Tag Archives: Enanta

Enanta. v. Pfizer: Alcohol analogy best for explaining 2≠1?

Enanta Pharma. Inc. v. Pfizer Inc., No. 2025-1427 (June 23, 2026)

“[Enanta] did not invent what they did not disclose.”

Boom!  Enanta’s provisional patent application did not disclose -NCH(O)-C1-alkyl. It only recited -NCH(O)-C2-C12-alky. Therefore, it did not support Enanta’s non-provisional patent disclosure of -NCH(O)-C1-C12-alkyl. In between filing dates, Pfizer disclosed Nirmatrelvir, a.k.a. PF-073213321 in Paxlovid®, comprising a -NCH(O)-CF3 substituent, which is a -NCH(O)-C1-alkyl. One carbon made a big difference.

The court followed this conclusion with the following analogy:

“The issue in this case is akin to asking whether a disclosure of ethanol, a two-carbon alcohol regularly consumed by people, would provide adequate written description support for methanol, a one-carbon alcohol that is highly toxic to people.”

Fair point. Chemistry is often understood as an unpredictable art. Simply adding one atom to a molecule can significantly change its activity.  Magic methyls are indeed real for various properties of various chemical compounds. Indeed, here the methanol/ethanol analogy was decidedly persuasive. Logically in this case, if ethanol (CH3CH2OH), which is a -C2-alkyl-OH, cannot support methanol (CH3OH), which is a -C1-alkyl-OH, then -NCH(O)-C2-C12-alkyl cannot support -NCH(O)-C1-C12-alkyl.

However, even though this methyl/ethyl alcohol analogy was persuasive, was it the best analogy? Perhaps not. It was definitely good enough. Such an analogy may be persuasive only in respect to which property you are observing. In this analogy, that property is toxicity. What about other properties? Is toxicity itself even the best property to focus on?  

I believe there are at least two reasons why the alcohol analogy may not be the best analogy.

  • Ethanol is toxic, albeit to a lesser degree than methanol.
  • Both ethanol and methanol are highly flammable.

Firstly, Ethanol is indeed toxic. Just ask someone who is drunk and praying to the porcelain god. Moreover, heavy long-term alcohol use is a common cause of cirrhosis.2 Granted, compared to ethanol, methanol is drastically more toxic. DO NOT DRINK METHANOL. You will go blind.3 Literally. My discomfort with the alcohol analogy is that it appears to imply that ethanol is as safe to drink as distilled water and has no toxicity at all, which is not accurate. Most skilled artisan parents would likely not give their toddlers shots of 100 proof vodka. Rum perhaps, if they are pirates.

Secondly, both ethanol and methanol are highly flammable. Let’s extend the analogy to even simpler molecules such as ethane (two carbons) and methane (one carbon). Both are also highly flammable. Just ask Blue Origin about their New Glenn Rocket.4 Granted, methanol and ethanol indeed have different burn rates depending on various factors such as concentration. However, though the lens of flammability, I think that a provisional application claiming a flammable composition and describing ethanol would likely provide adequate written description support for methanol.

Perhaps a good “magic methyl” analogy might be a better than the alcohol analogy. However, while it might be technically better, it might not be as persuasive the alcohol analogy. I fret that it may be dismissed as pedantic. Kudos to the alcohol analogy. It is simple to understand and impactful. Dare I say elegant? Most people, skilled in the art or not, understand the axiom of whisky good, wood alcohol bad.

  1. “Pfizer unveils its oral SARS-CoV-2 inhibitor” B. Halford, C&EN Global Enterprise 2021 99 (13), 7-7. DOI: 10.1021/cen-09913-scicon3. C&EN, 2021, 99 (13), p 7 April 12, 2021 ↩︎
  2. https://www.mayoclinic.org/diseases-conditions/cirrhosis/symptoms-causes/syc-20351487 ↩︎
  3. P.F. Suit, at al., “Methanol intoxication: clinical features and differential diagnosis” Cleve Clin J Med 1990; 57:464-471 https://www.ccjm.org/content/ccjom/57/5/464.full.pdf ↩︎
  4. https://apnews.com/article/blue-origin-rocket-explosion ↩︎

2 ≠ 1: Enanta v. Pfizer

The U.S. Court of Appeals for the Federal Circuit (CAFC) issued another decision where they held an applicant to a narrow interpretation of one their application’s definitions.  

In Enanta Pharmaceuticals. Inc. v. Pfizer Inc., No. 2025-1427 (June 23, 2026), the court held that Enanta’s U.S. Patent No. 11,358,953 (“the ’953 Patent”) could not claim the benefit of priority to U.S. Provisional Patent No. 63/054,048 because the patent’s recitation of “NCH(O)-C1-C12-alkyl” in its definition of “substituent” could not be supported by the provisional applications disclosure of only “-NCH(O)-C2-C12-alkyl” in its own definition. Without that priority, Pfizer’s disclosure of a species that falls within the Enanta’s generic claim is invalidating, novelty-defeating prior art.   

Timeline:

  • July 20, 2020: Enanta filed a provisional patent describing a substituent comprising a “C2-C12 alkyl.”
  • April 6, 2021: Pfizer disclosed PF-07321332 (nirmatrelvir), which has a “C1” for the substituent.
  • November 9, 2021: Enanta filed the non-provisional correcting C2 to C1.

Enanta argued that “C2” was merely a typographical error that a skilled artisan would recognize the error as such and clearly conclude that the inventor was in possession of the full range of “C2-C12.” The court disagreed saying that the “disclosure of one chemical compound, or integer in this case, cannot necessarily be a disclosure of another, even one close by structurally.” They backed it up with the analogy of “asking whether a disclosure of ethanol, a two-carbon alcohol regularly consumed by people, would provide adequate written description support for methanol, a one-carbon alcohol that is highly toxic to people.”

Fair point. Magic-methyls are indeed real for various properties of certain chemical compounds. However, I think there is more nuance to discuss. I also think there are other lenses that the court could have used to view the facts in this case.

I intend to follow up this post with further analysis of this case.

Check back soon!