Merck announces pause in enrolment for Phase 3 HIV PreP studies

Additional monitoring measures for study participants, have been recommended by the external Data Monitoring Committee.



Merck, which is known as MSD outside of the United States and Canada, announced yesterday that enrolment has been paused in the IMPOWER 22 (MK-8591-022) and IMPOWER 24 (MK-8591-024) Phase 3 clinical studies evaluating investigational, once-monthly oral islatravir (ISL), a nucleoside reverse transcriptase translocation inhibitor, for Pre-Exposure Prophylaxis (PrEP) in people at high risk of HIV-1 infection. This comes two weeks after the company’s decision to halt development of MK-8507 due to decrease of white blood cells in HIV patients from a combination therapy of MK-8507 and islatravir. Merck is stopping enrolment in the IMPOWER 22 and IMPOWER 24 trials, as recommended by the ISL PrEP external Data Monitoring Committee (eDMC), while the company undertakes additional analyses of these studies and other ongoing studies.


The study medication will continue to be given to those who have already enrolled in the studies. Merck is introducing additional monitoring procedures for research participants, including increasing the frequency of total lymphocyte and CD4+ T-cell assessments, based on the recommendations of the PrEP eDMC. The trial Investigators have been informed of these actions. “We continue to apply learnings from our HIV program as we move forward with the development of islatravir in both treatment and PrEP,” said Dr. Joan Butterton, vice president, infectious diseases, Global Clinical Development, Merck Research Laboratories. Adding that, “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.”