Enanta Pharmaceuticals faced a significant legal setback after the US Court of Appeals for the Federal Circuit affirmed summary judgment invalidating its patent on coronavirus inhibitors.
The Federal Circuit ruled the asserted patent was invalid as anticipated because its provisional application failed to provide adequate written description support for the claimed compound.
The case, Enanta Pharmaceuticals, Inc. v. Pfizer Inc., Case No. 25-1427, was decided on June 23, 2026, by Judges Lourie, Bryson, and Chen.
Enanta’s patent claimed priority to a provisional application filed on July 20, 2020, which the company argued should shield it from prior art disclosures made by Pfizer (PFE) in April 2021.
Pfizer’s April 2021 public disclosure described the compound that would eventually become part of its COVID-19 treatment, Paxlovid, predating Enanta’s nonprovisional application.
The central dispute involved a structural limitation concerning an alkyl substituent, where the provisional disclosed NHC(O)-C2-C12-alkyl while the asserted patent claimed NHC(O)-C1-C12-alkyl.
Enanta argued that the “C2” designation in the provisional application was an obvious drafting error that a person of ordinary skill in the art would naturally recognise and mentally correct.
The Federal Circuit rejected this approach, clarifying that the proper question was not whether a court could correct an alleged typographical error, but whether the provisional provided sufficient written description support for the later-claimed invention.
The Court stated plainly that “C2 is simply different from C1,” concluding that the provisional did not demonstrate that the inventors possessed the C1 embodiment at the time of filing.
The Federal Circuit also found that Enanta’s expert testimony failed to create a genuine issue of material fact, noting that generalised opinions on carbon chain nomenclature did not establish written description support for the specific claimed limitation.
The Court rejected Enanta’s reliance on cases addressing correction of errors in issued patents, explaining those authorities did not govern the priority analysis at issue here.
Because the asserted claims could not claim the provisional application’s filing date, Pfizer’s intervening public disclosure served as anticipatory prior art, rendering the patent invalid.
The ruling underscores how even small structural discrepancies in provisional patent applications can have significant consequences for priority claims and patent validity in pharmaceutical litigation.