Merck announced on June 8, 2026 that islatravir (MK-8591), an investigational oral nucleoside reverse transcriptase translocation inhibitor (NRTTI), met the primary virologic suppression endpoint in both Phase 3 trials in the ISLEND program — the most advanced clinical evaluation of a once-weekly antiretroviral regimen to date. Full efficacy and safety datasets were not released with the topline announcement; peer-reviewed publication is pending.

ISLEND-1 (NCT06630286) enrolled virologically suppressed adults living with HIV-1 and randomized them to an islatravir-based once-weekly oral regimen versus Biktarvy (bictegravir/emtricitabine/tenofovir alafenamide, B/F/TAF) — Gilead Sciences’ once-daily integrase inhibitor backbone widely considered the current clinical standard for maintenance therapy. ISLEND-2 (NCT06630299) evaluated a similar islatravir-based once-weekly regimen against a broader range of standard-of-care daily oral antiretroviral therapy regimens, expanding the comparator landscape.

Both trials met their primary virologic suppression endpoint. Merck did not release specific response rates or confidence intervals in its June 8 announcement; detailed data are expected at a forthcoming medical meeting.

Why weekly dosing matters

Daily oral antiretroviral therapy is transformative — but it imposes a lifelong adherence burden. Missed doses, even intermittently, risk viral rebound and the selection of resistance mutations in some regimens. Islatravir belongs to the NRTTI class; its prolonged intracellular half-life makes it pharmacologically suited for weekly dosing by maintaining sustained intracellular drug levels between doses. Approximately 1.2 million Americans and 39 million people globally live with HIV; a once-weekly oral option would meaningfully reduce the daily treatment reminder for those currently on daily therapy.

Safety context

Islatravir is investigational and is not approved by the FDA or any regulatory authority for HIV treatment. Earlier Phase 2 studies of islatravir in combination with other agents observed CD4+ T-lymphocyte count decreases in some participants, prompting study design modifications; the ISLEND Phase 3 program incorporates CD4+ monitoring. How this signal may affect the final benefit-risk profile will become clearer when full Phase 3 safety data are published.

Merck did not specify a regulatory submission timeline in the June 8 topline announcement.

This report is based on a corporate topline announcement; full data have not been published or peer-reviewed.