Enanta Pharmaceuticals (NASDAQ: ENTA) announced plans to advance its RSV antiviral candidate zelicapavir into a registrational Phase 2b/3 clinical trial in high-risk adults, following a completed End-of-Phase 2 meeting with the U.S. Food and Drug Administration.

The company said the adult trial is set to begin in the fourth quarter of 2026, with topline Phase 2b data expected in 2027. The randomized, double-blind, placebo-controlled global trial will enroll adult outpatients who test positive for RSV within three days of symptom onset and have at least one risk factor, including age 75 or older, chronic obstructive pulmonary disease, or congestive heart failure. The Phase 2b portion will include a minimum of 200 patients, with the Phase 3 portion targeting approximately 660 patients.

Enanta also plans to initiate a separate Phase 2b pediatric trial in the third quarter of 2026, with topline data also expected in 2027. The trial will enroll approximately 150 children aged 28 days to 36 months and will be conducted in Thailand in collaboration with the Penta Foundation and the AMS-PHPT Research Unit at Chiang Mai University.

In a prior Phase 2 trial of high-risk adults, zelicapavir showed a 6.7-day reduction in time to complete symptom resolution compared to placebo in the HR3 population. RSV-related hospitalization was observed in 5% of placebo patients versus 0% in the zelicapavir group. The drug has been administered to more than 700 subjects to date, according to the company.

Zelicapavir has received Fast Track designation from the FDA. The drug targets the RSV N-protein, a mechanism the company says differs from existing RSV fusion inhibitors.

According to the Centers for Disease Control and Prevention, RSV accounts for up to 6.8 million outpatient visits, approximately 370,000 hospitalizations, and 24,000 deaths annually in the United States.