Two African studies of pre-exposure prophylaxis (PrEP) provide additional evidence that antiretroviral (ARV) drugs can be used to prevent HIV transmission, according to early results from both clinical trials, released ahead of their full presentations at the 6th International AIDS Society Conference of HIV Pathogenesis, Treatment and Prevention next week in Rome.

Unlike the encouraging results from the iPrEx study, reported in November 2010 and enrolled men who have sex with men and transsexual women, the Partners PrEP study and TDF2 study focused on heterosexual men and women at risk of HIV Infection. 

“This is an extremely exciting finding for the field of HIV prevention,” said Jared Baeten, MD, of the University of Washington and a lead investigators of the Partners PrEP study, conducted in Kenya and Uganda. “Now, more than ever, the priority for HIV prevention research must be on how to deliver successful prevention strategies, like PrEP, to populations in greatest need.” 

“Our biggest challenge now is how do we move from research to getting things out to the general public where they’re most needed,” said Lynn Paxton, PhD, of the U.S. Centers of Disease Control and Prevention (CDC), according to the Washington Post. Paxton was a lead investigator of the TDF2 clinical trial, conducted in Francistown and Gaborne, Botswana.

The Partners PrEP study in enrolled 4,758 heterosexual couples in which one partner was HIV-positive and the other HIV-negative. The study was discontinued 18 months early, in light of an interim review suggesting favorable results.

The trial showed that both tenofovir (sold as Viread) and tenofovir plus emtricitabine (sold as Truvada), taken once daily by the uninfected partner, reduced the risk of HIV transmission by 62 percent and 73 percent, respectively, when compared to a placebo. The reported risk reductions were highly statistically significant, meaning they were too great to have occurred by chance, and both drugs were effective in both men and women.

A total of 78 infection were reported by the researchers: 18 in the Viread group, 13 in the Truvada group and 47 in the placebo group. The relatively low number of infections all three groups, given the size of the study, can partly be explained by the provision of HIV transmission counseling and condoms provided to all clinical trial participants.  

The study authors also noted remarkably high adherence rates. More than 97 percent of the prescribed doses were taken correctly and 95 percent of those who entered the study remained in the trial.

No significant safety problems were reported by the researchers.

The TDF2 study enrolled roughly 1,200 sexually active men and women between the ages of 18 and 39. While it was not designed as an efficacy trial—the goal was to look for potential safety problems—the researchers announced that Truvada reduced the risk of HIV infection in both men and women by nearly 63 percent, compared with those who received the placebo.

Interestingly, the results of the Partners PrEP study and TDF2 contrast those of the FEM-PReP study, which failed to demonstrate that Truvada was an effective PrEP strategy among at-risk women in Kenya, Tanzania and South Africa.

Complete results from the Partners PreP and TDF2 studies are to be presented in Rome and will likely be discussed in the context of similar studies, notably iPrEx and FEM-PREP.