Enanta Pharmaceuticals, Inc. v. Pfizer Inc.

United States Court of Appeals
for the Federal Circuit
______________________
ENANTA PHARMACEUTICALS, INC.,
Plaintiff-Appellant
v.
PFIZER INC.,
Defendant-Appellee
______________________
2025-1427
______________________
Appeal from the United States District Court for the
District of Massachusetts in No. 1:22-cv-10967-DJC, Judge
Denise J. Casper.
______________________
Decided: June 23, 2026
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BARBARA A. F IACCO, Foley Hoag LLP, Boston, MA, ar-
gued for plaintiff-appellant. Also represented by S PENSER
ANGEL , T AYLOR ROSS D AVIS , STUART K NIGHT , D ONALD ROSS
WARE, J EREMY YOUNKIN.
D AVID M. K RINSKY , Williams & Connolly LLP, Wash-
ington, DC, argued for defendant-appellee. Also repre-
sented by N ICHOLAS L OFTUS , BEN P ICOZZI , T HOMAS H.L.
SELBY , J ULIE T AVARES , CHRISTOPHER YEAGER.
______________________
Before L OURIE, BRYSON, and CHEN, Circuit Judges.
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ENANTA PHARMACEUTICALS, INC. v. PFIZER INC. 2
L OURIE, Circuit Judge.
Enanta Pharmaceuticals, Inc. (“Enanta”) appeals from
a decision of the United States District Court for the Dis-
trict of Massachusetts granting summary judgment that
all claims of Enanta’s U.S. Patent 11,358,953 (“the ’953 pa-
tent”) are invalid as anticipated by a public disclosure of a
compound within the scope of its claims. Enanta Pharms.,
Inc. v. Pfizer, Inc., No. 22-cv-10967-DJC, 2024 WL
5203036, at *9 (D. Mass. Dec. 23, 2024) (“Decision”).1 For
the following reasons, we affirm.
BACKGROUND
Enanta’s ’953 patent, which issued from a non-provi-
sional application filed on November 9, 2021, is directed to
compounds and methods of inhibiting coronavirus replica-
tion activity. ’953 patent col. 1 ll. 16–20. Claim 1 is repre-
sentative and recites:
1. A compound represented by Formula (VI-6a),
1 The parties do not specify which claims were inval-
idated. The district court determined that “[t]he ’953 pa-
tent is . . . invalid for anticipation,” Decision, 2024 WL
5203036, at *9, which we interpret as invalidating all
claims of the patent.
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ENANTA PHARMACEUTICALS, INC. v. PFIZER INC. 3
or a pharmaceutically acceptable salt thereof,
wherein
X is —CN; and
A is optionally substituted C1-C8 alkyl or optionally
substituted heteroaryl.
’953 patent col. 110 ll. 40–60 (emphasis added).
The ’953 patent states that it claims priority from
Enanta’s U.S. Provisional Patent Application 63/054,048
(“the ’048 provisional”), which has a priority date of July
20, 2020. See J.A. 92–166 (’048 provisional). The ’048 pro-
visional and the ’953 patent both provide a definition for
the term “substituted” and list several dozen chemical com-
pounds and moieties that qualify as “substituents.” J.A.
127–28; ’953 patent col. 69 l. 61–col. 71 l. 14. The defini-
tions provided in both are substantially the same, but with
one critical difference. The ’048 provisional recites “—
NHC(O)—C2-C12-alkyl,” whereas the ’953 patent recites “—
NHC(O)—C1-C12-alkyl.” J.A. 127; ’953 patent col. 70 l. 22.
The subscripted numbers identify the number of carbon at-
oms in the alkyl group: “C2-C12” denotes alkyl groups con-
taining a range of two to twelve carbon atoms, while “C1-
C12” additionally includes a one-carbon alkyl group.
On April 6, 2021, Pfizer Inc. (“Pfizer”) made publicly
available a presentation disclosing a protease inhibitor,
nirmatrelvir, which was eventually incorporated into
Pfizer’s Paxlovid® product to treat coronavirus infection.
See J.A. 2729–46. Nirmatrelvir has an “A” group substi-
tuted with a —NHC(O)—C1-alkyl group. See J.A. 2740.
Enanta contends that, on July 9, 2021, it realized that
the ’048 provisional contained a typographical error: The
“C2” in —NHC(O)—C2-C12-alkyl in the ’048 provisional re-
ally should have been a “C1.” Accordingly, on July 19, 2021,
Enanta filed a non-provisional application, following sev-
eral continuations one of which eventually issued as the
’953 patent, and listed the relevant substituent as —
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ENANTA PHARMACEUTICALS, INC. v. PFIZER INC. 4
NHC(O)—C1-C12 -alkyl, as opposed to —NHC(O)—C2 -C12-
alkyl that was listed in the ’048 provisional.
