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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]

                                       1
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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

                                       2
<PAGE>

                          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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<PAGE>

            (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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<PAGE>

      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

                                       4
<PAGE>

                              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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<PAGE>

                        (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

                                       6
<PAGE>

      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

                                       7
<PAGE>

      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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<PAGE>

      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

                                       9
<PAGE>

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

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<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]

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<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

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