Pre-Exposure Prophylaxis or PrEP

Pre-exposure prophylaxis (PrEP) involves putting HIV negative people on antiretroviral drugs (ARV) with the aim of protecting them from HIV infection. This blog looks at some of the pros and cons of PrEP.

Thursday, December 9, 2010

Experts More Muted About iPrEx Results Than Media

The iPrEx trial tested truvada as an oral pre-exposure prophylaxis for men who have sex with men (MSM). The results were moderate, showing a 44% efficacy. The result would have been far higher had adherence been higher. But if adherence was something that could be assured, high and consistent rates of condom use would make PrEP irrelevant.

From what I can work out also, the study did not test the partners of those who became infected with HIV during the trial. This makes the claim that those taking the drug were protected from sexual transmission, as opposed to some kind of non-sexual transmission, somewhat tenuous. This question is crucial and is still hanging over the equally hyped CAPRISA 004 trial, which tested the use of 1% tenofovir as a vaginal gel.

As a result of very low adherence to the drugs in the iPrEx trial, it is not possible to claim with any confidence that resistance will not develop where those taking the drug become infected with HIV, but where the infection is not detected in time. Most of those who seroconverted were not following adherence advice. HIV strains resistant to truvada were detected in two participants who entered the trial with HIV infection that was not detected until later.

The issue of how early HIV infection is detected in people taking PrEP is very important. How regular would testing need to be for the threat of resistance to be minimized? Most people never get tested. Some test once in their life. Very few test regularly. Would quarterly, or even yearly testing ever be logistically feasible or affordable?

Resistance is not just a danger for the person taking PrEP; resistant strains of HIV can be transmitted, perhaps even to people taking the same PrEP formulation. Worse still, resistant strains could infect large numbers in certain sexual networks, for example, where MSM are targeted as an especially high risk group.

Whatever about the use of this drug in rich countries, the feasibility of using it in developing countries seems pretty low. Levels of side effects were not high, but that's not so comforting given that adherence was so low. The drug itself is very expensive but the cost of regular testing of millions of people, even the very possibility of such an undertaking, makes it a luxury that few could afford. Dropping the price of the drug will not make the cost of large scale rollout of PrEP any more affordable.

Interestingly, it is reported in the appendix that rates of receptive intercourse dropped sharply in the first 12 weeks and stayed at about half what they were at the start. Use of condoms during receptive intercourse increased to a high level, also during the first 12 weeks, and stayed high for the rest of the trial. Similar patterns of protective behavior were noted in the CAPRISA 004 trial.

These findings suggest that even people thought to be at high risk of contracting HIV are amenable to taking precautions. Sadly, rollout of PrEP is not expected to include rollout of similar levels of support and monitoring found in clinical trials. But most sexually active people in some African countries seem to be at high risk of HIV infection and health facilities are unable to contain the endemic chest and diarrheal conditions that kill so many, let alone HIV.

Dr Joseph Sonnabend has a good critique of iPrEx which is worth reading in its entirety. It seems as if those who want to latch on to anything that can be dressed up as good news are being allowed free rein to do so. But those who treat the issue more thoughtfully, and that includes those involved in the trial, don't seem to be shouting from the rooftops.


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