The drugmaker halted a late-stage of its GVAX prostate cancer therapy after 20 more deaths were reported in patients on its treatment versus a placebo, sending its shares plummeting more than 75 percent to an all-time low, Reuters reports.

The cause for the “imbalance in deaths” has not yet been identified, but the biotech says an independent committee, which pointed out the death mismatch, reported no new safety issues for the therapy. One of two trials that were conducted had enrolled 408 patients and yielded 114 deaths - 67 were in the treatment arm and 47 in the control arm. (Here’s the statement).

Dendreon, by the way, is developing a similar med, Provenge, which has been mired in regulatory hurdles for years and has failed to show it significantly slowed the progress of advanced prostate cancer. However, a panel voted the vaccine was safe and effective, and an analysis of data also suggested Provenge increased survival time for patients whose cancer spread, but no longer respond to available treatments. (Back story). Both GVAX and Provenge are potential vaccines designed to stimulate the body’s immune system to fight an existing cancer.

“We are right now reviewing this disappointing development with our partner Takeda… We are reviewing all of our business operations with respect to any commensurate adjustments in the scope of those operations,” the biotech said on a conference call with analysts.