-
Federal Circuit Affirms Anticipation of COVID-19 Antiviral Patent, Holding That a Provisional’s “C2” Disclosure Cannot Support a “C1” Claim for Priority
06/30/2026On June 23, 2026, the U.S. Court of Appeals for the Federal Circuit (Judges Lourie, Bryson, and Chen) affirmed a decision of the U.S. District Court for the District of Massachusetts granting summary judgment that all claims of Enanta Pharmaceuticals, Inc.’s U.S. Patent No. 11,358,953 (“the ’953 patent”) are invalid as anticipated. Enanta Pharms., Inc. v. Pfizer Inc., No. 2025-1427 (Fed. Cir. June 23, 2026). The Court held that the ’953 patent could not claim priority to an earlier provisional application because the provisional lacked written description support for the claimed compound, leaving the patent anticipated by defendant’s intervening public disclosure of nirmatrelvir, an active ingredient in its Paxlovid® COVID-19 treatment.
The ’953 patent, which issued from a non-provisional application filed on November 9, 2021, is directed to compounds and methods of inhibiting coronavirus replication. The patent claims priority to a provisional application (“the ’048 provisional”), which had a priority date of July 20, 2020. Both the ’048 provisional and the ’953 patent define the term “substituted” and list dozens of chemical substituents, and the definitions are substantially the same but for one critical difference: the ’048 provisional recites the substituent “—NHC(O)—C2-C12-alkyl,” whereas the ’953 patent recites “—NHC(O)—C1-C12-alkyl.” Because the subscripts denote the number of carbon atoms, the provisional’s “C2-C12” covers alkyl groups of two to twelve carbons, while the patent’s “C1-C12” additionally includes a one-carbon alkyl group.
According to patentee, on July 9, 2021, it realized that the “C2” in the ’048 provisional was a typographical error that should have read “C1,” and it filed the non-provisional application listing “—NHC(O)—C1-C12-alkyl” ten days later. In the interim, however, on April 6, 2021, the defendant had publicly disclosed nirmatrelvir—a compound substituted with a —NHC(O)—C1-alkyl group. After being sued in June 2022, the defendant counterclaimed for invalidity and moved for summary judgment, arguing that the ’953 patent could not claim priority to the ’048 provisional and that its earlier disclosure of nirmatrelvir therefore anticipated the asserted claims. The district court agreed, concluding that the “C2” was not an obvious typographical error that it had the power to correct and that the change to “C1” impermissibly broadened the patent’s scope, defeating priority and rendering the claims anticipated.
On appeal, the Court began with “what this case is not.” It distinguished classic written description genus/species disputes such as Ariad Pharmaceuticals, Inc. v. Eli Lilly & Co. and In re Ruschig, explaining that the question here was narrower: whether the ’048 provisional’s disclosure of “—NHC(O)—C2-C12-alkyl” provided adequate written description support for the ’953 patent’s “—NHC(O)—C1-alkyl”—that is, whether “2” supports “1.” Citing Lockwood v. American Airlines, Inc., the Court reiterated that the written description standard of 35 U.S.C. § 112 governs entitlement to an earlier application’s filing date under 35 U.S.C. § 120. The Court also set aside the parties’ and the district court’s error-correction framing, explaining that neither In re Oda (governing the Patent Office’s reissue of patents with corrections of obvious errors under § 251 that add “no new matter”) nor Novo Industries, L.P. v. Micro Molds Corp. (governing judicial correction of patent errors not subject to reasonable debate) applied, because this case was not before the Patent Office and the existence of any error in the ’048 provisional was “certainly ‘subject to reasonable debate.’” Although the district court had not applied the written description standard, the Court applied it itself on de novo review.
Turning to the merits, the Court held that the ’048 provisional did not convey to a skilled artisan that the inventors possessed “—NHC(O)—C1-alkyl” as of the provisional’s filing date. “C2 is simply different from C1,” the Court reasoned, and a disclosed range of two to twelve carbon atoms “notably does not include an alkyl group with one carbon atom.” The Court rejected patentee’s reliance on an expert declaration identifying a purported inconsistency between the provisional’s numerical “C2-C12” and a corresponding written-out “one to twelve” range in the general definition of “alkyl.” That argument, the Court explained, addressed a purported error in the general definition of “alkyl,” not in the specific disclosure of “—NHC(O)—C2-C12-alkyl” at issue, and “[e]ntitlement to a filing date . . . extends only to that which is disclosed.” Observing that the specification was “highly detailed” and described dozens of moieties with care, the Court “respect[ed] applicants’ statements . . . that they invented what was specifically disclosed,” but concluded that “they did not invent what they did not disclose.” Illustrating the point, the Court analogized the dispute to “asking whether a disclosure of ethanol, a two-carbon alcohol regularly consumed by people, would provide adequate written description support for methanol, a one-carbon alcohol that is highly toxic to people.”
Finding no genuine dispute of material fact, the Court held that the ’048 provisional provided no written description support for the ’953 patent, that the patent therefore could not claim the provisional’s priority date, and that the defendant’s earlier disclosure of nirmatrelvir consequently anticipated the asserted claims.
The decision is a pointed reminder that a single character can decide a patent’s fate.
IP Litigation