Yesterday, Cell Genesys (NASDAQ:CEGE) delivered news that all biotech investors dread: It's halting a phase 3 study of its lead drug because of safety issues.

Shares of Cell Genesys promptly fell more than 70% on the news that patients taking its potential prostate cancer therapy, GVAX, plus a commonly used chemotherapy drug, Taxotere from Sanofi-Aventis (NYSE:SNY), died at a much higher rate than patients in the group taking just Taxotere.

The study was testing GVAX's effects on men with very late-stage and terminal prostate cancer, so deaths are unfortunately a fact of life in any clinical trial with this patient population. A data monitoring board recommended that Cell Genesys stop this phase 3 trial, dubbed VITAL-2. Why? Because, of the 114 patients who died in the study thus far, 67 of them had received GVAX compared to only 47 patients in the control group.

GVAX isn't dead as a potential treatment for prostate cancer despite the halting of VITAL-2. Cell Genesys is still crunching the numbers and there is the slim hope that there was some imbalance in the study (like if sicker patients were placed in the GVAX study group). But Cell Genesys investors shouldn't hold their breath, because drugmakers usually do a pretty good job of creating balanced studies in large trials like VITAL-2.

Cell Genesys had been on a bit of a roll in 2008 before yesterday. In March, it inked a partnership deal for GVAX with Japanese drug developer Takeda that brought it $50 million in cash upfront and potentially $270 million more in milestone payments (which it seems Cell Genesys won't be getting now). In June, Cell Genesys released more phase 1 results of GVAX in combination with ipilimumab, a drug candidate from Medarex (NASDAQ:MEDX).

Cell Genesys is still testing GVAX in another phase 3 study in a slightly different patient population and with different dosing. Earlier in the year that study, VITAL-1, was allowed to continue after a data monitoring board didn't find any safety issues that warranted stopping it.

Investors can't look too much into Cell Genesys' failed study and attempt to get any sort of read on how rival cancer vaccine developer Dendreon's (NASDAQ:DNDN) phase 3 results will turn out. Aside from being a good reminder of the risks involved in drug development (and what Dendreon's share price could look like if its lead drug fails), Cell Genesys uses a different technology than Dendreon and GVAX's results are no more applicable to Dendreon's Provenge than to any other prostate cancer treatment.

After yesterday's VITAL-2 safety issues, Cell Genesys also announced that it was going to look at the VITAL-1 GVAX data to see if that study should continue. Results from this "futility analysis" are expected to be out in "approximately" a month, according to Cell Genesys. But even if VITAL-1 is allowed to continue, it's going to be hard to have faith in GVAX's future after the developments with VITAL-2.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool's disclosure policy passes all its tests.