Cell Genesys ( CEGE) shares plummeted Wednesday after the company said it stopped a late-stage study for its prostate cancer vaccine G-VAX after a higher rate of chemotherapy patients taking the drug died than did those in a control group.

Shares plummeted 72.7% to 76 cents in recent trading.

The South San Francisco-based biotechnology company said it ended the trial, dubbed VITAL-2, on recommendation of an Independent Data Monitoring Committee (IDMC) after a routine safety review. The committee found that of 114 deaths to date, there were 67 deaths in the GVAX arm, vs. 47 in the control arm.

The primary endpoint in VITAL-2 was an improvement in survival. The trial was initiated in June 2005, and has enrolled 408 patients with hormone-refractory prostate cancer with metastatic disease who are symptomatic with cancer-related pain. It looked at GVAX plus chemotherapy, vs. a control arm of chemotherapy plus prednisone.

Cell Genesys said the cause for the imbalance has yet to be determined, and it has requested the IDMC also review its other late stage trial on GVAX, the ongoing VITAL-1 study. It expects the results of that futility analysis in roughly a month.

Cell Genesys has a development and commercialization collaboration with Japanese pharmaceutical company Takeda for GVAX in prostate cancer. The company ended the second quarter with $166 million in cash. "... with the cessation of VITAL-2, we expect to make commensurate adjustments to our business operations and we will provide further details regarding this in the near future," the company said in a press release Wednesday morning.

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