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Explore the latest in patent, trademark, and copyright cases on the Merchant & Gould LLP blog. Stay informed with expert insights on intellectual property law updates.

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Last week, the U.S. Court of Appeals for the Federal Circuit issued a precedential decision in<a href="https://www.cafc.uscourts.gov/opinions-orders/25-1427.OPINION.6-23-2026_2712960.pdf"> Enanta Pharmaceuticals, Inc. v. Pfizer Inc., No. 2025-1427 (June 23, 2026)</a>, affirming a district court’s grant of summary judgment that, because a specific claim term of Enanta’s U.S. Patent No. 11,358,953 was not supported by Enanta’s disclosure in an earlier provisional application, all claims of the ’953 patent are invalid as anticipated by Pfizer’s intervening public disclosure.

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In its opinion, the Federal Circuit explained what the case is not, namely, it is not a standard written description case, nor is it a standard case regarding the correction of errors in issued patents. The decision noted the following relevant facts:

On July 20, 2020, Enanta filed U.S. Provisional Patent Application No. 63/054,048, which included a definition that referenced “—NHC(O)—C2-C12-alkyl.”

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On April 6, 2021, Pfizer publicly disclosed the active ingredient in Paxlovid, which contains a one-carbon alkyl group. 

On July 9, 2021, Enanta realized that the ’048 provisional contained a typographical error and that the “C2” should have been “C1.”

On July 19, 2021, Enanta filed a non-provisional application listing the relevant substituent as “—NHC(O)—C1-C12-alkyl” rather than “—NHC(O)—C2-C12-alkyl,” which carried through several continuations to the ’953 patent.

In June 2022, Enanta sued Pfizer, alleging that Paxlovid infringed the ’953 patent. 

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At the district court, Pfizer had argued that the ’953 patent could not claim priority to the ’048 provisional because the provisional did not support the claimed one-carbon alkyl group. Enanta countered that the provisional’s “C2” was an obvious typographical error that should have read “C1,” so correcting it added no new matter. The district court concluded that the ’953 patent was not entitled to the ’048 provisional’s filing date, and therefore granted summary judgment of invalidity.

On appeal, the Federal Circuit noted that while the district court had focused on its power to correct a purported typographical error in the provisional application, the proper question is whether there was a genuine dispute of material fact as to whether the provisional application provided sufficient written description support for the invention claimed in the ’953 patent. The Court discounted Enanta’s expert testimony regarding the alleged typographical error, concluding there was no genuine dispute of material fact. The Court held that, “C2 is simply different from C1,” and the one-carbon alkyl group “was simply not disclosed in the ’048 provisional.”

C2 is Not C1: Federal Circuit Affirms Invalidity in Paxlovid Patent Dispute

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