Enanta Pharmaceuticals Inc saw its shares rise 2.9% in premarket trading Thursday after the company disclosed plans to advance its RSV antiviral candidate, zelicapavir, into later-stage clinical development.
The biotechnology firm said it will initiate a registrational Phase 2b/3 trial of zelicapavir in high-risk adults with confirmed RSV in the fourth quarter of 2026. Topline results from the Phase 2b portion of that study are expected in 2027. The move follows a successful End-of-Phase 2 meeting with the U.S. Food and Drug Administration.
In parallel, Enanta intends to open a pediatric Phase 2b trial of zelicapavir in the third quarter of 2026, with topline pediatric data also anticipated in 2027.
Trial design details for the adult registrational study were provided by the company. The Phase 2b/3 trial will enroll adult outpatients who test positive for RSV and have respiratory tract infection symptoms for no more than three days. To qualify, patients must be aged 75 years or older or have chronic obstructive pulmonary disease or congestive heart failure.
- The Phase 2b segment will include a minimum of 200 patients.
- The Phase 3 portion is planned to enroll approximately 660 patients.
Enanta said the primary endpoint for the registrational study will be time to complete resolution of all 13 RSV symptoms. A key secondary endpoint will assess hospitalization rate.
Enanta referenced results from an earlier Phase 2 trial in high-risk adults in which zelicapavir showed a 6.7-day improvement in time to complete symptom resolution for patients meeting the high-risk criteria of congestive heart failure, chronic obstructive pulmonary disease, or age 75 or older. In that trial, the zelicapavir-treated cohort had a 0% RSV-related hospitalization rate versus 5% for placebo.
For the pediatric program, the Phase 2b study will target about 150 participants aged 28 days to 36 months. Eligible infants and toddlers must test positive for RSV and have up to 72 hours of respiratory tract infection symptoms at enrollment.
Enanta noted that zelicapavir has received Fast Track designation from the FDA and has been well tolerated in more than 700 subjects to date. The company’s timeline sets out late-2026 initiation for adult registrational work, mid-2026 start for the pediatric Phase 2b, and topline data from the Phase 2b segments of both programs in 2027.
The announcement and regulatory progress appear to have been the immediate market catalysts for the premarket uptick in Enanta shares.
Key points
- Enanta will start a registrational Phase 2b/3 trial of zelicapavir in high-risk adults in Q4 2026, with Phase 2b topline data expected in 2027.
- A pediatric Phase 2b trial is planned for Q3 2026, enrolling roughly 150 participants aged 28 days to 36 months, with topline data expected in 2027.
- Zelicapavir has Fast Track designation and has been administered to more than 700 subjects with acceptable tolerability reported.
Sectors impacted - biotechnology, pharmaceuticals, and healthcare services, including hospital utilization and clinical research activity.
Risks and uncertainties
- Timing risk - The planned start dates (Q3 2026 for the pediatric trial and Q4 2026 for the adult registrational trial) and the expectation of topline Phase 2b data in 2027 are projected timelines and may change.
- Clinical outcome risk - While prior Phase 2 data in high-risk adults showed a 6.7-day improvement in symptom resolution and a 0% RSV-related hospitalization rate versus 5% for placebo, outcomes in larger registrational and pediatric trials may differ.
- Regulatory risk - Advancement to registrational status followed a successful End-of-Phase 2 meeting, but future regulatory assessment will depend on the results of the planned late-stage studies.
Bottom line - Enanta’s schedule to move zelicapavir into a registrational Phase 2b/3 trial for high-risk adults and to initiate a pediatric Phase 2b study sets a clear development path with topline Phase 2b data anticipated in 2027 for both programs. The market reaction was a modest premarket gain following the announcement.