Pfizer Inc said on Monday that U.S. regulators approved its AIDS drug Selzentry, the first in a new class of oral HIV medicines.

Selzentry is the first drug designed to keep the HIV virus that causes AIDS from entering healthy immune cells. Older AIDS medicines attack the virus itself.

The drug, also known as maraviroc, blocks the CCR5 co-receptor that serves as a main doorway for the HIV virus into immune cells.

The U.S. Food and Drug Administration approved Selzentry for use in patients who have tried other medicines and for those for whom a diagnostic test has confirmed their HIV strain is linked to the CCR5 receptor.

The world’s largest drugmaker said it expects the drug to be available next month.

Pfizer said in June that the FDA would approve the drug once certain conditions had been met.

The New York-based company is counting on new medicines like Selzentry to help drive profits as several blockbuster medicines lose patent protection and its top-selling Lipitor cholesterol treatment faces strong competition.

Industry analysts have projected annual Selzentry sales of about $500 million by 2011.