In Amgen the Federal Circuit held that prosecution history estoppel barred Amgen from using the doctrine of equivalents to cover salt combinations that were disclosed, but not claimed, in the patent, because the applicant had made arguments to the patent examiner stressing the patentability of the particular salt combinations recited in the claims.
Amgen owns a patent directed to the use of salt combinations in a method of purifying proteins. Coherus was accused of infringing the patent after filing an abbreviated Biologics License Application for a biosimilar of a protein-based drug owned by Amgen. In their aBLA disclosure, Coherus revealed a production method that involved purifying proteins with chromatography using a combination of salts that was disclosed, but not claimed, in the patent. Because Coherus’s aBLA used salt combinations, but not the specific combinations recited in the claims, Amgen asserted infringement under the doctrine of equivalents.
The doctrine of equivalents recognizes that language is often imperfect at describing an invention, and it is often possible to design around the literal wording of a patent claim with no more than insubstantial changes. The doctrine of equivalents protects patentees from accused infringers using products or processes that are identical or equivalent to what is claimed in the patent.
Prosecution history estoppel prevents a patentee from using the doctrine of equivalents to recapture subject matter disclaimed or surrendered by the patentee during the prosecution of the patent. Estoppel arises either where the patentee narrows the scope of a claim through amendment, or through arguments made to the patent examiner.
The district court dismissed Amgen’s claims for infringement as being barred by the doctrine of prosecution history estoppel. Amgen appealed to the Federal Circuit.
The Federal Circuit held that the district court did not err in dismissing Amgen’s claim, as during prosecution Amgen repeatedly argued that the particular combination of salts recited in the claim was not disclosed in the prior art. By doing so, Amgen surrendered unclaimed salt combinations to the public. Amgen also argued that the applicant raised several other arguments to distinguish the prior art in addition to the one relied on by the district court, and that they did not assert the particular salt combination in their final response that led to allowance. However, the Federal Circuit was not persuaded by either argument. Prosecution history estoppel can attach to each argument made in prosecution individually, and there is no requirement that prosecution history estoppel should apply only to the most recent submission made in prosecution.
Key takeaway: Prosecution history estoppel can attach to each argument made in prosecution, and is not limited to arguments made at the final stages of prosecution, unless some statement is made to clearly disavow a clear and unmistakable surrender of subject matter. Arguments made during prosecution to stress the importance and novelty of the particular limitations found in patent claims may be read by the courts to disclaim unclaimed alleged equivalents.