Merck Announces Pause in Enrollment for Two Phase 3 Clinical Trials of Investigational, Once-Monthly, Oral Islatravir for Pre-Exposure Prophylaxis (PrEP) of HIV-1 infection


December 6, 2021 5:00 pm ET

Enrolled Participants Will Continue to Receive Study Medicine

KENILWORTH, N.J.--(BUSINESS WIRE)-- Merck (NYSE: MRK), known as MSD outside the United States and Canada, today announced a pause in enrollment for 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.

At the recommendation of the ISL PrEP external data monitoring committee (eDMC), Merck is pausing enrollment for the IMPOWER 22 and IMPOWER 24 trials while the company conducts further analyses of these and other ongoing studies. Participants already enrolled in the trials will continue to receive the study medicine. Informed by the recommendations of the PrEP eDMC, Merck is implementing additional monitoring measures for study participants, including increasing the frequency of total lymphocyte and CD4+ T-cell assessments. 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. “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.”

About IMPOWER 22 (MK-8591-022) and IMPOWER 24 (MK-8591-024)

The IMPOWER 22 clinical trial is a Phase 3, randomized, active-controlled, double-blind clinical study to evaluate the efficacy and safety of oral islatravir once-monthly compared to once-daily emtricitabine/tenofovir disoproxil fumarate (FTC/TDF) as pre-exposure prophylaxis (PrEP) in cisgender women at high risk for HIV-1 Infection.

The IMPOWER 24 clinical trial is a Phase 3, randomized, active-controlled, double-blind clinical study to evaluate the efficacy and safety of oral islatravir once-monthly as PrEP compared to once-daily FTC/TDF or emtricitabine/tenofovir alafenamide (FTC/TAF) in cisgender men and transgender women who have sex with men, and are at high risk for HIV-1 infection.

About Islatravir (MK-8591)

Islatravir (MK-8591) is Merck’s investigational nucleoside reverse transcriptase translocation inhibitor under evaluation for the treatment and prevention of HIV-1.

Our Commitment to HIV

For more than 35 years, Merck has been committed to scientific research and discovery (R&D) in HIV. Today, we are developing a series of antiviral options designed to help people manage HIV and protect people from HIV, with the goal of reducing the growing burden of infection worldwide. We remain committed to working hand-in-hand with our partners in the global HIV community to address the complex challenges that impede progress toward ending the epidemic.

About Merck

For over 130 years, Merck, known as MSD outside the United States and Canada, has been inventing for life, bringing forward medicines and vaccines for many of the world’s most challenging diseases in pursuit of our mission to save and improve lives. We demonstrate our commitment to patients and population health by increasing access to health care through far-reaching policies, programs and partnerships. Today, Merck continues to be at the forefront of research to prevent and treat diseases that threaten people and animals – including cancer, infectious diseases such as HIV and Ebola, and emerging animal diseases – as we aspire to be the premier research-intensive biopharmaceutical company in the world. For more information, visit and connect with us on Twitter, Facebook, Instagram, YouTube and LinkedIn.

Forward-Looking Statement of Merck & Co., Inc., Kenilworth, N.J., USA

This news release of Merck & Co., Inc., Kenilworth, N.J., USA (the “company”) includes “forward-looking statements” within the meaning of the safe harbor provisions of the U.S. Private Securities Litigation Reform Act of 1995. These statements are based upon the current beliefs and expectations of the company’s management and are subject to significant risks and uncertainties. There can be no guarantees with respect to pipeline candidates that the candidates will receive the necessary regulatory approvals or that they will prove to be commercially successful. If underlying assumptions prove inaccurate or risks or uncertainties materialize, actual results may differ materially from those set forth in the forward-looking statements.

Risks and uncertainties include but are not limited to, general industry conditions and competition; general economic factors, including interest rate and currency exchange rate fluctuations; the impact of the global outbreak of novel coronavirus disease (COVID-19); the impact of pharmaceutical industry regulation and health care legislation in the United States and internationally; global trends toward health care cost containment; technological advances, new products and patents attained by competitors; challenges inherent in new product development, including obtaining regulatory approval; the company’s ability to accurately predict future market conditions; manufacturing difficulties or delays; financial instability of international economies and sovereign risk; dependence on the effectiveness of the company’s patents and other protections for innovative products; and the exposure to litigation, including patent litigation, and/or regulatory actions.

The company undertakes no obligation to publicly update any forward-looking statement, whether as a result of new information, future events or otherwise. Additional factors that could cause results to differ materially from those described in the forward-looking statements can be found in the company’s 2020 Annual Report on Form 10-K and the company’s other filings with the Securities and Exchange Commission (SEC) available at the SEC’s Internet site (

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Source: Merck & Co., Inc.

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