In Enanta Pharmaceuticals, Inc. v. Pfizer Inc., No. 2025-1427 (Fed. Cir. June 23, 2026), the Federal Circuit affirmed that a patent is not entitled to its provisional application’s priority date where the provisional application fails to provide adequate written description support for what is claimed in the patent.
Enanta accused Pfizer’s Paxlovid® of infringing U.S. Patent No. 11,358,953, which is directed to compounds and methods for inhibiting coronavirus replication. The ’953 patent issued from a non-provisional application filed on November 9, 2021 and claimed priority to a provisional application filed on July 20, 2020. The provisional application disclosed a substituent as —NHC(O)—C2–C12-alkyl, but the non-provisional recited —NHC(O)—C1–C12-alkyl. Between the two filings, Pfizer publicly disclosed nirmatrelvir, the active ingredient in Paxlovid®, which bears a C1-alkyl group.
The District Court granted summary judgment that the asserted claims were anticipated, holding that the change from C2 to C1 was not an obvious typographical error and impermissibly broadened the invention, so the patent could not claim the provisional application’s earlier priority date.
The Federal Circuit affirmed. It clarified that the dispositive question was not whether “C2” was a drafting error, but whether the provisional application demonstrated that the inventor possessed the later-claimed invention. The federal circuit determined a disclosure of C2–C12 alkyl does not provide written description support for C1–C12 alkyl, because C2 is simply different from C1. The Federal Circuit likened this distinction to disclosing ethanol, a two-carbon alcohol, versus methanol, a one-carbon alcohol. Rejecting Enanta’s expert testimony, the Federal Circuit concluded that the provisional application’s detailed disclosure of numerous chemical moieties did not support reading in a C1 group that was never disclosed. Because the ’953 patent was not entitled to the provisional application’s priority date, Pfizer’s intervening disclosure of nirmatrelvir constituted prior art, invalidating the claims of the ‘953 patent.
Authors: Hayley Yang and Nicholas Bougher (Summer Associates)
Edited by Kelly Hwang and Craig Drachtman