FOURTH AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT

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                                                                     Exhibit 4.2

                                                                  EXECUTION COPY

                         QUATRX PHARMACEUTICALS COMPANY
                                  AMENDMENT TO
                           FOURTH AMENDED AND RESTATED
                           INVESTORS' RIGHTS AGREEMENT

     THIS AMENDMENT TO FOURTH AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT
(the "AMENDMENT") is made and entered into as of this 25th day of May, 2005, by
and among QUATRX PHARMACEUTICALS COMPANY, a Delaware corporation (the
"COMPANY"), and the entities whose names are set forth on EXHIBIT A attached
hereto (referred to hereinafter collectively as the "INVESTORS" and each
individually as an "INVESTOR").

                                    RECITALS

     WHEREAS, the Company and certain of the Investors (the "PRIOR INVESTORS")
have entered into that certain Fourth Amended and Restated Investors' Rights
Agreement, dated as of November 22, 2004 (the "INVESTOR RIGHTS AGREEMENT"),
pursuant to which the Company and the Prior Investors made certain agreements
regarding registration rights and other matters set forth therein. Capitalized
terms used herein but not otherwise defined shall have the meaning given such
terms in the Investor Rights Agreement;

     WHEREAS, the Company proposes to issue up to 5,357,141 additional shares of
its Series D Convertible Preferred Stock, par value $0.01 per share ("SERIES D
PREFERRED") and 4,260,035 shares of its Series D-1 Convertible Preferred Stock,
par value $0.01 per share ("SERIES D-1 PREFERRED") to additional Investors (the
"ADDITIONAL INVESTORS");

     WHEREAS, Section 6.2 of the Investor Rights Agreement provides that the
Investor Rights Agreement may be amended and the observance thereof may be
waived only with the written consent of (i) the Company, (ii) the holders of at
least 60% of the outstanding shares of the Company's Series A Preferred and
Series B Preferred considered together as a single class (calculated on an as if
converted into Common Stock basis), (the "REQUISITE SERIES A AND B MAJORITY"),
(iii) the holders of at least 60% of the outstanding shares of Series C
Preferred as a separate class (the "REQUISITE SERIES C MAJORITY") and (iv) the
holders of at least 60% of the outstanding shares of Series D Preferred as a
separate class (the "REQUISITE SERIES D MAJORITY") (including for each purpose
any shares of Conversion Stock). Collectively, the Requisite Series A and B
Majority, the Requisite Series C Majority and the Requisite Series D Majority
are referred to herein as the "REQUISITE MAJORITIES";

     WHEREAS, the Company, the Prior Investors and the Additional Investors
desire to amend the Investor Rights Agreement as provided herein; and

     WHEREAS, the Investors executing this Amendment hold the Requisite
Majorities as of the date hereof.

     NOW, THEREFORE, for and in consideration of the mutual promises and
covenants set forth herein and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the parties to this
Amendment hereby agree as follows:

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                                    AMENDMENT

     1. Pursuant to Section 7.8 of the Investor Rights Agreement, upon its
execution of a counterpart signature page to the Investor Rights Agreement, each
of the Additional Investors shall be deemed an "Investor" under, and a party to,
the Investor Rights Agreement.

     2. SECTION 1.1 of the Investor Rights Agreement (Financial Information) is
hereby amended by deleting the first paragraph thereof in its entirety and
replacing it with the following:

     "The Company covenants and agrees that, commencing on the date of this
     Agreement, the Company will furnish to each Major Investor, as defined
     below, the information specified in this Section 1.1. For purposes of this
     Section 1.1 only, for so long as an Investor, together with its affiliates,
     holds not less than 500,000 shares of the Series A Preferred, the Series B
     Preferred, the Series C Preferred, the Series D Preferred, the Series D-1
     Preferred and/or the equivalent number (on an-as-converted basis) of shares
     of Common Stock of the Company ("COMMON STOCK") issued upon the conversion
     of such shares of Series A Preferred, Series B Preferred, Series C
     Preferred, Series D Preferred, or Series D-1 Preferred, such Investor shall
     be a "MAJOR INVESTOR"."

     3. SECTION 1.2 of the Investor Rights Agreement (Inspection) is hereby
amended by adding the following sentence at the beginning of such section:

     "For purposes of Section 1.2 only, a "MAJOR INVESTOR" means an Investor
     that, together with its affiliates, holds not less than 1,000,000 shares of
     the Series A Preferred, the Series B Preferred, the Series C Preferred, the
     Series D Preferred, the Series D-1 Preferred and/or the equivalent number
     (on an-as-converted basis) of shares of Common Stock issued upon the
     conversion of such shares of Series A Preferred, Series B Preferred, Series
     C Preferred, Series D Preferred, or Series D-1 Preferred."

     4. SECTION 2.1 of the Investor Rights Agreement (Registration Rights;
Definitions) is hereby amended by replacing each occurrence of the phrase
"Series A Preferred, Series B Preferred, Series C Preferred or Series D
Preferred" with the phrase "Series A Preferred, Series B Preferred, Series C
Preferred, Series D Preferred or Series D-1 Preferred."

     5. SECTION 3.1 of the Investor Rights Agreement (Pre-emptive Rights;
General) is hereby amended by replacing the phrase "Series A Preferred, Series B
Preferred, Series C Preferred and/or Series D Preferred" with the phrase "Series
A Preferred, Series B Preferred, Series C Preferred, Series D Preferred and/or
Series D-1 Preferred."

