The main purpose of the study is to evaluate the safety and tolerability of oral Islatravir (ISL) once monthly (QM) as Preexposure Prophylaxis (PrEP) in cisgender men who have sex with men (MSM) and transgender women (TGW) who have sex with men and who are at high risk of HIV-1 infection with 48 or 96 weeks of treatment and a minimum follow-up of 42 days.



Eligible Ages
Over 16 Years
Eligible Genders
Accepts Healthy Volunteers

Inclusion Criteria

  • Has confirmed Human Immunodeficiency Virus (HIV) uninfected based on negative HIV-1/HIV-2 test result before randomization - Is sexually active with male or transgender women (TGW) partners defined as having anal sexual intercourse with a man or TGW at least once in the past month - Is at high risk for sexually acquiring HIV-1 infection based on self-report of at least 1 of the following: a) Condomless receptive anal intercourse in the 6 months prior to screening occurring outside a mutually monogamous HIV seronegative concordant relationship b) More than 5 partners (anal intercourse) in the 6 months prior to screening c) Any unprescribed stimulant drug use in the 6 months prior to screening d) Rectal or urethral gonorrhea or chlamydia or incident syphilis in the 6 months prior to screening - Participants 16 or 17 years of age must weigh ≥35 kg. Enrollment for 16- to 17-year-old participants will begin only after completion of the Sentinel Cohort IA and review of IA results by the external data monitoring committee (eDMC) - Has no plans to relocate or travel away from the site for ≥4 consecutive weeks during study participation

Exclusion Criteria

  • Has hypersensitivity or other contraindication to any component of the study interventions as determined by the investigator - Has chronic HBV infection or past HBV infection which could indicate risk for Hepatitis B reactivation - Has known current or chronic history of liver disease or known hepatic or biliary abnormalities, unless the participant has stable liver function tests and no evidence of hepatic synthetic dysfunction - Has a history of malignancy within 5 years of screening except for adequately treated basal cell or squamous cell skin cancer or in situ anal cancers - Has a history or current evidence of any condition, therapy, laboratory abnormality or other circumstance that might, in the opinion of the investigator, confound the results of the study or interfere with the participant's participation for the full duration of the study, such that it is not in the best interest of the participant to enroll - Has taken cabotegravir, lenacapavir, or any other long-acting HIV prevention product at any time - Is currently receiving or is anticipated to require any prohibited therapies outlined in the study from 30 days prior to Day 1 through the duration of the study - Is currently participating in or has participated in an interventional or prevention clinical study with an investigational compound or device, within 30 days prior to Day 1 through the duration of the study - Has exclusionary laboratory values within 45 days prior to Day 1

Study Design

Phase 3
Study Type
Intervention Model
Parallel Assignment
Primary Purpose
Double (Participant, Investigator)
Masking Description
In Study Part 1, double-blind with in-house blinding is used. In Study Part 2, sponsor personnel not directly involved with blinded safety monitoring will be unblinded to participants' randomized study intervention in Part 1 (personnel involved with Part 2 will remain blinded). In Study Part 3, al participants, investigators, and Sponsor personnel are unblinded as to the participant's original randomized intervention group.

Arm Groups

ArmDescriptionAssigned Intervention
Islatravir (ISL) Once Monthly (QM) Group
Participants receive 60 mg tablet of ISL QM, orally plus Placebo to Emtricitabine/Tenofovir Disoproxil Fumarate (FTC/TDF) tablet once daily (QD) or Placebo to Emtricitabine/Tenofovir Alafenamide (FTC/TAF) tablet QD, orally for up to 24 months of treatment duration.
  • Drug: ISL
    ISL 60 mg tablet, QM, orally for up to 24 months
    Other names:
    • MK-8591
  • Drug: Placebo to FTC/TDF
    Placebo FTC/TDF 0 mg tablets QD, orally for up to 24 months
  • Drug: Placebo to FTC/TAF
    Placebo FTC/TAF 0 mg tablets QD, orally for up to 24 months
Active Comparator
Participants receive 200/245 mg or 200/300 mg of FTC/TDF combination tablet, QD, orally or 200/25 mg of FTC/TAF combination tablet, QD, orally at investigator's discretion plus Placebo to ISL tablet QM, orally for up to 24 months of treatment duration.
  • Drug: FTC/TDF
    Participants receive 200/245 mg of FTC/TDF combination tablet, QD, orally for up to 24 months
    Other names:
    • Truvada
    • Emtricitabine/Tenofovir Disoproxil Fumarate
  • Drug: FTC/TAF
    Participants receive 200/25 mg of FTC/TAF combination tablet, QD, orally for up to 24 months
    Other names:
    • Descovy
    • Emtricitabine/Tenofovir Alafenamide
  • Drug: Placebo to ISL
    Placebo ISL 0 mg tablets QM, orally for up to 24 months.

More Details

Active, not recruiting
Merck Sharp & Dohme LLC

Study Contact

Detailed Description

Based on laboratory findings of decreased lymphocyte and CD4+ T-cell counts across the islatravir program, dosing of blinded study intervention was halted on 13-Dec-2021. Blinded assessments conducted prior to then are designated as Study Part 1. During Study Part 2, participants from Part 1 were switched to PrEP therapy with emtricitabine/tenofovir disoproxil (FTC/TDF) or emtricitabine/tenofovir alafenamide (FTC/TAF) while continuing in the study. In Part 3, participants, investigators, and all Sponsor personnel are unblinded to participant's original randomized intervention group, and participants may continue to receive unblinded FTC/TDF or FTC/TAF. Screening and randomization of new participants have stopped, with no new participants being enrolled in Part 2 or Part 3.


Study information shown on this site is derived from ClinicalTrials.gov (a public registry operated by the National Institutes of Health). The listing of studies provided is not certain to be all studies for which you might be eligible. Furthermore, study eligibility requirements can be difficult to understand and may change over time, so it is wise to speak with your medical care provider and individual research study teams when making decisions related to participation.