In a recent precedential decision by the Federal Circuit, the Court delivered a cautionary reminder that errors or omissions in a provisional patent application can have significant consequences for a patent’s priority claim and ultimately its validity.
In Enanta Pharmaceuticals, Inc. v. Pfizer, Inc., No. 2025-1427 (Fed. Cir. June 23, 2026), the Federal Circuit affirmed summary judgment that Enanta’s ’953 patent (U.S. 11,358,953) was not entitled to the priority date of its ’048 provisional application (Application No. 63/054,048) because the ’048 provisional lacked adequate written description support for a chemical substituent claimed in the ’953 patent. As a result, Pfizer’s public disclosure of its protease inhibitor, nirmatrelvir (the active ingredient in Paxlovid®), anticipated the claims of the ’953 patent, rendering the patent invalid.
The dispute centered on a single difference in the disclosure of a chemical substituent. The ’048 provisional defined the substituent as “-NHC(O)-C2-C12-alkyl,” while the ’953 patent defined the substituent as “-NHC(O)-C1-C12-alkyl” with the only difference being the subscript of the C2 and C1 carbon atoms in the alkyl group: “C2-C12” denotes alkyl groups containing a range of two to twelve carbon atoms, while “C1-C12” additionally includes a one-carbon alkyl group. Although the difference was only in one subscript, the change expanded the claimed alkyl group to include one-carbon alkyl groups that were not expressly disclosed in the ’048 provisional.
Enanta argued that the “C2” in the ‘048 provisional was merely a typographical error, which should have been written as “C1,” and that a person of ordinary skill in the art would have understood it to mean “C1.” Enanta also relied on its expert’s declaration, which pointed out an inconsistency in the disclosure between the listed “C2-C12 alkyl” using numerals 2 and 12 and the corresponding written numbers of “one to twelve … carbon atoms.” The Court disagreed with both arguments.
The Court rejected Enanta’s expert’s declaration “because its expert points to a purported typographical error in the general definition of ‘alkyl,’ not in the specific disclosure of –NHC(O)-C2-C12-alkyl in the definition of ‘substituted.’” The Court further noted that Enanta’s expert failed to identify any information in the disclosure of –NHC(O)-C2-C12-alkyl to provide support for a C1 alkyl.
Finding no basis to treat the issue as a correction of an obvious typographical error, the Court applied the written description requirement for priority under 35 U.S.C. § 112. The Court emphasized that the ’953 patent is entitled to an earlier filing date only if the earlier application demonstrates that the inventors were in possession of the later-claimed invention. Here, the ’048 provisional expressly disclosed a C2-C12 alkyl range and not a C1-C12 alkyl range. Because a one-carbon alkyl group was not disclosed in the ’048 provisional, the Court held that the ’048 provisional did not support a one-carbon alkyl group.
This decision demonstrates the importance of ensuring fulsome and correct disclosures in provisional applications and underscores the fundamental principle of patent law that priority extends only to what was actually disclosed. In this case, a single difference in a subscript cost Enanta its priority date and ultimately resulted in the invalidation of the ’953 patent over intervening prior art. This case thus serves as an important reminder for careful review of patent applications prior to filing, as even a small error or ambiguity can lead to large consequences.
