In a precedential ruling on August 31, 2026, the U.S. Court of Appeals for the Federal Circuit affirmed a District of Delaware judgment that the claims of three Exelixis patents covering the cancer drug Cabometyx® (cabozantinib) were not invalid for lack of written description. The decision demonstrates that broad genus claims may satisfy the written description requirement where the specification shows possession of the claimed genus through disclosure of structural features common to the genus, despite disclosure of only a limited number of species. (Exelixis, Inc. v. MSN Laboratories Private Ltd., No. 2025-1236 (Fed. Cir. Aug. 31, 2026))
This ruling provides important guidance for patent drafting, prosecution, and lifecycle management strategies involving polymorph and salt-form inventions. This decision is particularly relevant to pharmaceutical and life sciences companies seeking patent protection for crystalline forms of small-molecule drugs.
Background
The Patents at Issue and the Evolution of Cabozantinib Polymorph Protection
Exelixis holds three patents – U.S. Patent Nos. 11,091,439, 11,091,440, and 11,098,015 (the “malate salt patents”) – that claim crystalline salts of cabozantinib (L)-malate, pharmaceutical compositions thereof, and methods of treatment, respectively. Exelixis also has a fourth patent, U.S. Patent No. 11,298,349, directed to low-impurity cabozantinib formulations.
Cabozantinib can exist in either an amorphous or a crystalline state. When crystalline, it can exist as multiple distinct crystalline forms, known as polymorphs. While the asserted claims covered crystalline cabozantinib (L)-malate salts generally, the patents disclosed only the N-1 and N-2 polymorphs. The Federal Circuit agreed with the district court that the claims were directed to the genus of crystalline cabozantinib (L)-malate salts, rather than any specific polymorph.
Exelixis was the first to identify crystalline cabozantinib (L)-malate polymorphs, discovering the N-1 and N-2 forms, submitting them to the FDA, and obtaining patents claiming those specific polymorphs (U.S. Patent Nos. 8,877,776 and 9,809,549). Additional crystalline forms were later identified by other companies, although the known universe of crystalline cabozantinib (L)-malate polymorphs consisted of a relatively small number of species. In 2019, Exelixis filed additional patent applications within the same patent family as its earlier polymorph patents, pursuing claims directed to the broader genus of crystalline cabozantinib (L)-malate salts, which later issued as the three malate salt patents asserted against MSN.
MSN’s Challenge Focused on Written Description Support for the Broad Genus Claims
Exelixis filed suit against MSN for patent infringement, alleging that MSN’s submission of an Abbreviated New Drug Application (ANDA) seeking approval to market a generic version of Cabometyx® infringed the patents.
MSN conceded infringement of the malate salt patents but argued that the asserted claims were invalid for lack of written description. Following a bench trial, the district court held that the asserted claims were not invalid, and MSN timely appealed to the Federal Circuit.
Key Issue – Written Description
Federal Circuit Finds Common Structural Features Were Sufficient
The Federal Circuit reaffirmed that, under Ariad Pharmaceuticals, Inc. v. Eli Lilly & Co., 598 F.3d 1336 (Fed. Cir. 2010), the written description requirement for a genus claim can be satisfied by disclosing either (1) a representative number of species falling within the scope of a claimed genus, or (2) structural features common to the genus that enable a person of ordinary skill in the art to visualize or recognize the members of the genus.
MSN argued that N-1 and N-2 were not representative of the claimed genus, because other crystalline forms, including MSN’s generic product, exhibited different physicochemical properties. The Federal Circuit rejected that argument, concluding that MSN had not shown why the differences in crystal packing and physicochemical properties undermined the specification’s disclosure of structural features common to the claimed genus.
Applying Ariad, the court held that the disclosure of the chemical name, formula, and crystallinity of cabozantinib (L)-malate was sufficient to identify structural features common to the claimed genus and satisfy the written description requirement.
Why This Matters for Life Sciences Patent Strategy
For pharmaceutical innovators and patent owners, this decision provides useful guidance for drafting and prosecuting patent applications directed to crystalline forms as part of a small-molecule patent strategy (see also Grünenthal GmbH v. Alkem Laboratories Ltd. (Fed. Cir. 2019) and Pharmacyclics LLC v. Alvogen, Inc. (Fed. Cir. 2022)). Although the malate salt patents disclosed only two polymorphs (N-1 and N-2), the Federal Circuit concluded that the specification adequately demonstrated possession of the broader genus of crystalline cabozantinib (L)-malate salts through the disclosure of common structural features.
The decision also underscores the importance of a layered claim and disclosure strategy that supports both specific polymorph claims and broader crystalline-salt genus claims. For applicants pursuing polymorph protection, the decision highlights the strategic value of drafting specifications that identify structural features common to a broader crystalline-salt genus, in addition to describing particular polymorphs. By disclosing structural features common to the genus, together with representative species and other supporting disclosures (such as XRPD data and preparation methods), applicants may demonstrate possession of a claimed crystalline-salt genus, thereby satisfying the written description requirement of 35 U.S.C. § 112(a).
Companies developing small-molecule therapies should consider whether existing and future patent filings adequately support both specific polymorph claims and broader crystalline-salt genus claims.
If you have any questions about how this Federal Circuit decision may affect your organization’s patent protection planning, please contact Christine Greene, William Scofield, Brian Trinque, or your regular Lathrop GPM attorney.