A new skin cancer treatment that promises to shrink tumors and prolong the lives of patients with a particular genetic mutation won speedy approval Wednesday, signaling U.S. regulators' enthusiasm for such targeted therapies.

Vemurafenib, from Roche Holding AG and Daiichi Sankyo Co., belongs to an emerging class of treatments that targets the molecular underpinnings of the cancer. These therapies offer the opportunity for significantly more effective treatment for patients who test positive for a genetic mutation driving the proliferation of their cancer cells. Patients would take the drugs only after a test showed they have the mutation.

The Food and Drug Administration's (FDA) approval of vemurafenib, which was submitted in May, came before an accelerated deadline in late October. The agency, which has faced criticism for moving slowly and cautiously while reviewing new medicines, also is expected to rapidly approve another treatment paired with a genetic test, a lung cancer therapy from Pfizer Inc. called crizotinib.

"If there is a group that a drug works really well in, there is some urgency getting it to them," said Robert Temple, a senior FDA official. Temple said it is easier for regulators to pass judgment on an experimental therapy that shows a strong benefit in select patients, and clinical trials do not need to be as large in order to provide evidence of that efficacy.

Vemurafenib stops a mutated gene called BRAF from its usual work of driving the proliferation of cancer cells. About half of metastatic melanoma patients have the mutation.

In clinical trials, the drug decreased the risk of death among this subset of patients by 64 percent compared with people using the standard chemotherapy. Almost 50 percent of the patients taking the pills saw their tumors shrink, compared with 5.5 percent who received chemotherapy.

The result "takes your breath away if you're a melanoma doc," said Anna Pavlick, director of the melanoma program at the New York University Cancer Institute who participated in vemurafenib's clinical trials. For four decades, she said, skin cancer patients whose tumors had spread to their lymph nodes and other organs were forced to rely on treatments such as chemotherapy that often failed.

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