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Why This Once-Promising HIV Candidate Could Be in Trouble

By Keith Speights and Brian Orelli, PhD – Dec 17, 2021 at 2:04AM

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It's too soon to know for sure what might happen with Merck's islatravir.

Merck (MRK 0.55%) recently paused enrollment for late-stage clinical studies evaluating islatravir as potential pre-exposure prophylaxis for HIV. In this Motley Fool Live video recorded on Dec. 8, Motley Fool contributors Keith Speights and Brian Orelli discuss whether this once-promising HIV candidate could be in trouble.

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Keith Speights: Speaking of Merck, there's more news for the big drugmaker. Last week, Merck paused the development of its experimental HIV therapy, MK-8507. This week, Merck has come back and announced that its pausing enrollment in phase 3 studies of islatravir for HIV pre-exposure prophylaxis. Is this once-promising HIV candidate in trouble, do you think?

Brian Orelli: Yes. Participants in this study are still getting the medication. Although positive enrollment on the patients there are participants that were enrolled are still getting your medication. [Editor's note: The U.S. Food and Drug Administration placed a clinical hold on the studies of islatravir after this video was recorded. Participants are not being dosed while the clinical hold remains in effect.] 

At first glance, it seemed like that big of a deal and if there was a major issue, they would've stopped dosing. The company said that the data monitoring committee wants the company to increase the frequency of measurements for total lymphocytes and for CD4 positive T-cells. They want to look at the immunological cells to see whether it's presumably causing them to go down although they didn't actually say that. Whether that's a rare event, more or less, we don't really know yet. It could be a commercial problem.

If the FDA recommends that people taking the drug need to get their immune systems checked out regularly. It wouldn't be so bad if it's a few times at the beginning of treatment and then you are cleared because it's not likely that you're going to develop the side effect later. It's only seen in people when they first start the treatment, then I think it's probably not going to be a big deal.

But keep in mind that this is a prophylaxis, so it's preventing and of course, there's already HIV prophylaxis drugs in the market from Gilead. If it requires regular monitoring every few weeks, people probably don't want to go get their blood drawn every few weeks just to stay on this drug if there's alternatives. I think that could be a major issue if the requirement is that you get blood monitoring every few weeks for the entire time that you're on it.

Brian Orelli, PhD has no position in any of the stocks mentioned. Keith Speights has no position in any of the stocks mentioned. The Motley Fool recommends Gilead Sciences. The Motley Fool has a disclosure policy.

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