In Amgen Inc. v. Coherus Biosciences Inc., No. 2018-1993 (Fed. Cir. July 29, 2019), the Federal Circuit affirmed the district court’s dismissal of Amgen’s infringement claim under the doctrine of equivalents. The asserted patent claimed certain methods of purifying proteins involving the use of any of three particular combinations of salts. Coherus’s acknowledged purification method employed a combination of salts but not one of particular three claimed combinations.

The Federal Circuit held that prosecution history estoppel barred Amgen from succeeding on its infringement claim under the doctrine of equivalents because, during prosecution, Amgen “clearly and unmistakably surrendered” salt combinations other than the three particular combinations recited in the claims. The file history showed that, in response to an obviousness rejection, Amgen had argued that the cited reference failed to disclose the particular salt combinations recited in Amgen’s claims. The argument was not the only one Amgen raised, nor was it included in the remarks that ultimately led to allowance. Nevertheless, the Court reasoned that each argument in support of patentability can create a separate estoppel, including assertions that were not actually required to secure allowance.