Last month, in Amgen Inc. v. Coherus BioSciences Inc., the Federal Circuit affirmed a decision by the U.S. District Court for the District of Delaware dismissing a complaint filed by Amgen Inc. and Amgen Manufacturing Ltd. against Coherus BioSciences Inc. for failure to state a claim.  Amgen had filed suit against Coherus for infringement of U.S. Patent No. 8,273,707.

The '707 patent is directed to methods of purifying proteins using hydrophobic interaction chromatography ("HIC"), in which a solid, hydrophobic matrix is used to separate proteins on the basis of hydrophobic interactions between the hydrophobic moieties of the protein and insoluble, immobilized hydrophobic groups on the matrix.  The process disclosed in the '707 patent increases an HIC column's dynamic capacity, which is the maximum amount of protein in a solution which can be loaded onto a column without significant leakage (or breakthrough) of the protein into the solution phase before elution.  Prior art HIC columns used buffers with higher salt concentrations to increase dynamic capacity, but such methods also resulted in protein instability, increased viscosity of a solution, increased formation of aggregates, protein loss due to dilution and filtration of the protein after elution from the column, and reduced protein purity.  According to the '707 patent, the claimed process "provides combinations of salts useful for increasing the dynamic capacity of an HIC column compared with the dynamic capacity of the column using separate salts alone."  Representative claim 1 recites:

1.  A process for purifying a protein on a hydrophobic 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, respectively, 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.

During prosecution of the application that issued as the '707 patent, the Examiner rejected the claims as obvious in view of U.S. Patent No. 5,231,178 ("Holtz").  Amgen responded to the rejection by arguing that Holtz neither taught nor suggested using combinations of salts nor the particular salts recited in the claims.  Amgen also noted that the claimed invention was directed to increasing the dynamic capacity of an HIC column, and that Holtz did not teach dynamic capacity.  In addition, Amgen provided a declaration from one of the inventors stating that using a sulfate/citrate or sulfate/acetate salt combination resulted in substantial increases in the dynamic capacity of a HIC column as compared to using a single salt.  After the Examiner again rejected the claims, Amgen reiterated that Holtz does not disclose a combination of salts and does not disclose enhancing the dynamic capacity of an HIC column, and also pointed out that "merely adding a second salt" would not result in the invention.  The Examiner thereafter allowed the claims.

In 2016, Coherus filed an abbreviated Biologic License Application ("aBLA") seeking FDA approval to market a biosimilar version of Amgen's pegfilgrastim product Neulasta.  After exchanging information pursuant to the Biologics Price Competition and Innovation Act ("BPCIA"), the parties determined that the '707 patent should be included in Amgen's infringement suit against Coherus.  In particular, the aBLA filed by Coherus indicates that its pegfilgrastim manufacturing process utilizes several chromatography steps, one of which involves a chromatography buffer containing a salt combination, albeit not one of the combinations specifically recited in the claims.

Based on Coherus' aBLA, Amgen filed suit against Coherus alleging infringement of the '707 patent under the doctrine of equivalents.  Coherus responded by moving to dismiss Amgen's complaint under Fed. R. Civ. P. 12(b)(6).  Noting that Amgen had distinguished Holtz by arguing that the reference did not disclose "one of the particular, recited combinations of salts," a magistrate judge recommended that Coherus' motion be granted because "prosecution history estoppel bars Amgen from now attempting to reassert surrendered ground involving other combinations of salts."  The District Court adopted the magistrate judge's recommendation and granted Coherus' motion to dismiss.  Amgen appealed the dismissal to the Federal Circuit.

In affirming the District Court's dismissal of Amgen's complaint, the Federal Circuit found that "during prosecution of the '707 patent, Amgen clearly and unmistakably surrendered salt combinations other than the particular combinations recited in the claims," and determined that "[p]rosecution history estoppel thus bars Amgen from succeeding on its infringement claim under the doctrine of equivalents."  Amgen, however, argued that during prosecution it had distinguished Holtz on the basis that this reference failed to disclose increasing dynamic capacity and failed to disclose any salt combinations at all.  The Federal Circuit noted that:

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 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."

Citing PODS, Inc. v. Porta Stor, Inc., 484 F.3d 1359, 1367 (Fed. Cir. 2007), the Federal Circuit also noted that "where 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."  The Court concluded that "Amgen did not rely on the combination of its asserted grounds to distinguish Holtz," and that "while Amgen did assert multiple reasons for why Holtz is distinguishable, our precedent instructs that estoppel can attach to each argument."  The Court therefore determined that in the instant case, "prosecution history estoppel applies to the 'particular combinations' ground regardless of the other two arguments Amgen made."

Amgen also argued that prosecution history should not apply in the instant case because the response filed prior to allowance of the claims did not contain the argument that Holtz failed to disclose the particular claimed salt combinations.  Explaining that "[t]here is no requirement that argument-based estoppel apply only to arguments made in the most recent submission before allowance," the Federal Circuit stated that "[w]e see nothing in Amgen's final submission that disavows the clear and unmistakable surrender of unclaimed salt combinations made in Amgen's [earlier] response."  The Federal Circuit therefore determined that the District Court did not err in determining that prosecution history estoppel barred Amgen from succeeding on its infringement claim under the doctrine of equivalents, and affirmed the District Court's order dismissing Amgen's complaint for failure to state a claim.

Amgen Inc. v. Coherus BioSciences Inc. (Fed. Cir. 2019)
Panel: Circuit Judges Reyna, Hughes, and Stoll
Opinion by Circuit Judge Stoll