In Endo Pharmaceuticals Inc. v. Teva Pharmaceuticals USA, Inc., No. 2017-1240 (Fed. Cir. Mar. 28, 2019), the Federal Circuit reversed a decision of invalidity under 35 U.S.C. § 101, holding instead that the method of treatment claims were patent eligible for claiming an application of a natural relationship and not the natural relationship itself.

Endo's asserted patent taught a method for adjusting the oxymorphone dose in patients with renal impairment. Actavis, a co-defendant in the case, moved to dismiss Endo's infringement claims, arguing that the claims were ineligible as directed to a natural law under 35 U.S.C. § 101. The district court dismissed after finding the claims ineligible using the two-step Alice/Mayo framework.

On appeal, the Federal Circuit held that the claims were not directed to a natural law because they required using the results of kidney function testing to adjust the oxymorphone dose administered. That is, the claims were "directed to a specific method of treatment for specific patients using a specific compound at specific doses to achieve a specific outcome." The Court rounded out its analysis by comparing the claims to method claims from other cases (CellzDirect, Ariosa, Mayo) and found there was "no room for a different outcome"—the claims were directed to patent eligible subject matter.

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