     6. SECTION 3.2 of the Investor Rights Agreement (Pre-emptive Rights; New
Securities) is hereby amended as follows:

          (A) Section 3.2(a) is amended and restated in its entirety to read
"shares of Conversion Stock;"

          (B) The following is added as Section 3.2(f):

               "(f) Shares of Common Stock, Series D Preferred and Series D-1
          Preferred


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          issued pursuant to that certain Exchange Agreement, made as of May
          ___, 2005, by and among the Company, Hormos Medical Corporation
          ("HORMOS"), and the shareholders of Hormos signatories thereto (the
          "EXCHANGE AGREEMENT")."

     7. SECTION 4.1 of the Investor Rights Agreement (Right to Purchase in
Connection with Initial Public Offering; Grant of Option) is hereby amended as
follows:

          (A) the phrase "Series A Preferred, Series B Preferred, Series C
Preferred and Series D Preferred" is replaced by the phrase "Series A Preferred,
Series B Preferred, Series C Preferred, Series D Preferred and Series D-1
Preferred";

          (B) the phrase "or issued to the Investor pursuant to the Exchange
Agreement" is inserted after "or the Series D Purchase Agreement" and before ",
as the case may be".

     8. SECTION 6.1 of the Investor Rights Agreement (Assignment), is hereby
amended by (i) adding the phrase ", 40,000 shares of Series D-1 Preferred" after
"40,000 shares of Series D Preferred" and (ii) by adding the following clause
(g) after clause (f) of the penultimate sentence thereof: "and (g) in the case
of a transfer by SITRA, a partnership or other investment vehicle in which SITRA
is a significant investor and that is managed by present or former personnel of
SITRA, provided that as a condition precedent to such transfer, SITRA has
delivered to the Company, if so requested by the Company, an opinion of U.S.
counsel that the transfer of the Company's securities held by SITRA to such
partnership or other investment vehicle is exempt from the registration
requirements under the Securities Act."

     9. SECTION 6.2 of the Investor Rights Agreement (Amendment of Rights), is
hereby amended by deleting the phrase "60% of the outstanding shares of Series D
Preferred as a separate class" and replacing it with "65% of the outstanding
shares of Series D Preferred as a separate class". In addition, the following
sentence shall be added to the end of Section 6.2: "Notwithstanding the
foregoing, the Company may amend EXHIBIT A hereto without the consent of the
Investors to add additional shares of Series D Preferred purchased under the
Amended and Restated Series D Preferred Stock Purchase Agreement."

     10. Exhibit A to the Investor Rights Agreement is hereby amended and
restated in its entirety by EXHIBIT A attached hereto.

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

     12. This Amendment shall become effective immediately upon execution by the
Company and the Requisite Majorities.

     13. Other than as set forth in this Amendment, all of the terms and
conditions of the Investor Rights Agreement shall continue in full force and
effect.

     14. This Amendment shall be governed by and construed exclusively in
accordance with the substantive laws of the State of Delaware as applied to
agreements among Delaware residents entered into and to be performed entirely
within Delaware excluding the body of law relating to conflict of laws and
choice of law.


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                            [SIGNATURE PAGE FOLLOWS]


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                                                                  EXECUTION COPY

     IN WITNESS WHEREOF, the parties hereto have caused this AMENDMENT TO FOURTH
AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT to be executed and delivered by
their respective officers hereunto duly authorized on the date first above
written.

COMPANY:

QUATRX PHARMACEUTICALS COMPANY


By: /s/ Robert L. Zerbe
    ---------------------------------
Print Name: Robert L. Zerbe
Title: CEO

PRIOR INVESTORS:

MPM BIO VENTURES III, L.P.              MPM BIOVENTURES III-QP, L.P.

By: MPM Bio Ventures III GP, L.P.,      By: MPM Bio Ventures III GP, L.P., its
its General Partner                     General Partner

By: MPM Bio Ventures III LLC, its       By: MPM Bio Ventures III LLC, its
General Partner                         General Partner


By: /s/  Nicholas J. Simon              By: /s/ Nicholas J. Simon
    ---------------------------------       ----------------------------------
Name: Nicholas J. Simon                 Name: Nicholas J. Simon
Title: Series A Member                  Title: Series A Member


MPM BIOVENTURES III PARALLEL FUND,      MPM BIOVENTURES III GMBH & CO.
L.P.                                    BETEILINGUNGS KG

By: MPM Bio Ventures III GP, L.P.,      By: MPM BioVentures III GP, L.P., in its
its General Partner                     capacity as the Managing Limited Partner

By: MPM Bio Ventures III LLC, its       By: MPM BioVentures III LLC, its General
General Partner                         Partner


By: /s/ Nicholas J. Simon               By: /s/ Nicholas J. Simon
    ---------------------------------       ----------------------------------
Name: Nicholas J. Simon                 Name: Nicholas J. Simon
Title: Series A Member                  Title: Series A Member


MPM ASSET MANAGEMENT INVESTORS 2003     MPM BIOVENTURES STRATEGIC FUND, L.P.
BVIII LLC

                                        By: MPM Bio Ventures III GP, L.P., its
By: /s/ Nicholas J. Simon               General Partner
    ---------------------------------
Name: Nicholas J. Simon                 By: MPM Bio Ventures III LLC, its
Title: Manager                          General Partner


                                        By: /s/ Nicholas J. Simon
                                            ----------------------------------
                                        Name: Nicholas J. Simon
                                        Title: Series A Member

               [SIGNATURE PAGE TO AMENDMENT TO FOURTH AMENDED AND
                      RESTATED INVESTORS' RIGHTS AGREEMENT]

<PAGE>

THOMAS WEISEL HEALTHCARE VENTURE PARTNERS, L.P.

By: Thomas Weisel Healthcare Venture Partners LLC,
its General Partner

By: Thomas Weisel Capital Management LLC,
its Managing Member


By: /s/ Casey M. Castelein
    ---------------------------------