Where a subsequent change alters the scope of what was disclosed—even by a single carbon atom—the change introduces subject matter for which the provisional provides no support.
The provisional application is frequently the first step in protecting an invention, prized for establishing an early priority date while deferring the cost and formality of a complete nonprovisional filing. Yet the benefit of that earlier filing date is not automatic. For a particular aspect, feature or limitation disclosed in a nonprovisional or international patent application under the Patent Cooperation Treaty to obtain the benefit of the earlier filing date of the provisional application, the provisional must satisfy the written description requirement of 35 USC § 112, and each application in the chain leading back to the earlier filing must demonstrate that the inventor was “in possession” of the claimed invention as of the filing date sought. The recent decision in Enanta Pharmaceuticals, Inc. v. Pfizer Inc., No. 2025‑1427, 2026 WL 1791832 (Fed. Cir. June 23, 2026), provides a classic example of the consequences that follow when the disclosure of a provisional application differs from what the nonprovisional ultimately claims, and how the resulting loss of the earlier priority date can transform an intervening disclosure into invalidating prior art.
Background and Case History
Enanta owns U.S. Patent No. 11,358,953 (the ’953 patent), which issued from a nonprovisional application filed November 9, 2021, as a continuation of a nonprovisional application filed October 21, 2021, with that application being a continuation of a nonprovisional application filed July 19, 2021.
The ’953 patent is directed to compounds and methods of inhibiting coronavirus replication activity. Representative claim 1 recites the following (with emphasis added):
1. A compound represented by Formula (VI-6a),
or a pharmaceutically acceptable salt thereof, wherein
X is —CN; and
A is optionally substituted C1-C8 alkyl or optionally substituted heteroaryl.
The face of the ’953 patent states a priority claim to U.S. Provisional Patent Application No. 63/054,048 (the ’048 provisional), filed July 20, 2020.
Both the ’048 provisional and the ’953 patent define the term “substituted” and provide possible chemical groups that could be substituents. The two definitions of “substituted” between the ’048 provisional and the ’953 patent are the same but for one difference, namely, the recitation of —NHC(O)—C2‑C12‑alkyl in the ’048 provisional versus —NHC(O)—C1‑C12‑alkyl in the ’953 patent. Compare ’048 provisional at 34, lines 25-26, with U.S. Patent No. 11,358,953 col. 70 l. 22. The subscripted numbers identify the number of carbon atoms present in the alkyl group such that “C2‑C12‑alkyl” designates an alkyl group having two to 12 carbon atoms, whereas “C1‑C12‑alkyl” is a broader recitation because such a designation includes one-carbon alkyl groups.
Through a publicly available presentation made on April 6, 2021—about three months prior to the filing date of the earliest nonprovisional application in the ’953 patent lineage, and about nine months after the ’048 provisional filing date—Pfizer made a public disclosure of a protease inhibitor, nirmatrelvir. In the context of Formula (VI-6a) of claim 1 of the ’953 patent, supra, nirmatrelvir includes an “A” group substituted with a —NHC(O)—C1‑alkyl group (where the alkyl group is further substituted with three fluorine atoms to yield a trifluoromethyl substituent).
Enanta filed a nonprovisional application on July 19, 2021, claiming priority to the ’048 provisional, but defining the “A” group substituted with —NHC(O)—C1‑C12‑alkyl rather than —NHC(O)—C2‑C12‑alkyl as in the ’048 provisional. This was done so accordingly, as the ’953 patent issued from the second continuation application from the July 19, 2021, nonprovisional application and recited —NHC(O)—C1‑C12‑alkyl instead of —NHC(O)—C2‑C12‑alkyl as in the ’048 provisional.
Enanta subsequently sued Pfizer alleging its drug Paxlovid for the treatment of COVID-19, which uses nirmatrelvir as one of the active pharmaceutical ingredients, infringes the claims of the ’953 patent. Pfizer moved for summary judgment with the assertion that the claims of the ’953 patent were invalid as anticipated by Pfizer’s April 6, 2021, nirmatrelvir presentation on the basis that the ’953 patent could not claim the benefit of the earlier filing date of the ’048 provisional because the ’048 provisional’s disclosure of —NHC(O)—C2‑C12‑alkyl failed to provide adequate support for the ’953 patent’s broader disclosure of —NHC(O)—C1‑alkyl. Enanta asserted that the changed recitation in the nonprovisional application and in the subsequent continuation application that issued as the ’953 patent merely corrected an obvious typographical error and did not add new matter to the application such that the ’953 patent could obtain the benefit of the earlier filing date of the ’048 provisional.
