Onyx News Slaps Adherents

That is, the slide doesn't make sense until you look at the drug's efficacy, which is also taking some hits. According to data from a phase II study released Monday, BAY 43-9006 wasn't very effective in shrinking tumors in kidney cancer patients.

Only 4% of patients in the study saw their tumors shrink by 50% or more. This 4% "partial response" rate is down from a 15% rate reported last June in a preliminary fashion at the annual meeting of the American Society of Clinical Oncology. In other words, as more patient data were analyzed, the partial response rate of BAY 43-9006 decreased, and by a fairly significant amount.

On its conference call Monday morning, Onyx executives downplayed these results, stating that the phase II study was not designed to measure tumor shrinkage. Instead, the phase II study was intended to determine whether BAY 43-9006 could slow or stop tumor growth, measured by the time it takes for a patient's tumor to progress.

Here, Onyx says the phase II study was positive. A higher percentage of patients taking BAY 43-9006 didn't see their tumors progress, or start growing again, at 12 weeks, compared with similar patients taking a placebo. Overall, patients taking the drug and a placebo had a "time to progression" (the time it takes a tumor to start growing again) of 169 days. Onyx says a historical search of other kidney cancer studies shows that patients given a placebo typically see their tumors progress after 60 to 90 days.

Onyx executives believe this data suggest that BAY 43-9006, by slowing or stopping tumors from growing, will lead kidney cancer patients to live longer. Prolonged survival is the primary endpoint of the ongoing phase III study, on which the companies expect to seek FDA approval.

But in order to be entirely comfortable with this phase II time-to- progression data and be confident that it will lead to a positive phase III study, it's important to know how much better patients taking BAY 43-9006 performed compared wiht patients taking placebos. At this point, Onyx and Bayer won't say, choosing to keep the data under wraps until they can be presented at a scientific meeting.