In June 2022, Enanta sued Pfizer, alleging that Pfizer’s
Paxlovid® product infringed the claims of the ’953 patent.
J.A. 533–44. Pfizer counterclaimed that the ’953 patent
was invalid, and subsequently filed a motion for summary
judgment that the asserted claims of the ’953 patent were
invalid as anticipated. Decision, 2024 WL 5203036, at *1.
Pfizer argued that the ’953 patent could not claim priority
from the ’048 provisional because the ’953 patent’s disclo-
sure of —NHC(O)—C1-alkyl was not supported by the ’048
provisional’s disclosure of —NHC(O)—C2-C12-alkyl, and
Pfizer’s intervening disclosure of nirmatrelvir anticipated
the asserted claims. See J.A. 2021–27. Enanta argued that
the ’953 patent is entitled to claim priority from the ’048
provisional because Enanta’s alteration to correct an obvi-
ous typographical error added no new matter not disclosed
in the ’048 provisional. See J.A. 4075–81.
The district court granted Pfizer’s motion, concluding
that the “C2” in —NHC(O)—C2-C 12-alkyl of the ’048 provi-
sional was not an obvious typographical error that it had
the power to correct, and therefore the change from —
NHC(O)—C2-C12 -alkyl in the ’048 provisional to —
NHC(O)—C1-C12 -alkyl in the ’953 patent impermissibly
broadened the scope of the patent such that the ’953 patent
was not entitled to priority from the ’048 provisional. De-
cision, 2024 WL 5203036, at *8. The district court thus
concluded that Pfizer’s disclosure of nirmatrelvir before the
’953 patent’s priority date anticipated the asserted claims.
Id. at *9.
Enanta timely appealed. We have jurisdiction under
28 U.S.C. § 1295(a)(1).
D ISCUSSION
We review a district court’s grant of summary judg-
ment according to the law of the regional circuit, here, the
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ENANTA PHARMACEUTICALS, INC. v. PFIZER INC. 5
First Circuit, which reviews a grant of summary judgment
de novo. AbbVie Deutschland GmbH & Co., KG v. Janssen
Biotech, Inc., 759 F.3d 1285, 1295 (Fed. Cir. 2014) (cita-
tions omitted) (applying First Circuit law). “In order to pre-
vail on a motion for summary judgment, the moving party
must show ‘that there is no genuine dispute as to any ma-
terial fact’ and that it ‘is entitled to judgment as a matter
of law.’” OneBeacon Am. Ins. Co. v. Com. Union Assur. Co.
of Can., 684 F.3d 237, 241 (1st Cir. 2012) (quoting Fed. R.
Civ. P. 56(a)). “A fact is material if it carries with it the
potential to affect the outcome of the suit under the appli-
cable law.” Santiago–Ramos v. Centennial P.R. Wireless
Corp., 217 F.3d 46, 52 (1st Cir. 2000) (internal quotation
marks and citation omitted). We apply Federal Circuit law
to substantive issues of patent law. ParkerVision, Inc. v.
Qualcomm Inc., 116 F.4th 1345, 1356 (Fed. Cir. 2024).
We start with what this case is not. This case does not
present the type of written description disputes addressed
in cases such as Ariad Pharms., Inc. v. Eli Lilly & Co.,
598 F.3d 1336 (Fed. Cir. 2010) (en banc) and In re Ruschig,
379 F.2d 990 (CCPA 1967). The issue in Ariad was
whether a patent specification provided adequate written
description support for a broad genus claim. 598 F.3d at
1341, 1354–55. And the issue in In re Ruschig was whether
a patent specification that disclosed a broad genus pro-
vided adequate written description support for a later-
claimed species. 379 F.2d at 993–96. Here, in contrast, the
dispute is whether there is adequate written description
support in the ’048 provisional’s disclosure of —NHC(O)—
C2-C12-alkyl for the ’953 patent’s disclosure of —NHC(O)—
C1-alkyl. That is, whether “2” provides adequate written
description support for “1.”
And, as we explained in Lockwood v. American Air-
lines, Inc., the written description standard applies for pur-
poses of priority. 107 F.3d 1565, 1571 (Fed. Cir. 1997).
That is, “[i]n order to gain the benefit of the filing date of
an earlier application under 35 U.S.C. § 120, each
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ENANTA PHARMACEUTICALS, INC. v. PFIZER INC. 6
application in the chain leading back to the earlier applica-
tion must comply with the written description requirement
of 35 U.S.C. § 112.” Id. (citation omitted); see also New
Railhead Mfg., LLC v. Vermeer Mfg. Co., 298 F.3d 1290,
1294 (Fed. Cir. 2002) (“[F]or [a] non-provisional utility ap-
plication to be afforded the priority date of [a] provisional
application, . . . the written description of the provisional
must adequately support the claims of the non-provisional
application.”).
