The US Court of Appeals for the Federal Circuit affirmed summary judgment that the asserted patent was invalid as anticipated because the provisional application to which it claimed priority did not provide adequate written description support for the claimed compound. The Court explained that the proper inquiry was whether the provisional demonstrated possession of the later-claimed subject matter, not whether a discrepancy in the provisional could be characterized as an obvious typographical error. Enanta Pharmaceuticals, Inc. v. Pfizer Inc., Case No. 25-1427 (Fed. Cir. June 23, 2026) (Lourie, Bryson, Chen, JJ.)

Enanta owns a patent direct to coronavirus inhibitors. The patent claimed priority to a July 20, 2020, provisional application. In April 2021, before Enanta filed a nonprovisional application correcting what it later characterized as a typographical error, Pfizer publicly disclosed the compound that ultimately became part of its COVID-19 treatment, Paxlovid®.

The dispute centered on a structural limitation defining an alkyl substituent. The provisional disclosed NHC(O)-C2-C12-alkyl while the asserted patent claimed NHC(O)-C1-C12-alkyl. According to Enanta, the “C2” designation in the provisional was an obvious drafting error that a person of ordinary skill in the art would recognize and mentally correct.

The district court granted summary judgment that the patent was anticipated because the asserted claims were not entitled to the provisional application’s filing date. Enanta appealed.
While the Federal Circuit affirmed, it clarified that the district court framed the issue incorrectly. Rather than asking whether a court could correct an alleged error in the provisional, the proper question was whether the provisional itself provided sufficient written description support for the later-claimed invention.

In clarifying the appropriate inquiry, the Federal Circuit rejected Enanta’s reliance on cases addressing correction of errors in issued patents and decisions concerning genus-species written description issues, explaining that those authorities did not govern the priority analysis presented here. Nor could the Court simply construe the provisional to correct the alleged mistake because the disclosure was subject to reasonable debate.

Applying the written description standard, the Federal Circuit concluded that “C2 is simply different from C1.” Because the provisional disclosed only the C2 structure, it did not demonstrate that the inventors possessed the later-claimed C1 embodiment as of the provisional filing date. The Court also found Enanta’s expert testimony insufficient to create a genuine issue of material fact, explaining that generalized opinions regarding carbon chain nomenclature and drafting errors elsewhere in the specification did not establish written description support for the specific claimed limitation.

Because the asserted claims were not entitled to the provisional application’s priority date, Pfizer’s intervening public disclosure constituted anticipatory prior art, rendering the patent invalid.