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of the available BRAF targets on cells, then it ought to make a significant impact on tumor shrinkage, Bollag says. But Plexxikon really only saw the tumor shrinkage benefits in patients when it was able to inhibit 80 to 90 percent of the chosen targets, Bollag says. “That surprised us,” he says. “If you inhibit 50 percent that should be enough. But it’s not enough.”
So while the researchers started seeing an effect at 240 milligram doses, the company ended up picking 960 milligrams as the ideal dose for future clinical trials, Bollag says.
More definitive results are expected to come fairly soon. Plexxikon and its partner, Roche, have completed enrollment in a mid-stage study of about 100 patients with metastatic melanoma and expect to have preliminary results from that trial by the end of the year. The companies are also recruiting people into a pivotal Phase III trial of about 700 patients, which will compare the Plexxikon drug to a common chemotherapy drug, dacarbazine. That study started enrollment in late 2009 and early 2010, Bollag says.
Promising Phase I results certainly are the kind of thing that can help speed up clinical trial enrollment, as patients with melanoma seek some kind of hope. That may help Plexxikon move this program along quickly, which it will have to do if it wants to hold onto the upper hand in the field. Novartis, GlaxoSmithKline, and South San Francisco-based Exelixis are all thought to be developing their own drugs to block the BRAF target, and other companies are also thought to have been encouraged to get into the race based on Plexxikon’s success so far, Bollag says. It’s safe to say that this is going to be an increasingly competitive niche to watch in the cancer drug development world.
“Our data has encouraged other companies that this is indeed an attractive target,” Bollag says. “Other efforts are escalating.”