Merck (MSD) has suspended subject enrolment in two Phase III clinical trials of an experimental oral drug, islatravir (MK-8591), being analysed as pre-exposure prophylaxis (PrEP) for individuals who are at increased risk of the human immunodeficiency virus (HIV)-1 infection.

An inhibitor of nucleoside reverse transcriptase translocation, islatravir is intended to be administered once every month.

The company decided to stop enrolment for the IMPOWER 22 and IMPOWER 24 trials based on the advice from the ISL PrEP external data monitoring committee (eDMC).

Investigators were informed of the latest development while subjects who are currently enrolled in the trial will be given the study drug.

Merck will also simultaneously carry out additional assessments of these trials, as well other trials underway.

Based on the PrEP eDMC recommendations, Merck is introducing further monitoring methods for trial subjects, including raising the rate of total lymphocyte and CD4+ T-cell analyses.

The randomised, double-blind, active-controlled IMPOWER 22 trial will assess the efficacy and safety of a one-monthly dose of oral islatravir versus a once-a-day dose of emtricitabine/tenofovir disoproxil fumarate (FTC/TDF) as PrEP in cisgender women at increased HIV-1 infection risk.

IMPOWER 24 is a randomised, double-blind, active-controlled trial that will also analyse the efficacy and safety of the oral drug given once a month as PrEP versus a single daily dose of FTC/TDF or emtricitabine/tenofovir alafenamide (FTC/TAF).

This trial will be carried out in cisgender men and transgender women.

Merck Research Laboratories Global Clinical Development infectious diseases vice-president Dr Joan Butterton said: “We continue to apply learnings from our HIV programme as we move forward with the development of islatravir in both treatment and PrEP.

“Merck remains committed to developing tools to help address unmet needs in HIV treatment and prevention to contribute to the global effort against the ongoing HIV epidemic.”

Last month, the company decided to stop participant dosing in the Phase II IMAGINE-DR trial of MK-8507 plus islatravir for the treatment of HIV-1 infection.