Dendreon (Nasdaq: DNDN) shares fell Thursday and Friday mostly because of data from potential competitor Johnson & Johnson (NYSE: JNJ). There was also a disclosure of an SEC investigation about its lowered guidance, but that seems to be a lesser concern than J&J's Zytiga stealing all of its potential customers away.

The data is from the American Society of Clinical Oncology abstract previewing Zytiga's use in pre-surgical prostate cancer patients. In the phase 2 trial, about 34% of patients taking Zytiga for six months saw their tumors eliminated or nearly eliminated. The trial didn't have a control arm, but that's impressive given the aggressiveness of the tumors. You couldn't get that level with the current available medication.

That current medication isn't Dendreon's Provenge though. These are patients earlier in the development of the disease; Provenge isn't used until the tumor metastasizes. Rather than competing, you could argue that these patients won't progress far enough to need Provenge, but I don't think there's enough data on these patients to say that. They might not need surgery either, but I didn't see Intuitive Surgical (Nasdaq: ISRG) tumbling on the news.

More likely, investors are worried that the data implies that Zytiga and Medivation's (Nasdaq: MDVN) MDV3100, which works in a somewhat similar way, are very potent and might compete directly with Provenge for metastatic patients. J&J has already said Zytiga is successful in this patient population and further details are expected at the ASCO meeting.

Dendreon's saving grace is the possibility that doctor won't pick one or the other, but treat patients with both Zytiga and Provenge. The biotech is running that trial now, testing the treatments in succession.

There's a hypothesis that immunotherapies might be good at mopping up after other therapies. It makes sense that the weakened cells would be good targets for the immune system, which is how Provenge works, but we'll have to wait and see the data.

Until investors are convinced that Provenge has a long-term role in the treatment paradigm, any good news by other prostate cancer treatments will be viewed as bad news for Dendreon. Buying at this level could produce some decent profits, just don't expect an immediate breakout.

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