More daily doses of Truvada (tenofovir/emtricitabine) per week are required for protection against vaginal exposure to HIV than for protection against anal exposure. This finding may help explain why, despite similar levels of adherence in clinical trials, women have not received as much protection against HIV from pre-exposure prophylaxis (PrEP) as men who have sex with men (MSM) have.

Publishing their findings in the Journal of Infectious Diseases, researchers studied human cells in test tubes to measure particular DNA materials in the cells, as well as to determine how much Truvada was required to protect the cells against HIV. They also studied Truvada levels in vaginal, cervical and rectal tissues in 47 HIV-negative women given the drug, and also studied the level of the key DNA materials in those cells.

Looking at both the test tube and the human results, the researchers created a mathematical model to predict the Truvada-to-DNA ratios in vaginal, cervical and rectal tissues. This model helped them calculate how many doses per week of Truvada were required to achieve protection against HIV in each cell type.

Protecting the female genital tract against HIV requires taking at least six daily tablets of Truvada per week, the researchers estimated. Protecting rectal cells, meanwhile, requires only two daily tablets per week. (Previous research has estimated that taking two to three tablets of Truvada per week confers some protection against rectal exposure to HIV, while taking four or more per week confers maximum protection.)

To read a press release about the study, click here.

To read the study abstract, click here.