<PAGE>
                                                                   EXHIBIT 10.37

                             [..**..] CERTAIN CONFIDENTIAL INFORMATION CONTAINED
                                  IN THIS DOCUMENT, MARKED BY BRACKETS, HAS BEEN
                                OMITTED AND FILED SEPARATELY WITH THE SECURITIES
                                  AND EXCHANGE COMMISSION PURSUANT TO RULE 24B-2
                             OF THE SECURITIES EXCHANGE ACT OF 1934, AS AMENDED.




              THIRD AMENDMENT TO THE LICENSE AGREEMENT (ENOXIMONE)
                                 BY AND BETWEEN
                          AVENTIS PHARMACEUTICALS INC.
                     (FORMERLY HOECHST MARION ROUSSEL, INC.)
                                       AND
                                  MYOGEN, INC.


         This Third Amendment to the License Agreement (Enoximone) by and
between Aventis Pharmaceuticals Inc. (formerly Hoechst Marion Roussel, Inc.
("HMR")) and Myogen, Inc. effective October 1, 1998 ("Third Amendment" and
"License Agreement" respectively) dated the 27th day of January 2005 and
effective the 1st day of January, 2005 (the "Effective Date of the Third
Amendment"), by and between Aventis Pharmaceuticals Inc., a Delaware corporation
with a principal office at 300 Somerset Corporate Blvd. Bridgewater, New Jersey
08807 ("Aventis"), and Myogen, Inc., a Delaware corporation with a principal
office at 7575 W. 103rd Avenue, suite 102, Westminster, CO 80021-5426 ("Myogen")
(each individually a "Party" or collectively "Parties").

                                   BACKGROUND

         HMR and Myogen entered into the License Agreement effective October 1,
1998 under which Myogen obtained an exclusive worldwide license under the HMR
Patents, HMR Know-how and Joint Patents to develop and commercialize Enoximone.
HMR also transferred to Myogen, on or about the Transfer Date, any and all of
HMR's rights under the existing HMR Trademarks in several countries around the
world, including the United Kingdom and the Benelux countries.

         Effective November 23, 1999, the Parties entered into the First
Amendment to the License Agreement (the "First Amendment") under which Myogen
granted to HMR the right to co-market the Product containing the oral form of
Enoximone ("Oral Enoximone") in Europe. This grant was terminated under the
Second Amendment to the License Agreement (the "Second Amendment"), which was
effective June 2, 2003. The Second Amendment also provided for a certain royalty
to be paid by Myogen to Aventis in the event the Net Sales of Oral Enoximone in
Europe reached or exceeded the threshold set forth in the amendment.

         Myogen is currently commercializing Perfan(R) i.v. (enoximone) ("Perfan
i.v.") in various countries around the world, including the United Kingdom,
Belgium and other countries in Europe. Myogen is also undertaking two pivotal
Phase III clinical trials for Oral Enoximone and, depending upon the outcome of
these studies, anticipates seeking marketing authorization approval in Europe.



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                                       [..**..] CONFIDENTIAL TREATMENT REQUESTED
<PAGE>


         Aventis and its Affiliates were acquired by Sanofi-Synthelabo, a
pharmaceutical company organized under the laws of France, in or about August
2004, and, shortly thereafter, Sanofi-Synthelabo changed its name to
sanofi-aventis. In approving the acquisition, the Commission des Communautes
Europeennes (the "EU Commission") requested that Aventis divest certain rights
related to Enoximone specifically in the United Kingdom and Belgium. The Parties
desire to modify the terms of the License Agreement, the First Amendment and the
Second Amendment (hereinafter collectively referred to as the "License
Agreement") in order to effectuate this request.

