IP Law Daily, PATENT—Fed. Cir.: Validity of three patents covering the crystalline malate salt of cabozantinib upheld, (Sep 2, 2026)

By Carolin Dennis, B.Sc., LL.B., LL.M.

The appellate court preserved the market exclusivity of the oncology company’s cabozantinib drug marketed under the brand name Cabometyx.

The U.S. Court of Appeals for the Federal Circuit affirmed the district court’s decision that three patents owned by Exelixis, Inc., which claimed cabozantinib malate salts compositions were not invalid because the district court applied the correct legal standard for written description for the asserted claims. However, the Federal Circuit dismissed the appeal regarding the validity dispute over one of Exelixis, Inc.’s patents covering low-impurity cabozantinib formulations, ruling the matter moot (Exelixis, Inc. v. MSN Laboratories Private Ltd., No. 25-1236 (Fed. Cir. Aug. 31, 2026)).

Background. Exelixis, Inc. (Exelixis) sells Cabometyx for the treatment of kidney cancer, liver cancer, and differentiated thyroid cancer and Cometriq for the treatment of metastatic medullary thyroid cancer. The active pharmaceutical ingredient (API) in Cabometyx and Cometriq is the (L)-malate salt of cabozantinib. MSN submitted Abbreviated New Drug Application (ANDA) No. 213878 to the U.S. Food and Drug Administration (FDA), seeking approval to manufacture and sell a generic version of Cabometyx. Exelixis brought a patent infringement action against MSN Laboratories Private Limited and MSN Pharmaceuticals, Inc. (together, MSN), asserting claim 3 of U.S. Patent No. 11,298,349 (the ’349 patent). The ’349 patent is directed to a pharmaceutical composition of cabozantinib (L)-malate that includes certain classes of excipients and is free of a harmful genotoxic impurity. In a second suit, Exelixis asserted a group of patents that shared the same specification—claim 4 of U.S. Patent Nos. 11,091,439 (the ’439 patent), claim 3 of U.S. Patent No. 11,091,440 (the ’440 patent), and claim 2 of U.S. Patent No. 11,098,015 (the ’015 patent) (collectively, the Malate Salt Patents). The Malate Salt Patent claims require a cabozantinib malate salt that is crystalline. The Malate Salt Patents and the ’349 patent have been listed in connection with Cabometyx in the FDA’s Orange Book. The district court found that the asserted claims of the ’439, ’440, and ’015 patents have adequate written description pursuant to 35 U.S.C. § 112(a). As to the ’349 patent, the district court found that MSN’s ANDA product did not infringe asserted claim 3. The district court also held that MSN failed to prove claim 3 was invalid. Accordingly, the district court entered final judgment against MSN as to the Malate Salt Patents, which the district court held infringed and not invalid, while it held the ’349 patent not infringed and not invalid.

MSN appealed asserting that the district court erred in finding written description support under 35 U.S.C. § 112(a) for claim 4 of the ’439 patent, claim 3 of the ’440 patent, and claim 2 of the ’015 patent. While MSN also initially challenged the district court’s finding of no inherency for claim 3 of the ’349 patent, it now urged the Federal Circuit to dismiss this portion of its appeal as moot and vacate the underlying district court order as to claim 3.

Malate Salt Patents—written description. The district court found that the Malate Salt Patents claimed a genus of crystalline cabozantinib (L)-malate and used the legal framework set forth in Ariad Pharms., Inc. v. Eli Lilly & Co., 598 F.3d 1336, 1350 (Fed. Cir. 2010)) and later applied in GlaxoSmithKline LLC v. Banner Pharmacaps, Inc., 744 F.3d 725 (Fed. Cir. 2014) to analyze the adequacy of the written description of the claimed genus. MSN contended that the district court legally erred by crediting allegedly cursory parts of the specification that fail to disclose structural features distinguishing the genus of crystalline cabozantinib (L)-malate salt such that a skilled artisan could visualize its members. The Federal Circuit was not persuaded because it found no clear error in the district court’s finding that disclosing the chemical name and formula of cabozantinib (L)-malate salt, as well as that the structure of the salt is crystalline, is an identification of the structural features possessed by members of the genus.

The Federal Circuit was also unpersuaded by MSN’s reliance on the purported “different densities, melting points, solubilities, hygroscopicity, vapor pressure, and stability” of the disclosed N-1 and N-2 polymorphs. According to MSN, these properties shared by N-1 and N-2 might not be shared by other species of the claimed genus and thus a person of ordinary skill in the art would not think that the inventor invented other species. However, MSN failed to explain, in the context of this particular invention, why these differences which, by the way, are unclaimed and merely listed in general terms showed that the district court clearly erred in its fact finding. According to the Federal Circuit, the district court reasonably relied on the specification’s disclosure of the chemical name and formula of cabozantinib (L)-malate salt and that the structure is crystalline as an identification of the structural features possessed by members of the genus.

MSN also contended that the district court erred by “assuming there are ‘more rigorous requirements for written description in support of functional claim language,’ which the court believed ‘require more disclosure to meet the written description requirement.’” However, the Federal Circuit noted that reading the district court’s order in full makes clear that the court was not using a lower standard for the structural claims than it would for functional ones. Instead, the district court was merely recognizing that when there is functional claiming, supplying adequate written description could be more challenging. Therefore, the Federal Circuit determined that the district court applied the correct legal standard for written description for the asserted structural claims and did not err in finding written description support for these claims.

The ’349 patent—obviousness. The Federal Circuit granted MSN’s motion to dismiss its appeal challenging the no inherency finding for claim 3 of the ’349 patent because it agreed with MSN that this portion of its appeal is now moot. Having determined that MSN’s appeal as to claim 3 is moot, the Federal Circuit noted that the appropriate resolution in this case is vacatur of the district court’s judgment of nonobviousness of claim 3. Therefore, because Exelixis’s unilateral decision to drop its cross-appeal frustrated MSN’s attempt to seek review on the merits of the district court’s no inherency finding, the Federal Circuit vacated the judgment as to claim 3.

Accordingly, the district court’s decision was affirmed in part, and dismissed and vacated in part.

The Case is No. 25-1236.

Judge: Stoll, K.

Attorneys: Thomas Saunders (Wilmer Cutler Pickering Hale and Dorr LLP) for Exelixis, Inc. Kurt A. Mathas (Winston Taylor LLP) for MSN Laboratories Private Ltd.

Companies: Exelixis, Inc.; MSN Laboratories Private Ltd.

Cases: Patent FedCirNews