CAFC Affirms Enanta Coronavirus Patent Invalid for Lack of Written Description Support in Provisional Application

The U.S. Court of Appeals for the Federal Circuit (CAFC) on Tuesday issued a decision in Enanta Pharmaceuticals, Inc. v. Pfizer Inc., affirming summary judgment that all claims of Enanta Pharmaceuticals, Inc.’s U.S. Patent 11,358,953 are invalid as anticipated. The court held that Enanta’s provisional application failed to provide adequate written description support under 35 U.S.C. Section 112 for a key chemical substituent claimed in the ‘953 patent. As a result, the patent could not claim the provisional’s earlier priority date and was anticipated by Pfizer’s prior disclosure of nirmatrelvir.

The ‘953 patent, filed in November 2021, covers compounds and methods for inhibiting coronavirus replication. The case turned on a single chemical notation difference between the patent and Enanta’s July 2020 provisional application. The provisional disclosed alkyl groups containing two to twelve carbon atoms, while the ‘953 patent expanded that range to include one-carbon alkyl groups. Pfizer’s nirmatrelvir contains a one-carbon alkyl group and was publicly disclosed in April 2021, making the priority date critical.

Enanta contended that on July 9, 2021, it realized the “C2” designation in the ‘048 provisional was a typographical error that should have read “C1,” and that the non-provisional application corrected that error. In June 2022, Enanta sued Pfizer, alleging that its Paxlovid product infringed the ‘953 patent’s claims. Pfizer counterclaimed invalidity and moved for summary judgment, arguing that the ‘953 patent could not claim priority from the ‘048 provisional because its disclosure of —NHC(O)—C2-C12-alkyl did not support the patent’s —NHC(O)—C1-alkyl limitation. Pfizer further argued that its intervening disclosure of nirmatrelvir therefore anticipated the asserted claims.

The U.S. District Court for the District of Massachusetts granted Pfizer’s motion in December 2024, concluding that the “C2” in the ‘048 provisional was not an obvious typographical error it had the power to correct. The court further held that changing C2 to C1 impermissibly broadened the patent’s scope, meaning the ‘953 patent was not entitled to the provisional application’s priority date. The district court then concluded that Pfizer’s earlier disclosure of nirmatrelvir anticipated the claims.

On appeal, the CAFC noted that the district court had focused on correction of a typographical error rather than the written description standard. Applying de novo review, the CAFC applied Section 112 directly. The court explained that a patent can claim the benefit of an earlier filing date under 35 U.S.C. Section 120 only if each application in the chain satisfies Section 112. That requirement is met only when the earlier application shows the inventor was in possession of the claimed invention at the earlier filing date.

The CAFC concluded the ‘048 provisional did not convey to a skilled artisan that the inventors possessed —NHC(O)—C1-alkyl at the time of filing. The court noted that C2 differs from C1 and that the provision explicitly disclosed a range of 2 to 12 carbon atoms, excluding a 1-carbon alkyl group.

The court also rejected Enanta’s expert declaration as insufficient to create a genuine dispute of material fact. Enanta’s expert pointed to an inconsistency in the ‘048 provisional, noting that “C2-C12 alkyl” conflicted with the surrounding text describing “one to twelve…carbon atoms.” He argued that a skilled artisan would view “C2” as a typographical error. The CAFC found this unpersuasive because the expert pointed to a purported error in the general definition, not in the specific —NHC(O)—C2-C12-alkyl disclosure. The court noted there was nothing in the specific substituent disclosure showing possession of a one-carbon group.

Moreover, the CAFC noted the provisional was highly detailed, listing many chemical moieties under the definition of “substituted.” As the court wrote, “we respect applicants’ statements in their specification that they invented what was specifically disclosed in the ‘048 provisional, but similarly we conclude that they did not invent what they did not disclose.”

The court also referenced In re Oda, in which the U.S. Court of Customs and Patent Appeals explained that the USPTO may reissue patents under Section 251 if no new matter is added and the error is obvious. Under Novo Industries, L.P. v. Micro Molds Corp., courts may correct issued patents only when the correction is not subject to reasonable debate. The CAFC found neither framework applicable, noting this case involved a provisional application and that the alleged error was “subject to reasonable debate.”

To illustrate the written description requirement, the court analogized the dispute to whether disclosure of ethanol, a two-carbon alcohol, would provide written description support for methanol, a distinct one-carbon alcohol. The court used this example to show that disclosure of one compound does not necessarily establish possession of another.

The CAFC held that because the ‘048 provisional did not disclose —NHC(O)—C1-alkyl, it provided no written description support for the ’953 patent, which therefore could not claim the provisional’s priority date. With priority unavailable, Pfizer’s April 2021 disclosure of nirmatrelvir anticipated the claims, and the district court’s judgment was affirmed.

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