The US Court of Appeals for the Federal Circuit affirmed that a patent’s specification provided adequate written description support for genus claims directed to crystalline cabozantinib (L)-malate salts because it disclosed structural features common to the claimed genus. Exelixis, Inc. v. MSN Laboratories Private Ltd., Case No. 25-1236 (Fed. Cir. Aug. 31, 2026) (Stoll, J.; Moore, C.J.; Moore, District J., sitting by designation).

Exelixis holds the New Drug Application for Cabometyx®, a cancer treatment containing cabozantinib (L)-malate. MSN sought US Food and Drug Administration approval to market a generic cabozantinib (L)-malate product. Exelixis asserted three related patents directed, respectively, to crystalline cabozantinib (L)-malate salts, pharmaceutical formulations containing those salts, and methods of treating cancer using the salts. The patents share a common specification. Exelixis also asserted a separate patent directed to cabozantinib (L)-malate compositions containing low levels of a genotoxic impurity. MSN sought FDA approval to market a generic cabozantinib (L) malate product.

MSN conceded infringement of the crystalline malate salt patents but argued that the asserted claims were invalid for lack of written description under 35 USC § 112(a). After a bench trial, the district court rejected that challenge. As to the low-impurity formulation patent, the district court found no infringement and concluded that MSN had failed to establish invalidity based on inherent obviousness. MSN appealed.

Addressing written description, the Federal Circuit applied the framework for genus claims set forth in its 2010 Ariad Pharmaceuticals. v. Eli Lilly & Co decision. A specification may demonstrate possession of a genus by disclosing either a representative number of species or structural features common to the genus that allow a skilled artisan to visualize or recognize its members.

The Federal Circuit found no clear error in the district court’s determination that the common specification adequately disclosed structural features of the claimed genus. In particular, the specification identified the chemical name and formula of cabozantinib (L)-malate and specified that the claimed salt had a crystalline structure. The claims were correspondingly limited to crystalline cabozantinib (L)-malate salts. The specification also disclosed processes for preparing crystalline and amorphous forms, although the Court noted that those disclosures were not dispositive of written description.

The Federal Circuit rejected MSN’s argument that the specification failed to describe the genus because different crystalline polymorphs could possess different properties, such as density, melting point, and solubility. The Court explained that those characteristics were not claimed and that MSN had not shown why differences in unclaimed properties undermined the district court’s finding that the specification identified structural features common to the claimed genus.

The Court therefore affirmed the district court’s determination that the crystalline malate salt claims satisfied the written description requirement.

The Federal Circuit separately addressed MSN’s appeal concerning the low-impurity formulation patent. The district court had found the asserted claim not infringed and not invalid, concluding that MSN failed to prove that the prior art inherently produced a formulation containing no more than the claimed level of a genotoxic impurity. Exelixis initially appealed the noninfringement ruling but later dismissed [...]

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