HIV-negative gay and bisexual men can lower their likelihood of acquiring the AIDS virus by taking an antiretroviral drug mix, concludes a study in which healthy men received either the medication or a placebo. The finding, published online November 23 in the New England Journal of Medicine, suggests that a preventive strategy might limit HIV spread, scientists say.

"These results represent a major advance in HIV-prevention research," says physician Kevin Fenton of the Centers for Disease Control and Prevention in Atlanta. "For the first time, we have evidence that a daily pill used to treat HIV is partially effective for preventing HIV among gay and bisexual men at high risk of infection." Fenton cautions, however, that the results don't warrant abandoning other proven prevention techniques.

Physician Robert Grant of the University of California, San Francisco and an international team of researchers recruited 2,499 men who have sex with men. Participants lived in the United States, Peru, Ecuador, South Africa, Brazil and Thailand. Half were randomly assigned to get a placebo and half got a similar-looking pill that contained two antiretroviral drugs, emtricitabine and tenofovir.

During a follow-up period that averaged slightly more than one year, 100 of the men became infected with HIV – 64 who took the placebo and 36 who got the drugs. Men assigned to the real drugs and who took them more than 90 percent of the time had the highest rate of protection, the data show.

In trials offering preventive drugs, there is always a risk that some participants' behavior will become more risky because they assume they are protected, says Connie Celum, a physician and epidemiologist at the University of Washington in Seattle who wasn’t involved in the new study. This can be offset by intense counseling and providing health care services, she says. "It helps to tell them they may be on a placebo."

High-risk behaviors actually decreased in both groups after enrollment, the authors note, possibly because participation included free condoms, counseling, regular HIV testing and treatment for other sexually transmitted diseases.

In the fight against HIV, Celum says, "we are enthusiastic about a role for preexposure prophylaxis." She is currently testing the strategy in a trial in East African heterosexual couples in which one spouse is HIV-positive but not the other. The HIV-negative spouses are being randomly assigned to get medication or a placebo, she says, along with counseling and other services, which appear to be indispensable. "No single strategy will have overwhelming efficacy," Celum predicts.

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