On June 23, 2026, the United States Court of Appeals for the Federal Circuit affirmed a district court decision that an Enanta Pharmaceuticals patent was invalid as anticipated by Pfizer’s public disclosure. The court held that Enanta’s provisional patent application did not sufficiently support the subject matter later claimed in the eventual patent and rejected patentee’s argument that the missing disclosure was a simple typographical error.
Provisional applications are often filed shortly before a public disclosure, such as a presentation at a conference, a big meeting with investors, or a publication in a journal. At times, the decision to file a patent application is a last-minute consideration and rushed. However, this case provides a pointed lesson in how such hastiness can have dire consequences, particularly in competitive areas of technology where multiple companies are striving to reach the same goals.
Here, Enanta filed a provisional patent application on July 20, 2020, disclosing a chemical composition where one of the substituents was identified as NHC(O)-C2-C12, whereas the later granted patent claims a composition where that substituent is identified as NHC(O)-C1-C12. Enanta first introduced the C1 substituent in its first non-provisional patent application one year after the provisional filing on July 19, 2021, which is perfectly permissible.
However, in the meantime at Pfizer, work was being carried out on the C1-substituent compound, and during the time between Enanta’s provisional and non-provisional patent application filings, Pfizer presented the C1-substituent compound at a presentation on April 6, 2021. This presentation created an intervening public disclosure. Pfizer went on to commercialize a compound identical to the claims, including the C1 substituent. Enanta sued Pfizer for infringement of that claim in its granted patent.
The issue in the case was whether the compound having the C1 substituent was sufficiently disclosed in the provisional application to support the later granted claims, which included the C1 compound. Enanta argued that the difference between NHC(O)-C2-C12 and NHC(O)-C1-C12 was a typographical error. Pfizer argued that “2” is expressly different from “1” and that “2” expressly excludes “1.”
Certain mistakes in patent claims can be corrected either through the U.S. Patent Office reissue process or by a court. In Novo Indus. L.P. v. Micro Molds Corp, 350 F.3d 1348, 1354 (Fed. Cir. 2003), the court set forth a two-part test for when it is appropriate for a court to make a correction in the claims. First, the correction must not be subject to reasonable debate based on consideration of the claim language and the specification, and second, the prosecution history must not suggest a different interpretation of the claims. In other words, the error must be evident from the face of the patent taken from the viewpoint of a person in the technical field of the invention. Here, the court held that this error was clearly subject to reasonable debate and therefore was not the type of error that could be corrected by a court. To explain its position, the court likened the distinction between a C1 compound and a C2 compound to the difference between methanol (a C1 compound, highly toxic to human ingestion) and ethanol (a C2 compound, commonly consumed alcohol). This example, according to the court, “illustrates why a disclosure of one chemical compound, or integer in this case, cannot necessarily be a disclosure of another, even one close by structurally.” Id. at 10.
Pfizer was particularly suited to attack the Enanta claim on the basis of a claim drafting error, because years earlier Pfizer itself was on the other end of such an error. Pfizer, Inc. v. Ranbaxy Labs, 457 F.3d 1284, 1292 (Fed. Cir. 2006). In the Pfizer case, Pfizer had asserted a patent against a generic drug maker alleging infringement, only to find that the asserted claim depended from a non-overlapping claim (i.e., the claim depended from claim 2 but should have depended from claim 1). Again, a “2” was mistakenly recited where the claim should have stated “1” and there too, the claim was held invalid. Although the statutory basis for invalidity was different, the coincidence is remarkable.
Patent applications are important company assets. It is critical that both a competent patent attorney and the inventor(s) work carefully to prepare and prosecute patent applications. Errors which may seem easily correctable may lead to patent invalidity if not addressed before litigation is initiated, and preferably during prosecution.
The case is Enanta Pharmaceuticals, Inc. v. Pfizer Inc. C.A. No. 2025-1427 __ F.4th__ (Fed. Cir., June 23, 2026). The case was before Circuit Judges Lourie, Bryson, and Chen, with the opinion authored by Circuit Judge Lourie.
For questions about this legal alert, please contact a member of the Davis Graham Intellectual Property & Transactions Group.