An FDA advisory panel on Wednesday voted in favor of Gilead Sciences Inc's combination drug to reduce the risk of sexually acquired HIV infection in men and transgender women who have sex with men.

The treatment, Descovy, is a combination of emtricitabine and tenofovir alafenamide, and is already approved to treat chronic HIV.

When asked to assess the efficacy of Descovy for pre-exposure prophylaxis (PrEP) of HIV in men and transgender women who have sex with men, a patient population that forms the largest component of the PrEP market, the panel voted 16-2 in favor.

The recommendation is a shot in the arm for Gilead as its third best-selling HIV drug, Truvada, faces exclusivity loss in the United States, beginning 2020.

Truvada is largely boosted by its PrEP use, SVB Leerink analyst Geoffrey Porges said in a note ahead of the committee's discussion. "Truvada grew at 15% last year, entirely due to PrEP."

Descovy's entry into the still underpenetrated PrEP market is vital to Gilead's attempts to retain its Truvada patient base after the drug goes off-patent, he said.

Truvada accounted for 20.5% of the drugmaker's total HIV franchise sales in 2018.

The panel's backing also comes as a possible launch of Descovy for PrEP faces opposition from patient advocacy groups.

They cite the scheduled U.S. launch next September of a generic version of the only currently approved prevention pill, Truvada, that could lower costs.

The panel, however, said there was not enough data to support the expansion of Descovy's label for use as a preventive HIV drug for cisgender women. The panel voted 10-8 against its approval in this patient population.

Cisgender women form less than 7% of PrEP use and the prospect of Descovy's label not including the population is not likely to impact the drug's sales, Jefferies analyst Michael Yee said in a note ahead of the panel's decision.

The FDA is yet to make a final decision on Descovy's approval for PrEP. While the agency is not bound to follow the advice of its advisory panels, it usually does so.

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