Furthermore, this case is unlike our line of cases con-
cerning the correction of errors in issued patents, as argued
by the parties and decided by the district court. In In re
Oda, our predecessor court explained that the United
States Patent and Trademark Office can reissue patents
with corrections pursuant to 35 U.S.C. § 251 that add “[n]o
new matter,” so long as one of skill in the art would con-
sider the error to be “obvious.” 443 F.2d 1200, 1203–04
(CCPA 1971); see 35 U.S.C. § 251 (permitting the Patent
Office to “[r]eissue . . . defective patents” with corrections).
But this case is not before the Patent Office, and a change
from C2 to C1 has not been shown here to be a correction of
an obvious error.
And, in Novo Industries, L.P. v. Micro Molds Corp., we
explained that a district court can “correct an error in a pa-
tent by interpretation of the patent where no certificate of
correction has been issued” only when “(1) the correction is
not subject to reasonable debate based on consideration of
the claim language and the specification and (2) the prose-
cution history does not suggest a different interpretation of
the claims.” 350 F.3d 1348, 1354 (Fed. Cir. 2003). The in-
stant case does not concern the power of a district court to
correct an error in an issued patent, and, even if it did, the
existence of an error in the ’048 provisional is certainly
“subject to reasonable debate.” Thus, neither Oda nor
Novo applies here.
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ENANTA PHARMACEUTICALS, INC. v. PFIZER INC. 7
As noted, the district court in this case determined that
the ’953 patent was not afforded the ’048 provisional’s filing
date because there was no obvious typographical error in
the ’048 provisional that it had the power to correct, and
the ’953 patent therefore broadened the scope of the patent
such that it could not claim priority from the ’048 provi-
sional. Decision, 2024 WL 5203036, at *6–8. Although the
district court focused on its power to correct a purported
typographical error and did not apply the written descrip-
tion standard, we proceed to do so. See Slip Track Sys. v.
Metal-Lite, Inc., 304 F.3d 1256, 1265 (Fed. Cir. 2002)
(“[B]ecause this court reviews de novo a grant of summary
judgment, it can apply the correct standard.”).
There being no error shown in an issued patent here,
the issue in this case is whether there was a genuine dis-
pute of material fact as to whether the ’048 provisional pro-
vided written description support for —NHC(O)—C1-alkyl
disclosed and claimed in the ’953 patent. Enanta does not
dispute that Pfizer’s disclosure of nirmatrelvir anticipates
the claims of the ’953 patent if the ’953 patent is not af-
forded a priority date before Pfizer’s disclosure. See gener-
ally Opening Br. 1–61; Response Br. 23. Rather, Enanta
argues that it presented a dispute of material fact, whether
a skilled artisan would recognize the “C2” in —NHC(O)—
C2-C12-alkyl of the ’048 provisional to be a typographical
error and therefore provided adequate written description
support for the ’953 patent’s disclosure of —NHC(O)—C1-
C12-alkyl, and that the district court therefore erred in
granting summary judgment. We disagree.
Whether the written description requirement of
35 U.S.C. § 112 is met in a given case is a question of fact.
Bradford Co. v. Conteyor N. Am., Inc., 603 F.3d 1262, 1268
(Fed. Cir. 2010). As noted, in order to be entitled to the
priority date of an earlier-filed application, each applica-
tion must comply with the written description requirement
of § 112, which “requires that the earlier application de-
scribe the later claimed invention, and do so in sufficient
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ENANTA PHARMACEUTICALS, INC. v. PFIZER INC. 8
detail that one skilled in the art can clearly conclude that
the inventor was ‘in possession’ of the claimed invention as
of the filing date sought.” Id. at 1269 (citation omitted).
We conclude that the ’048 provisional does not convey
to a skilled artisan that the inventors possessed —
NHC(O)—C1-alkyl at the time of the ’048 provisional’s fil-
ing date. The ’048 provisional and the ’953 patent recite
two different ranges of chemical moieties. The former re-
cites —NHC(O)—C2-C12-alkyl and the latter recites —
NHC(O)—C1-C12 -alkyl. C2 is simply different from C1, and
the ’048 provisional’s disclosure of a range of C2 to C12 pro-
vides no support for the ’953 patent’s disclosure of C1 be-
cause the ’048 application explicitly includes an alkyl group
with two to twelve carbon atoms (i.e., —NHC(O)—C2 -C12-
alkyl), and notably does not include an alkyl group with one
carbon atom (i.e., —NHC(O)—C1 -alkyl).
Enanta argues that its expert’s declaration creates a
genuine dispute of material fact that it disclosed and pos-
sessed —NHC(O)—C 1-alkyl at the time the ’048 provi-
sional was filed. We disagree.
