The Tangential Exception to Prosecution History Estoppel Should Not Be Viewed Rigidly

In Eli Lilly & Company v. Hospira, Inc., Nos. 2018-2128, 2018-2127 (Fed. Cir. Aug. 9, 2019), the Federal Circuit affirmed the district court’s application of the doctrine of equivalents and clarified that the tangential exception of prosecution history estoppel should not be viewed too rigidly.    

The dispute centered on whether Lilly’s claims at issue, which recited particular methods of treatment by administering pemetrexed disodium, covered administration of a different pemetrexed salt, pemetrexed ditromethamine, under DOE. During patent prosecution, Lilly submitted a claim amendment changing “an antifolate” to “pemetrexed disodium.” The Federal Circuit held that the amendment did not bar the application of DOE because narrowing “antifolate” to “pemetrexed disodium” could not possibly distinguish art referencing pemetrexed disodium in then-pending obviousness rejections. The court emphasized that when determining whether prosecution history estoppel applies to DOE, courts should analyze the context surrounding an amendment. In this case, the court concluded that the reason for the amendment was not to cede other, functionally identical, pemetrexed salts, and that it was unlikely that competitors would be justified in believing that equivalent pemetrexed salts would not infringe. The court refused to adopt a bright-line rule that the tangential exception does not apply where the reason for the amendment and the equivalent in question both related to the same claim element. Considering the prosecution history as a whole and the patent-at-issue itself, the court concluded that it was implausible that the reason for Lilly’s amendment was to surrender other pemetrexed salts.