Federal Circuit Rules Typographical Error in Provisional Application Spoils Priority Date, Invalidating Enanta Patent Against Pfizer's Paxlovid
核心洞察
The Federal Circuit affirmed that Enanta's US11358953 patent is not entitled to its July 2020 provisional priority date due to inadequate written description support.
The provisional application disclosed a substituent as —NHC(O)—C2–C12-alkyl, while the issued patent claimed —NHC(O)—C1–C12-alkyl, adding the one-carbon species matching nirmatrelvir.
Because the C1 group was never disclosed in the provisional filing, Pfizer's April 2021 public disclosure of nirmatrelvir constituted intervening prior art that anticipated the claims.
In a decision with significant implications for patent drafting in the pharmaceutical industry, the U.S. Court of Appeals for the Federal Circuit has affirmed that Enanta Pharmaceuticals' U.S. Patent No. 11,358,953 is invalid due to a critical discrepancy between its provisional and non-provisional applications. The ruling, handed down on June 23, 2026, underscores the unforgiving nature of written description requirements under 35 U.S.C. § 112.
The case, Enanta Pharmaceuticals, Inc. v. Pfizer Inc., No. 2025-1427, centered on Enanta's allegations that Pfizer's COVID-19 (搜索) antiviral Paxlovid infringed the '953 patent, which is directed to compounds and methods for inhibiting coronavirus replication.
The Critical Discrepancy
The '953 patent issued from a non-provisional application filed on November 9, 2021, and claimed priority to a provisional application filed on July 20, 2020. In the provisional application, a key substituent was disclosed as —NHC(O)—C2–C12-alkyl, describing a two-to-twelve carbon alkyl chain. However, the non-provisional application and resulting patent recited the substituent as —NHC(O)—C1–C12-alkyl, expanding the range to include a one-carbon alkyl group.
Between the two filings, Pfizer publicly disclosed nirmatrelvir, the active ingredient in Paxlovid, which bears a C1-alkyl group. This intervening disclosure became the central issue in the priority dispute.
The District Court and Federal Circuit Analysis
The District Court granted summary judgment that the asserted claims were anticipated, holding that the change from C2 to C1 was not an obvious typographical error and impermissibly broadened the invention. Consequently, the patent could not claim the provisional application's earlier priority date.
The Federal Circuit affirmed. The court clarified that the dispositive question was not whether "C2" was a drafting error, but whether the provisional application demonstrated that the inventor possessed the later-claimed invention. The Federal Circuit determined that a disclosure of C2–C12 alkyl does not provide written description support for C1–C12 alkyl, because C2 is simply different from C1.
In a striking analogy, the Federal Circuit likened this distinction to disclosing ethanol, a two-carbon alcohol, versus methanol, a one-carbon alcohol. The court rejected Enanta's expert testimony, concluding that the provisional application's detailed disclosure of numerous chemical moieties did not support reading in a C1 group that was never disclosed.
Implications for Patent Practice
Enanta argued that the "C2" in its provisional was an obvious typographical error that should have read "C1," and that correcting it added no new matter. The Federal Circuit applied the written description standard de novo, even though the district court had treated the case as one about its power to correct an error. The court's answer to the question "Does '2' describe '1'?" was an unequivocal no.
Because the '953 patent was not entitled to the provisional application's July 2020 priority date, Pfizer's intervening disclosure of nirmatrelvir in April 2021 constituted prior art, invalidating the claims of the '953 patent.
The ruling serves as a cautionary tale for pharmaceutical patent practitioners: even a single-character discrepancy between a provisional and non-provisional application can prove fatal to patent validity when it affects the scope of chemical subject matter disclosed.