ENANTA PHARMACEUTICALS, INC. v. PFIZER INC.

Authored by: Jeremy J. Gustrowsky

In a decision highlighting the unforgiving nature of the written description requirement, the Federal Circuit affirmed a summary judgment ruling that Enanta Pharmaceuticals’ U.S. Patent 11,358,953 is invalid as anticipated by Pfizer’s public disclosure of nirmatrelvir, the active ingredient in Paxlovid®. The case turned on a one-character difference between Enanta’s provisional application and its later-issued patent.

Enanta’s ’953 patent, directed to compounds for inhibiting coronavirus replication, claimed priority to U.S. Provisional Patent Application 63/054,048, filed in July 2020. Both documents listed dozens of chemical moieties under a definition of “substituted,” but with one critical discrepancy: the provisional recited “—NHC(O)—C2-C12-alkyl” (covering alkyl groups with two to twelve carbon atoms), while the later non-provisional changed this to “—NHC(O)—C1-C12-alkyl” (additionally including a one-carbon version). Enanta claimed it discovered this typographical error in July 2021 and filed the corrected non-provisional shortly thereafter. Unfortunately for Enanta, Pfizer had publicly disclosed nirmatrelvir, which contains an —NHC(O)—C1-alkyl group, in April 2021. After Enanta sued for infringement of Paxlovid® in June 2022, Pfizer moved for summary judgment of invalidity, arguing the ’953 patent could not claim priority to the provisional.

The Federal Circuit took care to distinguish this case from the typical written description case law line. Unlike Ariad or In re Ruschig, this case did not involve whether a specification supported a broad genus or a later-claimed species. Instead, the question was simply whether a disclosure of “2” provides written description support for “1.” The court also distinguished its case law on correcting errors in issued patents, noting that In re Oda concerns reissue proceedings before the Patent Office, and Novo Industries permits district court correction of patent errors only when the correction is “not subject to reasonable debate.” Here, the alleged error existed in a provisional application, not an issued patent, and the existence of an error was itself debatable.

Applying the written description standard de novo, the court concluded that the provisional did not convey to a skilled artisan that the inventors possessed the C1 version of the compound. The provisional explicitly disclosed a range of two to twelve carbon atoms and notably excluded a one-carbon alkyl group. Enanta’s expert pointed to an inconsistency elsewhere in the provisional, where the definition of “alkyl” listed “C2-C12 alkyl” alongside written text describing “one to twelve” carbon atoms, arguing this mismatch demonstrated a typographical error that infected all later uses of “C2-C12 alkyl” throughout the document. The court rejected this argument because the expert pointed to a purported error in the general definition of “alkyl,” not in the specific disclosure of —NHC(O)—C2-C12-alkyl under the definition of “substituted.”

Judge Lourie offered a memorable illustration of the principle: the issue is “akin 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.” Even structurally similar compounds cannot necessarily substitute for one another in a written description analysis. The provisional specification was “highly detailed,” carefully cataloguing dozens of specific chemical moieties, and the court declined to expand its disclosure beyond what was actually written.

Because the provisional did not disclose —NHC(O)—C1-alkyl, the ’953 patent could not claim its priority date, and Pfizer’s intervening disclosure of nirmatrelvir anticipated the claims. The decision is a reminder that priority claims rise or fall on what is actually disclosed in the earlier filing, and that expert testimony about purported typographical errors elsewhere in a document will likely not rescue a claim from an anticipation finding where the specific subject matter at issue was never described.