While genus claims encompass a broader range than species claims, these claims are often vulnerable to challenges as to whether there was adequate written description across the breadth of the genus claimed.

A goal of patent practitioners during the prosecution of a patent application before the United States Patent and Trademark Office is to lawfully obtain the broadest possible legal scope of protection for the underlying invention through granted claims that can withstand a validity challenge. While genus claims encompass a broader range than species claims, these claims are often vulnerable to challenges as to whether there was adequate written description across the breadth of the genus claimed. Patent claims directed to a genus of chemical compounds must be supported by a specification that allows a person skilled in the art to ascertain and distinguish which species fall within the genus from those that do not. Under the written description requirement of 35 USC § 112(a), that obligation may be discharged either by disclosing a representative number of species within the genus or by delineating structural features common to the genus members that set them apart from other materials. The rigor of the inquiry scales with the nature, complexity and predictability of the technology at issue. In Exelixis, Inc. v. MSN Laboratories Private Ltd., No. 2025-1236, (Fed. Cir. Aug. 31, 2026), the Federal Circuit confirmed that when a genus claim is anchored to well-understood structural attributes—the chemical name, formula and crystalline character of a pharmaceutical salt—the patentee need not exhaustively catalog every polymorph or predict the individual physical properties of undisclosed forms to satisfy the written description requirement.

Background and Case History

Exelixis holds the New Drug Application for Cabometyx, whose active pharmaceutical ingredient is cabozantinib (L)-malate, indicated for the treatment of kidney, liver and differentiated thyroid cancer. Exelixis previously investigated freebase cabozantinib and filed patent applications claiming the same. Exelixis further investigated cabozantinib salts that would be suitable for commercial development.

Salts can take the form of a crystalline or amorphous salt. Both crystalline and amorphous salts of a particular compound share the same chemical name and formula but differ in their arrangement of molecules. Amorphous salts have a random arrangement of molecules whereas crystalline salts have an ordered and repeated arrangement of molecules that give rise to a unique crystal structure. The arrangement of molecules can be distinct between crystalline salts of the same compound, giving rise to polymorphs. Although polymorphs share identical chemical composition, they differ in the three-dimensional packing of their molecules within the crystal lattice and can exhibit different physical properties such as melting point, solubility, stability and hygroscopicity.

Exelixis’s investigation of other cabozantinib salts led to the discovery of two crystalline solid forms or polymorphs, referred to as N-1 and N-2, which were demonstrated to have similar properties. Exelixis filed for and obtained the grant of U.S. Patent Nos. 8,877,776 and 9,809,549 directed to crystalline cabozantinib (L)-malate forms N-2 and N-1, respectively. Exelixis later obtained asserted U.S. Patent Nos. 11,091,439, 11,091,440 and 11,098,015 (collectively “malate salt patents”),[1] which are broadly directed to crystalline forms of cabozantinib malate, pharmaceutical formulations thereof, and methods of treating cancer using the same. Each of the asserted patents shares a common specification with U.S. Patent Nos. 8,877,776 and 9,809,549. In addition to providing six examples for preparing both crystalline and amorphous forms, the specification describes the N-1 and N-2 polymorphs in detail.

MSN subsequently filed an Abbreviated New Drug Application seeking FDA approval for generic cabozantinib (L)-malate tablets, using its own polymorphic form designated “form S.” Exelixis sued MSN, asserting that its form S infringed the malate salt patents. Although MSN conceded infringement of the malate salt patents, it further argued that the asserted claims were invalid for lack of written description under 35 USC § 112(a).

The district court held that there was written description support for the malate salt patents. Applying the framework established in Ariad Pharmaceuticals, Inc. v. Eli Lilly & Co., 598 F.3d 1336 (Fed. Cir. 2010) (en banc), the court found that the specification adequately demonstrated possession of the claimed genus by identifying the key structural feature—the chemical formula and structure of crystalline cabozantinib (L)-malate—and that a person skilled in the art could readily distinguish the crystalline form from the amorphous form. The court further noted that the specification described the processes of making the crystalline and amorphous forms of cabozantinib malate. MSN appealed to the United States Court of Appeals for the Federal Circuit.

The Federal Circuit’s Decision

On appeal, MSN challenged the sufficiency of the disclosure relied on by the district court in holding that the malate salt patents had adequate written description. MSN also asserted that the properties of the N-1 and N-2 polymorphs disclosed in the specification may not be shared by other members within the genus such that the inventors could not have purported to invent these other members. Finally, MSN argued that the district court applied a more lenient written description standard to structural claims than to functional claims.

The Federal Circuit, in affirming the district court’s holding that the malate salt patents had adequate written description, reaffirmed the Ariad framework that a genus claim may be sustained by a specification that discloses either a representative number of species within the genus or structural features common to the genus members that enable a skilled reader to visualize or recognize them. The specification may accomplish this through structure, formula, chemical name, physical properties or other identifying characteristics sufficient to differentiate the genus from other materials.

