The Federal Circuit’s precedential decision in Exelixis, Inc. v. MSN Laboratories Private Ltd., Case No. 25-1236 (Fed. Cir. Aug. 31, 2026), affirmed a bench-trial finding that patents claiming crystalline cabozantinib (L)-malate satisfied the written-description requirement even though the specification expressly characterized only two crystalline polymorphs. The court concluded that the chemical name and formula of cabozantinib (L)-malate, together with the requirement that the material be crystalline, supplied structural features common to the claimed genus that allowed skilled artisans to recognize its members.

The decision gives meaningful force to the structural-features pathway identified in Ariad Pharmaceuticals, Inc. v. Eli Lilly & Co., but its limits are important. Written description is a question of fact, and the Federal Circuit was reviewing the district court’s findings after a bench trial for clear error. The court therefore did not establish that identifying a chemical compound and its crystalline state will invariably support claims covering every polymorph. Rather, Exelixis shows how Ariad can support a comparatively small, structurally defined genus where the specification identifies characteristics that distinguish genus members from other materials.

The court also addressed a separate appellate issue with consequences beyond pharmaceutical patents. After Exelixis dismissed a cross-appeal challenging noninfringement of a different patent, the Federal Circuit concluded that MSN’s corresponding validity appeal had become moot and vacated the district court’s judgment of nonobviousness as to the disputed claim. That portion of the opinion illustrates how appellate decisions concerning infringement, validity, and cross-appeals can affect whether an unreviewed district court judgment survives for use in later litigation.

Background of the Dispute

Exelixis holds the New Drug Application for Cabometyx, which contains cabozantinib (L)-malate and is indicated for treatment of kidney, liver, and differentiated thyroid cancers. During development, Exelixis investigated different salt and solid-state forms of cabozantinib.

That distinction matters in pharmaceutical development because the same chemical compound can exist in amorphous or crystalline form. In an amorphous material, molecules lack a regular repeating arrangement. Crystalline material has an ordered molecular structure, and the same compound can sometimes crystallize in multiple arrangements known as polymorphs. Those polymorphs can share the same chemical name and formula while exhibiting different physical properties.

Exelixis identified two crystalline forms of cabozantinib (L)-malate, designated N-1 and N-2, and obtained patents specifically directed to those forms. It later obtained U.S. Patent Nos. 11,091,439, 11,091,440, and 11,098,015, collectively referred to by the court as the Malate Salt Patents. The ‘439 patent claims crystalline cabozantinib (L)-malate salts, the ‘440 patent claims pharmaceutical formulations containing those salts, and the ‘015 patent claims methods of treating cancer using them.

MSN sought FDA approval for a generic cabozantinib (L)-malate product using another polymorph, Form S, on which MSN had obtained its own patent. In the resulting Hatch-Waxman litigation, MSN conceded infringement of the three Malate Salt Patents but argued that the asserted claims were invalid because Exelixis had not adequately described the broader genus of crystalline cabozantinib (L)-malate. After a bench trial, the district court rejected that written-description challenge, and MSN appealed.

Ariad Provides Two Paths to Describing a Genus

The Federal Circuit began with Ariad Pharmaceuticals, Inc. v. Eli Lilly & Co., the en banc decision that remains the foundation of modern written-description doctrine. Section 112(a) requires the specification, viewed from the perspective of a person of ordinary skill in the art, to demonstrate that the inventor actually possessed the claimed invention when the application was filed. The amount of disclosure required depends on the nature and scope of the claims and the complexity and predictability of the technology.

For genus claims, Ariad identifies two alternative ways of making that showing. A patent may disclose a representative number of species within the genus, or it may disclose structural features common to members of the genus that allow a skilled artisan to visualize or recognize those members. Ariad also explains that a genus can be defined through characteristics such as structure, formula, chemical name, physical properties, or other properties that distinguish it from other materials.

The standard of review is important to understanding Exelixis. Written-description adequacy is a question of fact, and because the district court decided the issue after a bench trial, the Federal Circuit reviewed its factual findings for clear error. The appellate question was therefore not whether the Federal Circuit would independently have found the disclosure sufficient in the first instance, but whether the district court clearly erred in finding that Exelixis had adequately described the claimed genus.

