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. About 50 percent to 60 percent of patients who have been treated with other AIDS drugs meet that definition, the FDA said.

Selzentry will come with a black box warning, the strongest for prescription drugs, about the risk of liver toxicity, the FDA said.

The drug's label also will mention the possibility of heart attacks under a list of warnings and precautions, the agency said.

The most common problems reported in studies of patients who took Selzentry included cough, fever, upper respiratory tract infections, rash, abdominal pain and dizziness.

Pfizer, the world's largest drug maker, expects Selzentry to be available next month.

In the company's studies, about 45 percent of patients who took maraviroc had their HIV virus suppressed to undetectable levels. The same was true for 23 percent who got a placebo. All patients also were treated with a cocktail of the best available drugs and had tried other regimens.

Selzentry's safety and effectiveness have not been established in adults or children who have never been treated with other HIV drugs, the FDA said, adding the drug has not been studied in pregnant women.

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

The New York-based company is counting on new medicines such as 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.

Pfizer shares rose 2.6 percent to close at $24.11 on the New York Stock Exchange.