Shares of Onyx Pharmaceuticals Inc. plunged after it and partner Bayer Pharmaceuticals Corp. said Monday they will use ongoing late-stage clinical trial data to seek marketing approval of their kidney cancer treatment rather than using mid-stage clinical trial results.

Shares of Onyx fell $11.02, or 27 percent, at $29.79 Nasdaq. American depositary receipts of Bayer AG, Bayer Pharmaceuticals parent company, fell 32 cents, or 1.1 percent, to $27.85.

The decision to use the mid-stage data to support late-stage clinical trial data, which is still being enrolled, puts a possible U.S. launch date into 2006, later than if the companies would have started the filing process with mid-stage data. The Phase III study is expected to enroll more than 800 patients with the primary endpoint being overall patient survival.

The Phase II clinical study showed that patients treated with the drug BAY 43-9006 were more likely not to have progression of their disease than those on placebo during 12 weeks of treatment.

The companies said 70 percent of patients in the study had tumor shrinkage or disease stabilization but that the study had been designed to look tumor stabilization, not tumor shrinkage.