         NOW, THEREFORE, in consideration of the mutual promises, covenants and
agreements hereinafter set forth, the sufficiency of which is hereby
acknowledged, and intending to be legally bound, the Parties to this Third
Amendment mutually agree as follows:

1.       INTERPRETATION. Unless the context of this Third Amendment otherwise
requires, (i) words of any gender include each other gender; (ii) words using
the singular or plural number also include the plural or singular number,
respectively; (iii) the terms "hereof," "herein," "whereby" and derivative or
similar words refer to this Third Amendment; and (iv) the term "Article" refers
to the specified Article of this Third Amendment. Whenever this Third Amendment
refers to a number of days, such number shall refer to calendar days unless
otherwise specified. Capitalized terms not otherwise defined in this Third
Amendment shall have the meaning set forth in the License Agreement. To the
extent there are any conflicts or inconsistencies between this Third Amendment
and the License Agreement, this Third Amendment shall control. Except as
expressly modified by this Third Amendment, the terms and conditions of the
License Agreement shall remain unchanged and in full force and effect.

2.       LIMITED EUROPEAN COUNTRIES; TERMINATED COUNTRIES.

         (a) The term "Limited European Countries" means Austria, Denmark,
Finland, France, Germany, Greece, Ireland, Italy, Luxembourg, Netherlands,
Portugal, Spain, and Sweden. The term "Limited European Country" means a country
within the Limited European Countries.

         (b) The term "Terminated Countries" as defined under Sections 1.36,
9.3.1 and 9.3.2 shall not include the United Kingdom and Belgium.

3.       NO ROYALTIES IN U.K. AND BELGIUM. Notwithstanding anything in the
License Agreement or this Third Amendment to the contrary, as of the Effective
Date of the Third Amendment, Myogen shall have no obligation to pay earned
royalties on any Product, whether on Net Sales or otherwise, in the United
Kingdom and Belgium. For purposes of clarity, as of the Effective Date of the
Third Amendment, Myogen has no further obligation to pay royalties to Aventis
for sales of Perfan i.v. or Oral Enoximone within the United Kingdom and
Belgium. Myogen also has no obligation, as of the Effective Date of the Third
Amendment, to report Net Sales of any Product to Aventis in the United Kingdom
and Belgium. In addition, Myogen has no obligation under Section 3.8.2 of the
License Agreement to deliver to Aventis a report containing information with
respect to Regulatory Filings in the United Kingdom and Belgium or to maintain
records under Section 3.8.3 with respect to Development activities in the United
Kingdom and Belgium.



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                                       [..**..] CONFIDENTIAL TREATMENT REQUESTED
<PAGE>

4.       SECTION 4.1 AND SECTION 2.3.2.

         (a) The Parties acknowledge and agree that Myogen has, prior to the
Effective Date of the Third Amendment, made all payments and taken all other
actions required by Sections 4.1(a) through 4.1(e) of the License Agreement.

         (b) The Parties acknowledge and agree that Section 4.1(f) of the
License Agreement only obligates Myogen to pay Aventis royalties on Net Sales of
Perfan i.v. in the Limited European Countries. Section 4.1(f) does not obligate
Myogen to pay royalties on the Net Sales of Oral Enoximone in the Limited
European Countries. The Parties further acknowledge and agree that (i) Section
4.2 of the License Agreement (as amended and restated below) only concerns and
relates to the sale of any Product, including Perfan i.v. and Oral Enoximone, in
any Territory other than Europe, and (ii) Section 4.3 of the License Agreement
(as amended and restated below) only concerns and relates to the sale of any
Additional Product (as defined below) in the Limited European Countries.

         (c) Section 4.1(f) of the License Agreement is amended and restated to
read as follows:

                  "(f) Beginning on the Effective Date and ending October 1,
         2011, Myogen will pay to Aventis, on a quarterly basis within 60 days
         after the end of each calendar quarter, periodic payments equal to
         [..**..] of Net Sales of Perfan i.v. in the Limited European Countries
         that occurred in the immediately preceding calendar quarter."

         (d) Section 2.3.2 of the License Agreement is amended and restated to
read as follows:

         "2.3.2   SUBLICENSE FEES AND PAYMENTS.

                  (a) At such time that Sublicense Fees and Payments (as defined
         below) exceed [..**..], Myogen shall pay to Aventis [..**..] of the
         Sublicense Fees and Payments in excess of the initial [..**..]. Myogen
         shall pay Sublicense Fees and Payments to Aventis within ten (10) days
         of Myogen's receipt of the corresponding payment from the applicable
         Sublicensee (as defined below).

