Federal Circuit affirms invalidation of Enanta’s Paxlovid-related patent: finds “typo” defense insufficient to fix written description gap in provisional filing

Context: Enanta Pharmaceuticals, Inc. sued Pfizer Inc. on June 21, 2022 in the U.S. District Court for the District of Massachusetts, just one week after the patent issued, alleging that Pfizer’s COVID-19 antiviral Paxlovid infringed Enanta’s U.S. Patent No. 11,358,953 (“Functionalized peptides as antiviral agents”), which is directed to compounds and methods for inhibiting coronavirus replication. Enanta sought damages and ongoing royalties rather than an injunction, asserting at the time that it did not intend to impede Paxlovid’s production or sale (June 21, 2022, Enanta Pharmaceuticals, press release). Pfizer countered that the ‘953 patent could not claim priority back to Enanta’s underlying provisional application and that Pfizer’s own pre-filing public disclosure of the active ingredient in Paxlovid, nirmatrelvir, anticipated the claims. On December 23, 2024, Judge Denise J. Casper of the District of Massachusetts agreed, granting summary judgment that the ‘953 patent is invalid. Enanta appealed.