Gay and bisexual men and transwomen who take their pre-exposure prophylaxis (PrEP) medications as few as four times per week may still have strong protection against becoming infected with HIV, according to a study published in the September 12 issue of Science Translational Medicine.
The published analysis, led by Peter Anderson, from the University of Colorado in Denver, actually took data from two unrelated studies. The first was the iPrEx randomized placebo controlled PrEP study in men who have sex with men (MSM) and transgender women, which found that the antiretroviral drug Truvada (tenofovir disoproxil fumarate and emtricitabine) could cut the risk of HIV infection overall by 44%. When drug levels were compared between those who became infected and those who did not, researchers found that having detectable levels of Truvada decreased HIV risk even further—by 90%.
The second study, STRAND, was designed to measure drug concentrations in both blood and peripheral blood mononuclear cells (PBMCs) from various Truvada dosing strategies. When Anderson and his colleagues over-laid the STRAND results with the portion of the iPrEX participants in whom drug levels were measured, they were able to generate new estimates for how protective three dosing schemes would be: two, four and seven doses per week.
What Anderson’s team found was that drug concentration levels in PBMCs from twice weekly dosing was associated with a 76 percent reduction in HIV risk, while four times per week generated a reduction of 96 percent, and perfect adherence (every day) was associated with a 99 percent decreased risk of infection.
The results do have to be interpreted with caution. First, tenofovir (part of Truvada) concentrates heavily in the rectal tissue. This means that these estimates of protection from various dosing schemes are most applicable to people whose primary risk activity is unprotected anal sex. We can’t extrapolate the results for the risk of receptive vaginal sex, insertive vaginal or anal sex or risk through the sharing of injection equipment.
Secondly, these results don’t automatically mean that “disco” dosing strategies, where a person simply takes Truvada during a weekend of partying will be effective. In fact, until the IPERGAY study is complete, which is testing various intermittent dosing strategies, we can’t recommend that people do anything other than attempt to take their Truvada daily for PrEP.
What the results do indicate, however, is that perfect adherence may not be the necessity that some of PrEP’s strongest critics have asserted.