  United States Court of Appeals
      for the Federal Circuit
                 ______________________

  AMGEN INC., AMGEN MANUFACTURING LTD.,
              Plaintiffs-Appellants

                            v.

           COHERUS BIOSCIENCES INC.,
                Defendant-Appellee
              ______________________

                       2018-1993
                 ______________________

    Appeal from the United States District Court for the
District of Delaware in No. 1:17-cv-00546-LPS, Chief Judge
Leonard P. Stark.
                 ______________________

                  Decided: July 29, 2019
                 ______________________

     NICHOLAS P. GROOMBRIDGE, Paul, Weiss, Rifkind,
Wharton & Garrison LLP, New York, NY, argued for plain-
tiffs-appellants. Also represented by JENNIFER GORDON,
GOLDA LAI, PETER SANDEL, JACOB WHITT, JENNIFER H. WU;
LOIS M. KWASIGROCH, KIMBERLIN L. MORLEY, WENDY A.
WHITEFORD, Amgen Inc., Thousand Oaks, CA.

    ADAM G. UNIKOWSKY, Jenner & Block LLP, Washing-
ton, DC, argued for defendant-appellee. Also represented
by BRADFORD PETER LYERLA, AARON A. BARLOW, LOUIS
FOGEL, SUSAN O’BRIEN, Chicago, IL.
                ______________________
2                    AMGEN INC. v. COHERUS BIOSCIENCES INC.




    Before REYNA, HUGHES, and STOLL, Circuit Judges.
STOLL, Circuit Judge.
    Amgen Inc. and Amgen Manufacturing Ltd. (collec-
tively, “Amgen”) sued Coherus BioSciences Inc. for patent
infringement in the District of Delaware. The district court
dismissed Amgen’s complaint for failure to state a claim,
and Amgen appeals. Because prosecution history estoppel
bars Amgen from succeeding on its infringement claim un-
der the doctrine of equivalents, we affirm the order of the
district court.
                        BACKGROUND
                              I
     Recombinant therapeutic proteins are a class of bio-
logic medicines that are manufactured inside living cells.
Before a protein can be therapeutically useful, it must first
be purified from contaminants. Amgen’s U.S Patent
No. 8,273,707 claims methods of purifying proteins using
hydrophobic interaction chromatography (“HIC”). A HIC
column contains a solid, hydrophobic matrix and “is used
to separate proteins on the basis of hydrophobic interac-
tions between the hydrophobic moieties of the protein and
insoluble, immobilized hydrophobic groups on the matrix.”
’707 patent col. 1 ll. 36–39. In a HIC purification, a buff-
ered salt solution containing the desired protein and asso-
ciated impurities is first poured onto a HIC column. Id.
at col. 1 ll. 40–41. This is known as the “loading” step. The
salt in the buffer exposes the hydrophobic regions of the
protein and causes them to adsorb (i.e., attach) onto the hy-
drophobic groups on the column matrix. See id. at col. 1
ll. 41–44. The impurities are then washed out of the col-
umn with a buffered salt solution while the desired protein
remains attached to the matrix. See id. at col. 4 ll. 27–29.
Finally, molecules of the desired protein are detached (or
“eluted”) by pouring a buffer solution with a lower salt con-
centration through the column. See id. at col. 1 ll. 44–49.
AMGEN INC. v. COHERUS BIOSCIENCES INC.                      3