Enanta’s expert points out that, before providing the
definition for “substituted,” the ’048 provisional defines “al-
kyl” as “saturated straight- or branched-chain hydrocarbon
radicals,” and explains that “‘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 re-
spectively.” J.A. 124 (emphasis added). That is, he asserts
that there is an inconsistency between the listed “C2-C12
alkyl” using numerals 2 and 12 and the corresponding writ-
ten numbers of “one to twelve . . . carbon atoms.” And, ac-
cording to Enanta’s expert, “a person of ordinary skill in
the art would understand that where there is a mismatch
between a numeral and a written number, . . . the numeral
is likely to contain [the] typographical error;” thus “the
structure of the definition of ‘alkyl’ in the provisional
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ENANTA PHARMACEUTICALS, INC. v. PFIZER INC. 9
application confirms that the typographical error is in the
phrase C2-C12 alkyl,’ not the written number.” J.A. 4270.
The expert then opined that this typographical error in the
definition of “alkyl” shows that any later use of “C2-C12 al-
kyl” in the ’048 provisional’s specification, such as the dis-
closure of —NHC(O)—C2-C12 -alkyl, was likely also a
typographical error and should have been “—NHC(O)—C1-
C12 alkyl.” J.A. 4271.
Enanta’s argument is unpersuasive because its expert
points to a purported typographical error in the general
definition of “alkyl,” not in the specific disclosure of —
NHC(O)—C2-C12 -alkyl in the definition of “substituted.”
Compare J.A. 124 with J.A. 127. Although the expert also
offers an opinion that “a person of ordinary skill in the art
would understand that other references to ‘C2-C12 alkyl’ . . .
are likely to be erroneous as well,” J.A. 4271, the expert
points to nothing in the disclosure of —NHC(O)—C2 -C12-
alkyl in particular to show that a skilled artisan would un-
derstand the disclosed C2-C12 alkyl to provide support for
C1 alkyl.
As we have explained, “[e]ntitlement to a filing date . . .
extends only to that which is disclosed,” Lockwood,
107 F.3d at 1571–72, and —NHC(O)—C1-alkyl was simply
not disclosed in the ’048 provisional. An expert opinion on
a typographical error contained elsewhere in the ’048 pro-
visional does not render the former fact disputed. Cf. id. at
1572 (“It is not sufficient for purposes of the written de-
scription requirement of § 112 that the disclosure, when
combined with the knowledge in the art, would lead one to
speculate as to modifications that the inventor might have
envisioned, but failed to disclose. Each application in the
chain must describe the claimed features.”); Omega Eng’g,
Inc. v. Raytek Corp., 334 F.3d 1314, 1332 (Fed. Cir. 2003)
(“[E]xtrinsic evidence cannot be used to vary the plain lan-
guage of the patent document.”). There is no genuine issue
of material fact here.
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ENANTA PHARMACEUTICALS, INC. v. PFIZER INC. 10
Enanta points to several other opinions of its expert for
support that the C2 in —NHC(O)—C2-C12-alkyl was likely
to have been a typographical error. See Opening Br. 52–
56. For example, Enanta’s expert pointed out that “C1-C12
alkyl” is listed under the definition of “substituted,” along
with other substituents with a C1 alkyl group, and a skilled
artisan would therefore recognize that any substituent con-
taining a “C2-C12 alkyl” group was a typographical error
that should have been “C1-C12 alkyl.” J.A. 4271. Again,
these arguments do not concern the actual substituent at
issue, and do not change the fact that the ’048 provisional
specifically discloses only —NHC(O)—C2-C12-alkyl. In-
deed, the ’048 provisional specification is highly detailed,
providing dozens of specific chemical moieties presumably
described with great care under the definition of “substi-
tuted.” J.A. 127–28. We respect applicants’ statements in
their specification that they invented what was specifically
disclosed in the ’048 provisional, but similarly we conclude
that they did not invent what they did not disclose.
The issue in this case is akin to asking whether a dis-
closure of ethanol, a two-carbon alcohol regularly con-
sumed by people, would provide adequate written
description support for methanol, a one-carbon alcohol that
is highly toxic to people. That example illustrates why a
disclosure of one chemical compound, or integer in this
case, cannot necessarily be a disclosure of another, even
one close by structurally.
The ’048 provisional did not disclose —NHC(O)—C1 -
alkyl. It therefore provided no written description support
for the ’953 patent, so the ’953 patent cannot be afforded
the ’048 provisional’s priority date. The district court
therefore properly granted summary judgment that the
’953 patent claims were anticipated by Pfizer’s disclosure
of nirmatrelvir.
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CONCLUSION
We have considered Enanta’s remaining arguments
but find them unpersuasive. For the foregoing reasons, we
affirm.
AFFIRMED
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