Enanta Pharmaceuticals (NASDAQ: ENTA) lost its patent infringement appeal against Pfizer Inc. after the United States Court of Appeals for the Federal Circuit ruled on June 23, 2026, that the claims of U.S. Patent No. 11,358,953 were invalid.

The Federal Circuit's decision affirmed a December 2024 summary judgment ruling by the United States District Court for the District of Massachusetts. Enanta had filed the appeal seeking damages for alleged infringement related to the manufacture, use, and sale of Pfizer's COVID-19 antiviral Paxlovid (nirmatrelvir tablets; ritonavir tablets).