The US Court of Appeals for the Federal Circuit expanded the Markush doctrine, determining that the claim language “comprising . . . [at least] . . . a group consisting of . . .” absent some basis or extrinsic evidence for limiting the group, such a group could capture an alleged infringement  having an additional component  present, as long as the additional component is “functionally similar” to the component identified in the Markush group limitation. ,Amgen Inc. v. Amneal Pharm. LLC, Case No. 18-2414 (Fed. Cir. Jan. 7, 2020) (Lourie, J).

This case involves an abbreviated new drug application (ANDA) litigation over Amgen’s Sensipar®, a rapid dissolution formulation of cinacalcet hydrochloride. To overcome a US Patent and Trademark Office rejection in an office action citing prior art, Amgen filed an amendment narrowing the cinacalcet formulation limitation and later accepted the examiner’s claim amendment that modified the binder claim elements of the formulation into a Markush format. The district court found that the examiner’s reliance on the “combination of components” in the notice of allowance underscored that the Markush groups were added for patentability. The district court found that:

• Amneal’s and Piramal’s ANDA product did not infringe because it did not meet other claim limitations, even though its formulation used crospovidone (a variation of povidone claimed in the Markush group).

• Zydus’s ANDA product literally infringed claim 1 of the asserted patent.

Amgen appealed.

On appeal, the Federal Circuit explained that the district court’s constructions of the Markush group were incorrect because it excluded formulations with additional unlisted ingredients—binders, disintegrants or otherwise. The Court noted that “the claim requires only that [hydroxypropyl methylcellulose (HPMC)] be present, not that HPMC’s physical characteristics or function be unaffected by additional ingredients,” and asked the lower court to determine (on remand) whether Amneal’s formulation contained “from about 1% to about 5% by weight” of HPMC, irrespective of the HPMC’s pairing with polyethylene glycol.

With respect to Piramal, whose formulation contained pregelatinized starch as a binder (not as a diluent as claimed in the Markush group), the Federal Circuit affirmed that prosecution history estoppel applied to Amgen’s doctrine of equivalents argument because Amgen failed to carry its burden to demonstrate that the examiner’s amendment was made for a reason unrelated to patentability. Thus, the Court agreed that Amgen surrendered equivalent but unclaimed binders and disintegrants.

With respect to Zydus, the Court affirmed that Zydus infringed because its ANDA stated that the pregelatinized starch in Zydus’s formulation functioned as a diluent and fell within the scope of the asserted claim.

In sum, the Federal Circuit vacated and remanded the district court’s decision and instructed the district court to evaluate whether Amneal’s product met the disintegrant limitation when applying the corrected construction for the Markush claim elements.

Practice Note: The Federal Circuit recognized that claims written in a “comprising . .  [at least] . . . a group consisting of . . .” Markush format also include functionally similar elements to the claimed limitations within the Markush group but can be limited by prosecution history estoppel. In other words, if “comprising” is used as the claim transition before a Markush group, the scope of that Markush group may reach beyond the literally stated elements to functionally similar elements that are recognized by the art. When using a Markush claim format, practitioners therefore should use “comprising” before reciting the limitations in a Markush group.