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CROI 2012 Seattle: Prevention Study Results Yield More Good News

By Enid Vázquez

May/June 2012

After two years of landmark studies, HIV prevention continued to dominate research news at CROI.

The Partners PrEP study enrolled nearly 5,000 heterosexual couples where one partner had HIV and the other didn't. "PrEP" stands for pre-exposure prophylaxis (prevention), and is taken by HIV-negative people to prevent sexual transmission of HIV.

Final primary results from Partners PrEP showed a 67% reduction in risk of HIV infection with Viread and a 75% reduction with Truvada (considered not to be a statistically significant difference). The two HIV medications were compared against placebo (fake pill) in couples in Kenya and Uganda.

"They worked approximately the same," said presenter Jared Baeten, M.D., M.P.H., of the University of Washington in Seattle, in a press conference. "This was definitive protection for people at high risk of HIV because of a known HIV-positive partner." He added that the results were consistent no matter the viral load in the positive partner. For the HIV-negative partners who took the prevention medications, side effects included mild fatigue and nausea.

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Asked about treatment as prevention (TasP), in which successful anti-HIV treatment of a positive partner has been shown to greatly reduce transmission of HIV to a negative partner, Baeten said a need for PrEP remains. Some HIV-positive individuals prefer to delay therapy, he said, but at the same time, heterosexual couples are concerned about conceiving children. Still, "we've been very aggressive about making sure infected partners are treated when eligible." In some resource-poor countries, HIV-positive people are not eligible for government-sponsored antiviral treatment until their medical condition meets certain requirements, such as a CD4+ T-cell count under 200. "We can use PrEP as a bridge until a partner starts treatment," he said.

Of the 27 individuals who seroconverted in Partners PrEP, none developed drug resistance to Viread or Truvada, the drugs studied for PrEP. The development of drug resistance is a concern for individuals taking PrEP who become infected because HIV treatment is heavily dependent on the use of Viread or Truvada (in wealthier countries, at least). But more and more, prevention studies with HIV medications are showing that where they fail, it's because the drugs are not being taken as prescribed. A person's virus can develop drug resistance when medications are being taken inconsistently (missing doses).

Deborah Donnell, Ph.D., of the University of Washington and the Fred Hutchinson Cancer Research Center in Seattle, illustrated this issue with a Partners PrEP sub-study showing that only nine of the 27 infected individuals had any tenofovir in their blood at all at the time of seroconversion. (Tenofovir is the generic name of Viread, and it is also found in Truvada.) "Tenofovir detected in blood was highly correlated with protection from HIV infection," she said. "I think our results are clearly proof of concept that daily use of tenofovir can substantially reduce the risk of HIV infection."

Lut Van Damme, M.D., of Family Health International, illustrated the same concept with the FEM-PrEP study, which disappointed HIV advocates when a daily Truvada pill failed to prevent HIV in women. However, Van Damme reported that the medication was found in blood levels of less than half of the 33 infected women who had been given it, as well as in less than half of the uninfected women in a matched control group. According to the FEM-PrEP abstract, studies will need to focus on what determines adherence to preventative medicine in people at high risk of infection. Also of importance in this study was that the infected women perceived themselves to be at "little or no risk of HIV infection." They were actually at high risk of infection and didn't know it.

There was more welcome data on the potential problem of resistance. The CAPRISA 004 research team reported no resistance in either the blood or the genital tracts of women who became infected during the study. "This is a good news result," said Will Fischer of the Los Alamos National Laboratory. "There was only one case of high-level resistance and this was from a woman on placebo, so it was presumably a transmitted resistance [she picked up a drug-resistant virus from a sex partner]." CAPRISA 004 used a tenofovir gel applied vaginally.

"I guess this decade will be remembered as the tenofovir decade," said Linda-Gail Bekker of the University of Capetown, while co-facilitating a session on the use of HIV drugs for the prevention of sexually transmitted HIV. She noted that her research team has used tenofovir in all of its prevention studies.

Other encouraging tenofovir data: a small study from the Microbicide Trials Network with a re-formulated tenofovir gel was found to be safe and acceptable as a rectal microbicide used daily for seven days. A previous formulation was safe but bothersome. The new reduced-glycerin formulation of 1% tenofovir gel is moving into a Phase 2, eight-week study.

Tenofovir and Truvada are ahead of the game now, but other HIV antivirals are also being tested for prevention. A small, early study with the new non-nucleoside rilpivirine (Edurant) was shown to penetrate vaginal and rectal tissue and fluids in an amount needed to inhibit HIV infection. Rilpivirine was used in a long-acting, injectable form in this research (used intramuscularly). The use of the HIV entry inhibitor drug Selzentry (maraviroc) is also being explored with a vaginal ring that releases maraviroc into the genital tract. The hope is that such medications will help counter the adherence problems found with prevention research to date.

Said Susan Buchbinder, of the San Francisco Department of Public Health and the University of California, San Francisco, "Lots of data was presented this morning -- [showing that] HIV infections occur during periods of low blood exposure." See her talk on the possibilities for intermittent PrEP (not taken daily) in the Wednesday symposium "Next Steps in Using ARV for Prevention" at www.retroconference.org, along with a talk from Baeten, "What Can the Twisted Tale of PrEP Results Teach Us?" and two other presentations.


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