant, setting forth in
reasonable detail the method of calculation of each.

      5. Transfers, etc.

      (a) At any time prior to the Expiration Date, the Warrants shall only be
transferable to Affiliates (as defined below) (each transferee shall be referred
to as a "Permitted Transferee"); and, provided further, that any such transfer
does not increase the total number of holders of Warrants by more than one
(taking into account all prior transfers of any one Warrant) and will not
increase the total number of stockholders of the Company by more than one
(taking into account all prior transfers of any one Warrant). For purposes of
this Warrant, "Affiliate" of a Holder shall mean any of the stockholders,
subsidiaries, officers, directors, members or partners of such Holder, and any
other corporation, partnership or other business entity which directly or
indirectly controls, is controlled by or is under common control with, such
Holder.

      (b) Any transfer pursuant to Section 5(a) shall be effected upon surrender
of this Warrant with a properly executed assignment (in the form attached hereto
as Exhibit B) at the principal office of the Company (or, if another office or
agency has been designated by the Company for such purpose, then at such other
office or agency).

      6. Notices.

      (a) All notices, requests, consents, and other communications under this
Warrant shall be in writing and shall be deemed delivered (i) seven business
days after being sent by registered or certified mail, return receipt requested,
postage prepaid or (ii) three business days after being sent via a reputable
nationwide overnight courier service guaranteeing next business day delivery, in
each case to the intended recipient as set forth below:

      If to the Company, at 10 K Gill Street, Woburn, Massachusetts 01801,
Attention: Chief Financial Officer, or at such other address as may have been
furnished in writing by the Company to the Holder, with a copy to Hale and Dorr
LLP, 650 College Road East, Princeton, New Jersey 08540, Attention: Richard S.
Mattessich, Esq.; or

      If to a Holder, at 11 North Washington Street, Suite 310, Rockville,
Maryland 20850, Attention: Joseph P. Clancy, or at such other address as may be
furnished in writing by the Holder to the Company, with a copy to Curtin, Law,
Roberson, Dunigan & Salans, 900 M Street, NW, Washington, DC 20036, Attn: Thomas
Dunigan, Esq.

                                       5
<PAGE>

      (b) Either party may give any notice, request, consent or other
communication under this Warrant using any other means (including, without
limitation, personal delivery, messenger service, telecopy, first class mail or
electronic mail), but no such notice, request, consent or other communication
shall be deemed to have been duly given unless and until it is actually received
by the party for whom it is intended. Either party may change the address to
which notices, requests, consents or other communications hereunder are to be
delivered by giving the other party notice in the manner set forth in this
Section.

      7. Legends. Each certificate evidencing the Shares issued upon exercise of
this Warrant, or transfer of such Shares (other than a transfer registered under
the Securities Act, or any subsequent transfer of Shares so registered) shall be
stamped or imprinted with a legend substantially in the following forms:

      THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
      UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"), OR THE
      SECURITIES LAWS OF ANY STATE. THE SECURITIES MAY NOT BE TRANSFERRED EXCEPT
      PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER SUCH ACT AND
      APPLICABLE STATE SECURITIES LAWS OR PURSUANT TO AN APPLICABLE EXEMPTION
      FROM THE REGISTRATION REQUIREMENTS OF SUCH ACT AND SUCH LAWS.

      8. Fractional Shares. No fractional Shares will be issued in connection
with any exercise hereunder. Instead, the Company shall pay to such Holder an
amount in cash equal to any fractional share to which such Holder would be
entitled, multiplied by the Fair Market Value of a Share, as determined in good
faith by the Company's Board of Directors. For purposes of this Section 8, the
Fair Market Value of one Share shall equal: (a) the average of the closing sale
price of the Common Stock (or any other security for which this Warrant is then
exercisable) as quoted on the Nasdaq Stock Market or in the Over-The-Counter
Market Summary or the closing price quoted on any national securities exchange
on which such securities are listed, whichever is applicable, as reported in The
Wall Street Journal for the ten trading days immediately prior to the date of
determination of Fair Market Value (or, if no sales take place on any such
trading day, the average of the closing bid and asked prices on such trading
day); or (b) if the Common Stock (or any other security for which this Warrant
is then exercisable) is not quoted on the Nasdaq Stock Market or
Over-The-Counter or on a national securities exchange, the Fair Market Value of
a Share shall be established in good faith by the Company's Board of Directors.

