The One-Carbon Mistake That Cost A Patent Its Priority Date
Nobody likes the grammar police. Patent lawyers, unfortunately, may be the exception.
In Enanta Pharmaceuticals, Inc. v. Pfizer Inc. No. 2025-1427 (Fed. Cir. June 23, 2026), one character did a lot of damage. The difference between C2 and C1 cost Enanta the priority date of its provisional application—and led the Federal Circuit to affirm that the challenged claims were anticipated.
Turns out proofreading really can be worth millions.
But the case raises a more interesting question: Even if Enanta had gotten that one number right, did its provisional actually demonstrate possession of the later-claimed invention?
Enanta’s patent (U.S. Patent No. 11,358,953) covered compounds for inhibiting coronavirus replication and claimed priority to a provisional application filed in July 2020.
For non-patent folks, a priority claim is basically a way of saying: “Don’t judge my patent as of today, judge it as of that earlier filing date.” That can be hugely important, because anything that became public after the earlier date may not count against you. But there’s a price of admission: the earlier application must actually describe the invention you are later claiming. You don’t get to time-travel unless the invention was disclosed at the time of filing.
For Enanta, that date mattered because Pfizer subsequently disclosed nirmatrelvir, the antiviral component of Paxlovid.
And this was not an academic distinction. Paxlovid became one of the world’s most important COVID-19 treatments and generated billions in sales for Pfizer. Whether Enanta could reach back to its 2020 provisional therefore had enormous commercial consequences.
The problem for Enanta was tiny—literally one character.
The provisional disclosed: —NHC(O)—C2-C12 alkyl
The later patent disclosed: —NHC(O)—C1-C12 alkyl
For the non-chemists, the alphabet soup is less scary than it looks. C1 means one carbon. C2 means two. So C2-C12 starts at two carbons, while C1-C12 also captures the one-carbon option.
In other words, changing a 2 to a 1 added one additional possibility. Unfortunately for Enanta, that one possibility mattered.
Enanta argued that the “2” was essentially a typo and that a person of ordinary skill would have understood C1 was intended. The Federal Circuit disagreed.
The question wasn’t whether the typo could be corrected. It was whether the provisional itself provided written-description support for the later claims.
It didn’t. As the court put it, “C2 is simply different from C1.” Without that support, Enanta lost its 2020 priority date, Pfizer’s intervening disclosure became prior art, and the claims were anticipated.
Score one for the grammar police.
What If Enanta Had Written C1?
Assume the provisional had said C1-C12 from the beginning. Problem solved? Maybe not.
Enanta’s provisional disclosed a huge universe of compounds, formulas, subgenera, and possible substituents. The later claims required making a particular series of selections from that universe. And that’s where things get interesting.
Once Pfizer disclosed nirmatrelvir, everyone knew which combination mattered. But knowing the answer and then finding the individual pieces in an earlier application isn’t necessarily the same as having described that answer in the first place.
Think of it like a recipe. If I hand you a cookbook with hundreds of ingredients and endless substitutions, and somewhere inside are flour, butter, sugar, eggs, and chocolate chips, have I really disclosed a chocolate-chip cookie recipe?
Then someone else invents a particularly valuable chocolate-chip cookie. Can I point back to my cookbook and say: “See? Flour was on page 12. Butter was on page 37. Chocolate chips were on page 82. I had the recipe all along.”
Maybe.
But merely having all the ingredients somewhere in the kitchen doesn’t necessarily mean you possessed the recipe. That’s the more interesting written-description issue lurking behind Enanta.
The Real Lesson of Enanta
The Federal Circuit didn’t need to decide that broader question. The C2/C1 problem was enough. The provisional specifically disclosed C2-C12 and did not disclose the one-carbon group required by the later patent. But the counterfactual is what makes the case useful for prosecutors.
The easy lesson is: Proofread your provisional applications. The grammar police finally have their Federal Circuit case.
The more important lesson is: Don’t confuse disclosing all the pieces with demonstrating possession of every possible combination of those pieces.
Broad disclosure is valuable. Alternatives are valuable. Markush groups are valuable.
But a strong provisional should also identify meaningful combinations—preferred embodiments, express combinations, examples, dependent claims, or other indications of which pieces actually belong together.
Because years later, when a commercially important embodiment appears, it may not be enough to point to five different places in a 100-page provisional and say: “See? It was all in there.”
Having the ingredients is not always the same as having the recipe. And sometimes, one little typo is enough to keep anyone from reaching the recipe question at all.