The district court granted Pfizer's motion for summary judgement, holding that the “correction” of —NHC(O)—C2‑C12‑alkyl in the ’048 provisional to —NHC(O)—C1‑C12‑alkyl in the subsequent nonprovisional application and continuation application that issued as the ’953 patent was not a correction of an obvious typographical error, that the court did not have the power to make such correction, that such a correction would impermissibly broaden the scope of the ’953 patent and that the ’953 patent was not entitled to claim the benefit of the earlier filing date of the ’048 provisional. Accordingly, the ’953 patent claims enjoyed only the July 19, 2021, filing date of the first nonprovisional application in the ’953 patent lineage, and the court arrived at this conclusion through analysis of a court’s power to correct an obvious error.
Because the ’953 patent could not claim priority to the ’048 provisional, Pfizer's disclosure of nirmatrelvir before the July 19, 2021, priority date of the ’953 patent claims (for at least the disclosure of the disputed recitation of a —NHC(O)—C1‑alkyl) anticipated the ’953 patent. Accordingly, the court held that Pfizer's disclosure of nirmatrelvir before the ’953 patent's priority date anticipated the asserted claims, granting Pfizer’s motion for summary judgment as to invalidity of the asserted ’953 patent. Enanta then appealed to the Federal Circuit.
The Federal Circuit's Decision
The court first distinguished the underlying issues from the body of current written description case law drawn to the determination of sufficient written description for a broad genus and underlying species. See generally Ariad Pharms., Inc. v. Eli Lilly & Co., 598 F.3d 1336 (Fed. Cir. 2010) (en banc) (addressing adequate written description for a broad genus); In re Ruschig, 379 F.2d 990 (C.C.P.A. 1967) (addressing support of a later-claimed species drawn from a broad genus). Rather, the instant dispute is whether there is sufficient written description in the ’048 provisional reciting “—NHC(O)—C2‑C12‑alkyl” to demonstrate that the inventors had possession of “—NHC(O)—C1‑alkyl” as disclosed in the ’953 patent.
In applying the correct written description standard in its de novo review of the district court’s holding, which in contrast relied on the power to correct an obvious error, the Federal Circuit reiterated that for a later disclosure to claim priority to an earlier application, each application in the chain must satisfy the written description requirement. Lockwood v. Am. Airlines, Inc., 107 F.3d 1565, 1571 (Fed. Cir. 1997) (“[T]o gain the benefit of the filing date of an earlier application under 35 U.S.C. § 120, each application in the chain leading back to the earlier application must comply with the written description requirement of 35 U.S.C. § 112.”). Thus, in the case of a nonprovisional application seeking to obtain the benefit of the earlier priority date of provisional application, “the written description of the provisional [application] must adequately support the claims of the nonprovisional application.” New Railhead Mfg., LLC v. Vermeer Mfg. Co., 298 F.3d 1290, 1294 (Fed. Cir. 2002).
The court affirmed the district court’s grant of summary judgment holding that the ’953 patent could not obtain the benefit of the priority date of the ’048 provisional. The two documents are drawn to two different ranges of alkyl moieties, the ’048 provisional being directed to —NHC(O)—C2‑C12‑alkyl and the ’953 patent being drawn to —NHC(O)—C1‑C12‑alkyl. There is no support in the ’048 provisional’s disclosure of —NHC(O)—C1‑C12‑alkyl for a —NHC(O)—C1‑alkyl.
Enanta relied on an expert declaration to assert that there was a genuine dispute of material fact. In particular, Enanta’s expert relied on the definition of “alkyl” in the ’048 provisional, which states:
The term “alkyl” as used herein, refers to saturated, straight- or branched-chain hydrocarbon radicals. “C1-C4 alkyl,” “C1-C6 alkyl,” “C1-C8 alkyl,” “C2-C12 alkyl,” “C2-C4 alkyl,” or “C3-C6 alkyl,” refer to alkyl groups containing from one to four, one to six, one to eight, one to twelve, 2 to 4 and 3 to 6 carbon atoms respectively. Examples of C1-C8 alkyl radicals include, but are not limited to, methyl, ethyl, propyl, isopropyl, n-butyl, tert-butyl, neopentyl, n-hexyl, heptyl and octyl radicals.
’048 provisional at 31, lines 19-24 (emphasis added).