      9. Representations and Warranties of the Company. The Company hereby
represents and warrants to the Holder as follows:

      (a) This Warrant has been duly authorized and executed by the Company and
is a valid and binding obligation of the Company enforceable in accordance with
its terms;

                                       6
<PAGE>

      (b) The Company shall use its best efforts to ensure that the Shares have
been duly authorized and reserved for issuance by the Company and, when issued
in accordance with the terms hereof, will be validly issued, fully paid and
nonassessable, and free from all preemptive rights, rights of first refusal or
first offer, taxes, liens and charges of whatever nature; and

      (c) The execution and delivery of this Warrant are not, and the issuance
of the Shares upon exercise of this Warrant in accordance with the terms hereof
will not be, inconsistent with the Company's Certificate of Incorporation, as
then in effect, and the Company's by-laws (the "By-Laws"), as then in effect.

      10. Representations and Warranties by the Holder. The Holder represents
and warrants to the Company as follows:

      (a) This Warrant is being acquired for its own account, for investment and
not with a view to, or for resale in connection with, any distribution thereof
within the meaning of the Securities Act. Upon exercise of this Warrant, the
Holder shall, if so requested by the Company, confirm in writing, in a form
reasonably satisfactory to the Company, that the Shares issuable upon exercise
of this Warrant are being acquired for investment and not with a view toward
distribution or resale that would violate the Securities Act.

      (b) The Holder understands that the Warrant and the Shares have not been
registered under the Securities Act by reason of their issuance in a transaction
exempt from the registration and prospectus delivery requirements of the
Securities Act pursuant to Section 4(2) thereof, and that they must be held by
the Holder indefinitely, and that the Holder must therefore bear the economic
risk of such investment indefinitely, unless a subsequent disposition thereof is
registered under the Securities Act or is exempted from such registration.

      (c) The Holder has such knowledge and experience in financial and business
matters that it is capable of evaluating the merits and risks of the purchase of
this Warrant and the Shares purchasable pursuant to the terms of this Warrant
and of protecting its interests in connection therewith.

      (d) The Holder is able to bear the economic risk of the purchase of the
Shares pursuant to the terms of this Warrant.

      (e) The Holder is an "accredited investor" within the meaning of Rule 501
of Regulation D under the Securities Act.

      11. Rights of Stockholders. Subject to Section 4 hereof, no Holder, as
such, shall be entitled to vote or receive dividends or be deemed the holder of
the Shares or any other securities of the Company that may at any time be
issuable on the exercise hereof for any purpose, nor shall anything contained
herein be construed to confer upon the Holder, as such, any of the rights of a
stockholder of the Company or any right to vote for the election of directors or
upon any matter submitted to stockholders at any meeting thereof, or to give or
withhold consent to any corporate action (whether upon

                                       7
<PAGE>

any recapitalization, issuance of stock, reclassification of stock, change of
par value, consolidation, merger, conveyance, or otherwise) or to receive notice
of meetings, or to receive dividends or subscription rights or otherwise until
this Warrant shall have been exercised and the Shares purchasable upon the
exercise hereof shall have become issuable, as provided herein.

      12. Expiration of Warrant. This Warrant shall expire and shall no longer
be exercisable as of 5:00 p.m., Eastern Time, on October 30, 2008.

      13. Miscellaneous.

      (a) This Warrant shall be governed by and construed for all purposes under
and in accordance with the laws of the State of Delaware without regard to
principles of conflicts of law.

      (b) The headings in this Warrant are for purposes of reference only, and
shall not limit or otherwise affect any of the terms hereof.

      (c) The representations, warranties, covenants and conditions of the
respective parties contained herein or made pursuant to this Warrant shall
survive the execution and delivery of this Warrant.

      (d) The terms of this Warrant shall be binding upon and shall inure to the
benefit of any successors or assigns of the Company and of the Holder and of the
Shares issued or issuable upon the exercise hereof.

