A case note on the Federal Circuit’s August 31, 2026 decision affirming the validity of Exelixis’s crystalline cabozantinib (L)-malate claims and vacating the district court’s judgment on the ’349 patent as moot.
Exelixis, Inc. v. MSN Laboratories Private Ltd. is being discussed as a case about whether disclosure of two polymorphs can support claims covering an entire genus of crystalline forms.
That is part of the story, but it is not quite the question the Federal Circuit decided.
The court did not hold that N-1 and N-2 were representative species of the claimed genus. In fact, it expressly declined to reach that question. Instead, the Federal Circuit affirmed the district court’s finding that the specification itself identified the structural features common to the claimed genus: the chemical identity of cabozantinib (L)-malate and its crystalline structure.
That distinction is important because Ariad provides more than one route to written-description support for a genus claim. The decision turned on the structural-features route, not on whether two disclosed polymorphs were representative of the broader genus.
The Patents and the Dispute
Exelixis markets Cabometyx®, which contains cabozantinib (L)-malate. A salt may exist in amorphous or crystalline form, and a crystalline material may exist in different polymorphs — forms that have the same chemical formula but different internal molecular arrangements and potentially different physical properties.
Exelixis’s earlier patents claimed two specific polymorphs, N-1 and N-2. In a 2015 NDA submission, Exelixis informed the FDA that cabozantinib (L)-malate had been found to exist in two neat, closely related crystalline solid forms, N-1 and N-2, with similar properties, and that no other crystalline forms had been identified.
The patents at issue in the appeal — the ’439, ’440, and ’015 patents, referred to as the Malate Salt Patents — shared a specification with the earlier N-1 and N-2 patents, but their claims were broader. For example, claim 4 of the ’439 patent covered the (L)-malate salt where the salt “is crystalline.” MSN’s generic product contained a different polymorph, form S, for which MSN held its own patent. MSN conceded infringement of the Malate Salt Patents and instead challenged their validity under 35 U.S.C. § 112(a), arguing that the specification did not provide adequate written description support for the breadth of the claims.
The ’349 patent involved a separate issue. It covered cabozantinib (L)-malate compositions that were “essentially free” of a genotoxic synthesis impurity. That patent was litigated on inherency and infringement issues unrelated to the written-description dispute and is addressed separately below.
The Governing Written-Description Standard
The parties agreed that the governing framework came from Ariad Pharmaceuticals, Inc. v. Eli Lilly & Co., 598 F.3d 1336 (Fed. Cir. 2010).
For a genus claim, adequate written description may be established through either:
- disclosure of a representative number of species falling within the scope of the genus; or
- disclosure of structural features common to the members of the genus, in sufficient detail that a person of ordinary skill in the art can “visualize or recognize” the members of the genus.
Ariad also requires a sufficiently precise definition — by structure, formula, chemical name, physical properties, or other characteristics — to identify the species falling within the genus and distinguish the claimed genus from other materials.
Written description is a question of fact. Following a bench trial, the Federal Circuit therefore reviewed the district court’s findings for clear error. That standard of review was significant here because the Federal Circuit was not deciding the written-description issue from scratch. The question was whether the district court’s findings were clearly erroneous.
MSN’s Argument
MSN argued that the specification did not adequately describe the full scope of the claimed crystalline genus.
Its principal point was that N-1 and N-2 themselves differed in properties such as density, melting point, solubility, hygroscopicity, vapor pressure, and stability. If the two disclosed polymorphs could differ in these properties, MSN argued, disclosure of those two forms could not provide a basis for predicting or visualizing the undisclosed members of the genus.
MSN relied on ICU Medical, Inc. v. Alaris Medical Systems, Inc., Tronzo v. Biomet, Inc., and Eli Lilly & Co. v. Teva Pharmaceuticals USA, Inc., where genus claims failed written description because the claims extended beyond what the specifications actually described.