MSN focused heavily on the proposition that N-1 and N-2 could not represent every crystalline form because different polymorphs may have different crystal structures and physical properties. The Federal Circuit concluded that the district court did not need to decide whether those two forms constituted a representative number of species because Exelixis had satisfied Ariad‘s alternative structural-features route.

The specification identified cabozantinib (L)-malate by chemical name and formula and further described the claimed material as crystalline. A skilled artisan could distinguish crystalline cabozantinib (L)-malate from amorphous material. The claims likewise required cabozantinib (L)-malate having a crystalline structure, leading the Federal Circuit to emphasize that the claims were no broader than the written description on that point. The specification also disclosed processes for making the invention, although the court made clear that this additional disclosure was not independently dispositive.

GSK, Rather Than AbbVie, Supplied the Better Analogy

The court’s reliance on GlaxoSmithKline LLC v. Banner Pharmacaps, Inc. helps explain both the holding and its limits. In GSK, the Federal Circuit found adequate written-description support for a genus of complexes formed between dutasteride and solvent molecules because the patent identified structural characteristics that allowed skilled artisans to recognize members of the genus.

The Exelixis claims shared an important feature with GSK. They were not defined by achieving a specified therapeutic result, stability level, solubility threshold, or other performance characteristic. The claimed genus was cabozantinib (L)-malate in crystalline form. As the Federal Circuit emphasized, the claims contained no performance property requiring the specification to establish that undisclosed species would achieve a particular functional result.

That distinction reduced the significance of MSN’s evidence that different polymorphs could have different densities, melting points, solubilities, hygroscopicity, vapor pressures, or stability. Those properties may be highly important when selecting a pharmaceutical form for commercial development, but they were not claim limitations. The district court did not rely on those properties to identify members of the genus. Instead, it relied on the compound’s chemical identity and crystalline state, and the Federal Circuit found no clear error in doing so.

The court contrasted that circumstance with AbbVie Deutschland GmbH & Co. v. Janssen Biotech, Inc. There, the patents described a comparatively narrow group of structurally similar antibodies while the claimed genus extended to materially different antibodies. The accused antibodies shared only about 50 percent sequence similarity with the disclosed antibodies, compared with approximately 90 percent similarity among the disclosed examples themselves. The evidence therefore showed a meaningful structural divide between what the inventors described and what they later claimed.

The scale of the genus also mattered. The district court had found, without challenge on appeal, that the maximum potential size of a pure polymorph genus was fourteen forms. The Federal Circuit expressly contrasted that bounded universe with genus cases potentially encompassing vast numbers of species. The combination of a recognizable structural definition, alignment between the specification and claim language, and a relatively limited potential genus makes Exelixis significant without turning it into a broad retreat from the Federal Circuit’s demanding genus-claim jurisprudence.

Why Other Structural-Claim Cases Did Not Control

MSN also relied on ICU Medical, Inc. v. Alaris Medical Systems, Inc., Tronzo v. Biomet, Inc., and Eli Lilly & Co. v. Teva Pharmaceuticals USA, Inc. for the proposition that structurally defined claims can still fail written description. The Federal Circuit agreed with the premise but concluded that the cases involved a different problem: the claims reached subject matter that the specifications did not actually describe.

In ICU Medical, the specification disclosed medical valves containing spikes, while later claims attempted to encompass spikeless valves. The problem was not simply that the patent disclosed too few species. The specification did not describe a valve operating without the very feature omitted from the broader claims.

Tronzo presented a similar mismatch. The patent repeatedly distinguished prior-art artificial hip cups and emphasized the advantages of a conical cup, but later asserted claims that were generic as to cup shape. The disclosure therefore pointed toward a narrower structure rather than demonstrating possession of the broader claim.

In Eli Lilly, the claims covered particle-size characteristics both before and after pharmaceutical formulation, while the specification addressed only the pre-formulation condition. Again, the claimed subject matter extended beyond what the specification actually described.

These cases also answer a possible overreading of Exelixis. The Federal Circuit did not hold that structurally defined claims receive a less rigorous written-description standard than functional claims. It instead recognized that functional genus claims can make the possession inquiry particularly difficult because a specification must support a class defined by achieving a function or result. Structural claims remain subject to the same fundamental requirement: the specification must demonstrate possession of what the patent claims.

