July 22, 2015
Written by Gus Cairns
This post originally appeared on NAM’s website.
The pharmacodynamics (drug absorption and elimination rates) of the two drugs that comprise Truvada may favour the efficacy of intermittent pre-exposure prophylaxis (PrEP) more than previously thought, at least when it comes to protection from transmission via anal sex, the Eighth International AIDS Society Conference on HIV Pathogenesis, Treatment and Prevention (IAS 2015) heard on Monday.
A new study, HPTN 067 or ADAPT, received a lot of attention at the conference and one of its main conclusions was that its participants found it easier to adhere to PrEP if it was taken daily than when it was taken every few days or only when sex was anticipated.
Despite this, we have evidence from the only randomised controlled trial of intermittent PrEP so far conducted, Ipergay, that this way of taking PrEP can be highly effective. In Ipergay, a two-pills-before-sex-plus-two-pills-after regimen of intermittent PrEP proved to be just as effective at preventing HIV – stopping 86% of potential infections – as the daily regimen in a near-simultaneous study of daily PrEP, PROUD. The question delegates were asking at IAS this week was: Why did Ipergay work?
Adherence in Ipergay was good, but not spectacular: 81% of participants said in self-interview that they used PrEP the last time they had sex, but only 53% said they took all doses as required. Detectable plasma levels of tenofovir, the longer-lasting of the two drugs in Truvada, were found in samples from 82 to 91% of participants in their two-monthly visits. Tenofovir levels indicated that 80 to 85% of Ipergay participants had taken PrEP at least once in the week before a sample was taken.
Was this enough to protect? Jean-Michel Molina, Ipergay’s principal investigator, said that in previous studies, 96% protection from HIV was reached after just three doses of Truvada PrEP and 99% after five doses. Adequate concentrations in blood of both of the drugs in Truvada, tenofovir and emtricitabine, were reached as soon as two hours after taking a pill but that while similar levels of emtricitabine were reached in rectal secretions in the same timespan, tenofovir did not become detectable in rectal secretions until 24 hours after the first dose. This may mean, Molina commented, that emtricitabine offers the sole protection against HIV very early on after a first dose.