MSN also argued that the district court had effectively applied a different written-description standard to structural genus claims than to functional genus claims.
The District Court’s Finding
The district court did not find written-description support on the basis that N-1 and N-2 were representative species.
Instead, it identified what it considered to be the key structural feature of the genus: cabozantinib (L)-malate having a crystalline structure.
The court reasoned that a skilled artisan would be able to identify whether a polymorph was crystalline and distinguish crystalline from amorphous cabozantinib. It relied on the specification’s disclosure of the chemical formula and structural state of the material and analogized the case to GlaxoSmithKline LLC v. Banner Pharmacaps, Inc., 744 F.3d 725 (Fed. Cir. 2014).
The Federal Circuit found no clear error in that analysis.
The court summarized the point directly:
“The claims are no broader than the written description, as the claims require cabozantinib (L)malate salt with a crystalline structure.”
The Federal Circuit also relied on GSK, noting that the claims there did not impose a performance property and therefore did not raise the same concern about whether the specification adequately described a particular functional result. The same was true here: the asserted claims required the salt to be crystalline, but did not require it to exhibit a particular physical or functional property.
The Court Did Not Decide Whether N-1 and N-2 Were Representative Species
This is perhaps the most important point to preserve when characterizing the decision.
The Federal Circuit expressly stated that the district court did not need to analyze whether N-1 and N-2 constituted a representative number of species, and that there was therefore no need for the Federal Circuit to reach that inquiry.
The case was resolved under the other Ariad route: whether the specification disclosed structural features common to the members of the claimed genus.
Accordingly, the decision should not be characterized as holding that two polymorphs are enough to represent a genus of crystalline forms. That was not the basis of the decision.
The Differences Between N-1 and N-2
MSN’s argument concerning the differences in properties between N-1 and N-2 was therefore not rejected because the Federal Circuit concluded that polymorph properties are irrelevant to written description.
The problem was more specific.
The district court had not relied on the properties of N-1 and N-2 to identify other members of the genus. Instead, it had relied on the specification’s disclosure of the chemical name and formula of cabozantinib (L)-malate and the fact that the structure was crystalline.
The Federal Circuit noted that MSN did not dispute the accuracy of that disclosure. Its argument went to whether the disclosure was sufficient. But MSN did not explain why, in the context of this particular invention, the differences in properties between N-1 and N-2 undermined the structural identification on which the district court relied.
That makes the holding narrower than a broader statement that differences in polymorph properties do not matter under § 112(a).
The decision does not establish that physical-property differences among polymorphs are generally irrelevant to written description. It establishes that, where the district court relied on structural features rather than the properties of disclosed species, an argument directed to those property differences does not demonstrate clear error without connecting the differences to the particular structural analysis on which the finding rested.
The Fourteen-Form Genus
Another factual finding played a supporting role.
The district court found — and MSN did not challenge on appeal — that the maximum potential size of any pure polymorph genus was fourteen forms.
The Federal Circuit referred to that finding when distinguishing AbbVie Deutschland GmbH & Co., KG v. Janssen Biotech, Inc., 759 F.3d 1285 (Fed. Cir. 2014). In AbbVie, the claimed antibody genus was substantially broader and less predictable, including accused antibodies sharing only 50% sequence similarity with the disclosed antibodies.
The fourteen-form finding therefore provided context for the court’s assessment of the claimed genus. But the opinion does not establish fourteen as a numerical threshold or safe harbor for written description.
Genus size was not an independent test. The relevant question remained whether the specification provided a sufficiently precise structural definition to identify the claimed genus and distinguish it from other materials.
Why MSN’s Other Cases Did Not Control
The Federal Circuit did not disagree with the principle illustrated by ICU Medical, Tronzo, and Eli Lilly: a claim can fail written description when its scope extends beyond what the specification actually describes.
The court distinguished those cases based on the relationship between the claimed subject matter and the disclosure.
