CAFC Holds Prosecution History Estoppel Bars Patentee’s Infringement Claim

Last week, the United States Court of Appeals for the Federal Circuit issued an opinion in Amgen Inc. v. Coherus Biosciences Inc., No. 18-1993 (July 29, 2019). Amgen owns U.S Patent No. 8,273,707 for methods of purifying proteins. The invention disclosed combinations of salts for enhancing protein recovery as compared with the use of separate salts alone. Amgen’s patent claims required a salt combination chosen from citrate, sulfate, or acetate salts. During prosecution, Amgen’s claims were rejected as obvious in view of a prior art patent to Holtz. The examiner noted that Holtz disclosed several salts that could improve protein recovery, and asserted that routinely optimizing Holtz would have resulted in Amgen’s invention. Amgen responded that its patent claims recited a particular combination of salts and that Holtz did not teach or suggest either a combination of salts generally or the particular combinations of salts recited in Amgen’s claims. Amgen further emphasized that the particular combination of salts improved manufacturing cost-effectiveness. Amgen included a supporting inventor declaration stating that using a sulfate/citrate or sulfate/acetate salt combination resulted in substantial increases in protein recovery at a reduced cost compared to using a single salt. The patent claims were allowed to issue.

Coherus subsequently sought to market a biosimilar version of an Amgen therapeutic protein. Amgen sued Coherus for infringement. Because Coherus’s process did not use any of the specific combinations recited in the Amgen patent claims, Amgen alleged infringement under the doctrine of equivalents. Coherus moved to dismiss Amgen’s complaint under Rule 12(b)(6). The district court granted Coherus’s motion, noting that the Amgen patent’s prosecution history revealed Amgen clearly and unmistakably surrendered claim scope in its arguments to overcome Holtz. Amgen appealed.

The Federal Circuit affirmed, holding that argument-based prosecution history estoppel applied because Amgen clearly and unmistakably surrendered unclaimed salt combinations during prosecution. The Court noted that Amgen emphasized “particular” salt combinations in its responses to the Patent Office, and indeed referred to its particular salts three times in the span of two pages for just one Office Action response. And Amgen’s supporting inventor declaration discussed the same combinations recited in Amgen’s claims. The Court explained that, according to applicable precedent, asserting multiple grounds for distinguishing a prior art reference does not avoid estoppel; rather, estoppel attaches to each such ground.

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