“Usually, a decreasing salt gradient is used to elute pro-
teins from a column. As the ionic strength decreases, the
exposure of the hydrophilic regions of the protein increases
and proteins elute from the column in order of increasing
hydrophobicity.” Id. at col. 1 ll. 45–49.
    During the loading step, only a finite amount of protein
can bind to the matrix. If too much protein is loaded on the
column, “‘breakthrough’ or loss of protein to the solution
phase before elution” will occur. Id. at col. 3 ll. 40–41. The
’707 patent claims a process that reduces breakthrough, or
in other words, increases the “dynamic capacity” of a HIC
column. Dynamic capacity refers to “the maximum amount
of protein in solution which can be loaded onto a column
without significant breakthrough or leakage of the protein
into the solution phase of a column before elution.” Id.
at col. 3 l. 65–col. 4 l. 3.
    Prior art methods of increasing a HIC column’s dy-
namic capacity included using a higher salt concentration
in the buffer solution. See id. at col. 3 ll. 37–38. This re-
sulted in other problems, however, as “high salt can be det-
rimental to protein stability. High salt increases the
viscosity of a solution, results in increased formation of ag-
gregates, results in protein loss due to dilution and filtra-
tion of the protein after elution from the column, and can
lead to reduced purity.” Id. at col. 3 ll. 41–45. Instead of
increasing the concentration of a single salt, the ’707 inven-
tion:
    provides combinations of salts useful for increasing
    the dynamic capacity of an HIC column compared
    with the dynamic capacity of the column using sep-
    arate salts alone. These combinations of salts al-
    low for a decreased concentration of at least one of
    the salts to achieve a greater dynamic capacity,
    without compromising the quality of the protein
    separation.
4                      AMGEN INC. v. COHERUS BIOSCIENCES INC.




Id. at col. 2 ll. 9–15. All of the ’707 claims require a salt
combination chosen from one of three pairs: citrate and sul-
fate, citrate and acetate, or sulfate and acetate. Repre-
sentative claim 1 recites:
    1. A process for purifying a protein on a hydropho-
    bic interaction chromatography column such that
    the dynamic capacity of the column is increased for
    the protein comprising
        mixing a preparation containing the protein
    with a combination of a first salt and a second salt,
        loading the mixture onto a hydrophobic inter-
    action chromatography column, and eluting the
    protein,
        wherein the first and second salts are selected
    from the group consisting of citrate and sulfate, cit-
    rate and acetate, and sulfate and acetate, respec-
    tively, and
        wherein the concentration of each of the first
    salt and the second salt in the mixture is between
    about 0.1 M and about 1.0.
Id. at col. 15 ll. 8–18.
                              II
     During prosecution, the examiner rejected the then-
pending ’707 claims as obvious in view of U.S. Patent
No. 5,231,178 (“Holtz”). J.A. 174–75. The examiner noted
that Holtz disclosed several salts for improving hydropho-
bic interactions between a protein and the column matrix.
J.A. 174. According to the examiner, it would have been
obvious for a person of ordinary skill to routinely optimize
Holtz to achieve the claimed invention. J.A. 175.
    On January 26, 2011, Amgen responded to the exam-
iner’s rejection, pointing out that “the pending claims recite
a particular combination of salts. No combinations of salts
AMGEN INC. v. COHERUS BIOSCIENCES INC.                      5



[are] taught nor suggested in the Holtz et al. patent, nor
[are] the particular combinations of salts recited in the
pending claims taught nor suggested in this reference.”
J.A. 182. Amgen further noted that the claimed invention
is directed to increasing dynamic capacity of a HIC column
and Holtz does not teach dynamic capacity at all. See id.
It also attached a declaration from ’707 patent inventor
Anna Senczuk (“Declaration”) for support. The Declaration
states that the inventors discovered that using a sul-
fate/citrate or sulfate/acetate salt combination resulted in
substantial increases in the dynamic capacity of a HIC col-
umn as compared to using a single salt. See J.A. 187 ¶ 3.
It further explains that using a sulfate/citrate, sulfate/ace-
tate, or acetate/citrate combination reduced purification
costs on a commercial scale as compared to using only a
single salt. See J.A. 187–88 ¶ 4. The Declaration did not
discuss any salt pairs other than sulfate/citrate, sulfate/ac-
etate, and acetate/citrate—the only claimed pairs in the
’707 patent. Amgen’s response highlighted the particular
salt pairs disclosed in the Declaration:
    As pointed out in paragraph 4 of the Declaration,
    “The improvement resulting from the use of dual
    salts in HIC goes beyond merely optimizing a col-
    umn to best suit a particular protein. Use of this
    particular combination of salts greatly improves
    the cost-effectiveness of commercial manufacturing
    by reducing the number of cycles required for each
    harvest and reducing the processing time for each
    harvest.”
J.A. 183 (emphasis added) (quoting J.A. 188 ¶ 4).
    On April 7, 2011, the examiner again rejected the
claims. The examiner stated that “[a]pplicant contends
that the instant claims recite a particular combination of
salts. However, the examiner contends that the cited ref-
erence does disclose salts used in a method of purification”
and that adjustment of conditions was within the skill of
6                   AMGEN INC. v. COHERUS BIOSCIENCES INC.