The Decision Does Not Create a Safe Harbor for Polymorph Genus Claims

The practical significance of Exelixis lies partly in what the court did not hold. Disclosure of one or two polymorphs does not automatically support claims covering every crystalline form of the same compound.

Several features made the claims comparatively defensible. The specification identified a specific chemical compound. It distinguished crystalline from amorphous material. Skilled artisans could determine whether material possessed that characteristic. The potential genus was relatively small. Most importantly, the claims did not require undisclosed polymorphs to possess particular physical or pharmaceutical properties.

Future disputes are therefore likely to turn on whether a purported common structural feature genuinely identifies the claimed genus or merely describes a characteristic too general to demonstrate possession. A feature shared across an enormous universe of possibilities may provide little assistance in identifying the claimed invention, even though every genus member technically possesses it. The Federal Circuit’s written-description cases continue to ask whether the disclosure gives skilled artisans a meaningful way to recognize the territory the inventor claims to have possessed.

For patent drafting, the decision reinforces the value of expressly identifying the structural characteristics that unite a claimed genus and explaining how skilled artisans can distinguish members from nonmembers. For challengers, merely showing that undisclosed species have different properties may not be enough. The stronger argument connects those differences to the claim language and explains why they demonstrate that the specification’s stated common features do not actually establish possession across the claimed scope.

Mootness and Vacatur of the ‘349 Patent Judgment

The appeal also involved U.S. Patent No. 11,298,349, directed to cabozantinib (L)-malate pharmaceutical compositions containing no more than a specified amount of a genotoxic impurity. The district court found that MSN did not infringe claim 3 and also rejected MSN’s obviousness challenge, including its contention that a prior-art manufacturing process inherently produced material satisfying the impurity limitation.

MSN appealed the adverse validity ruling, while Exelixis initially cross-appealed the finding of noninfringement. Exelixis later dismissed its cross-appeal before filing its first appellate brief, making the noninfringement judgment final. MSN nevertheless continued to pursue its validity appeal until the Federal Circuit directed the parties to address Article III jurisdiction, after which MSN moved to dismiss that portion of its appeal as moot and requested vacatur.

The court began with Cardinal Chemical Co. v. Morton International, Inc., which establishes that a validity challenge does not automatically become moot merely because a patent has been found not infringed. A continuing validity controversy may remain after noninfringement. But Cardinal Chemical also recognizes that a material change in circumstances can terminate that controversy.

The Federal Circuit concluded that this case had reached that point. Exelixis argued that an appellate ruling on validity could have collateral consequences in litigation involving U.S. Patent No. 12,128,039, a continuation of the ‘349 patent. The court found those potential consequences too speculative and hypothetical to provide MSN with the concrete redressable injury required for Article III jurisdiction. The continuation patent also lacked the limitation on which the district court’s noninfringement judgment concerning claim 3 of the ‘349 patent had been based.

The remaining question was what should happen to the district court’s favorable judgment of nonobviousness. Applying United States v. Munsingwear, Inc., U.S. Bancorp Mortgage Co. v. Bonner Mall Partnership, and Arizonans for Official English v. Arizona, the Federal Circuit concluded that vacatur was appropriate when appellate review had been frustrated by the unilateral action of the party that prevailed below.

Exelixis had prevailed on validity, while MSN sought appellate review of that adverse ruling. Exelixis’s dismissal of its cross-appeal ultimately eliminated the controversy that would have permitted review. The Federal Circuit therefore vacated the district court’s judgment of nonobviousness as to claim 3, preventing MSN from being bound by a judgment that it had sought, but lost the opportunity, to challenge on appeal.

The procedural lesson is narrower than saying dismissal of an infringement cross-appeal necessarily eliminates a validity appeal. Cardinal Chemical forecloses that generalization. But parties deciding whether to abandon appellate issues should consider how doing so may affect Article III jurisdiction over related rulings and whether mootness could result in vacatur of a judgment that otherwise would have survived.

Practical Implications

For pharmaceutical patent owners, Exelixis provides useful support for genus claims directed to a bounded class of solid-state forms when the specification identifies a chemical structure and physical condition that skilled artisans can recognize. The case confirms that Ariad‘s structural-features route is not merely theoretical. A patentee need not always establish possession by demonstrating that the disclosed examples constitute a representative cross-section of the genus.

