* Bolsters hopes for new way to slow AIDS pandemic
* Gilead pills cut heterosexual infection 62-73 pct
* Studies tested drugs in Kenya, Uganda, Botswana
* WHO says findings "could have enormous impact" (Adds comment from US CDC, investigators)
By Ben Hirschler
LONDON, July 13 (Reuters) - AIDS drugs designed to treat HIV can also be used to reduce dramatically the risk of infection among heterosexual couples, two studies conducted in Africa showed for the first time on Wednesday.
The findings add to growing evidence that the type of medicines prescribed since the mid-1990s to treat people who are already sick may also hold the key to slowing or even halting the spread of the sexually transmitted disease.
The research involving couples in Kenya, Uganda and Botswana found that daily AIDS drugs reduced infection rates by an average of at least 62 percent when compared with placebo.
"Effective new HIV prevention tools are urgently needed and these studies could have enormous impact in preventing heterosexual transmission," Margaret Chan, director-general of the World Health Organization (WHO), said in a statement.
She said the United Nations health agency would now work with countries to use the new findings to implement better protection strategies.
The U.S. Centers for Disease Control and Prevention plans to review the data and issue its own guidelines -- taking into account issues such as whether behavior may change when people know they are taking a drug that reduces infection risk, said Dr. Jonathan Mermin, CDC director HIV/AIDS prevention.
In its first official guidance on the topic, the CDC said in January that only high-risk gay and bisexual men should use a daily AIDS pill to protect themselves from the virus.
The larger of the two new studies examined 4,758 "discordant" couples in Kenya and Uganda in which one partner was HIV-positive and one was negative. Those negative partners taking Gilead Sciences Inc's tenofovir, or Viread, had on average 62 percent fewer infections.
For couples on Truvada -- another Gilead drug combining tenofovir and emtricitabine -- the infection risk was cut by an estimated 73 percent in the clinical trial, which was led by researchers at the University of Washington.
Dr. Jared Baeten, principal investigator of the study funded by the Bill & Melinda Gates Foundation, said real-life application of the results has not been determined. But Baetennoted that "discordant" couples who want to have children may now have an alternative to condoms.
The second study, involving just over 1,200 sexually active men and women in Botswana, found those on daily Truvada reduced their risk of HIV infection by 62.6 percent.
Lead investigator, Dr. Michael Thigpen, said Truvada proved to be safe and effective. He said the drug -- along with things like male circumcision, topical microbicide gels and condoms -- could be another tool for preventing the spread of HIV.
The idea of such "pre-exposure prophylaxis," known as PrEP, has gained traction in the past year, following results of other research showing a fall in infection rates among gay men taking AIDS drugs.
However, PrEP took a knock earlier this year when another study failed to demonstrate a protective effect in high-risk women. The latest strong evidence is likely to restore confidence in the approach.
Around 33 million people worldwide have the human immunodeficiency virus (HIV) that causes AIDS, most living in Africa and Asia. Only about half know their HIV status, and the WHO hopes that news of an effective approach to prevention will encourage more people to get tested.
Michel Sidibe, head of the UN's program on HIV/AIDS, said the new studies "could help us to reach the tipping point in the HIV epidemic."
The larger study, conducted in Kenya and Uganda, had been scheduled to run until late 2012 but its placebo arm was stopped early because the evidence of efficacy was so strong.
Results of the Botswana study, led by the CDC, had been due to be unveiled next week at an international AIDS congress in Rome but were released ahead of time to coincide with the University of Washington research.
AIDS drugs are available as generics in many poor countries at prices as low as 25 U.S. cents a tablet, according to the WHO. Prices could fall further and supplies increase following an agreement by Gilead, the leading maker of HIV drugs, to share intellectual property rights on its medicines in a new patent pool. The California-based group Tuesday became the first drugmaker to sign up to the Medicines Patent Pool.
Additional reporting by Deena Beasley Reporting by Ben Hirschler; Editing by Will Waterman