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said late Wednesday that the Food and Drug Administration has delayed making a final decision on maraviroc, the company's experimental HIV/AIDS drug.

Pfizer received an FDA "approvable letter," which is a regulatory ruling that signals companies must take extra steps to gain final approval for drugs. The company didn't provide details on the FDA's reasons for the letter, but said it continues to talk to the regulators "to address outstanding questions and finalize the product labeling as soon as possible."

Maraviroc is aimed at patients who have developed a resistance to existing medications. In April, a panel of outside medical advisors to the FDA

unanimously recommended that the drug be approved.

These experts also suggested that Pfizer conduct additional tests to assess the drug's effectiveness on a broader population than the drug giant had surveyed. In Pfizer's clinical trials, 90% of patients were male and 83% were Caucasian.

Pfizer says maraviroc would be the

first new class of pills for HIV, or human immunodeficiency virus, in more than 10 years.

Pfizer made its announcement after markets had closed. In regular trading, the stock lost 50 cents, or 1.9%, to close at $25.71.