FDA Approves Merck's ENFLONSIA for Preventing RSV in Infants

Merck, recognized globally as MSD except in the United States and Canada, recently announced a significant milestone: the U.S. Food and Drug Administration (FDA) has given its approval to ENFLONSIA™ (clesrovimab-cfor). This new preventive treatment is designed to protect newborns and infants from respiratory syncytial virus (RSV) disease affecting the lower respiratory tract, particularly during their first RSV season. ENFLONSIA offers a novel preventive approach, thanks to its long-acting monoclonal antibody formulation, which provides protection throughout a typical five-month RSV season with a consistent dose of 105 mg, irrespective of the infant's weight. RSV seasons generally extend from autumn until spring.

Dr. Octavio Ramilo, heading the Department of Infectious Diseases at St. Jude Children’s Research Hospital, highlighted the significance of this development. RSV is a predominant cause of hospitalizations among infants in the U.S., often leading to severe conditions such as bronchiolitis and pneumonia. ENFLONSIA not only simplifies dosing but also demonstrates robust clinical evidence indicating a substantial decrease in the incidence of RSV-related illnesses and hospital admissions. This makes it a promising intervention for safeguarding infants against RSV.

However, caution is advised for infants with a history of significant hypersensitivity reactions, including anaphylaxis, related to any ENFLONSIA component. The recent approval draws on findings from the pivotal CLEVER trial (MK-1654-004), which examined the effects of a single dose of ENFLONSIA on both preterm and full-term infants, from birth to one year of age. The study achieved its primary and key secondary endpoints, providing substantial evidence for the drug's effectiveness.

Support for this approval also comes from the Phase 3 SMART trial (MK-1654-007). This trial assessed the safety and efficacy of ENFLONSIA in comparison to palivizumab, specifically in infants at an elevated risk of severe RSV disease. Dr. Dean Y. Li, president of Merck Research Laboratories, emphasized ENFLONSIA's potential as a crucial preventive measure for both healthy and at-risk infants entering their initial RSV season. Merck is committed to ensuring that ENFLONSIA is available in the U.S. before the commencement of the upcoming RSV season, aiming to alleviate the considerable impact of this widespread seasonal infection on families and healthcare systems.

The Centers for Disease Control and Prevention’s (CDC’s) Advisory Committee on Immunization Practices is scheduled to convene later this month to deliberate and potentially recommend the use of ENFLONSIA for infants. Ordering is expected to start in July, with deliveries anticipated before the onset of the 2025-2026 RSV season.

ENFLONSIA is distinctly formulated for non-weight-based dosing and is intended to be administered to infants born during the RSV season starting at birth. For those born outside this period, administration should occur before the onset of their first RSV season. In cases where infants undergo cardiac surgery during or before their first RSV season, an additional dose is recommended post-surgery once the infant is stable.

In clinical trials, ENFLONSIA has shown a safety profile comparable to placebos and other treatments like palivizumab, especially when administered in tandem with routine childhood vaccinations. Common side effects include mild to moderate injection-site reactions and rashes. The rigorous CLEVER and SMART trials underscore the clinical promise of ENFLONSIA as an effective preventive tool against RSV in infants, providing the healthcare community with a new strategy to combat this challenging infection.