On June 23, the U.S. Court of Appeals for the Federal Circuit issued its decision in Enanta Pharmaceuticals, Inc. v. Pfizer Inc., No. 2025-1427, refusing to revive U.S. Patent No. 11,358,953 (“the ’953 patent”) and affirming the district court’s grant of summary judgment of invalidity. The case arose from Enanta’s 2022 infringement action alleging that Pfizer’s blockbuster COVID-19 antiviral Paxlovid (nirmatrelvir tablets; ritonavir tablets) infringed claims directed to coronavirus protease inhibitors. The federal circuit’s ruling underscores that errors in the priority chain — even those a patent holder characterizes as typographical — can fatally undermine patent validity when intervening prior art fills the gap.

Background and Procedural History

Enanta filed its infringement complaint in the U.S. District Court for the District of Massachusetts in June 2022, shortly after the USPTO issued the ’953 patent based on a July 2020 application describing coronavirus protease inhibitors. Enanta publicly stated that it sought damages only and did not intend to seek an injunction or impede Paxlovid’s availability.

Pfizer moved for summary judgment of invalidity, arguing that the ’953 patent was not entitled to the benefit of Enanta’s earlier provisional application filing date. The crux of the dispute was a change between the provisional and non-provisional applications in the definition of “substituted:” the provisional specified C₂-C₁₂ alkyls, while the issued patent recited C₁-C₁₂ alkyls. This distinction was material because Enanta’s infringement theory depended on nirmatrelvir containing a C₁ alkyl group.

In December 2024, Judge Denise J. Casper granted Pfizer’s motion, holding that the broadened claim scope was not supported by the provisional disclosure and therefore the patent could not claim the provisional’s priority date. Enanta argued the C₂-to-C₁ change corrected an “obvious typographical error,” but the court rejected that characterization, reasoning that while courts may correct obvious errors in patent documents under narrow circumstances, they cannot use claim construction to remedy a substantive mistake that should properly be addressed through USPTO procedures such as reissue.

Without the provisional’s priority date, intervening Pfizer disclosures — including a 2021 American Chemical Society publication describing nirmatrelvir and a Pfizer patent application filed before the ’953 patent application but after Enanta’s provisional — constituted invalidating prior art. The district court accordingly entered summary judgment of invalidity, rendering Enanta’s infringement motion moot.

The Federal Circuit’s Decision

After oral argument on May 11, the federal circuit affirmed, rejecting Enanta’s arguments and declining to revive the patent. The decision confirms that the federal circuit will not use error-correction doctrines to expand a patent’s priority entitlement where the claimed scope in the issued patent exceeds what was described in the provisional application.

Key Implications

This decision is particularly significant in life sciences patenting, where provisional applications are often filed rapidly (such as here, during a public health emergency) and where intervening disclosures from competitors and collaborative research networks can quickly populate the prior art landscape. A single definitional discrepancy between a provisional and a non-provisional application can open a gap through which invalidating prior art enters, collapsing the entire enforcement effort.

Notably, Enanta has pursued parallel enforcement of related European Patent No. EP 4 051 265 before the Unified Patent Court (Action No. 35071/2025), filed in August 2025. The U.S. outcome does not automatically dictate foreign results, but it may affect settlement dynamics and damages negotiations globally.

Practical Advice for Clients

1. Rigorously Audit Provisional-to-Non-Provisional Continuity Before Filing and Enforcement.

Patent applicants and litigants should conduct a methodical, claim-by-claim comparison of definitions, ranges, Markush groups, species, and genus coverage between provisional applications and subsequently filed non-provisional applications. Do not assume that a later “clarification” or broadening will automatically receive the provisional’s filing date. Any discrepancy should be identified and resolved — either by narrowing claims to match provisional support or by acknowledging the later effective filing date — before the patent is asserted.

2. Address Material Errors Through USPTO Mechanisms Before Litigation Hardens Positions.

If a material drafting error is discovered post-issuance, evaluate corrective procedures such as reissue (35 U.S.C. § 251), certificates of correction, or ex parte reexamination at the earliest opportunity. The Enanta decision reinforces that courts will not rewrite patent text to preserve priority; the proper forum for substantive corrections is the USPTO. Waiting until litigation to argue that a discrepancy is merely a “typo” substantially increases the risk of an adverse outcome.

3. Map Accused Products Against Each Priority Document and Intervening Prior Art.

In due diligence and pre-litigation assessments, do not analyze infringement solely against issued claims. Instead, trace each asserted claim back through the priority chain and identify any intervening publications, patent applications, or public disclosures that could become prior art if the earliest priority date is lost. This exercise is particularly critical in fast-moving fields — such as antivirals during a pandemic — where competitors may publish rapidly.

4. Assess Multi-Jurisdictional Enforcement Independently.

A U.S. priority or validity loss does not necessarily determine outcomes in foreign jurisdictions, including the UPC, EPO, or national courts. However, a significant adverse ruling in one jurisdiction can affect settlement leverage, narrative, and damages calculations elsewhere. Global enforcement strategies should account for jurisdiction-specific priority rules and evaluate each patent family member on its own merits.