Practical Guidance for Pharmaceutical Method-of-Use Patent Claims After Teva v. Eli Lilly and In re Xencor

Introduction

In our companion article, Pharmaceutical Method-of-Use Claims After Teva v. Eli Lilly and In re Xencor: Written Description, Enablement, and the Known-Compound Genus, we analyzed two recent Federal Circuit decisions that offer complementary guidance on a recurring question in patent law: what must a patent specification disclose to satisfy written description and enablement under 35 U.S.C. § 112 when the claimed invention is the new use of a genus of known compounds or compositions?

In Teva Pharm. Int’l GmbH v. Eli Lilly & Co., 172 F.4th 1367 (Fed. Cir. 2026), the Federal Circuit reversed JMOL of invalidity for claims directed to reducing the incidence of, or treating, headache with humanized anti-CGRP antagonist antibodies. The court held that where the composition genus is well-known in the art, the specification and evidentiary record establish that a person of ordinary skill in the art (POSA) would know how to make or obtain genus components, and the record supports a finding that all claimed members work for the specific therapeutic use, written description and enablement may be satisfied even without exhaustive disclosure of every genus member. The court credited jury findings that were supported by Lilly’s own Inter Partes Review (IPR) admissions, expert testimony, and documentary evidence, all of which established that the antibody genus and humanization methods were well-established.

In In re Xencor, Inc., 130 F.4th 1350 (Fed. Cir. 2025), the Federal Circuit affirmed a written-description rejection for claims directed to “treating a patient” with modified anti-C5 antibodies bearing Fc substitutions M428L/N434S. The court held that broad, unlimited treatment language that is untethered to any particular disease, patient population, or outcome expands the written-description burden beyond what a sparse specification can bear. The applicant’s failure to establish through credible evidence that the anti-C5 antibody genus was well-known, combined with the absence of any treatment data or working example, proved fatal.

Together, these decisions establish that the validity of method-of-use claims employing known compounds turns on specificity, evidentiary support, and standard of review. This article distills the practical drafting and prosecution lessons that flow from that framework.

Drafting and Prosecution Lessons

1. Claim the New Use with Specificity

Practitioners should identify the disease or condition, patient population, dosing or route where relevant, and therapeutic outcome or biomarker. They should avoid undifferentiated “treating a patient” language unless the specification truly supports all patients and all diseases. The contrast between “reducing incidence of or treating headache” and “treating a patient” was not cosmetic. It determined whether the court evaluated the disclosure against a specific therapeutic use or against treatment across all patients and diseases.

2. Document Known-in-the-Art Status

If the compound or composition genus is known and not itself the invention, practitioners should document that status in the specification and prosecution record with concrete citations and examples. The specification should include representative species, commercially available materials, known sequences or deposits, and methods of making or modifying the compositions. Teva’s specification cited prior-art antibodies and methods, and Lilly’s own IPR statements corroborated their well-known status.

3. Explain Why the Known Genus Works in the New Method

The specification should include mechanism, common functional property, data, prophetic examples, or rationale demonstrating why all claimed members work for the specific use. Teva turned on the finding that all humanized anti-CGRP antagonist antibodies treat headache. Xencor failed because the specification did not establish that any anti-C5 antibody, whether modified or otherwise, actually treated any patient.

4. Avoid Relying Solely on Claim Format

Jepson format makes the preamble limiting and requires written description support for the alleged prior-art context. Under Xencor, a patentee cannot obtain a Jepson claim with a preamble asserting prior art if the specification and record do not establish that the prior art is, in fact, well-known. Means-plus-function format under § 112(f) narrows a limitation to disclosed corresponding structure and equivalents, as provided by MPEP § 2181, but practitioners must actually disclose that structure and should not assume that § 112(f) solves written description problems elsewhere in the claim.

5. Preserve the Evidentiary Record

Admissions that a genus is well-known may help in validity disputes but may also be used by adversaries. Lilly’s IPR statements that anti-CGRP antagonist antibodies were well-known undermined its written description challenge. Practitioners should build a record on POSA knowledge, routine methods, and whether all claimed members work for the claimed use. They should also be aware that broad statements in IPR or prosecution may be deployed later in litigation or vice versa.

Repurposed Drugs

For repurposed or repositioned drugs, these cases offer cautious support for method-of-use claims where the drug or compound class is known and the new use is narrow and well-supported. The Teva framework applies most naturally to the drug-repurposing context, where the molecule is known and the invention is the discovery that it treats a different condition.

Several key considerations improve the possibility of success. The specific therapeutic indication should be identified with particularity. The specification should provide mechanistic rationale for why the known compound works in the new indication. The application should include in vitro, in vivo, clinical, or mechanistic evidence demonstrating the new therapeutic use. The compound’s known-in-the-art status should be documented with citations to prior art, commercial availability, or known sequences and structures. Finally, practitioners should focus on claiming what the evidence supports, because “treating a patient” without disease limitation invites the same problem that proved fatal in Xencor.

Repurposed-drug patents do not excuse failure to describe the claimed therapeutic use. Broad treatment claims require broad treatment evidence.

AI-Assisted Method-of-Use Inventions

As drug discovery continues to evolve with the application of artificial intelligence and machine-learning models, such models will increasingly be used to identify new therapeutic uses for known compounds. Where AI predicts that a known compound class is effective for a specific disease, the resulting patent application faces the same written description and enablement requirements that Teva and Xencor illuminate.

Based on the teaching of Teva and Xencor, patent drafters should address the following considerations. The AI methodology should be disclosed in sufficient detail to make predictions credible, including model type, training data characterization, and validation approach. The biological rationale supporting the AI prediction should be explained, tying computational output to mechanistic understanding. The application should provide representative experimental confirmation or robust prophetic examples that demonstrate the predicted therapeutic effect. Claims should track no broader than the prediction plus evidence reasonably supports so that the application establishes possession and enablement of the claimed method. Finally, practitioners should document that the compound genus is known, if it is, with the same rigor that Teva’s specification and record demonstrated for anti-CGRP antagonist antibodies.

Applications relying on AI predictions alone, without experimental confirmation or strong mechanistic reasoning, may face meaningful risk under the Xencor rationale. The ARP found that listing disease classes without data was inadequate, and AI-predicted indications without confirmation data would likely face the same conclusion.

Conclusion

Teva v. Eli Lilly and In re Xencor together establish that method-of-use claims employing known compounds occupy a spectrum. At one end, claims directed to a specific, well-supported therapeutic use of a demonstrably known composition genus can survive written description and enablement challenges even without exhaustive disclosure of every genus member. At the other end, claims directed to undifferentiated “treatment” of any patient with any disease, using a genus whose well-known status is merely asserted, will fail.

The practical lesson is clear: specificity in claiming and evidentiary rigor in establishing POSA knowledge are the twin pillars of valid method-of-use claims for known compounds. Practitioners who heed both decisions will be better positioned to draft claims that survive validity challenges in prosecution and litigation alike.