In a June 23, 2026, precedential ruling, the U.S. Court of Appeals for the Federal Circuit affirmed a decision by the U.S. District Court for the District of Massachusetts invalidating Enanta Pharmaceuticals’ U.S. Patent No. 11,358,953. The court held that Enanta could not rely on its provisional application filing date because the provisional application did not provide written description support for subject matter later included in the issued patent.
The decision reinforces the importance of carefully reviewing provisional applications before filing. Even modest changes between a provisional application and later-filed applications can affect priority, particularly where an intervening disclosure occurs before the later application is filed.
Background
Enanta’s ’953 patent is directed to compounds and methods for inhibiting coronavirus replication activity. The patent claims priority to a July 20, 2020 provisional application. Both the provisional application and the later-issued patent included definitions of “substituted” and listed various chemical moieties that could qualify as substituents.
The key difference between the provisional application and the issued patent was modest but unfavorable. The provisional application listed a critical substituent as “—NHC(O)—C2-C12-alkyl,” while the non-provisional application was updated to list the corresponding substituent as “—NHC(O)—C1-C12-alkyl.” The C1-C12 range includes one-carbon alkyl groups that were not included in the C2-C12 range disclosed in the provisional application.
After Enanta filed its provisional application, but before the later application leading to the ’953 patent, Pfizer publicly disclosed nirmatrelvir. Importantly, nirmatrelvir includes a C1 alkyl-containing substituent. Enanta later sued Pfizer, alleging that Pfizer’s product Paxlovid® infringed the ’953 patent. Pfizer counterclaimed that the patent was invalid as being anticipated by Pfizer’s intervening disclosure by arguing that Enanta was not entitled to the provisional filing date. This is because the provisional application did not provide written description support for the later, crucial C1 alkyl disclosure.
Key Issues
Written Description and Priority
The Federal Circuit framed the issue as one of written description. The Court reaffirmed the need of written description in priority documents to support a priority claim. To obtain the benefit of an earlier filing date, the earlier application must describe the later-claimed invention in sufficient detail to show that the inventors were in possession of the claimed subject matter as of the earlier filing date.
The court held that Enanta’s provisional application did not support the later C1 alkyl disclosure. As the court explained, “C2 is simply different from C1.” Because the provisional application expressly disclosed a C2-C12 alkyl range, and did not disclose the relevant C1 alkyl substituent, the later-issued patent could not rely on the provisional filing date.
Typographical Error Argument
Enanta argued that the C2 language in the provisional application was a typographical error and that the substituent should have been listed as C1-C12 alkyl. The Federal Circuit was not persuaded. The court noted that Enanta’s expert pointed to a possible inconsistency in the general definition of “alkyl,” but not in the specific C2-C12 substituent disclosure at issue.
The court also distinguished cases involving correction of errors in issued patents. The issue was not whether a court or the U.S. Patent and Trademark Office could correct an obvious error, but whether the provisional application provided written description support for the later C1 alkyl disclosure. The court concluded that it did not.
Anticipation
Because the ’953 patent was not entitled to the provisional filing date, Pfizer’s public disclosure of nirmatrelvir became intervening prior art. Enanta did not dispute that Pfizer’s disclosure anticipated the claims if the patent was not entitled to the provisional application filing date. The Federal Circuit therefore affirmed the summary judgment of invalidity. Enanta is likely to seek further review, including panel rehearing, rehearing en banc, or Supreme Court review. Absent a change in outcome on further review, however, the Federal Circuit’s decision leaves the ’953 patent claims invalid as anticipated.
Implications of the Ruling
This decision underscores the importance of treating provisional applications with the same care as non-provisional applications, particularly for chemical and pharmaceutical inventions. Even small changes to chemical definitions, substituent lists, Markush groups, or numerical ranges can affect whether later claims are entitled to an earlier filing date.
The ruling also highlights the risk of relying on later corrections or expert testimony to address perceived errors in a priority application. Entitlement to priority depends on what the earlier application explicitly discloses, not what the applicant later intended to disclose.
Applicants should carefully review provisional applications before filing, particularly definitions, substituent lists, examples, and claim-like embodiments. Where possible, applicants should include explicit fallback positions and ensure that the provisional application supports the full intended scope of the claims.
If you have questions about the impact of this ruling on current or future patent strategy, please contact the authors or your regular Lathrop GPM attorney.