In noting the inconsistency in the reciting “C2-C12 alkyl” and “one to twelve,” Enanta argued that a person of ordinary skill in the art would understand that there is a mismatch in the definition of alkyl and that the numerical representation, versus the written out representation, is more likely to be incorrect. Based on this inconsistency, Enanta further argued that it follows that each representation of “C2-C12 alkyl” throughout the ’048 provisional must also be incorrect and should instead recite “C1-C12 alkyl” such that the ’048 adequately supports recitation of a —NHC(O)—C1‑alkyl.
The court disagreed with Enanta’s position because Enanta focused on the general definition of an alkyl group and not the disclosure of the specific substituent at issue, namely —NHC(O)—C2‑C12‑alkyl within the definition of “substituted.” Id. at 34, lines 25-26. Simply put, the disputed —NHC(O)—C1‑alkyl does not appear at all in the disclosure of the ’048 provisional and thus, is not disclosed. Even though the definition of “substituted” includes C1-C12 alkyl groups as further argued by Enanta, the court emphasized that the focus is on the specific substituent at issue, —NHC(O)—C1‑C12‑alkyl, which does not appear, and only —NHC(O)—C2‑C12‑alkyl is disclosed. Because Enanta did not disclose —NHC(O)—C1‑alkyl, it could not have purported to invent an embodiment having a —NHC(O)—C1‑alkyl and be in possession of such at the time of the ’048 provisional.
The court analogized its position in a comparison of methanol, a one-carbon (i.e., C1) alcohol, with ethanol, a two-carbon (i.e., C2) alcohol, in particular, whether the disclosure of ethanol would also support a disclosure of methanol. Though noting the structural similarity of merely one carbon atom between methanol and ethanol, the court responded in the negative, noting that despite structural similarity, the former is toxic whereas the latter is consumable. Thus, the disclosure of one chemical compound does not automatically extend sufficient written description to the disclosure of structurally similar compounds, especially given the stark differences in properties.
Holding that the —NHC(O)—C1‑alkyl was not disclosed in the ’048 provisional such that the ’953 patent could not obtain the benefit of its priority date, the court found no issue of material fact and affirmed the district court’s grant of summary judgment that the ’953 patent was anticipated by Pfizer’s prior public disclosure of nirmatrelvir and thus invalid.
Ironically, the USPTO made a typographical error in printing the subscripts for a separate, nonrelevant substituent group when the ’953 patent was issued.
Conclusion and Practice Points
Enanta is a pointed reminder that the benefit of a provisional's filing date is earned through disclosure, not assumed by the mere act of filing; to secure priority, the provisional must satisfy the written description requirement of Section 112 for each feature later claimed in the nonprovisional and must demonstrate that the inventor was in possession of that subject matter as of the provisional's filing date.
While a provisional application allows applicants to obtain the benefit of an earlier priority date, the filing of a provisional application should not involve the submission of a barebones placeholder that the applicant hopes to remedy at a later date (i.e., at the time of the filing of a nonprovisional or international patent application under the Patent Cooperation Treaty). Applicants should seek to provide as complete and robust a disclosure as possible at the time of the provisional application. Where a subsequent change alters the scope of what was disclosed—even by a single carbon atom—the change introduces subject matter for which the provisional provides no support.
Practitioners drafting in the chemical arts should be especially attentive to numerical ranges and genus definitions; small differences—C1 versus C2, an endpoint included or omitted—carry decisive legal significance, and the disclosure of one integer or species will not necessarily support another, even a structurally adjacent one. The provisional should affirmatively disclose the full intended range, including its endpoints. Applicants should also consider explicitly disclosing the specific species and preferred embodiments they may ultimately wish to claim.
Internal consistency matters. Mismatches between numerals and spelled‑out numbers, or between a general definition and a specific list of substituents, invite costly disputes that—as Enanta shows—will not necessarily be resolved in the patentee's favor, even through expert testimony. Expert opinion regarding an error located elsewhere in the document cannot manufacture a genuine factual dispute or supply a disclosure that is absent from the specific limitation at issue; entitlement to a filing date extends only to that which is actually disclosed.
Finally, Enanta underscores the cost of a lost priority date. In Enanta, an intervening third‑party disclosure, namely Pfizer’s disclosure of nirmatrelvir, became invalidating prior art. Care in preparing a robust, complete and internally consistent provisional application that fully describes the subject matter to be claimed remains the surest protection against unexpected intervening disclosures.
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