WASHINGTON (Reuters) - The quest for a cream or gel to prevent AIDS infection has narrowed to using powerful HIV pills that are already on the market, scientists say.

AIDS experts have long been searching for a microbicide -- a cream, gel or vaginal ring that women or men could use as a chemical shield to protect themselves from sexual transmission of the deadly and incurable virus.

Several substances have been tried unsuccessfully but experiments presented this week at the Conference on Retroviruses and Opportunistic Infections, a scientific meeting of AIDS experts, suggested HIV drugs might hold the key to making such gels work.

"The next wave of compounds is all going to be based on antiretroviral drugs," Dr. John Moore of Weill Cornell Medical College in New York told reporters.

Moore's team tested Pfizer's new drug maraviroc, sold under the brand name Selzentry. It is in a new class of drugs called CCR5 entry inhibitors, designed to stop the human immunodeficiency virus from getting into human cells using a type of cellular doorway or receptor named CCR5.

"The CCR5 inhibitors are compelling candidates as an alternative because these drugs are not being used for treatment in, for example, Africa," Moore said.

That means there is less risk of resistance developing -- when viruses evolve to get around the effects of drugs.

Moore's team took a unique approach to formulating their experimental microbicide using Selzentry.

"We found a friendly physician, scrounged a tablet, ground it up," Moore said. "I assure you it actually works very well," he told the San Francisco meeting.

Tests in monkeys showed it would protect a female from sexual transmission for about four hours. "You couldn't apply these gels in the morning and have protection in the evening," Moore said.

A vaginal ring with a time-release formula may work better for longer-term protection, Moore said.

The approach is affordable, he said. "A single maraviroc tablet, about 300 mg, retailing for about $15 on the Internet, contains enough drug to fully protect around 15 macaques. That is broadly going to be applicable to women."

Laura Guay of the Elizabeth Glaser Pediatric AIDS Foundation said the approach sounds reasonable. Her group supports the development of microbicides to protect women and by extension their children.

"The hope is by putting antiretrovirals into the microbicide, you can prevent the virus from either entering or replicating," she said in a telephone interview.

Last year researchers found Gilead Sciences Inc.'s drug Truvada also might work as a microbicide. But a gel made by Massachusetts-based Indevus Pharmaceuticals that did not include an HIV drug failed in human trials.

The AIDS virus, which infects 33 million people globally and has killed 25 million, is mostly passed sexually. In Africa women account for more new cases than men and are often infected by their husbands.

Abstinence and condom use are not options for women trying to have children, but a microbicide would be. Microbicides using HIV drugs would represent a large new market for the companies that make the drugs, which are currently now used only to treat infection.

(Editing by Alan Elsner)