an ordinary artisan. J.A. 949. On August 22, 2011, Amgen
replied to the examiner’s rejection and reiterated that
Holtz does not disclose a combination of salts and does not
disclose enhancing the dynamic capacity of a HIC column.
See J.A. 160–61. Amgen pointed out that choosing a work-
ing salt combination was a “lengthy development path” and
that “merely adding a second salt” would not result in the
invention. J.A. 162. The examiner then allowed the
claims.
                            III
    In August 2016, Coherus filed an abbreviated Biologic
License Application (“aBLA”) seeking FDA approval to
market a biosimilar version of Amgen’s pegfilgrastim prod-
uct Neulasta. Pegfilgrastim is a recombinant therapeutic
protein that stimulates the production of neutrophils, a
type of white blood cell. The parties exchanged information
as required by the Biologics Price Competition and Innova-
tion Act and determined that the ’707 patent should be in-
cluded in Amgen’s infringement suit. Coherus’s aBLA
revealed that Coherus’s manufacturing process contains
several chromatography steps used to purify pegfilgrastim.
One of the steps involves a chromatography buffer contain-
ing a salt combination, but not one of the specific combina-
tions recited in the claims.
    On May 10, 2017, Amgen sued Coherus for infringing
the ’707 patent based on Coherus’s aBLA. Amgen alleged
infringement under the doctrine of equivalents because the
salt combination used in Coherus’s process did not match
any of the three expressly claimed salt combinations in the
’707 patent. See J.A. 109–10 ¶ 50. Coherus then moved to
dismiss Amgen’s complaint under Federal Rule of Civil
Procedure 12(b)(6).
    The magistrate judge issued a Report and Recommen-
dation (“Report”), recommending that Coherus’s motion to
dismiss be granted. Amgen Inc. v. Coherus Biosciences Inc.,
No. 17-cv-546-LPS-CJB (D. Del. Dec. 7, 2017) (D.I. 50);
AMGEN INC. v. COHERUS BIOSCIENCES INC.                     7



J.A. 12–30. The magistrate judge noted that, during pros-
ecution, Amgen distinguished Holtz by arguing that Holtz
did not disclose “one of the particular, recited combinations
of salts.” 1 J.A. 24. Based on this, the magistrate judge de-
termined that Amgen “clearly and unmistakably—and in-
deed, repeatedly—indicated to competitors that it
surrendered processes using combinations of salts different
from the ‘particular combinations of salts recited in the
. . . claims[.]’” J.A. 23. The Report concluded that “prose-
cution history estoppel bars Amgen from now attempting
to reassert surrendered ground involving other combina-
tions of salts.” J.A. 28.
     The district court adopted the magistrate judge’s Re-
port and granted Coherus’s motion to dismiss. See Amgen
Inc. v. Coherus Biosciences Inc., No. 17-cv-546-LPS-CJB,
2018 WL 1517689, *1 (D. Del. Mar. 26, 2018) (“Decision”).
It held that “[t]he prosecution history, namely, the pa-
tentee’s correspondence in response to two office actions
and a final rejection, shows a clear and unmistakable sur-
render of claim scope by the patentee.” Id. at *2. The dis-
trict court further held that, by disclosing but not claiming
the salt combination used by Coherus, Amgen had dedi-
cated that particular combination to the public. Id. at *3.
It concluded that the dedication-disclosure doctrine formed
an independent basis on which to dismiss Amgen’s in-
fringement claim. See id. Amgen appeals. We have juris-
diction under 28 U.S.C. § 1295(a)(1).




    1   The magistrate judge also noted that, while di-
rected to a different salt combination, Amgen made the
same argument—that Holtz did not disclose the claimed
“particular combination” of salts—during prosecution of
the parent patent. J.A. 21–22.
8                   AMGEN INC. v. COHERUS BIOSCIENCES INC.




