Enanta Pharmaceuticals, Inc. v. Pfizer Inc., Appeal No. 2025-1427 (Fed. Cir. Jun. 23, 2026)

In our Case of the Week, the Federal Circuit affirmed summary judgment invalidating all claims of Enanta’s Patent No. 11,358,953 as anticipated by Pfizer’s public disclosure of nirmatrelvir, a key component of Pfizer’s “Paxlovid” COVID treatment drug.  The court held the ’953 patent was not entitled to the priority date of Enanta’s July 20, 2020 provisional application, and that without that priority, Pfizer’s April 6, 2021 public presentation of nirmatrelvir anticipated the asserted claims.  The ’953 patent claimed coronavirus protease inhibitor compounds defined by “Formula (VI-6a)” with “substituent” limits including “X = CN” and “A” as optionally substituted alkyl or heteroaryl.  The dispute centered on the definition of permissible substituents for “A,” specifically a –“NHC(O)-C1-alkyl” group found in Pfizer’s nirmatrelvir.  Enanta’s provisional application listed -NHC(O)-C2-C12-alkyl, while the later ’953 patent listed -NHC(O)-C1-C12-alkyl—i.e., a difference of one carbon atom.

Enanta sued Pfizer in June 2022 for infringement based on Paxlovid’s nirmatrelvir.  Pfizer countered that the ’953 patent was invalid for anticipation because it was not entitled to the provisional’s priority date.  The district court granted summary judgment to Pfizer, finding no obvious typographical error permitting correction from C2 to C1.  On appeal, the Federal Circuit held the written description standard governs entitlement to priority under 35 U.S.C. § 120.  The court emphasized that each application in the priority chain must show the inventor possessed the later-claimed subject matter.

The central question was whether the ’048 provisional’s disclosure of -NHC(O)-C2-C12-alkyl provided written description support for -NHC(O)-C1-alkyl.  The court held that C2 is different from C1, and the C2-C12 range excludes a one-carbon alkyl group.  Because the provisional expressly disclosed C2-C12 and not C1, it did not convey possession of C1 to a skilled artisan.  Enanta’s expert argued a typographical inconsistency in the general “alkyl” definition suggested C1 had been intended in the provisional application.  The court found this unpersuasive because the alleged error was in a general definition, not in the specific “substituted” list using -NHC(O)-C2-C12-alkyl.  The expert identified no specific disclosure tying the C2-C12 substituent entry to C1, and speculation cannot supply written description.  The court reiterated that entitlement to a filing date extends only to what is actually disclosed.  The specification at issue contained many carefully enumerated substituents, reinforcing that any omissions were intentional or, at a minimum, controlling.  The court analogized that disclosing ethanol does not disclose methanol, underscoring that adjacent carbon counts can be materially different.  Because the provisional lacked C1 support, the ’953 patent could not claim its July 2020 priority date.  Pfizer’s April 6, 2021 disclosure of nirmatrelvir with a -NHC(O)-C1-alkyl “A” group therefore anticipated the claims.

The full opinion can be found here.

By Jason Evans

This article summarizes aspects of the law and does not constitute legal advice. For legal advice with regard to your situation, you should contact an attorney.