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Taking PrEP Does Not Increase Drug Resistance Risk

By Warren Tong

May 30, 2014

With the recent U.S. Centers for Disease Control and Prevention's guidelines recommending pre-exposure prophylaxis (PrEP) for individuals in groups at high risk for HIV, there has been some concern over the possibility of developing drug resistance if one were to seroconvert while on PrEP. This risk of developing resistance does not appear to be greater for those on PrEP who may not be adhering properly, according to iPrEx study results published in the Journal of Infectious Diseases.

The iPrEx study followed about 2,500 individuals and found an average 44% risk reduction among gay and transgender women taking tenofovir/emtricitabine (Truvada) as PrEP versus a placebo. Moreover, among those who had detectable levels of medicine in their blood (meaning they were adhering consistently), the risk was reduced by as much as 92%.

The researchers wanted to investigate whether poor adherence to PrEP could lead to resistance to emtricitabine or tenofovir, both of which are part of first-line HIV regimens. They used phenotypic and genotypic resistance assays to test for drug-resistant virus in people who had seroconverted while enrolled in the study. reported:

There were 141 HIV infections. These included ten patients with unrecognised acute HIV infection at the time of randomisation. Two of these patients were in the treatment arm. Both developed resistance to FTC.

There were 131 post-randomisation seroconversions, 48 of which involved patients taking FTC/tenofovir.

None of the patients infected post randomisation had FTC- or tenofovir-selected resistance mutations or reduced phenotypic susceptibility to these drugs.

Two of the patients who seroconverted while in the tenofovir/emtricitabine arm had minor variant emtricitabine-associated resistance, though both below 1%. Therefore, the authors concluded that, "Drug resistance was rare in iPrEx on-study FTC/TDF-randomized seroconverters, and only as low-frequency minor variants," further noting that poor adherence did not result in drug resistance.

Warren Tong is the research editor for and

Follow Warren on Twitter: @WarrenAtTheBody.

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