                  (b) Myogen shall make certain that any sublicense agreement,
         under which a Sublicensee receives (x) a sublicense under Myogen's
         license under this Agreement in the United Kingdom and/or Belgium and
         (y) one or more other country in the Territory, clearly and equitably
         aggregates Sublicense Fees, Signing Fees, Milestone Payments and Equity
         Payments (as those terms are defined below) between the United Kingdom
         and/or Belgium (as the case may be) and such other country or countries
         in the Territory.

                  (c) Definitions.


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                                       [..**..] CONFIDENTIAL TREATMENT REQUESTED
<PAGE>


                           (i) "Sublicense Fees and Payments" mean the aggregate
                  of Sublicense Fees, Signing Payments, Milestone Payments and
                  Equity Payments to Myogen from Sublicensees. Sublicense Fees
                  and Payments shall not include any Sublicense Fees, Signing
                  Fees, Milestone Payments and Equity Payments that Myogen
                  receives from Sublicensees specifically related to the United
                  Kingdom and Belgium.

                           (ii) "Sublicense Fees" mean any lump sum fees paid by
                  a Sublicensee to Myogen.

                           (iii) "Signing Fees" means fees received by Myogen
                  from a Sublicensee upon signing of the sublicense agreement.

                           (iv) "Milestone Payments" means payments received by
                  Myogen from a Sublicensee that are payable upon achievement of
                  a particular milestone or goal.

                           (v) "Equity Payments" means amounts received by
                  Myogen for investments by the Sublicensee in Myogen to obtain
                  an equity position in Myogen in connection with Myogen
                  sublicensing activity.

                           (vi) "Sublicensee" means a Third Party that enters
                  into a sublicense agreement with Myogen providing a sublicense
                  to such Third Party under Myogen's license provided by this
                  Agreement.

                  (d) Notwithstanding any other provision contained in this
         Agreement, Sublicense Fees and Payments, Sublicense Fees, Signing Fees
         and Milestone Payments shall not include (i) any royalties on sales of
         Product received by Myogen from the Sublicensee, (ii) any payments to
         Myogen by the Sublicensee for research, development, clinical,
         manufacturing and regulatory work performed by Myogen during the term
         of the sublicense agreement (the "Myogen Sublicensee Services"),
         provided any such payments to Myogen by the Sublicensee for the Myogen
         Sublicensee Services are under the sublicense agreement and made at
         fair market value, and (iii) any loans provided by the Sublicensee to
         Myogen, provided Myogen is contractually obligated to fully repay such
         loans with interest at fair market rates."

5.       ROYALTIES IN TERRITORIES OTHER THAN EUROPE.  Section 4.2 of the License
Agreement is amended and restated to read as follows:

         "4.2     TERRITORY OTHER THAN EUROPE.

                  (a) Except as otherwise set forth in Sections 4.2(b) through
         (f) below, with respect to markets in the Territory other than the
         Limited European Countries and in consideration of the rights granted
         by Aventis under this Agreement, and provided that an HMR Patent is
         issued in one or more such markets containing a claim which would, but
         for a license granted in the Agreement, be infringed by Myogen's
         activities relating to such Product, Myogen will pay to Aventis, on a
         quarterly basis within sixty (60) days



                                       4

                                       [..**..] CONFIDENTIAL TREATMENT REQUESTED
<PAGE>


         after the end of each calendar quarter, periodic payments equal to
         [..**..] of Net Sales in such markets that occurred in the immediately
         preceding calendar quarter.

                  (b) Except as otherwise provided in Sections 4.2(c) or (d)
         below, in the event that in any country other than a Limited European
         Country there is no claim of an issued and unexpired HMR Patent which
         would, but for a license granted in the Agreement, be infringed by
         Myogen's activities relating to a Product or such HMR Patent expires in
         such country, the Net Sales royalty percentage set forth in Section
         4.2(a) above relating to such Product shall be reduced in such country
         to [..**..] of the Net Sales royalty percentage set forth in Section
         4.2(a) but shall in no event be less than [..**..] until the tenth
         (10th) anniversary of the first commercial sale of the relevant Product
         in that country, after which time all royalty obligations in that
         country relating to such Product shall cease (subject to Sections
         4.2(c) and (d) below).