                       DISCUSSION
                             I
    We review an order dismissing a complaint for failure
to state a claim under the law of the regional circuit, here
the Third Circuit. McZeal v. Sprint Nextel Corp., 501 F.3d
1354, 1355–56 (Fed. Cir. 2007). The Third Circuit reviews
challenges to a dismissal for failure to state a claim de
novo. Sands v. McCormick, 502 F.3d 263, 267 (3d Cir.
2007). “In evaluating the propriety of the dismissal, we ac-
cept all factual allegations as true, construe the complaint
in the light most favorable to the plaintiff, and determine
whether, under any reasonable reading of the complaint,
the plaintiff may be entitled to relief.” Id. at 267–68.
“Whether prosecution history estoppel applies, and thus
whether the doctrine of equivalents is available for a par-
ticular claim limitation, is a question of law reviewed
de novo.” Spectrum Pharm., Inc. v. Sandoz Inc., 802 F.3d
1326, 1337 (Fed. Cir. 2015).
                             II
    We agree with the district court that, during prosecu-
tion of the ’707 patent, Amgen clearly and unmistakably
surrendered salt combinations other than the particular
combinations recited in the claims. Prosecution history es-
toppel thus bars Amgen from succeeding on its infringe-
ment claim under the doctrine of equivalents.
    “Prosecution history estoppel applies as part of an in-
fringement analysis to prevent a patentee from using the
doctrine of equivalents to recapture subject matter surren-
dered from the literal scope of a claim during prosecution.”
Trading Techs. Int’l, Inc. v. Open E Cry, LLC, 728 F.3d
1309, 1322 (Fed. Cir. 2013). Prosecution history estoppel
can occur in two ways: “either (1) by making a narrowing
amendment to the claim (‘amendment-based estoppel’) or
(2) by surrendering claim scope through argument to the
patent examiner (‘argument-based estoppel’).” Conoco, Inc.
AMGEN INC. v. COHERUS BIOSCIENCES INC.                     9



v. Energy & Envtl. Int’l, L.C., 460 F.3d 1349, 1363
(Fed. Cir. 2006). To invoke argument-based estoppel, “the
prosecution history must evince a clear and unmistakable
surrender of subject matter.” Id. at 1364 (quoting Deering
Precision Instruments, LLC v. Vector Distribution Sys.,
Inc., 347 F.3d 1314, 1326 (Fed. Cir. 2003)).
    “[W]here a patent applicant sets forth multiple bases
to distinguish between its invention and the cited prior art,
the separate arguments [can] create separate estoppels as
long as the prior art was not distinguished based on the
combination of these various grounds.” PODS, Inc. v. Porta
Stor, Inc., 484 F.3d 1359, 1367 (Fed. Cir. 2007) (internal
quotation marks omitted) (quoting Southwall Techs., Inc.
v. Cardinal IG Co., 54 F.3d 1570, 1581–83 (Fed. Cir.
1995)). “[C]lear assertions made during prosecution in sup-
port of patentability, whether or not actually required to
secure allowance of the claim, may also create an estop-
pel . . . [t]he relevant inquiry is whether a competitor
would reasonably believe that the applicant had surren-
dered the relevant subject matter.” Id. at 1368 (internal
quotation marks omitted).
    We hold that argument-based prosecution history es-
toppel applies here because Amgen clearly and unmistak-
ably surrendered unclaimed salt combinations during
prosecution. In its January 6, 2011 response, Amgen dis-
tinguished Holtz on the basis that Holtz did not teach or
suggest the “particular combinations of salts” recited in
Amgen’s claims. J.A. 182. Indeed, Amgen emphasized
“particular” and referred to its particular salts three times
in the span of two pages. See J.A. 182–83. The Declaration
attached to Amgen’s response also highlights and discusses
the same particular combinations recited in Amgen’s
claims. For example, the Declaration refers to sulfate/cit-
rate and sulfate/acetate as “particular dual salt combina-
tion[s]” that resulted in increased dynamic capacity as
compared to a single salt. J.A. 187. It also explains that
using a sulfate/citrate, sulfate/acetate, or acetate/citrate
10                   AMGEN INC. v. COHERUS BIOSCIENCES INC.




