PrEP Needs a Week to Offer Full Protection; Efficacy Lingers for Several Days

PrEP Needs a Week to Offer Full Protection; Efficacy Lingers for Several Days

When a person starts tenofovir/emtricitabine (Truvada) as a method of HIV prevention, how long does it take for the drug to reach protective levels in the bloodstream or body tissues that could be exposed to HIV? And how long would a person remain protected if he or she stopped taking the drug?

At the 15th International Workshop on Clinical Pharmacology of HIV and Hepatitis Therapy in Washington, D.C., researchers presented findings on how long it took to reach levels that were likely protective. They also looked at how long the drug would remain at this level if daily dosing stopped.

In a report for NATAP on the conference presentation, Mark Mascolini explained that the researchers drew upon findings from iPrEx, the first placebo-controlled trial to show the efficacy of oral tenofovir/emtricitabine as pre-exposure prophylaxis (PrEP) in HIV-negative men who have sex with men (MSM) and transgender women who have sex with men. In iPrEx, investigators had identified the level of drug needed in the peripheral blood mononuclear cells (PBMCs) to reach EC90, meaning that the risk of HIV was 90% lower compared to placebo.

So how long would it take to get to EC90, and how long would it take to drop below that level once PrEP was stopped?

To find out, 21 healthy, HIV-negative volunteers were enrolled in the intensive pharmacokinetic study in which they took tenofovir/emtricitabine for 30 days and then took nothing for another 30 days afterwards. Blood was collected at the time of their first dose and at days 3, 7, 20 and 30 while still on PrEP, and then five, 15 and 30 days post-PrEP. They also underwent a rectal biopsy while they were taking tenofovir/emtricitabine.

Upon analysis, researchers concluded that "high PrEP efficacy and high rectal tissue concentrations can be achieved in MSM after approximately one week of daily dosing," according to the study abstract. "[Data] also suggest that following daily dosing to steady state in MSM, a high level of protection may persist for several days after the last dose taken."

As reported by Mascolini, "The University of Colorado researchers who conducted this study cautioned that the EC90 rests solely on data from MSM and transgender women enrolled in iPrEx. Jonathan Schapiro, a member of the Pharmacology Workshop organizing committee, noted that factors unmeasured in this analysis could contribute to protection and counseled against telling PrEP users that the 7 days to protection and 6-day duration are absolute values that apply to everyone."

The trial included 11 men and 10 women, and no differences were observed by gender.

The U.S. Centers for Disease Control and Prevention (CDC) released its clinical practice guidelines on PrEP just days before the 15th International Workshop on Clinical Pharmacology of HIV and Hepatitis Therapy.

In the PrEP 101 webpage that accompanies the guidelines, the CDC gives this answer to the question of how long a person has to be taking PrEP before it is effective:

Scientists do not yet have an answer on how long it takes PrEP to become fully effective after you start taking it. Some studies suggest that if you take PrEP every day, it reaches its maximum protection in blood at 20 days, in rectal tissue at about 7 days, and in vaginal tissues at about 20 days. Talk to your health care provider about when PrEP might be effective for you.

The guidelines also note that "no data are yet available about intracellular drug concentrations in penile tissues susceptible to HIV infection to inform considerations of protection for male insertive sex partners."

Julie "JD" Davids is the managing editor for and