The opinion should not, however, be read as making the number or diversity of disclosed species irrelevant. Where a supposed common structural characteristic does little to distinguish the claimed genus from a much larger universe, representative species, additional structural guidance, or other evidence of possession may still be necessary. The Federal Circuit’s express attention to the comparatively small potential polymorph genus makes that limitation especially significant.

For patent challengers, the decision illustrates the importance of connecting scientific differences to the actual claim boundaries. Evidence that polymorphs have different commercially significant properties is not necessarily evidence of inadequate written description when those properties are absent from the claims. The invalidity theory must explain why the differences demonstrate that the patentee did not possess the structural territory it ultimately claimed.

The facts also illustrate a separate commercial point concerning patent strategy. MSN had obtained its own patent on Form S but nevertheless conceded infringement of Exelixis’s broader Malate Salt Patents. A patent on a new species or improvement does not itself provide freedom to practice that invention if an earlier patent contains broader claims covering it. Patentability and freedom to operate remain distinct inquiries, a distinction that can become commercially significant when later-developed pharmaceutical forms fall within earlier genus claims.

Finally, the vacatur ruling demonstrates that appellate strategy can affect more than the immediate dispute. Parties considering whether to dismiss infringement claims, validity challenges, or cross-appeals should evaluate the consequences for related judgments, continuation patents, and parallel litigation before assuming that a favorable district court ruling will remain intact.

Conclusion

Exelixis v. MSN Laboratories reinforces an important part of Ariad‘s written-description framework. Section 112(a) does not invariably require disclosure of enough individual species to represent an entire genus. Common structural features can establish possession when they meaningfully define the genus and allow skilled artisans to recognize its members.

The decision nevertheless remains closely tied to its facts and procedural posture. The genus was comparatively small, the claims tracked structural characteristics found in the specification, no functional performance limitation expanded the required showing, and the Federal Circuit reviewed the district court’s factual findings only for clear error. Those features distinguish Exelixis from cases involving enormous chemical or biological genera in which broadly stated common characteristics provide little evidence of what the inventor actually possessed.

Key Takeaways

• Ariad provides alternative routes for genus written description: representative species or common structural features that allow skilled artisans to recognize members of the genus.
• Written-description adequacy is a factual question. In Exelixis, the Federal Circuit reviewed the district court’s bench-trial findings for clear error, which limits how categorically the decision should be read.
• A chemical name and formula combined with a crystalline-state limitation supported the district court’s finding of adequate structural definition on the particular record before the court.
• Differences in unclaimed physical properties among polymorphs do not necessarily defeat written description unless those differences show that the claimed genus is not actually described or possessed.
• Structural genus claims are not subject to a lower written-description standard, although functional claiming can make the possession inquiry more difficult.
• Exelixis is materially narrower than cases involving vast genera. The Federal Circuit specifically noted the district court’s unchallenged finding that the potential pure polymorph genus was comparatively small.
• A patent on a later species, such as MSN’s Form S, does not itself establish freedom to operate under an earlier, broader genus patent.
• Noninfringement does not automatically moot a validity appeal under Cardinal Chemical, but a subsequent loss of a concrete controversy can do so.
• When appellate review is frustrated by the unilateral action of the party that prevailed below, vacatur can eliminate an otherwise surviving district court judgment.

Related Analysis

• Duke v. Sandoz: When a Genus Claim Becomes a Forest Without Blaze Marks examines the opposite side of the written-description boundary, where purported common structural features failed to guide skilled artisans from a vast disclosed chemical genus to the narrower claimed subgenus.
• Brita v. ITC: Functional Genus Claims Fail Written Description and Enablement provides a useful contrast because the claims in Brita defined their breadth through functional performance rather than the comparatively concrete structural characteristics accepted in Exelixis.
• In re Xencor: Written Description and Preamble Limitations illustrates the heightened written-description problems that can arise in biotechnology genus claims when a limited disclosure does not demonstrate possession of the full claimed class.
• Agilent v. Synthego: Federal Circuit Clarifies Written Description and Motivation to Combine complements Exelixis by showing that § 112 requires possession of the claimed invention without imposing disclosure requirements untethered from the actual claim scope.
• Dental Monitoring v. Align: Federal Circuit Requires Written-Description Support for AIA Provisional Prior Art addresses written description in a different statutory context but reinforces the continuing importance of demonstrating possession at the relevant filing date.

By Charles Gideon Korrell