Enanta to Start Phase 2b/3 RSV Trial in Adults, Phase 2b in Kids

On June 18, 2026, Enanta Pharmaceuticals announced the advancement of zelicapavir, its N-protein inhibitor for respiratory syncytial virus (RSV), into late-stage clinical development following a successful End-of-Phase 2 meeting with the FDA. The company plans to initiate a single registrational Phase 2b/3 clinical trial in high-risk adults (those with COPD, CHF, or aged 75+) in Q4 2026. The Phase 2b portion of this trial, involving approximately 200 patients, is designed to confirm treatment effect and primary endpoints before proceeding to the Phase 3 portion of 660 patients. Additionally, Enanta will initiate a Phase 2b trial in pediatric patients (28 days to 36 months old) in Q3 2026. Previous Phase 2 data in high-risk adults showed zelicapavir reduced the time to complete symptom resolution by approximately 7 days and resulted in a 0% hospitalization rate compared to 5% for placebo. Both the adult Phase 2b and pediatric Phase 2b trials are expected to yield topline data in 2027. Enanta estimates the global market opportunity for an RSV antiviral exceeds $2 billion.