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The ‘long tail’ problem: forthcoming injected-PrEP efficacy trial delayed due to persistence of drug in some volunteers
Gus Cairns, 2016-11-11 07:10:00

A study presented at last month’s HIV Research for Prevention (HIVR4P) conference in Chicago shows that in a minority of subjects who were given an experimental injectable drug as HIV pre-exposure prophylaxis (PrEP), the drug was still measurable in their body a full year after their last injection.

In the ECLAIR study, levels of the drug cabotegravir were above the lower limit of quantification in 14 out of 86 participants (16%) a year after their last injection, but below the IC90 – the level that, in treatment, slows HIV replication by 90%.

Obviously if people stop having PrEP injections they become vulnerable to HIV unless they start oral PrEP. In addiiton, though, the ‘long tail’ in some people means that there is a lengthy period during which, if they catch HIV, they could develop drug resistance. Drug resistance only arises in situations like this when there is some drug in the body but not enough to fully suppress an infection. 

Because of this unexpected persistence of cabotegravir, the study’s principal investigator, Raphael Landovitz, in answer to a question told delegates that ECLAIR was being extended by a few months to find out how long measurable drug levels persisted.

This would have the effect of delaying HPTN 083, the phase III study planned to measure the efficacy of cabotegravir in preventing HIV (ECLAIR, a phase II study, was designed to measure safety and drug absorption, not efficacy). “We hope it will only be delayed slightly,” he told delegates.