Examiner Amendments Can Trigger Prosecution History Estoppel

| Brandon G. SmithKendall Loebbaka

Amgen, Inc. v. Amneal Pharmas. LLC et al

Before Newman, Lourie, and Taranto. Appeal from the U.S. District Court for the District of Delaware.

Summary: An examiner amendment may give rise to prosecution history estoppel, even when the examiner suggests the amendment in the absence of a pending office action.

Amgen sued multiple parties for infringement of a patent directed to pharmaceutical compositions useful for treating secondary hyperparathyroidism in adult patients with chronic kidney disease. After a bench trial, the district court held that some parties infringed, but others did not because the claims contained Markush groups of binders and disintegrants. The district court concluded that the Markush group made the entire claim closed ended with regard to the binders and disintegrants such that the presence of a binder or disintegrant in an alleged infringing product, if not recited in the Markush group, supported a finding of non-infringement. The district court also determined that prosecution history estoppel created by an Examiner’s Amendment, precluded the use of the doctrine of equivalents (“DOE”) with regard to the Markush groups.

The claim at issue used an open ended “comprising” preamble but contained two Markush groups. Each Markush group recited “at least one [binder/disintegrant] selected from the group consisting of …” The Federal Circuit reversed, determining that though one of the compounds in the Markush group must be present for infringement, other binder/disintegrant compounds were still permissible in an infringing product because of the plain language of the claim and the open ended preamble. Accordingly, the Federal Circuit remanded infringement to the district court.

The Federal Circuit also upheld the district court’s ruling that prosecution history estoppel barred the use of DOE as it related to the Markush groups. The claims as originally filed did not contain the Markush groups. In response to an obviousness rejection, Amgen made certain amendments to the claims. In a subsequent interview, prior to issuing any further office action, the examiner proposed the Markush groups as an Examiner’s Amendment. Amgen agreed, and the claims were then allowed. Eight months later, in continued examination, Amgen filed a statement that the Examiner’s Amendment was not made to overcome a prior art rejection. Amgen argued that the presumption of prosecution history estoppel was overcome because the addition of the Markush groups was not necessary to secure allowance. The Federal Circuit rejected this argument because its implication was that the Examiner’s Amendment served no purpose, which was “at best unpersuasive.” The Federal Circuit also rejected the argument that Amgen’s comments on the Examiner’s Amendment during prosecution overcame the presumption because those comments were made well after the claims were allowed and were simply “conventional boilerplate” that provided no insight into the reasons for the Examiner’s Amendment.

Editor: Paul Stewart