The U.S. Court of Appeals for the Federal Circuit (CAFC) issued another decision where they held an applicant to a narrow interpretation of one their application’s definitions.  

In Enanta Pharms. Inc. v. Pfizer Inc., No. 2025-1427 (June 23, 2026), the court held that Enanta’s U.S. Patent No. 11,358,953 (“the ’953 Patent”) could not claim the benefit of priority to U.S. Provisional Patent No. 63/054,048 because the patent’s recitation of “NCH(O)-C₁-C₁₂-alkyl” in its definition of “substituent” could not be supported by the provisional applications disclosure of only “-NCH(O)-C₂-C₁₂-alkyl” in its own definition. Without that priority, Pfizer’s disclosure of a species that falls within the Enanta’s generic claim is invalidating, novelty-defeating prior art.   

Timeline:

• July 20, 2020: Enanta filed a provisional patent describing a substituent comprising a “C₂-C₁₂ alkyl.”
• April 6, 2021: Pfizer disclosed PF-07321332 (nirmatrelvir), which has a “C₁” for the substituent.
• November 9, 2021: Enanta filed the non-provisional correcting C₂ to C₁.

Enanta argued that “C₂” was merely a typographical error that a skilled artisan would recognize the error as such and clearly conclude that the inventor was in possession of the full range of “C₂-C₁₂.” The court disagreed saying that the “disclosure of one chemical compound, or integer in this case, cannot necessarily be a disclosure of another, even one close by structurally.” They backed it up with the analogy of “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.”

Fair point. Magic-methyls are indeed real for various properties of certain chemical compounds. However, I think there is more nuance to discuss. I also think there are other lenses that the court could have used to view the facts in this case.

I intend to follow up this post with further analysis of this case.

Check back soon!