                  (c) Subject to Sections 4.2(d) and (e) below, in the event
         that prior to the tenth (10th) anniversary of the first commercial sale
         of a relevant Product in any country other than a Limited European
         Country, an HMR Patent is issued in such country containing a claim
         which would, but for a license granted in the Agreement, be infringed
         by Myogen's activities relating to such Product, then, from that date
         forward, the royalty percentage relating to such Product in such
         country shall increase to the Net Sales royalty percentage as set forth
         under Section 4.2(a) above (that is, to [..**..] of Net Sales) and
         Myogen shall pay this increased royalty to Aventis in such country as
         set forth in Section 4.2(a) until such time as (i) there is no claim of
         an issued and unexpired HMR Patent in such country which would, but for
         a license granted in the Agreement, be infringed by Myogen's activities
         relating to such Product, or (ii) such HMR Patent expires, after which
         time all royalty obligations in that country relating to such Product
         shall cease. In the event that either of the contingencies set forth in
         (i) or (ii) in the preceding sentence occur prior to the tenth (10th)
         anniversary of the first commercial sale of a relevant Product in such
         country, then the Net Sales royalty percentage, from such contingency
         date, shall be at the rate set forth in Section 4.2(b) above until such
         tenth (10th) anniversary.

                  (d) Notwithstanding the foregoing and the existence of any HMR
         Patent issued in a country that is not a Limited European Country
         containing a claim which would, but for a license granted in the
         Agreement, be infringed by Myogen's activities relating to such Product
         in such country, in the event that the marketing of a generic version
         of such Product in such country is otherwise prohibited by applicable
         law, rule or regulation (as determined in good faith by the Parties),
         then until the earlier of (i) the date on which the marketing of a
         generic version of the Product in such country is no longer prohibited
         by applicable law, rule or regulation, or (ii) one (1) year from the
         date on which the relevant HMR Patent was issued in such country, the
         Net Sales royalty percentage in such country set forth in Section
         4.2(a) above relating to such Product shall be reduced by [..**..] but
         shall in no event be less than [..**..], after which time all royalty
         obligations relating to such Product in such country, if any, shall be
         governed by Sections 4.2(a), (b) and (c) above and Sections 4.2(e) and
         (f) below, as applicable.



                                       5

                                       [..**..] CONFIDENTIAL TREATMENT REQUESTED
<PAGE>


                  (e) Notwithstanding any other provision contained in this
         Section 4.2, in the event there is a claim of an issued and unexpired
         HMR Patent in any country other than a Limited European Country which
         would, but for a license granted in the Agreement, be infringed by
         Myogen's activities, but, notwithstanding, a generic version of a
         Product is marketed in such country, the royalty percentage under
         Section 4.2(a) for that Product will be reduced, in that country, to
         [..**..] of the Net Sales royalty percentage set forth in Section
         4.2(a) but shall in no event be less than [..**..].

                  (f) The Parties acknowledge that the reductions in royalty
         rates in Sections 4.2(b), (c), (d) and (e) above are not cumulative so
         that there shall not be more than one reduction in royalty rates at any
         one time in any country based upon any of the occurrences in Sections
         4.2(b), (c), (d) and (e). By way of example, in the event that there is
         no relevant HMR Patent in a country other than a Limited European
         Country and the royalty rate is [..**..] the rate set forth in Section
         4.2(a) above, and, some time thereafter, a law comes into effect in
         such country prohibiting the marketing of a generic version of such
         Product, the royalty rate shall not otherwise be further reduced."

6.       ROYALTIES IN LIMITED EUROPEAN COUNTRIES. Section 4.3 of the License
Agreement is amended and restated to read as follows:

         "4.3     ROYALTIES IN LIMITED EUROPEAN COUNTRIES.

