Enanta Pharmaceuticals v. Pfizer — Federal Circuit Kills Paxlovid Patent Over Single-Carbon Priority Gap

Background

Enanta Pharmaceuticals, Inc. spent years developing antiviral compounds to fight coronavirus infections. In July 2020, it filed a provisional patent application disclosing a class of compounds that could inhibit coronavirus replication — including substituents defined as “—NHC(O)—C2-C12-alkyl” (an alkyl group with two to twelve carbon atoms). In November 2021, Enanta filed a non-provisional application that became U.S. Patent 11,358,953. But the non-provisional contained a subtle change: the same substituent was now listed as “—NHC(O)—C1-C12-alkyl” — adding one-carbon alkyl groups that the provisional had not included.

In April 2021 — after the provisional but before the non-provisional — Pfizer publicly disclosed nirmatrelvir, the active ingredient in its Paxlovid® antiviral drug. Nirmatrelvir contains a one-carbon (C1) alkyl group within the relevant substituent. In June 2022, Enanta sued Pfizer in the District of Massachusetts, alleging Paxlovid infringed the ‘953 patent. Pfizer responded that the patent was invalid: if the patent could not claim the provisional’s July 2020 priority date, then Pfizer’s own April 2021 nirmatrelvir disclosure was prior art that anticipated all of Enanta’s claims.

The district court agreed with Pfizer and granted summary judgment of invalidity, concluding that the C2-to-C1 change was not an obvious typographical error and therefore prevented the non-provisional from claiming the provisional’s priority date. Enanta appealed to the Federal Circuit.

The Court’s Holding

The Federal Circuit affirmed, but clarified the legal framework. Writing for Judges Lourie, Bryson, and Chen, Judge Lourie held that the correct standard is the written description requirement under 35 U.S.C. § 112 — not the district court’s “obvious typographical error” analysis borrowed from reissue practice. To claim the benefit of an earlier provisional, a patent must show that the provisional adequately describes the later-claimed invention, meaning a skilled artisan would conclude the inventors possessed the claimed subject matter at the time of the provisional filing.

The court was direct: “C2 is simply different from C1.” The provisional “explicitly includes an alkyl group with two to twelve carbon atoms (i.e., —NHC(O)—C2-C12-alkyl), and notably does not include an alkyl group with one carbon atom.” The later patent’s C1 disclosure was new matter — not supported by the provisional. Enanta’s expert argued that a separate part of the provisional’s general “alkyl” definition contradictorily described “C2-C12 alkyl” in words as “one to twelve carbon atoms,” suggesting the numeral “2” was itself a typo. The Federal Circuit rejected this: the alleged inconsistency appeared in a different portion of the specification, not the specific substituent at issue. Expert speculation about errors elsewhere in the document does not create a genuine fact dispute about a specific, unambiguous disclosure.

The court offered a memorable analogy: asking whether a disclosure of ethanol (a two-carbon alcohol safe for consumption) provides written description support for methanol (a one-carbon alcohol that is highly toxic) illustrates exactly why a disclosure of one chemical compound cannot necessarily support a structurally adjacent but different one.

Key Takeaways

• Provisional applications are read literally: Numerical chemical ranges in provisional applications are taken at face value. A provisional disclosing C2–C12 alkyl does not support C1–C12 alkyl in the later patent, even if the patentee claims a typo.
• Written description governs priority disputes: The standard for whether a non-provisional can claim a provisional’s priority date is the § 112 written description requirement — not the “obvious typographical error” doctrine from patent reissue practice under § 251.
• Expert declarations about errors elsewhere fall short: Pointing to an inconsistency in a different part of the specification does not create a genuine fact dispute about a specific, unambiguous disclosure in another part of the document.
• Drug races amplify the stakes: In competitive pharmaceutical research, a competitor disclosure that lands between a provisional and non-provisional filing can be fatal to an entire patent if the provisional did not precisely capture the relevant compound class.

Why It Matters

This is a cautionary tale for pharmaceutical innovators — particularly those racing to patent novel drug candidates against competitors working the same compound class. Enanta’s patent on coronavirus antivirals was invalidated not because its underlying science was flawed, but because a single subscript in its 2020 provisional application read “2” instead of “1.” When Pfizer publicly disclosed nirmatrelvir (the active ingredient in Paxlovid®) before Enanta’s corrected non-provisional was filed, that single-digit gap became fatal to the patent.

For patent practitioners, the decision reinforces that provisional applications deserve the same careful drafting scrutiny as non-provisional filings. In fast-moving fields like antiviral drug development, companies must ensure their provisional applications fully capture every aspect of the compound class they intend to claim — because there may be no second chance once a competitor’s disclosure enters the public record. No amount of “it was just a typo” argumentation, even backed by expert testimony, will substitute for unambiguous written description in the provisional itself.