With respect to MSN’s challenge as to the sufficiency of the disclosure relied on by the district court, the Federal Circuit observed that multiple Ariad factors—structure, formula and chemical name—were satisfied. The court found no clear error in the district court’s determination that disclosing the chemical name and formula of cabozantinib (L)-malate salt and specifying that the structure is crystalline constituted adequate identification of the structural features shared by the claimed genus. The claims were no broader than the written description because they require a cabozantinib (L)-malate salt possessing a crystalline structure. The court also endorsed the district court’s analogy to GlaxoSmithKline LLC v. Banner Pharmacaps, Inc., 744 F.3d 725 (Fed. Cir. 2014), where describing a complex of dutasteride and solvent molecules was held to be an adequate identification of structural features common to the claimed genus, and like the claims at issue, the instant claims impose no performance-property requirement—the salt need not perform a particular function or produce a particular result—thereby eliminating any concern about insufficient structural or process descriptions to support such a property.

The Federal Circuit was also unpersuaded by MSN’s contention that the N-1 and N-2 polymorphs exhibit different properties from other possible polymorphs, noting that these property differences were unclaimed and described only in general terms, and MSN failed to explain, in the specific context of this invention, why those differences demonstrated that the district court committed clear error. Critically, the district court had not relied on the properties of N-1 and N-2 to identify other polymorphs but rather it had relied on the chemical name, formula and crystalline character of the salt. The court also took note of the district court’s unchallenged finding that the maximum potential size of the crystalline polymorph genus is 14 forms, suggesting that the concerns attendant to vast-genus claims do not arise in this context. There was accordingly no need to analyze the alternative representative-number-of-species prong of Ariad, since the structural-features prong was satisfied.

The court disagreed with MSN’s assertion that the district court applied a more lenient standard as to satisfying written description for structural claims. Rather, from the district court’s opinion, it was evident that the district court was simply recognizing—as Ariad itself acknowledged—that furnishing adequate written description can be more demanding when claims employ functional language to define the boundaries of the genus as there may be questions as to whether the inventors have demonstrated a sufficient number of species to adequately support the entire functional genus.

Accordingly, because the instant claims are no broader than what the specification describes, crystalline cabozantinib malates, which share a chemical name and formula with amorphous salts, can be readily distinguished from amorphous analogs, the Federal Circuit affirmed the district court’s holding of adequate written description.

Conclusion and Practice Points

Exelixis reinforces the principle that written description of a genus claim defined by structural attributes does not require an exhaustive enumeration of every species within the genus. Where the specification identifies shared structural features—chemical name, formula and crystalline character—sufficient to enable a person skilled in the art to recognize genus members, the requirement is satisfied even if the specification discloses only a subset of possible members of the genus.

However, the requisite amount of description necessary is a fact-intensive analysis and is evaluated on a case-by-case basis. There is no established minimum threshold or magic number that must be reached. For example, in contrast to Exelixis, the asserted patent disclosing and exemplifying 26 antibodies in Amgen Inc. v. Sanofi, 598 U.S. 594 (2023) was held invalid. While the issue in Amgen involved enablement, the asserted claims in Amgen were functional claims and recited what the claimed antibodies did versus linking structural features of the claimed antibodies to the epitope binding function. Thus, despite disclosing 13 times as many examples as compared to Exelixis, the patent at issue in Amgen was held invalid because the scope of the claimed genus (antibodies binding to the PCSK9 epitope) was overly broad and encompassed the disclosed 26 antibodies as well as a multitude of antibodies yet to be discovered that are also able to accomplish binding to the epitope, such that ascertaining and discovery of such antibodies would be tantamount to undertaking a research project requiring undue experimentation.

While the chemistry itself in Exelixis such as the core chemical structure of cabozantinib (L)-malate and the limited number of possible crystal structures limited the scope of the possible number of genus members, this may not always be the case and practitioners often seek a scope of protection that is broader than that of two specific examples. This can be accomplished through establishing a structure-function relationship by tying associated function with a structural element. Having sufficiently disclosed and claimed the structural features that give rise to the beneficial functions of the claimed embodiments, reasonable inferences as to other members of the genus can be made to broaden the scope of the genus. Other patent family members of those patents at issue in Amgen claimed the PCSK9-binding antibodies via claiming the specific amino acid sequences responsible for binding to the epitope (i.e., complementary determining regions (CDR) of the heavy and light chains). To not limit the scope of protection to the specific CDRs and to expand the genus of antibodies, reasonable inferences could be made during the drafting process such as those based on knowledge of conservative amino acid substitutions that retain biological function to broaden the genus of CDRs that could bind to the target epitope and the overall scope of antibodies.[2]

For More Information

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Notes

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