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The New York drug maker is expected to announce today that it will license its new medicine, Selzentry, to a nonprofit that investigates ways to turn HIV medicines for infected patients into vaginal substances to prevent transmission to women during sex. The partnership offers a low-risk way for Pfizer to find out if the medicine could become a frequently taken drug, while potentially offering an empowering concept to women in the developing world.

HIV preventives have proven elusive, with researchers and advocates still recovering from last year's collapse of Merck & Co.'s once-promising vaccine trial. And Pfizer's new venture with the International Partnership for Microbicides is a long shot that relies on an unproven theory. But with some 33 million people infected with HIV, the virus that causes AIDS, the enormous health and financial stakes continue to drive the hunt for treatments.

Pfizer's drug was approved last year for patients who have undergone other HIV treatment. Pfizer is now giving the IPM a license to try to turn the medicine into a vaginal gel, ring or film that might prevent transmission.

The Pfizer drug already has a safety portfolio approved by the Food and Drug Administration, potentially making it easier to get through testing in a new form. And the way Selzentry works, by blocking the virus from infecting healthy cells, could make it more appropriate for prevention than medicines that prevent already-diseased cells from replicating.

Jack Watters, Pfizer's vice president for external medical affairs, is hopeful about Selzentry's prospects as a preventive therapy, but he says "we're a long way from proving that."

Some drugs are used to both prevent and treat diseases such as malaria and tuberculosis, and there has been hope that existing HIV medicines can somehow do the same. The topical use is preferred to daily pills, because less medicine gets into the bloodstream, potentially making it safer for long-term use, and it can be concentrated where the disease enters the body, says Zeda Rosenberg, IPM's chief executive.

Selzentry could be particularly well-suited for HIV prevention. Because the drug interrupts the virus's ability to penetrate a healthy cell at an entry point called CCR5, it is only approved for patients who have one strain of the disease. But that receptor is the primary one responsible for HIV transmission, says John Mellors, chief of infectious diseases at the University of Pittsburgh's medical school, who has done consulting for Merck & Co., Abbott Laboratories, Gilead Sciences Inc. and the IPM.

But the microbicides' promise is clouded. Last year, one of the most advanced microbicide trials was halted when early data indicated that more women became infected with HIV than those using a placebo.