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Pembrolizumab (Keytruda) has won FDA approval for treating advanced or unresectable melanoma in patients not responding to other therapies, .

The monoclonal antibody drug inhibits the PD-1 pathway, which prevents immune-mediated killing of melanoma cells. Pembrolizumab thus makes it possible for anti-tumor T cells to attack melanoma tumors. It's the first approved product with this mechanism of action.

Some oncologists have already called this approach "a major breakthrough" for metastatic melanoma.

"Keytruda is intended for use following treatment with ipilimumab," sold as Yervoy, the FDA noted in announcing the approval. In patients with the V600 BRAF mutation, patients should also have received a BRAF inhibitor before starting pembrolizumab.

The agency explained that this was an accelerated approval and that "improvement in survival or disease-related symptoms has not been established" for the drug. Under such approvals, the drug's sponsor -- Merck & Co., in this case -- must perform additional studies to confirm a clinical benefit. The FDA announcement did not indicate what specifically it would require from Merck.

Pembrolizumab's efficacy was demonstrated in uncontrolled studies involving 173 patients; the studies had reductions in tumor volume as the primary outcomes. In those receiving the recommended dose of 2 mg/kg, 24% showed tumor shrinkage, the FDA said. A 10-mg/kg dose had a similar level of efficacy.

"The effect lasted at least 1.4 to 8.5 months and continued beyond this period in most patients," the agency said.

Experience with the drug in 411 patients provided the safety data. Fatigue, cough, nausea, pruritus, rash, anorexia, constipation, arthralgia, and diarrhea were the most common side effects, according to the FDA. Less frequently, severe immune-mediated adverse effects involving the lungs, colon, liver, and endocrine glands were seen.

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