In ICU Medical, the claims covered spikeless valves even though the specification disclosed valves with spikes. In Tronzo, the claims covered generic hip-socket cup shapes even though the specification specifically distinguished other shapes and identified one conical shape as advantageous. In Eli Lilly, the claims addressed post-formulation particle size while the specification addressed only pre-formulation particle size.
Here, the Federal Circuit found a different relationship. The claim required a crystalline cabozantinib (L)-malate salt, and the specification described that same material in terms of its chemical identity and crystalline structure.
The court therefore found no broader subject matter in the claims than the structural subject matter described in the specification.
MSN’s argument that the district court had applied a heightened standard to structural genus claims also failed. The Federal Circuit understood the district court to be recognizing a point already reflected in Ariad: written-description concerns can be particularly acute where a genus is defined functionally. That did not mean that structural genus claims are subject to a different legal standard; it reflected the different written-description issues that may arise from the way a genus is defined.
The ’349 Patent: A Separate Procedural Holding
The ’349 patent involved a different issue altogether.
Claim 3 covered cabozantinib (L)-malate compositions “essentially free” of a genotoxic impurity, which the specification defined as 200 ppm or less. The district court found that the prior-art Brown process did not inherently produce such a composition and separately found no infringement.
MSN appealed the validity finding. Exelixis had initially cross-appealed the infringement finding but dismissed that cross-appeal before briefing.
That dismissal left the non-infringement judgment intact. As a result, even if MSN prevailed on validity, the outcome of the case would not change: the claim was not infringed.
The Federal Circuit therefore held that MSN’s appeal of the validity finding was moot. Exelixis argued that MSN nevertheless retained an interest because of related litigation involving continuation patent ’039, but the court found that interest too speculative and hypothetical to establish standing.
Applying United States v. Munsingwear, Inc., 340 U.S. 36 (1950), the Federal Circuit dismissed that portion of the appeal and vacated the district court’s judgment of no invalidity as to claim 3 of the ’349 patent.
This procedural holding is separate from the written-description analysis of the Malate Salt Patents and should not be treated as part of that substantive holding.
What the Decision Does — and Does Not — Establish
Read closely, Exelixis v. MSN supports several narrower propositions.
- A structural genus claim may be supported by disclosure of the chemical identity and a defining structural state. Here, the specification identified cabozantinib (L)-malate and its crystalline structure, and the claims did not impose an additional functional or performance limitation.
- The representative-species route is not the only way to establish written description for a genus. The Federal Circuit did not decide whether N-1 and N-2 were representative. It resolved the case under the structural-features route.
- Differences in properties among disclosed species do not necessarily defeat written description. Where the district court’s finding rests on structural features rather than those properties, an appellant must explain why the property differences undermine that particular analysis.
- A bounded genus can provide useful context. The finding that the pure polymorph genus could contain at most fourteen forms helped distinguish this case from cases involving much broader and less predictable genera. It was not treated as a numerical safe harbor.
- The decision does not weaken the cases cited by MSN. ICU Medical, Tronzo, Eli Lilly, and AbbVie were distinguished based on the relationship between the scope of the claims and what the respective specifications actually disclosed.
For polymorph claiming, the practical point is therefore narrower than the headline that “two polymorphs support a fourteen-form genus.”
The Federal Circuit upheld claims directed to crystalline cabozantinib (L)-malate because the court found that the specification disclosed the structural features defining the claimed genus and that the claims did not require a particular performance property.
That does not mean that disclosure of a chemical formula and crystallinity will necessarily satisfy § 112(a) for every polymorph genus. Where the claims depend on properties that vary among polymorphs, or where a functional limitation defines the boundaries of the genus, the writtendescription inquiry may be materially different.
The significance of Exelixis v. MSN lies in that distinction: the written-description analysis depends not simply on how many species the specification discloses, but on what the claim requires and what the specification identifies as common to the claimed genus.
Reference: Exelixis v. MSN Judgement
