“2” Does Not Provide Written Description Support for “1”: Federal Circuit Affirms District Court’s Invalidation of Patent

In Enanta Pharms., Inc. v. Pfizer Inc., No. 25-1427, (Fed. Cir. June 23, 2026), the Federal Circuit affirmed the district court’s grant of summary judgment invalidating U.S. Patent No. 11,358,953 (“the ’953 patent”).

The ’953 patent issued from a nonprovisional application filed on November 9, 2021, and claims priority to U.S. Provisional Patent App. No. 63/054,048 (“the ’048 provisional”) with a priority date of July 20, 2020. Notably, the ’048 provisional discloses a two-to-twelve alkyl group (“C₂-C₁₂-alkyl”) whereas the ’953 patent discloses a one-to-twelve alkyl group (“C₁-C₁₂-alkyl”). Between the filing of the ’048 provisional and the ’953 patent, Pfizer publicly disclosed an inhibitor with a C₁-alkyl structure.

Enanta sued Pfizer, alleging that Pfizer’s Paxlovid® product, a coronavirus treatment, infringed the ’953 patent. On summary judgment, Pfizer argued that the ’953 patent was invalid because there was no written description support for the C₁-alkyl structure in the ’048 provisional, and thus, Pfizer’s intervening public disclosure anticipated the ’953 patent claims.

Enanta agreed that the claims would be anticipated if the priority claim failed, but argued the Court could should correct what it characterized as a typographical error in the ’048 provisional. The district court disagreed and granted summary judgment for Pfizer.

The Federal Circuit explained that the issue turned on whether the ’048 provisional provided adequate written description support for the claimed C₁-alkyl substituent. Enanta argued an inconsistency in the ’048 provisional’s definition of “alkyl” purportedly showed that references to “C₂-C₁₂-alkyl” were typographical errors that should have been read as “C₁-C₁₂-alkyl.” The Court disagreed, concluding that an alleged error elsewhere in the specification did not create a factual dispute that the specific disclosure of the C₂-C₁₂-alkyl group was mistaken.