Spotlight Center on HIV Prevention Today

PrEP Adherence Issues Kick Off CROI 2014

March 3, 2014

PrEP Adherence Issues Kick Off CROI 2014

What do people need to be able to adhere to daily oral PrEP? And how can scientists understand, measure, and support adherence in clinical trials?

These questions were at the heart of a workshop that kicked off Day 1 of the 21st Conference on Retroviruses and Opportunistic Infections.

David Bangsberg of Massachusetts General Hospital Center for Global Health described the wildly different estimates of "efficacy," or how well PrEP worked in a trial's controlled setting, among the various studies of daily oral PrEP.

"It's striking that, depending upon the study, we could get ranges of efficacy estimates from 'nothing' to 'quite good,'" Bangsberg said. (Estimates of efficacy address how well a product works in a clinical trial; effectiveness, on the other hand, refers to how well the product works in "real world" settings.)

Differences in population, HIV exposure routes (i.e., anal sex, vaginal sex, and injecting drugs), biological factors, and rates of participant drop-out are potential sources for the "heterogeneity," or diversity, in estimates of PrEP efficacy, Bangsberg noted. However, he added, "as the dust has settled on interpreting these trials, I think the most likely explanation is that this wide heterogeneity in efficacy estimates is due to differences in adherence between these studies."

Support for this idea comes from blood sample data: The lower the proportion of trial participants with detectable levels of the study drug in their blood, the lower the efficacy estimate for the trial. For example, in the VOICE study of PrEP for women, analysis of blood samples from 773 participants revealed that 58% of those in the oral tenofovir (Viread) arm had no detectable tenofovir in any sample. The oral tenofovir arm of the trial was halted early due to lack of ability to demonstrate efficacy.

This excerpt was cross-posted with the permission of Read the full article.

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