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.