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The ‘long tail’ problem: injected-PrEP trial will be extended due to persistence of drug in companion study
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 or start/continue another method of HIV prevention. In addition, 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 a companion study to ECLAIR, HPTN 077, would be extended by a 24 weeks to find out how long measurable drug levels persist. In HPTN 077, cabotegravir is being given either as 800mg injections every twelve weeks, or 600mg every eight weeks, so it may be possible to find out if smaller and more frequent injections may shorten the 'tail'.

This may slightly delay the start of the planned phase III efficacy study, HPTN 083, which is planned to measure the efficacy of cabotegravir in preventing HIV (ECLAIR and HPTN 077, as phase II studies, are designed to measure safety and drug absorption, not efficacy). “We hope HPTN 083 will only be delayed slightly,” Landovitz told delegates, "and we still hope to have it up and running by the end of the year. People enrolled in HPTN 083 are being informed of the ECLAIR findings and being re-consented to allow for any variations being needed in the trial protocol.