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A small drafting mistake can have major consequences in patent law. That was the lesson from Enanta Pharmaceuticals, Inc. v. Pfizer Inc., where the Federal Circuit held in a precedential opinion that Enanta’s patent could not rely on the filing date of its earlier provisional application. Because the provisional application, as specifically written, did not clearly describe the version of the invention later claimed in the issued patent, Pfizer’s intervening public disclosure of nirmatrelvir—the active ingredient in Paxlovid—constituted prior art that anticipated and invalidated Enanta’s claims.

The issue turned on a single carbon atom! Enanta’s provisional application described a chemical substituent as having a C2-C12 alkyl group, meaning a chain of two to twelve carbon atoms. But the later-issued patent claimed a C1-C12 alkyl group. This was an attempt to cover a one-carbon chain. That one-carbon version was critical for Enanta because it would encompass Pfizer’s nirmatrelvir.

Enanta argued that the C2 in the provisional application was just an obvious typo and should have been read as C1. The Federal Circuit was not persuaded. The court focused on the written description requirement under 35 U.S.C. § 112, which provides that to claim the benefit of an earlier filing date, the earlier application must show that the inventor actually possessed the claimed invention at that time. In other words, the application must describe the invention clearly enough that a skilled reader would understand the inventor had already conceived of it.

The court concluded that a disclosure of C2-C12 did not describe C1. Even if C1 was structurally similar to C2, that proximity alone was insufficient. Patent priority depends on what the document objectively says, not what the inventor later explains was intended. The court was especially skeptical because Enanta’s expert could say only that the C2 language was “likely” a mistake—not that C1 was necessarily disclosed in the provisional application.

The court used a simple chemistry analogy to make its point. The court reasoned that disclosing ethanol, a two-carbon alcohol found in alcoholic beverages, does not automatically disclose methanol, a one-carbon alcohol that is highly toxic. Even though the two molecules are structurally related and differ by a single carbon atom, they are not the same. Likewise, a range of C2-C12 does not disclose the missing C1 compound.

The practical takeaway is straightforward. Patent applications, particularly those in the chemical field, must be drafted with care, because compounds with similar names or structures can have vastly different properties and utilities. Seemingly small structural differences or typographical errors can be legally decisive. A later correction may not prevail to preserve priority to the original filing date.