RSV Drug Zelicapavir Shows Promise Despite Missing Primary Goal

Enanta Pharmaceuticals is advancing zelicapavir, an oral treatment for RSV, into Phase III trials for high-risk adults. This decision follows promising secondary efficacy data from a Phase IIb study, even though the primary endpoint was not met. The study, presented at ESCMID Global 2026, investigated zelicapavir in adults aged 65 or older with specific underlying conditions like COPD or CHF.

While the primary goal of time to resolution of four lower respiratory tract symptoms was missed, zelicapavir showed statistically significant improvements in other symptom resolution measures. Notably, the drug candidate was associated with faster resolution of 13 RSV symptoms and improved quality of life parameters, with the most high-risk patients experiencing the greatest benefits. Additionally, a lower RSV-associated hospitalization rate was observed in the zelicapavir group compared to placebo.

The drug candidate exhibited a favorable safety and tolerability profile. Treatment-emergent adverse events were similar between groups, but serious adverse events and those leading to discontinuation were notably lower with zelicapavir. Enanta plans to initiate a Phase III study in Q4 2026.

Zelicapavir is also being evaluated in pediatric trials, suggesting a broad application for the drug. Enanta's development pipeline also includes another RSV therapeutic candidate, EDP-323, currently in Phase II. The company anticipates a strong market position in RSV therapeutics.