combination (the only claimed combinations) resulted in
reduced commercial manufacturing costs as compared to
using only a single salt. See J.A. 187–88. Notably,
Amgen’s response to the examiner’s office action quotes the
Declaration’s conclusion that “[u]se of this particular com-
bination of salts greatly improves the cost-effectiveness of
commercial manufacturing by reducing the number of cy-
cles required for each harvest and reducing the processing
time for each harvest.” J.A. 183 (quoting J.A. 188 ¶ 4).
Amgen’s response and Declaration do not mention any salt
combinations other than those claimed. 2          Based on
Amgen’s statements during prosecution, we agree with the
district court’s conclusion that “a competitor would reason-
ably believe” that Amgen surrendered unclaimed salt com-
binations. See PODS, 484 F.3d at 1368.
     Amgen argues that it did not distinguish Holtz on the
basis that Holtz failed to disclose the particular claimed
combinations, but rather, it distinguished Holtz on the ba-
sis that Holtz failed to disclose increasing dynamic capacity
and failed to disclose any salt combinations at all. See Ap-
pellant Br. 30–36. According to Amgen, its statement re-
garding the “particular combinations” of salts “simply
observes (correctly) as a factual matter that Holtz does not
disclose using combinations of salts in the first instance,”
and thus does not clearly and unmistakably surrender un-
claimed salt pairs. Id. at 35. We disagree.
     In its January 6, 2011 response, Amgen asserted three
bases for distinguishing Holtz: (1) “[n]o combinations of
salts [are] taught nor suggested in the Holtz et al. patent”;
(2) “nor [are] the particular combinations of salts recited in


     2  That Amgen made the same “particular combina-
tion” argument with respect to the same prior art reference
as to salts claimed in the parent patent further reflects
Amgen’s emphasis on the particular claimed combinations.
See J.A. 1240.
AMGEN INC. v. COHERUS BIOSCIENCES INC.                       11



the pending claims taught nor suggested in [Holtz],”; and
(3) “[t]here is no description or suggestion in Holtz et al. for
the use of any combination of salts to increase the dynamic
capacity of a HIC.” J.A. 182. So while Amgen did assert
multiple reasons for why Holtz is distinguishable, our prec-
edent instructs that estoppel can attach to each argument.
“[W]here a patent applicant sets forth multiple bases to dis-
tinguish between its invention and the cited prior art, the
separate arguments [can] create separate estoppels as long
as the prior art was not distinguished based on the combi-
nation of these various grounds.” PODS, 484 F.3d at 1367
(internal quotation marks omitted) (quoting Southwall
Techs., 54 F.3d at 1581–83). Amgen did not rely on the
combination of its asserted grounds to distinguish Holtz, so
prosecution history estoppel applies to the “particular com-
binations” ground regardless of the other two arguments
Amgen made.
    Amgen also argues that prosecution history estoppel
does not apply because its August 22, 2011 response—the
response after which the claims were ultimately allowed—
did not contain the argument that Holtz failed to disclose
the particular claimed salt combinations. See Appellant
Br. 38–40. According to Amgen, the arguments made in its
last response prior to allowance “must be the focus of any
argument-based estoppel analysis.” Id. at 40. Our case
law does not support this argument. We recognize that
Amgen did not include the “particular combinations”
ground in its August 22, 2011 response to the patent office.
See J.A. 160–62. This does not mean, however, that
Amgen’s prior statements are erased. There is no require-
ment that argument-based estoppel apply only to argu-
ments made in the most recent submission before
allowance. “[C]lear assertions made during prosecution in
support of patentability, whether or not actually required
to secure allowance of the claim, may also create an estop-
pel[.]” PODS, 484 F.3d at 1368 (quoting Southwall Techs.,
54 F.3d at 1583). We see nothing in Amgen’s final
12                  AMGEN INC. v. COHERUS BIOSCIENCES INC.




submission that disavows the clear and unmistakable sur-
render of unclaimed salt combinations made in Amgen’s
January 6, 2011 response.
    Because we hold that prosecution history estoppel ap-
plies, we do not reach the issue of whether Amgen dedi-
cated unclaimed salt combinations to the public.
                       CONCLUSION
     We have considered Amgen’s remaining arguments
and find them unpersuasive. The district court did not err
in determining that prosecution history estoppel bars
Amgen from succeeding on its infringement claim under
the doctrine of equivalents. Accordingly, we affirm the dis-
trict court’s order dismissing Amgen’s complaint for failure
to state a claim.
                       AFFIRMED