                  (a) Except as otherwise set forth in Sections 4.3(b) through
         (f) below, beginning two (2) years after the date on which a Product
         that has not been approved in any country in Europe as of the Effective
         Date of the Third Amendment (an "Additional Product"), including but
         not limited to Oral Enoximone but excluding Perfan i.v., is approved in
         a Limited European Country (such two (2) year anniversary, the "Limited
         European Payment Initiation Date"), and provided that an HMR Patent is
         issued in one or more Limited European Countries containing a claim
         which would, but for a license granted in the Agreement, be infringed
         by Myogen's activities relating to such Additional Product, Myogen will
         pay to Aventis on a quarterly basis within sixty (60) days after the
         end of each calendar quarter, periodic payments equal to (i) [..**..]
         of Net Sales (the "Net Sales Royalty Percentage") of the Additional
         Products less than [..**..] during the calendar year in such Limited
         European Countries, and (ii) [..**..] on incremental Net Sales of the
         Additional Products that equal or exceed [..**..] in a calendar year in
         such Limited European Countries.

                  (b) Except as otherwise provided in Sections 4.3(c) or (d)
         below, in the event that in a Limited European Country there is no
         claim of an issued and unexpired HMR Patent which would, but for a
         license granted in the Agreement, be infringed by Myogen's activities
         relating to such Additional Product or such HMR Patent expires in a
         Limited European Country, the Net Sales Royalty Percentages set forth
         in Section 4.3(a) above relating to such Additional Product shall be
         reduced in such Limited European Country to [..**..] of the Net Sales
         Royalty Percentages set forth in Section 4.3(a) until the tenth (10th)
         anniversary of the Limited European Payment Initiation Date, after
         which



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                                       [..**..] CONFIDENTIAL TREATMENT REQUESTED
<PAGE>


         time all royalty obligations in that Limited European Country relating
         to such Additional Product shall cease (subject to Sections 4.3(c) and
         (d) below).

                  (c) Subject to Sections 4.3(d) and (e) below, in the event
         that prior to the tenth (10th) anniversary of the Limited European
         Payment Initiation Date, an HMR Patent is issued in a Limited European
         Country containing a claim which would, but for a license granted in
         the License Agreement, be infringed by Myogen's activities relating to
         such Additional Product, then, from that date forward, the royalty
         percentage relating to such Additional Product in such country shall
         increase to the Net Sales Royalty Percentages as set forth under
         Section 4.3(a) above and Myogen shall pay this increased royalty to
         Aventis in such Limited European Country as set forth in Section 4.3(a)
         until such time as (i) there is no claim of an issued and unexpired HMR
         Patent in such Limited European Country which would, but for a license
         granted in the Agreement, be infringed by Myogen's activities relating
         to such Additional Product, or (ii) such HMR Patent expires, after
         which time all royalty obligations in that Limited European Country
         relating to such Additional Product shall cease. In the event that
         either of the contingencies set forth in (i) or (ii) in the preceding
         sentence occur prior to the tenth (10th) anniversary of the Limited
         European Payment Initiation Date, then the Net Sales Royalty
         Percentages, from such contingency date, shall be at the rate set forth
         in Section 4.3(b) above until such tenth (10th) anniversary.

                  (d) Notwithstanding the foregoing and the existence of any HMR
         Patent issued in a Limited European Country containing a claim which
         would, but for a license granted in the Agreement, be infringed by
         Myogen's activities relating to such Additional Product in such
         country, in the event that the marketing of a generic version of such
         Additional Product in such Limited European Country is otherwise
         prohibited by applicable law, rule or regulation (as determined in good
         faith by the Parties), then until the earlier of (i) the date on which
         the marketing of a generic version of the Additional Product in such
         Limited European Country is no longer prohibited by applicable law,
         rule or regulation, or (ii) three (3) years from the Limited European
         Payment Initiation Date, the Net Sales Royalty Percentages relating to
         such Additional Product in such country set forth in Section 4.3(a)
         above shall be reduced to [..**..] of the Net Sales Royalty Percentages
         set forth in Section 4.3(a) (that is, to [..**..] under Subsection
         4.3(a)(i) and to [..**..] under Subsection 4.3(a)(ii)), after which
         time all royalty obligations in such Limited European Country, if any,
         shall be governed by Sections 4.3(a), (b) and (c) above and Sections
         4.3(e) and (f) below, as applicable.

