FDA Accepts Biologics License Application for Infant RSV Antibody

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The FDA has accepted a biologics license application (BLA) for Merck’s clesrovimab (MK-1654), an investigational antibody to protect infants against respiratory syncytial virus (RSV), according to a recent press release. The FDA has also set a Prescription Drug User Fee Act date (PDUFA) for June 10, 2025.

Clesrovimab is an investigational extended half-life monoclonal antibody designed to be administered as a single dose, regardless of weight. While vaccines take longer to provide immunity and typically offer longer-lasting protection, monoclonal antibodies provide immediate but shorter-term protection, usually lasting only a few months, according to the National Institutes of Health.

This application approval is based on the results of the Phase 2b/3 CLEVER trial (MK-1654-004) and results from the ongoing Phase 3 SMART trial evaluating the effectiveness of clesrovimab compared to Merck’s competition, palivizumab, in children with a heightened risk of RSV. In the CLEVER trial, clesrovimab reduced RSV-associated hospitalizations by 84.2% compared to placebo, according to study data presented at ID Week 2024 in Los Angeles.

RSV is a contagious virus responsible for seasonal infections like the flu. Children and older adults are especially susceptible. In the United States, an estimated 58,000 – 80,000 children under the age of five are hospitalized every year due to RSV infections. In adults over 65, RSV-related hospitalizations range from 60,000 to 160,000 each year, according to the Centers for Disease Control and Prevention.

Globally, RSV is the leading cause of hospitalization for otherwise healthy children under a year old, potentially causing respiratory conditions such as bronchiolitis and pneumonia, which are responsible for approximately 3.6 million hospitalizations and 101,000 deaths a year worldwide in children under the age of five.

In October 2024, RSV vaccine Abrysvo was approved by the FDA to prevent lower respiratory tract disease caused by RSV in adults ages 18 to 59. Pregnant women from weeks 32 to 36 of gestation can also receive Abrysvo and the infant will be protected from birth to six months of age.

“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,” Paula Annunziato, M.D., vice president of clinical development at Merck Research Laboratories, said in the release.