via, by Liz Highleyman

The 2 drugs in the Truvada pill — tenofovir and emtricitabine — reach different concentrations in human cervical, vaginal, and rectal mucosa tissues and fluids, according to new research published in the December 7, 2011, issue of Science Translational Medicine. Lower drug levels in the female genital tract suggest that women may need higher doses to achieve a prophylactic effect, which may help explain conflicting results from some recent biomedical HIV prevention trials.

A series of large trials over the past 2 years have produced mixed findings about the benefits and risks of pre-exposure prophylaxis (PrEP), or use of antiretroviral drugs by HIV negative people in an effort to prevent infection.

The iPrEx study of gay and bisexual men in several countries and the TDF2 trial of heterosexual women and men in Botswana both showed that daily oral tenofovir/emtricitabine dramatically reduced the risk of HIV infection when given along with risk-reduction counseling, free condoms, and other prevention services.

The Partners PrEP trial of serodiscordant heterosexual couples found that both daily tenofovir/emtricitabine and oral tenofovir alone reduced the risk of infection, by 73% and 62%, respectively.

In contrast, the FEM-PrEP trial did not find a prevention benefit of daily oral tenofovir/emtricitabine for heterosexual women in Kenya, South Africa, and Tanzania; that trial was halted this past April after an interim review showed a similar number of new HIV infections in the tenofovir/emtricitabine and placebo arms.

Most recently, the VOICE trial, looking at women in South Africa, Uganda, and Zimbabwe, halted its oral tenofovir monotherapy arm in September after an interim analysis found that the study could not demonstrate that it was more effective than placebo. But another study arm testing tenofovir/emtricitabine was  allowed to continue, suggesting the combination performed better in the interim analysis.

The reasons for these conflicting results are not clear, but researchers have noted that, overall, tenofovir-based PrEP appears to work somewhat better for men than for women, leading some to speculate that the drugs may not behave the same at different anatomical sites.

Kristine Patterson and Myron Cohen from the University of North Carolina Chapel Hill and colleagues designed a study to look at pharmacological properties of tenofovir and emtricitabine in genital and colon-rectal mucosal tissue from 15 healthy HIV negative volunteers, 8 men and 7 women.

Cohen was the principle investigator of the HPTN 052 study — presented to much fanfare at the International AIDS Society conference this summer in Rome — which showed that if the HIV positive partner in a serodiscordant couple started immediate ART upon diagnosis regardless of CD4 cell count, the risk of HIV transmission was reduced by 96%; HPTN was mostly conducted in low- and middle-income countries, however, and use of tenofovir/emtricitabine was uncommon (10% overall, but zero at several sites).

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