Merck's investigational prophylactic long-acting monoclonal antibody, clesrovimab (MK-1654), has reached a significant regulatory milestone with the FDA's acceptance of its Biologics License Application (BLA). Designed to protect infants from respiratory syncytial virus (RSV) during their first RSV season, clesrovimab could offer a new approach to preventing this common and potentially severe infection. The FDA has set a Prescription Drug User Fee Act (PDUFA) target action date of June 10, 2025.
The BLA is supported by data from the Phase 2b/3 CLEVER trial and interim results from the ongoing Phase 3 SMART trial. The CLEVER trial evaluated a single dose of clesrovimab in healthy preterm and full-term infants, while the SMART trial assessed the safety and efficacy of clesrovimab versus palivizumab in infants and children at increased risk for severe RSV disease. Data from these trials, presented at IDWeek in October 2024, demonstrated a significant reduction in RSV-associated medically attended lower respiratory infections (MALRI) requiring at least one indicator of lower respiratory infection (LRI) or severity compared to placebo through day 150 post-dose.
Clinical Efficacy and Safety
Results from the CLEVER trial showed a 60.4% reduction in the incidence of RSV-associated MALRI requiring at least one indicator of LRI or severity compared to placebo through day 150 post-dose (95% CI: 44.1, 71.9, P < 0.001). Furthermore, clesrovimab reduced RSV-associated hospitalizations and RSV-associated LRI hospitalizations by 84.2% compared to placebo (95% CI: 66.6, 92.6, P < 0.001). In the SMART trial, clesrovimab demonstrated a comparable safety profile to palivizumab, with no drug-related serious adverse events reported.
Addressing Unmet Needs in RSV Prevention
RSV is a highly contagious virus causing seasonal infections, posing a significant burden on infants and older adults worldwide. It is the leading cause of hospitalization for healthy infants under one year old globally and a major cause of death in low- and middle-income countries. RSV can lead to severe respiratory conditions such as bronchiolitis and pneumonia, resulting in an estimated 3.6 million hospitalizations and 101,000 deaths annually in children under five.
Paula Annunziato, SVP, infectious diseases and vaccines, global clinical development, Merck Research Laboratories, stated, "Despite recent advances in RSV prevention, unmet needs remain for additional effective interventions to help protect infants and continue to help address the burden RSV places on families and the healthcare system. This regulatory milestone, along with promising results from our pivotal studies demonstrating efficacy in the prevention of RSV disease, marks important progress toward our goal of having clesrovimab available in time for the 2025-26 RSV season."
Potential Market Impact
If approved, Merck anticipates clesrovimab will be available for ordering by physicians and healthcare administrators by July 2025, positioning it for the 2025 RSV season. Clesrovimab is designed to be administered as a single dose, regardless of weight, offering direct, rapid, and durable protection through an infant's first RSV season against mild, moderate, and severe RSV. This single-dose approach could provide a competitive advantage over existing options like AstraZeneca and Sanofi’s Beyfortus (nirsevimab), which was approved in the United States in July 2023.
Competition in the RSV Landscape
While other RSV preventatives and vaccines are available, including Pfizer’s Abrysvo and GSK’s Arexvy for older adults and maternal immunization, clesrovimab's single-dose administration could streamline the prevention process. The potential approval and launch of clesrovimab before the 2025-26 RSV season would allow for proper market preparation and integration into healthcare systems, strengthening Merck's infectious disease portfolio and potentially driving new growth.
