New York - Pfizer halted a late-stage study of its second-best selling drug, Lyrica, as a possible treatment for a nerve pain in HIV patients.

The world's largest drugmaker said Friday it was studying the drug in patients with HIV neuropathy, which is nerve damage characterized by burning pain that usually starts in the feet. An early analysis of study data showed that pain symptom improvements were nearly identical to those patients treated with a placebo.

No safety concerns were raised in the study, and Pfizer said it will continue to analyze the results.

Lyrica is already approved in the United States to treat diabetic nerve pain, pain after shingles, fibromyalgia and for some seizures in adults with epilepsy. It brought in $955 million in sales in the first quarter for the New York drugmaker, behind only cholesterol fighter Lipitor's $1.4 billion.

Pfizer lost the U.S. patent protecting Lipitor last fall, and Lyrica could overtake the drug to become Pfizer's top seller, especially after several generic drugmakers enter the market at the end of this month with their own versions of Lipitor.

Lyrica's revenue increased 16 percent in the recently completed first quarter, while Lipitor sales plunged 42 percent.

Shares of Pfizer Inc. fell 23 cents to $22.38 in afternoon trading while the Standard and Poor's 500 index dropped more than 1 percent.