Merck to unveil Week 24 data on weekly islatravir/ulonivirine at AIDS 2026
The Phase 2b readout tests whether a once-weekly pill can hold viral suppression as well as daily BIC/FTC/TAF, Merck's established switch regimen.
Executive Summary
- Merck will present the first primary efficacy and safety data from a trial testing whether a once-weekly two-drug pill can replace a daily standard regimen in people already suppressed on treatment.
- The readout compares maintenance of viral suppression, treatment discontinuation for side effects, and overall adverse events between the weekly regimen and the incumbent daily pill, the bar any dosing-frequency switch has to clear.
- The trial enrolled to its target and closed recruitment on schedule, and its completion timeline moved earlier rather than slipping, pointing to a study that ran without disruption ahead of this presentation.
- The mechanism sits in a sparsely populated corner of an otherwise dense HIV treatment field, with few programs testing this same target at this stage, positioning the result as an early signal for how far reduced-dosing regimens can go rather than a contest against a crowded set of rivals.
The presentation
Merck said it will present primary Week 24 efficacy and safety data from the Phase 2b MK-8591B-060 trial (NCT06891066), which switches virologically suppressed adults from daily bictegravir/emtricitabine/tenofovir alafenamide (BIC/FTC/TAF) to once-weekly oral islatravir and ulonivirine. The data will be delivered as an oral late-breaker abstract at the International AIDS Conference in Rio de Janeiro, alongside separate Week 48 data on Merck's islatravir/lenacapavir combination from the Phase 3 ISLEND-1 and ISLEND-2 trials. Dr. Eliav Barr, Merck's chief medical officer for global clinical development, said the conference will showcase "the potential of once-weekly oral treatments in combination with islatravir". MerckMerck to Present New Data on Daily, Weekly, and Monthly Options Across its HIV Treatment and Prevention Pipeline at AIDS 2026Jul 15, 2026
Probability of SuccessBased on the AppliedXL Probability of Success model. For more information about the methodology, read the research here.
What the trial tests
MK-8591B-060 registers three primary endpoints: the percentage of participants discontinuing treatment due to adverse events, the percentage with HIV-1 RNA at or above 50 copies/mL at Week 24, and the overall percentage experiencing an adverse event. The trial is randomized, with an active-comparator arm continuing daily BIC/FTC/TAF against two experimental arms of the weekly regimen. Because the comparator, BIC/FTC/TAF, is an established suppressive regimen and islatravir already anchors an FDA-approved combination, the trial's job is to show the weekly pill reproduces suppression rates already achieved daily, not to establish a new treatment effect. NCT06891066A Study of Islatravir (ISL) and Ulonivirine (ULO) Once Weekly (QW) in Virologically Suppressed Adults With Human Immunodeficiency Virus Type 1 (HIV-1) (MK-8591B-060)NCT06891066
Trial conduct
The trial enrolled 150 participants against an anticipated target of 150, a flat result the operational model classifies as typical and not a departure from plan. It moved from Recruiting to Active, not recruiting in August 2025, and its primary completion date shifted earlier twice, from October 2027 to September 2027 and then to August 2027, rather than slipping later. Those changes reflect a study that finished enrollment and tightened its timeline, consistent with a trial proceeding on schedule into this Week 24 readout. NCT06891066A Study of Islatravir (ISL) and Ulonivirine (ULO) Once Weekly (QW) in Virologically Suppressed Adults With Human Immunodeficiency Virus Type 1 (HIV-1) (MK-8591B-060)NCT06891066
Where it sits competitively
The trial's target, HIV-1 reverse transcriptase, and its nucleoside reverse transcriptase translocation inhibitor mechanism class are shared by a wide roster of approved and investigational HIV therapies, including Gilead Sciences' bictegravir, ViiV Healthcare's cabotegravir and dolutegravir, and Merck's own doravirine and islatravir/lenacapavir programs. Within that broader field, only one other Phase 2 trial studies this specific target-indication pairing, indicating that once-weekly reverse-transcriptase-based regimens remain an early-stage subset of an otherwise mature mechanism class. Merck's own islatravir/lenacapavir combination is already generating Phase 3 Week 48 data at the same conference, giving the company two parallel readouts on reduced-dosing-frequency regimens built around the same backbone molecule.
The company's recent track record
Merck's FDA approval of IDVYNSO, a once-daily doravirine/islatravir combination shown non-inferior to BIKTARVY and other antiretroviral regimens, gives the company a recent precedent for islatravir-based regimens clearing a regulatory bar on non-inferiority to standard-of-care therapy. That approval sets the template this Week 24 data will be read against: whether extending islatravir-based dosing from once-daily to once-weekly preserves the same suppression and tolerability profile. MerckMerck to Present New Data on Daily, Weekly, and Monthly Options Across its HIV Treatment and Prevention Pipeline at AIDS 2026Jul 15, 2026
This analysis was produced using AI-assisted reporting systems, AppliedXL data, and official public records. These systems undergo editorial review, quality checks, and regular audits by human experts. Errors may still occur, as with any automated system. Always consult the linked primary sources. Read our AI Editorial Policy.