In a recent decision, the U.S. Court of Appeals for the Federal Circuit upheld the invalidation of Enanta Pharmaceuticals Inc.’s U.S. Patent No. 11,358,953, which Enanta had asserted against Pfizer Inc. regarding its COVID-19 antiviral, Paxlovid. The ruling centered on a discrepancy involving a single carbon atom in the patent’s chemical description.
Enanta’s ‘953 patent, issued on June 14, 2022, claimed priority from a provisional application filed on July 20, 2020. The provisional application described a chemical group as containing two to twelve carbon atoms (“C2-C12”), whereas the later ‘953 patent expanded this range to include one to twelve carbon atoms (“C1-C12”). This seemingly minor amendment became pivotal in the legal proceedings.
On April 6, 2021, Pfizer publicly disclosed nirmatrelvir, a protease inhibitor integral to Paxlovid, which features a chemical group with a single carbon atom. Enanta alleged that Pfizer’s product infringed its ‘953 patent. However, Pfizer contended that the ‘953 patent was invalid because the provisional application did not provide adequate written description support for the “C1” inclusion, and thus, the patent could not claim the earlier priority date. Consequently, Pfizer’s disclosure of nirmatrelvir anticipated the claims of the ‘953 patent, rendering them invalid.
The District Court for the District of Massachusetts granted summary judgment in favor of Pfizer, concluding that the change from “C2” to “C1” was not a correctable typographical error and that the ‘953 patent was not entitled to the provisional application’s priority date. Enanta appealed this decision.
On June 23, 2026, the Federal Circuit affirmed the lower court’s ruling. The appellate court emphasized that for a patent to claim priority from an earlier application, the earlier application must satisfy the written description requirement of 35 U.S.C. § 112. The court found that the provisional application did not adequately support the later inclusion of the “C1” group, and therefore, the ‘953 patent could not claim the earlier priority date. As a result, Pfizer’s prior disclosure of nirmatrelvir anticipated the claims of the ‘953 patent, leading to its invalidation.
This case underscores the critical importance of precision in patent drafting, particularly in provisional applications. Even minor discrepancies can have significant legal ramifications, potentially affecting the validity and enforceability of a patent.