*Text courtesy Reuters

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(Reuters) - Two large clinical trials in Africa are ramping up to test the effectiveness of a vaginal ring that releases an HIV-fighting drug for a month or more, offering women at high risk a discreet way to protect themselves from the virus that causes AIDS.

The studies will test the effectiveness of a vaginal ring containing the antiretroviral drug dapivirine in thousands of women in several African countries to evaluate its ability to prevent new HIV infections and its long-term safety.

If effective, the ring will add "a long-acting, female-initiated technology to the existing toolkit of HIV prevention options," said Dr. Zeda Rosenberg, chief executive officer of International Partnership for Microbicides (IPM), a nonprofit group founded by Rosenberg which is developing the ring.

Because it only needs to be replaced once a month, the ring may help address some of the problems with getting women to consistently use vaginal gels each time they have sex, Rosenberg said during a briefing at the International AIDS Conference in Washington.

Irregular use is thought to be the reason a large study of the microbicidal gel containing the anti-HIV drug tenofovir failed to prevent infections in women in sub-Saharan Africa.

IPM has a royalty-free licensing agreement with Johnson & Johnson's Janssen unit in Ireland to use its dapivirine antiretroviral product in gel and ring forms to prevent HIV infections in low and middle income countries.

Dapivirine is part of a class of antiretroviral drugs that have long been used to treat HIV and prevent mother-to-child transmission of the virus.

NIH PARTNERSHIP

The IPM study will enroll 1,650 women aged 18 to 45, who will be randomly assigned to use the ring or a placebo in four sites in South Africa, with plans to expand to sites in Rwanda and Malawi.

It is being conducted in partnership with the U.S. National Institutes of Health-backed Microbicide Trials Network, which just started enrolling women in a separate trial called ASPIRE.

"Developing scientifically proven forms of HIV prevention that women can control is essential," said Dr. Anthony Fauci, director of the NIH's National Institute of Allergy and Infectious Disease.

"Because the vaginal ring is a long-acting intervention, it has a potential added benefit in that women may find it relatively easy to use."

The ASPIRE study will test the ring in 3,476 women aged 18 to 45 in Malawi, South Africa, Uganda, Zambia and Zimbabwe.

Women in the studies will be offered condoms and counseling on HIV prevention, and taught how to insert the vaginal ring. At monthly visits, researchers will keep track of whether women are still using the ring and give them a replacement.

Those women who become pregnant during the study will discontinue use of the ring, and their safety and that of their child will continue to be monitored.

Dr. Saidi Kapiga of the London School of Hygiene and Tropical Medicine, who is coordinating the ring study in Africa and has also conducted tests of vaginal gels for HIV protection, said there are already signs that women prefer the new option.

"It is acceptable," Kapiga told the briefing. "The fact that they use it only once in four weeks was a major advantage."

Both trials are designed to detect at least a 60 percent reduction in HIV risk, but researchers said they hope for even better results, which are expected in 2015.

"If proven to be effective, I think this will really revolutionize prevention for women," Dr. Sharon Hillier, who heads the Microbicide Trials Network at the University of Pittsburgh School of Medicine, told the briefing.

(This story corrects Rosenberg's first name to Zeda in paragraph 3 and number of women in study to 1650 in paragraph 8)

(Reporting by Julie Steenhuysen; Editing by Michele Gershberg and Eric Walsh)