Biotechnology company Cytokinetics Inc. said Monday it will assume responsibility for continuing development of two cancer treatment compounds under an amended agreement with GlaxoSmithKline.

The company announced in June that it was taking responsibility for further development of SB-743921 from its partner GlaxoSmithKline.

The revised agreement also assigns responsibility for development of ispinesib, or SB-715992, to Cytokinetics.

Glaxo has the option to resume responsibility for some or all of the development and marketing activities associated with the two drugs.

The companies have been collaborating in developing cancer treatments for five years and have extended the relationship for an additional year in June. The collaboration has already moved two drug candidates into clinical trials.

Cytokinetics said it is considering plans to conduct in 2007 a focused development program to supplement a broad series of Phase I and Phase II clinical trials on ispinesib. The trials, previously sponsored by GlaxoSmithKline, focused on lung and breast cancer patients. Cytokinetics' clinical trials would focus on breast cancer patients.

Shares of Cytokinetics fell 33 cents, or 4.3 percent, to close at $7.29 on the Nasdaq. The stock has traded between $5.20 and $9.35 over the last 52 weeks.

Shares of GlaxoSmithKline finished 17 cents lower at $51.48 on the New York Stock Exchange.