People taking either daily or intermittent pre-exposure prophylaxis (PrEP) to prevent HIV infection had few side effects and relatively good adherence to these regimens, according to data presented Monday, July 19, at the XVIII International AIDS Conference (IAC) being held July 18 to 23 in Vienna.

Experts hold great hope for PrEP as an HIV prevention strategy. Currently a number of PrEP trials are ongoing, testing either Viread (tenofovir) or Truvada (tenofovir plus emtricitabine) in a variety of dosing strategies. Basic safety and adherence data are now beginning to roll in, with efficacy data expected by the end of 2010.

At the IAC, researchers in Kenya and the United States offered preliminary results from a trial that is comparing either daily Truvada PrEP or intermittent Truvada PrEP with a placebo. PrEP is being studies in three risk-groups: men who have sex with men (MSM), female sex workers (FSW) and in HIV-negative heterosexuals with an HIV-positive partner (serodiscordant couples). Thus far, early data are available on 67 MSM, 5 FSW and 32 serodiscordant couples.

Those taking either Truvada or a placebo intermittently were asked to take their study drugs on Mondays and Fridays and within two hours after having insertive sex. Adherence was measured using both self-reporting and an electronic memory cap monitor on pill bottles.

Based on these preliminary data, both PrEP strategies appear to be safe and well tolerated. All side effects reported so far have been classified as mild to moderate, and nearly all were judged by researchers as not likely related to Truvada. There have also been no cases of kidney problems, which can occur with Truvada.

Adherence was very good with daily PrEP use, and moderately good with intermittent use. Adherence rates among daily PrEP users was 84 percent among MSM and FSWs, and 100 percent among the HIV-negative partner in serodiscordant couples. Rates in those taking PrEP intermittently varied, depending on the dose. Among MSM and FSWs, 66 percent took their Monday and Friday doses correctly, but only 25 percent were able to take their post-sex dose. In the serodiscordant couples, the Monday and Friday doses were taken correctly 91 percent of the time, but the post-sex dose was taken correctly only 50 percent of the time. This is concerning, as monkey studies have revealed that the post-sex dose in intermittent strategies is very important in preventing transmission.

The number of people included in this analysis is quite small, and the data are preliminary, so it isn't yet possible to draw firm conclusions. Nevertheless, the authors conclude that, “These preliminary, blinded data suggest that intermittent and daily [Truvada] have similarly acceptable safety profiles.”

“Adherence to fixed doses was comparable to daily dosing,” they continue. “However, post-coital dosing may be challenging in these settings.”