The investigational prophylactic monoclonal antibody met its primary endpoints, reducing infections up to day-150.
Clesrovimab (MK-1654), Merck’s investigational prophylactic monoclonal antibody designed to protect infants from respiratory syncytial virus (RSV) disease has met its primary safety and efficacy endpoints in the Phase IIb/Phase III clinical trial, MK-1654-004 (
MK-1654-004 is a double-blind, randomized, placebo-controlled study to evaluate the safety and efficacy of clesrovimab in healthy preterm and full-term infants. Participants received either a single dose of clesrovimab or placebo. The primary endpoints included the incidence of participants with RSV-associated medically attended lower respiratory infection (MALRI) from day-1 (post dose) to day-150 as compared to placebo and safety.
“RSV is highly contagious and can cause inflammation in the airways of infants leading to difficulty breathing. As a widespread illness globally, RSV is the leading cause of hospitalization for healthy infants,” Paula Annunziato, MD, senior vice president, infectious diseases and vaccines, global clinical development, Merck Research Laboratories said in a press release. “We are encouraged by these findings and look forward to working with regulators to provide a new option to help address the impact of RSV on infants and their families."
Merck plans to present more detailed findings from the study at an upcoming scientific conference.
In the RSV treatment space, Merck is competing with Sanofi and AstraZeneca’s Beyfortus (nirsevimab). In May 2023, Sanofi shared data from the HARMONIE Phase IIIb clinical trial (
Data from the randomized open-label study showed an 83.21% (95% CI 67.77 to 92.04; P<0.001) reduction in hospitalizations due to RSV-related LRTD. Data also showed that nirsevimab reduced the incidence of hospitalizations due to severe RSV-related LRTD (patients whose oxygen level is under 90% and require oxygen supplementation) by 75.71% (95% CI 32.75 to 92.91; P<0.001).
These results were collected in a real-world setting during the 2022-2023 RSV season with more than 8,000 infants being enrolled across 250 sites in France, Germany, and the United Kingdom.
“This winter saw higher rates of RSV-related infant hospitalizations than during pandemic or pre-pandemic years. The HARMONIE data demonstrate the real-world impact nirsevimab has on pediatric hospitalizations, and illustrate its importance for infants, their families, and public health,” Thomas Triomphe, executive vice president, vaccines, Sanofi said in an earlier press release.
In July 2023, the FDA approved nirsevimab for the prevention of RSV LRTD in newborns and infants born during or entering their first RSV season, and for children up to 24 months of age who remain vulnerable to severe RSV disease through their second RSV season.3
1. Merck Announces Topline Results from Phase 2b/3 Trial of Clesrovimab (MK-1654), an Investigational Respiratory Syncytial Virus Preventative Monoclonal Antibody for Infants. News release. Merck. July 23, 2024. Accessed July 24, 2024.
2. Press Release: Nirsevimab delivers 83% reduction in RSV infant hospitalizations in a real-world clinical trial setting. News release. Sanofi. May 12, 2023. Accessed July 24, 2024.
3. Beyfortus approved in the US for the prevention of RSV lower respiratory tract disease in infants. News release. AstraZeneca. July 17, 2023. Accessed July 24, 2024.
