Experts concur that event-related oral PrEP probably won’t work for women

Six to seven doses a week may be needed for full protection

Two presentations at the recent International AIDS Society Conference on HIV Science (IAS 2017) in Paris told delegates that both trial results and analysis of drug levels supported the idea that event-related, “on demand” pre-exposure prophylaxis (PrEP) might not be sufficiently powerful to prevent HIV infection in women and trans men via vaginal sex.

Infections and adherence in women on PrEP

Professor Bob Grant, principal investigator of the pioneering iPrEx PrEP study in gay men and transgender women, said that although that there was abundant evidence that PrEP did protect cisgender women from infection, it looked like six to seven doses a week, i.e. daily PrEP and good adherence, would be needed to fully protect them.

Grant said that whereas in studies of PrEP in gay men and transgender women there had never been a case of HIV infection in a trial participant who had drug levels consistent with even moderate adherence to PrEP, there were cases in women.

He cited 24 cases of HIV seroconversion in women in PrEP trials where drug levels were measured either shortly before infection, shortly afterwards, or both. In nine cases drug levels either before or after infection were consistent with the women taking four to five doses of PrEP a week. In another three cases levels both before and after infection were consistent with taking four to five pills a week. This is a level that has been associated with almost complete protection in men who have sex with men (MSM). No infection has been seen in women whose drug levels were consistent with their taking six or seven pills a week.



The last part of the large intestine just above the anus.


An umbrella term for people whose gender identity and/or gender expression differs from the sex they were assigned at birth.

deoxyribonucleic acid (DNA)

The material in the nucleus of a cell where genetic information is stored.


Refers to the mouth, for example a medicine taken by mouth.

on demand

In relation to pre-exposure prophylaxis (PrEP), on-demand dosing is an alternative term for event-based dosing. See ‘event based’.

Grant said that the few studies of intermittent or on-demand PrEP found levels of adherence considerably below this, and lower than people's adherence to daily PrEP. The Kenya/Uganda iPrEP study randomised heterosexual serodiscordant couples, female sex workers and MSM to take either daily PrEP or a twice-weekly regular dose plus an immediate post-sex dose if sex happened.

In the heterosexual couples, most of the negative partners took both the daily and the twice-weekly doses correctly, with adherence of 97% and 91% respectively, but they struggled to take the post-sex dose, with adherence of 45%.

In MSM and female sex workers, daily PrEP adherence was adequate (83%, equivalent to nearly six doses a week), but adherence to twice-weekly PrEP was only 55% and to the post-sex dose 26%. “Being away from home” was the most common reason given for missing the post-sex dose, followed by “just forgetting” – as one of the MSM in the study said, people tend to fall asleep after sex.

Another study of intermittent PrEP that included women was the ADAPT study, which has already been covered here. In brief however, the women in the study, who were at its Cape Town site, had just-about-adequate adherence to daily PrEP (75% – just above five pills a week) but only 56% to a twice-weekly regular dose and 52% to ‘event-driven’ PrEP, which was one pill before and one after sex.

Drug levels in animal studies

Angela Kashuba of the University of North Carolina looked at the biological reasons PrEP adherence might need to be higher and more consistent in women. It has been known for some time that intracellular tenofovir levels in the female genital tract after a single dose of tenofovir/emtricitabine are 100 times lower than in rectal tissue, and emtricitabine levels 50 times lower.

Studies in monkeys show that after repeated doses, tenofovir levels in the female genital tract actually reach levels considerably higher than in the rectum – but only by about the eighth day; peak drug levels are reached in rectal tissue after just one or two doses. Emtricitabine levels, on the other hand, never reach a level in vaginal tissues anything as high as in rectal tissues; even after eight days, levels in the vagina are seven times lower than in the rectum.

Kashuba advanced a possible biological mechanism behind this. The nucleoside reverse transcriptase inhibitor (NRTI) drugs like tenofovir and emtricitabine work by replacing the bases – the individual ‘letters’ in the DNA of HIV – with ‘fake bases’ that act as stop commands for further lengthening of the DNA chain. However, they have to compete with the real bases circulating in the blood and present in the cell.

Kashuba said that in vaginal tissue, levels of two natural bases, deoxyadenosine triphosphate (dATP), and deoxycytidine triphosphate (dCTP) are 50-100 times higher than they are in rectal tissue. In terms of the ratio of dATP to the PrEP drug, this is the same in both rectal and vaginal tissues, with 1.8  times more emtricitabine than dATP.

However, the ratio of tenofovir to dATP is about 350 times higher in rectal than vaginal tissues; tenofovir is 17 times higher than dATP in rectal tissues, but 20 times lower in vaginal. Drug levels of tenofovir and emtricitabine are both related to dATP and dCTP levels; the higher the natural base level, the lower the drug levels.

What this means is that optimal drug levels (i.e. ones that should guarantee 100% efficacy in preventing infection) both take longer to reach in the female genital tract and that some women never develop them. Optimal rectal concentrations of tenofovir plus emtricitabine, taken together, could probably be reached in 100% of people by the second dose of Truvada whereas it takes seven doses of Truvada for 85% of women to reach optimal concentrations in vaginal tissue – some never do.

Furthermore, inside the cell, the optimal levels of intracellular tenofovir remain in rectal tissues for at least five days after the last dose of PrEP, and are still at 90% of optimum after ten days. In the vagina on the other hand, intracellular tenofovir levels start falling immediately and are at 85% of optimum after three days (emtricitabine levels fall much more quickly, within hours).


What does this imply for PrEP in women?

It shows that there is no reason oral PrEP should not work well in women having vaginal sex, but only as long as they take PrEP daily and have pretty good adherence of 6-7 pills a week. It means that women may need to start taking PrEP several days in advance of possible HIV exposure, though nothing like as long as the three weeks or so suggested by some recommendations.

It also implies that, for extra reassurance, women may want to combine oral PrEP with other prevention methods both old (condoms) and new (vaginal rings, etc).

It also implies that more research needs to be done into women and PrEP – the dATP and dCTP levels, for instance, imply that inflammatory responses and gene activation might influence drug levels in the female genital tract, and thought needs to be given to drugs that could work in other sites such as the lymph nodes.


Grant RM. Review of clinical studies of intermittent PrEP. 9th International AIDS Society Conference on HIV Science, Paris, symposium presentation MOSY0804, July 2017.

Download the presentation slides from the conference website.

Kashuba ADM. Does pharmacology support on demand PrEP? 9th International AIDS Society Conference on HIV Science, Paris, symposium presentation MOSY0803, July 2017.

Download the presentation slides from the conference website.

Watch the webcast of this session on YouTube.