                  (e) Notwithstanding any other provision contained in this
         Section 4.3, in the event there is a claim of an issued and unexpired
         HMR Patent in a Limited European Country which would, but for a license
         granted in the Agreement, be infringed by Myogen's activities related
         to an Additional Product, but, notwithstanding, a generic version of
         such Additional Product is marketed in such country, the royalty
         percentage under Section 4.3 (a) for that Additional Product will be
         reduced, in that Limited European Country, to [..**..] of the Net Sales
         Royalty Percentages set forth in Section 4.3(a) (that is, to [..**..]
         under Subsection 4.3(a)(i) and to [..**..] under Subsection
         4.3(a)(ii)).


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<PAGE>


                  (f) The Parties acknowledge that the Net Sales Royalty
         Percentages reductions in Sections 4.3(b), (c), (d) and (e) above are
         not cumulative so that there shall not be more than one reduction in
         royalty rates at any one time in any Limited European Country based
         upon any of the occurrences. By way of example, in the event that there
         is no relevant HMR Patent in a Limited European Country and the Net
         Sales Royalty Percentages are [..**..] the rates set forth in Section
         4.3(a) above, and, some time thereafter, a law comes into effect in
         such country prohibiting the marketing of a generic version of such
         Additional Product, the Net Sales Royalty Percentages shall not be
         otherwise further reduced."

7.       SECTION 4.4.  Section 4.4 of the License Agreement is amended and
restated as follows:

         "4.4     INTENTIONALLY OMITTED."

8.       MYOGEN COOPERATION DUE TO TERMINATION FOR MATERIAL BREACH. Section
9.4(b)(iv) of the License Agreement is amended and restated as follows:

                  "(iv) Myogen shall cooperate in the transfer to Aventis of all
     NDAs, Drug Approval Applications and Regulatory Approvals related to
     Enoximone, and shall take such other actions and execute such other
     instruments, assignments and documents as may be necessary to effect the
     transfer of rights hereunder to Aventis. Notwithstanding the preceding,
     Myogen shall have no obligation to transfer to Aventis any NDAs, Drug
     Approval Applications and Regulatory Approvals related to Enoximone in the
     United Kingdom and Belgium."

9.       HMR TRADEMARKS IN THE UNITED KINGDOM AND BELGIUM. Article IX of the
License Agreement is amended to add the following Section 9.8:

                  "9.8 HMR TRADEMARKS IN THE UNITED KINGDOM AND BELGIUM.
     Notwithstanding anything to the contrary in the Agreement, Aventis'
     transfer under Section 4.1(c) of the Agreement of the HMR Trademarks in the
     United Kingdom and the Benelux countries (Belgium, the Netherlands and
     Luxembourg) is, as of the Effective Date of the Third Amendment, perpetual
     and irrevocable. Under no circumstance, including termination of the
     Agreement, whether due to Material Breach of the agreement by Myogen or
     otherwise, shall Myogen be obligated to transfer to Aventis its rights
     under the HMR Trademarks in the United Kingdom and the Benelux countries.
     It is understood between the Parties that the rights granted hereunder are
     limited to the United Kingdom and the Benelux countries and consequently,
     unless otherwise specifically permitted under the Agreement, Myogen shall,
     except as otherwise permitted by the Agreement, refrain from (i) using the
     HMR Trademarks or any other similar trademarks outside of the United
     Kingdom and the Benelux countries, (ii) making applications to register
     these trademarks or any other similar trademarks outside the United Kingdom
     and the Benelux countries, or (iii) challenge the rights Aventis or any of
     its Affiliates may have to these trademarks outside the United Kingdom and
     the Benelux countries."



