The Food and Drug Administration (FDA) has placed clinical holds on studies evaluating islatravir (MK-8591), an investigational nucleoside reverse transcription translocation inhibitor, for the treatment and prevention of HIV-1 infection.

The clinical holds were based on observations from trials that showed a decrease in total lymphocyte and CD4+ T-cell counts among patients treated with islatravir. While the clinical holds are in place, no new studies may be initiated. The FDA has placed 6 studies on full clinical hold and 7 studies on partial clinical hold. 

Study participants who were receiving the oral or implant formulations of islatravir for HIV-1 pre-exposure prophylaxis (PrEP), as well as those who received injectable islatravir for treatment and prophylaxis, will no longer receive the study drug. An FDA-approved PrEP therapy will be offered to participants in the PrEP studies.

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Patients enrolled in clinical trials evaluating the combination of doravirine and islatravir will continue to receive study medication. While the partial clinical hold is in place, no new participants will be screened or randomized in the doravirine/islatravir studies for HIV-1 treatment. 

A full list of the clinical trials that have been placed on hold can be found here.

“We are grateful to the participants and the study investigators for their ongoing contributions to this research,” said Dr Joan Butterton, vice president, infectious diseases, Global Clinical Development, Merck Research Laboratories. “Merck continues to investigate the potential of islatravir and nucleoside reverse transcriptase translocation inhibitors and remains committed to helping to address unmet needs in HIV treatment and prevention.”

Gilead and Merck have also decided to stop dosing all participants in a phase 2 study ( Identifier: NCT05052996) evaluating an oral-weekly combination of Merck’s islatravir and Gilead’s lenacapavir in people living with HIV who are virologically suppressed on antiretroviral therapy.


Merck announces clinical holds on studies evaluating islatravir for the treatment and prevention of HIV-1 infection. News release. Merck. December 13, 2021. Accessed December 14, 2021.

This article originally appeared on MPR