<PAGE>
                                                             Exhibit 10.23


[*] 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 406 OF THE SECURITIES EXCHANGE ACT OF
    1933, AS AMENDED.


[CYTOKINETICS LOGO]                   280 East Grand Avenue
                                      South San Francisco, CA 94080
                                      Tel (650) 624-3000 Fax (630) 624-3010


July 28, 2003

Glaxo Group Limited, doing business as GlaxoSmithKline
709 Swedeland Road
King of Prussia, Pennsylvania 19406
Attn: Pradip K. Bhatnagar, Ph.D., Director, Genetic & Discovery Alliances

RE: COMPOUND CRITERIA UNDER THAT CERTAIN COLLABORATION AND LICENSE AGREEMENT BY
AND BETWEEN GLAXO GROUP LIMITED, A GLAXOSMITHKLINE COMPANY, ("GSK") AND
CYTOKINETICS, INC. ("CK") OF EVEN DATE JUNE 20, 2001 (THE "COLLABORATION
AGREEMENT").

Dear Pradip:

Pursuant to this letter amendment to the Collaboration Agreement (the "Compound
Criteria Letter Amendment"), GSK and CK desire to revise the Compound Criteria
applicable under the Collaboration Agreement as approved by the JRC on
September 21, 2001, all on the terms set forth herein.

Now therefore, GSK and CK agree as follows:

     1.   All capitalized terms not defined herein shall have the meaning
          ascribed to them in the Collaboration Agreement.

     2.   For purposes of the Collaboration Agreement, "Compound Criteria" shall
          mean, effective as of the Effective Date, those criteria set forth in
          the revised Exhibit 1.17 attached hereto and incorporated by
          reference. For clarity, it is understood that the Compound Criteria
          may only be modified or amended pursuant in a writing signed by both
          Parties referencing the Collaboration Agreement and expressly
          modifying or amending the Compound Criteria.

     3.   Except as specifically modified or amended hereby, the Collaboration
          Agreement shall remain in full force and effect and, as modified or
          amended, is hereby ratified, confirmed and approved. No provision of
          this Compound Criteria Letter Amendment may be modified or amended
          except expressly in a writing signed by both Parties nor shall any
          terms be waived except expressly in a writing signed by the Party
          charged therewith. This Compound Criteria Letter Amendment shall be
          governed in accordance with the laws of the [*], without regard to
          principles of conflicts of laws.


* Certain information on this page has been omitted and filed separately with
  the Commission. Confidential treatment has been requested with respect to the
  omitted portions.


<PAGE>

Pradip K. Bhatnagar, Ph.D.
July 28, 2003
Page Two


Please sign and return two copies of this letter if you agree to the foregoing
terms.

Sincerely,

/s/ Robert I. Blum
-------------------------
Robert I. Blum
Senior Vice President, Finance and Corporate Development
Chief Financial Officer
Cytokinetics, Inc.

Agreed and accepted:

GLAXO GROUP LIMITED

/s/ Pradip K. Bhatnagar
--------------------------

Name: Pradip K. Bhatnagar

Title: Director

cc: Vice President, Business Development, Glaxo Group Limited, doing
    business as GlaxoSmithKline
    Senior Vice President and Assistant General Counsel-R&D Legal Operations,
    GlaxoSmithKline Corporate Legal Department
    Kenneth A. Clark, Esq., Wilson Sonsini Goodrich & Rosati Professional
    Corporation


<PAGE>
                                  EXHIBIT 1.17
                          [Revised September 21, 2001]

                               Compound Criteria
                        (effective as of June 20, 2001)

[*]

* Certain information on this page has been omitted and filed separately with
  the Commission. Confidential treatment has been requested with respect to the
  omitted portions.