      (e) This Warrant and the other documents delivered pursuant hereto
constitute the full and entire understanding and agreement between the parties
with regard to the subjects hereof and thereof.

      (f) The Company shall not, by amendment of its Certificate of
Incorporation or By-Laws, or through any other means, directly or indirectly,
avoid or seek to avoid the observance or performance of any of the terms of this
Warrant and shall at all times in good faith assist in the carrying out of all
such terms and in the taking of all such action as may be necessary or
appropriate in order to protect the rights of the Holder against impairment.

      (g) Upon receipt of evidence reasonably satisfactory to the Company of the
loss, theft, destruction or mutilation of this Warrant and, in the case of any
such loss, theft or destruction, upon delivery of an indemnity agreement
reasonably satisfactory in form and amount to the Company or, in the case of any
such mutilation, upon surrender and cancellation of such Warrant, the Company,
at its expense, will execute and deliver to the Holder, in lieu thereof, a new
Warrant of like date and tenor.

      (h) This Warrant and any provision hereof may be amended, waived or
terminated only by an instrument in writing signed by the Company and the
Holder.

                                       8
<PAGE>

      (i) The Warrant Shares shall be registered under the Securities Act to the
extent provided in the certain Registration Rights Agreement, of even date
herewith, by and between the Company and the Holder of this Warrant.

                            [Signature Page Follows]

                                       9
<PAGE>

      IN WITNESS WHEREOF, the Company has caused this Warrant to be signed by
its duly authorized officer.

                                       Predix Pharmaceuticals Holdings, Inc.

                                       By: /s/ NIK HAYES
                                           ------------------------------------
                                           Name: Nik Hayes
                                           Title: Chief Financial Officer

Acknowledged and Accepted:

RRD INTERNATIONAL, LLC

By: /s/ JOSEPH P. CLANCY
    -------------------------------
Name: Joseph P. Clancy
Title: Chief Executive Officer
Address: 11 North Washington Street, Ste 310
         Rockville, MD 20850

                                       10
<PAGE>

                                                                       EXHIBIT A

                               NOTICE OF EXERCISE

TO: Predix Pharmaceuticals Holdings, Inc.
    Attention: Chief Financial Officer

      1. The undersigned hereby elects to purchase _________________ shares of
Common Stock of Predix Pharmaceuticals Holdings, Inc. pursuant to the terms of
this Warrant, and tenders herewith payment of the purchase price of such shares
in full.

      2. Please issue a certificate or certificates representing said securities
in the name of the undersigned or in such other name as is specified below:

                              ___________________________
                                        (Name)

                              ___________________________

                              ___________________________
                                       (Address)

      3. The undersigned hereby represents and warrants that the aforesaid
securities are being acquired for the account of the undersigned for investment
and not with a view to, or for resale, in connection with the distribution
thereof, and that the undersigned has no present intention of distributing or
reselling such shares and all representations and warranties of the undersigned
set forth in Section 10 of the attached Warrant are true and correct as of the
date hereof.
                                           ___________________________
                                           (Signature and Date)

                                       11
<PAGE>

                                                                       EXHIBIT B

                               FORM OF ASSIGNMENT

                  (To be executed by the Holder if such Holder
                        desires to transfer the Warrant)

FOR VALUE RECEIVED __________________________________ (the "Transferor") hereby
sells, assigns and transfers unto _____________________________ (the
"Transferee")

                  (Please print name and address of transferee)

this Warrant, together with all right, title and interest therein, and does
hereby irrevocably constitute and appoint ___________________________ as its
Attorney to transfer such Warrant on the books of Predix Pharmaceuticals
Holdings, Inc., with full power of substitution. The Transferor has provided a
written instrument to the Company notifying the Company of such transfer and
pursuant to which the Transferee hereunder has agreed in writing to be bound by
the terms of the Warrant dated October 30, 2003 issued by Predix Pharmaceuticals
Holdings, Inc., a Delaware corporation, a copy of which has been provided to the
Transferee by the Transferor.

Dated:                                 Signature ______________________________
                                       (Signature must conform in all respects
                                       to name of Holder as specified on the
                                       face of the Warrant)

                                       _________________________________________
                                       (Insert Social Security or other
                                       Identifying Number of Holder)

                                       12

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