The US Court of Appeals for the Federal Circuit affirmed a district court’s claim construction and grant of a preliminary injunction, concluding that an accused infringer’s proposed construction would have produced the “highly improbable” result of excluding the patentee’s Orange Book-listed drug from the scope of the asserted claims. Otsuka America Pharmaceutical, Inc. v. Hetero Labs Limited, Case No. 25-2016 (Fed. Cir. July 1, 2026) (Bryson, Stoll, JJ.) (Dyk, J., dissenting in part)

Otsuka sued Hetero Labs, alleging that Hetero Labs’ generic drug product would infringe Otsuka’s patent rights. Otsuka moved for a preliminary injunction to prevent Hetero Labs from entering the market. The district court granted the motion, concluding that Otsuka was likely to succeed on its infringement claim. Hetero Labs appealed, arguing that the district court erred in construing the asserted claims.

The central dispute concerned how to calculate the patent’s claimed weight ratio. The representative claim requires administering dextromethorphan in combination with quinidine, with the proviso that the “weight to weight ratio of dextromethorphan to quinidine is 1:0.5 or less.” The district court construed “dextromethorphan” and “quinidine” to include their pharmaceutically acceptable salts. Under that construction, the claimed ratio is calculated using the full weight of the compounds as administered – for example, the full weight of dextromethorphan hydrobromide rather than only the dextromethorphan active moiety. Hetero Labs, by contrast, argued that when the compounds are administered as salts, the ratio must be calculated using only the weight of the active moieties.

The Federal Circuit affirmed the district court’s construction, concluding that the intrinsic evidence supported construing “dextromethorphan” and “quinidine” to include both the free base compounds and their pharmaceutically acceptable salts. The Court found that the patent claims, specification, and prosecution history consistently used those terms to encompass the compounds in the form in which they are administered. The Court also found that extrinsic evidence supported its construction. Relying in part on its 2007 decision in Osram GmbH v. ITC, the Court explained that Hetero Labs’ proposed construction would produce the “highly improbable” result of excluding Otsuka’s Orange Book-listed Nuedexta product – the very product the patent was intended to protect – from the scope of the claims. The Court also rejected Hetero Labs’ indefiniteness argument, concluding that the claims were not rendered indefinite simply because they encompassed formulations using either free bases or pharmaceutically acceptable salts.

Judge Dyk, dissenting in part, concluded that the district court’s construction was inconsistent with both the intrinsic evidence and the purpose of the claimed invention. According to Judge Dyk, the invention depended on maintaining a specific ratio of dextromethorphan to quinidine to ensure that:

• Dextromethorphan achieved a therapeutic effect
• Quinidine slowed the metabolism of dextromethorphan
• Patients were not exposed to excessive amounts of quinidine

In his view, calculating the claimed ratio using the full weight of pharmaceutically acceptable salts rather than only the active moieties distorted that relationship by incorporating the weight of carrier ions that have no therapeutic significance. Judge Dyk also disagreed with the majority’s reliance [...]

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