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<PAGE>


10.      HMR PATENTS, JOINT PATENTS AND HMR KNOW-HOW IN THE UNITED KINGDOM AND
BELGIUM.  Article IX of the License Agreement is amended to add the following
Section 9.9:

                  "9.9 HMR PATENTS, JOINT PATENTS AND HMR KNOW-HOW IN THE UNITED
     KINGDOM AND BELGIUM. Notwithstanding anything to the contrary in the
     Agreement, the license grants set forth in Sections 2.1 and 2.2 of the
     Agreement under the HMR Patents, Joint Patents and the HMR Know-how in the
     United Kingdom and Belgium are perpetual and irrevocable and shall survive
     termination of the Agreement. Section 2.3.1 shall not restrict Myogen from
     freely sublicensing the HMR Patents, Joint Patents and the HMR Know-how in
     the United Kingdom and Belgium. Nothing in the Agreement (including nothing
     in Section 6.6) shall restrict Myogen from freely assigning its rights
     under the HMR Patents, Joint Patents and the HMR Know-how to the extent,
     but only to the extent, the HMR Patents, Joint Patents and the HMR Know-how
     relate to the United Kingdom and Belgium."

11.      TERMINATION WITHOUT CAUSE (INCLUDING DENIAL OF PHASE III TRIALS). The
Parties acknowledge and agree that Myogen received consent from the FDA as
referenced in Section 9.5.1(b) of the License Agreement and that, therefore,
Myogen may not terminate the License Agreement pursuant to Section 9.5.1(b).
Except as expressly modified by this Third Amendment, the terms and conditions
of Section 9.5 of the License Agreement, including, without limitation, Myogen's
right to terminate the License Agreement without cause pursuant to Section
9.5.1(a), shall remain unchanged and in full force and effect.

12.      EU COMMISSION APPROVAL; CONDITION PRECEDENT. This Third Amendment and
its terms and conditions, including the license grant and corresponding transfer
of rights and assets hereunder, are subject to approval by the EU Commission.
Myogen agrees to use reasonable commercial efforts to cooperate with the EU
Commission and with Aventis in order to effectuate such approval and clearance
of the transaction contemplated under this Third Amendment in accordance with
the terms of this Third Amendment. The Parties shall diligently take all further
action reasonably necessary or useful and shall cooperate with each other in
order to carry out the purpose of this Third Amendment, including, but not
limited to, diligently undertaking any action reasonably necessary to satisfy as
quickly as possible the condition precedent referred to in this Section 12. To
the extent separate or particular documents or instruments are mutually agreed
by the Parties to be required or any other steps are reasonably necessary or
beneficial in connection therewith, the Parties agree to use reasonable
commercial efforts to execute such documents or instruments and to consummate
such steps promptly. In the event that the EU Commission informs sanofi-aventis
that it will not approve this Third Amendment, then the Parties, in good faith,
will use reasonable commercial efforts to promptly modify this Third Amendment
and to undertake any other steps so as to expeditiously obtain the approval of
the EU Commission. Notwithstanding any other provision contained in this Third
Amendment (including, without limitation, this Section 12), Myogen shall not be
required to take any action or agree to any material modification or material
amendment to the License Agreement or this Third Amendment or take any action
which requires Myogen to incur any material expense or make any material payment
not expressly contemplated by this Third Amendment or the License Agreement. In
the event that the Parties have timely and in good faith undertaken such actions
and the EU Commission continues to refuse to approve this Third Amendment, then
either Party may terminate this Third Amendment upon five (5) days prior written
notice and this Third



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<PAGE>


Amendment shall be null and void. This Third Amendment, the transaction
contemplated herein and the transfer of ownership to the corresponding rights
and assets to Myogen shall have been approved by the European Commission, in
accordance with Articles 16 and 17 of the commitments signed by
Sanofi-Synthelabo on April 23, 2004 (Case M. 3354), as properly evidenced by
Aventis.







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                                       [..**..] CONFIDENTIAL TREATMENT REQUESTED
<PAGE>


         IN WITNESS WHEREOF, the Parties have caused this Third Amendment to be
executed by their duly authorized representatives as of the date first set forth
above.


MYOGEN , INC.


By: /s/ J. William Freytag
    ---------------------------

Name: J. William Freytag, Ph.D.

Title: CEO & President




AVENTIS PHARMACEUTICALS INC.



By: /s/ Juergen Lasowski
    ---------------------------

Name: Juergen Lasowski

Title: VP Business Dev. & Strategy





By: /s/ Gregory Irace
    ---------------------------

Name: Gregory Irace

Title: Senior Vice President, Finance



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                                       [..**..] CONFIDENTIAL TREATMENT REQUESTED