Detectable amounts of the HIV pre-exposure prophylaxis (PrEP) drugs tenofovir disproxyl fumarate in the plasma of individuals who took the medication was strongly associated with increased protection from HIV acquisition, results from the phase 3 Partners PrEP study indicate.

“Most of those who took pre-exposure prophylaxis appeared to have high and consistent adherence,” Deborah Donnell, PhD, of the Fred Hutchinson Cancer Research Center and the University of Washington, and colleagues wrote.

The researchers randomly assigned the uninfected partner from 4,758 HIV serodiscordant couples to once-daily tenofovir disproxyl fumarate (TDF) alone, TDF with emtricitabine (FTC/TDF) or matching placebo.

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Participants were followed monthly for safety and to monitor for HIV seroconversions for up to 36 months. Blood samples were stored at months one and three, and quarterly thereafter, and at any visit in which a participant tested positive for HIV.

Based on blood concentrations of >40 ng/mL, the estimated protective effect was 91% among patients assigned to the FTC/TDF combination (95% CI: 47%-98%; P=0.008), compared with 88% (95% CI: 60%-96%; P<0.001) among those assigned to TDF alone.

“This high level of protection was attributed to consistently high drug concentrations from the majority of participants throughout follow-up,” the researchers wrote.

Eighty-nine percent of controls had PrEP concentrations greater than 40 ng/mL at the one-month follow-up; 75% maintained the concentration at one year. Additionally, 96% of controls and 65% of cases had greater than 80% treatment adherence during these same follow-up visits.

Older age and shorter time on study were associated with tenofovir levels greater than 40ng/mL, according to the researchers.


  1. Donnell D. J Acquir Immune Defic Syndr. 2014; doi:10.1097/QAI.0000000000000172.


See full study for full list of researchers’ relevant financial disclosures.