With some analyses generating signs of efficacy, the biotech made the case that the data support further development of the direct-acting antiviral. A phase 2b trial of Enanta Pharmaceuticals’ respiratory syncytial virus (RSV) drug candidate has missed its primary endpoint. But, with other analyses generating signs of efficacy, the biotech made the case that the data support further development of the oral, direct-acting antiviral zelicapavir. Investigators randomized 186 patients to receive zelicapavir or placebo once daily for five days. Enanta later narrowed its efficacy population to include just the 175 patients who tested positive for RSV at a central laboratory. After the treatment period, the biotech tracked participants for a further 28 days to assess the effect of zelicapavir. The time to resolution of RSV lower respiratory tract disease (LRTD) symptoms was statistically no better on zelicapavir than on placebo, causing the trial to miss its primary endpoint. Enanta reported a 0.5-day improvement over placebo in time to the complete resolution of the four LRTD symptoms in the efficacy population. Other analyses linked zelicapavir to bigger improvements in time to symptom resolution. In the efficacy population, all 13 RSV symptoms cleared up 2.2 days faster in the treatment cohort. When looking at all 29 parameters in a patient-reported respiratory infection tool, Enanta found symptoms resolved 3.6 days faster on zelicapavir.  The biotech also shared analyses of how zelicapavir compared to placebo in patients who had congestive heart failure, chronic obstructive pulmonary disease or who were aged 75 years or older. In total, 81% of participants met one of the criteria for inclusion in the subgroup analyses.   Limiting assessments to those patients, Enanta linked zelicapavir to a three-day improvement in the time to resolution of LRTD symptoms. Time to resolution of the 13 RSV symptoms was 6.7 days shorter on the study drug in the subpopulation. The difference when looking at all 29 parameters was 7.2 days.  Enanta shared data on a secondary endpoint that used another scale and on the rates of hospitalization—1.7% for zelicapavir versus 5% for placebo—to further support its argument that the results are positive. That conclusion informed comments by Scott Rottinghaus, M.D., chief medical officer of Enanta, about the next steps. “We believe the totality of these data provides strong rationale for further clinical advancement of zelicapavir,” Rottinghaus said in a statement. “Importantly, we identified multiple potential registrational endpoints for a phase 3 trial.” In a note to clients, Evercore analyst Jonathan Miller lauded the drug's “robust benefit” and stated that “no one in [the] RSV space has ever shown benefit like this with an antiviral before.” Shares in Enanta surged by about 40% to $11.03 by early afternoon on Monday from a closing price of $7.90 on Friday.    Enanta's Press Release (Sept. 29, 2025): https://ir.enanta.com/news-releases/news-release-details/enanta-pharmaceuticals-reports-positive-topline-results-its   

6.7-Day Improvement in Time to Complete Resolution of All RSV Symptoms for Patients with Chronic Obstructive Pulmonary Disease (COPD), Congestive Heart Failure (CHF), or Age ≥75 Statistically Significant Improvement in Patient Global Impression of Severity Score Lower Hospitalization Rate for...

A phase 2b trial of Enanta Pharmaceuticals’ respiratory syncytial virus (RSV) drug candidate has

Shares in Enanta Pharma were sliding this morning after it reported results of a phase 2b trial of its RSV antiviral zelicapavir.

Stay updated with Enanta Pharmaceuticals (ENTA) news: Press releases, clinical trial updates, and partnership announcements in antiviral drug development.

Enanta Pharmaceuticals’ respiratory syncytial virus (RSV) antiviral