Merck's (NYSE:MRK) new HIV drug Isentress (also known by the generic name raltegravir) is like a big "Do not enter" sign, and a Food and Drug Administration advisory panel seems to think the sign is big enough.

On Wednesday, the advisory panel voted unanimously to recommend approval of the drug. The results of the vote weren't a big surprise; the FDA had posted its highly favorable review of the drug on its website on Friday.

The FDA will get to make the final ruling, but usually follows the panel's recommendation; Merck expects to hear from the agency by mid-October. Merck is asking for accelerated approval, under which it would gain marketing approval with less clinical data, but the company must continue clinical trials to gain full approval.

The drug would be the first in a class of HIV drugs called integrase inhibitors that fight HIV by inhibiting a protein required for the virus to enter cells. Gilead Sciences' (NASDAQ:GILD) GS 9137, which showed positive results in a phase 2 clinical trial earlier this year, is Merck's closest competitor in this new class of drugs. GlaxoSmithKline (NYSE:GSK) and Bristol-Myers Squibb (NYSE:BMY) also have integrase inhibitors that are farther behind in the clinic.

The drug will also compete against Pfizer's (NYSE:PFE) Selzentry, which received FDA approval last month. Selzentry also works by stopping the virus from going into cells, but it does so by blocking one of the human proteins that the virus uses to enter the cell rather than inhibiting the viral proteins, as the integrase inhibitors do.

Because it uses a different mechanism to inhibit the virus, Isentress works well in patients who have become immune to established drugs. In fact, Merck asked the FDA to approve the drug for use in "treatment experienced" patients. Merck has only tested the drug in patients who had failed many therapies, so it might gain approval with a more limiting label of patients who have failed therapies.

The advisory committee discussed at length the safety of Isentress, especially the increased number of patients with cancer while on the drug. If the trend continues in future clinical trials, Isentress might not be the blockbuster Merck hopes it will be, especially with competitors on its tail.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any companies mentioned in this article. The Fool has a disclosure policy.