Key Takeaway: A specification may support claims broader than the specific examples it describes when it identifies features across the claimed genus.
In Exelixis, Inc. v. MSN Laboratories Private Ltd., a central issue before the Federal Circuit was whether claims directed to the genus of crystalline cabozantinib (L)-malate had adequate written description support when the specification described only two polymorphs (unique crystalline structures) of the salt, N-1 and N-2.[1] Exelixis held the New Drug Application (NDA) for Cabometyx®, which contains cabozantinib (L)-malate as its active pharmaceutical ingredient, and reported both the N-1 and N-2 polymorphs in its NDA submission. Exelixis owned the patents asserted in the case, which included the genus claims requiring the cabozantinib (L)-malate salt to be crystalline without specifying a particular polymorph.
MSN filed an Abbreviated New Drug Application (ANDA) seeking approval for generic cabozantinib (L)-malate tablets using crystalline Form S, a different polymorph of the same salt that was not disclosed in the asserted patents. Litigation followed, and Exelixis asserted its broader genus claims against MSN. Although MSN conceded that its generic product fell within the claim language, it argued that the claims were invalid for lack of written description. Specifically, MSN argued that the disclosure of only the N-1 and N-2 crystalline forms did not show possession of the broader genus covering other crystalline forms. It also relied on potential differences in the physical properties of thepolymorphs to argue that N-1 and N-2 could not support the broader claims.
The district court rejected MSN’s written description challenge, and the Federal Circuit affirmed. Under Ariad Pharmaceuticals, Inc. v. Eli Lilly & Co., written description support for a genus may be shown by disclosing either a representative number of species or structural features common to the genus that allow one of skill in the art to recognize its members.[2] Here, the chemical name and formula of cabozantinib (L)-malate, together with the fact that the salt was crystalline rather than amorphous, identified structural features shared by members of the genus. Although not dispositive of written description, the court noted that the specification disclosed examples for making the invention.
Potential differences in the physical properties of the polymorphs did not change the analysis because those properties were not required by the claims. Rather, the claims required crystalline cabozantinib (L)-malate without specifying a particular polymorph or requiring properties like solubility or melting point. In addition, the district court found that the genus could include no more than 14 polymorphs, a finding MSN did not challenge.[3] Accordingly, the case did not present the same concerns as genus claims covering a potentially large number of species. Exelixis shows how the Ariad framework may apply when a specification describes certain polymorphs but also identifies structural features shared across the broader claimed genus.
Co-author: Nicholas Mszar (Pending California Bar Admission)
Editor: Brenden S. Gingrich, Ph.D.
[1] Exelixis, Inc. v. MSN Labs. Priv. Ltd., No. 25-1236 (Fed. Cir. Aug. 31, 2026).
[2] Ariad Pharms., Inc. v. Eli Lilly & Co., 598 F.3d 1336, 1350 (Fed. Cir. 2010) (en banc).
[3] Exelixis, Inc. v. MSN Lab’ys Priv. Ltd., No. 22-228-RGA, 2024 WL 4491176, at *11 (D. Del. Oct. 15, 2024), aff’d, No. 25-1236, slip op. at 12 (Fed. Cir. Aug. 31, 2026).
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