Watertown, Mass. — Enanta Pharmaceuticals said it is advancing zelicapavir into a registrational Phase 2b/3 clinical trial in adults at high risk of severe outcomes from respiratory syncytial virus infection after a successful End-of-Phase 2 meeting with the U.S. Food and Drug Administration.
The company also said it plans to begin a Phase 2b trial of zelicapavir in pediatric patients with RSV.
Zelicapavir is an investigational oral antiviral being developed for the treatment of RSV. Enanta said the adult Phase 2b portion is expected to begin in the fourth quarter of 2026, with topline data expected in 2027. The pediatric Phase 2b trial is expected to begin in the third quarter of 2026, with topline data also expected in 2027.
“We are thrilled to move zelicapavir into a single Phase 2b/3 registrational clinical trial in adults with RSV at high risk of progressing to severe disease, following a productive and collaborative End-of-Phase 2 meeting with the FDA. This progress, alongside the planned initiation of our pediatric Phase 2b trial, provides the opportunity for two key Phase 2b datasets next year in two important patient populations and underscores zelicapavir’s potential to become the first antiviral therapy for RSV infection,” said Jay R. Luly, Ph.D., President and Chief Executive Officer of Enanta Pharmaceuticals.
Luly said RSV creates a significant global disease burden and that there are no available treatment options. The company estimates that an oral RSV antiviral for children and high-risk adults could represent a global market opportunity of more than $2 billion, with a potential total addressable population of more than 3 million patients in the United States alone.
“The recent availability of at-home RSV diagnostics is expected to further drive disease awareness and increase earlier diagnosis. As we advance into late-stage development, we are well-positioned to continue our leadership in RSV and remain urgently focused on delivering zelicapavir to patients in need of treatment,” Luly said.
The adult Phase 2b/3 trial will be a randomized, double-blind, placebo-controlled, multicenter global study evaluating zelicapavir in adult outpatients who test positive for RSV and have had respiratory tract infection symptoms for no more than three days.
Eligible patients will have at least one risk factor, including being 75 years of age or older, having chronic obstructive pulmonary disease or having congestive heart failure. Enanta said these risk factors match the HR3 population in its previous Phase 2 RSVHR trial, where the company observed a one-week reduction in time to complete symptom resolution and a reduction in hospitalization.
Patients in the adult trial will receive 800 mg of oral zelicapavir or placebo once daily for seven days and will be evaluated for 28 days after treatment. The primary endpoint is time to complete resolution of all 13 RSV symptoms, as measured by the RiiQ tool. A key secondary endpoint will evaluate hospitalization rate, with other secondary endpoints measuring clinical efficacy, antiviral activity and safety.
The Phase 2b portion will include at least 200 patients and is designed to confirm the primary endpoint and treatment effect size in the targeted population. The Phase 3 portion is expected to include about 660 patients.
“We now have a clear and efficient pathway to advance zelicapavir into a registrational program through a single Phase 2b/3 trial. The Phase 2b part is designed to confirm the primary endpoint and treatment effect size in the targeted population and will provide a key dataset to further strengthen the body of evidence supporting the Phase 3 portion of the study. We look forward to beginning the trial in the fourth quarter of 2026, with data from the Phase 2b portion expected in 2027,” said Scott T. Rottinghaus, M.D., Chief Medical Officer of Enanta Pharmaceuticals.
Rottinghaus said the company is also advancing zelicapavir in children through a Phase 2b trial to be conducted with the Penta Foundation and the AMS-PHPT Research Unit at Chiang Mai University.
“We are also excited to advance zelicapavir development in children, with a Phase 2b trial to be conducted in collaboration with the Penta Foundation, a leading organization dedicated to pediatric infectious disease research, and the internationally recognized AMS-PHPT Research Unit at Chiang Mai University. This trial will build on the encouraging findings from our first-in-pediatrics study and allow us to assess zelicapavir’s treatment effect on symptom resolution using RESOLVE-P, our proprietary tool being developed for use as a registrational endpoint. We plan to initiate this pediatric study in the third quarter of 2026, with topline data expected in 2027,” Rottinghaus said.
The pediatric Phase 2b trial will be a double-blind, placebo-controlled, multicenter study evaluating zelicapavir in hospitalized and non-hospitalized children with up to 72 hours of respiratory tract infection symptoms who test positive for RSV.
The study is expected to enroll about 150 participants, ages 28 days to 36 months. Participants will receive 5 mg/kg of zelicapavir if they are younger than 12 months or 7.5 mg/kg if they are at least 12 months old, or placebo, once daily for seven days. They will be evaluated for 28 days after treatment.
The primary endpoint is time to complete resolution of clinical signs of RSV, as measured by the RESOLVE-P clinical scoring scale. Secondary endpoints will evaluate additional clinical efficacy measures, antiviral activity and safety.
Enanta said the pediatric trial will be conducted in Thailand in collaboration with the Penta Foundation and the AMS-PHPT Research Unit at Chiang Mai University, which has experience conducting infectious disease trials and access to clinical sites in regions where RSV season begins in the summer.
