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Introduction

Two recent Federal Circuit decisions 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? Teva Pharm. Int’l GmbH v. Eli Lilly & Co., 172 F.4th 1367 (Fed. Cir. 2026), and In re Xencor, Inc., 130 F.4th 1350 (Fed. Cir. 2025), reach different outcomes on superficially similar facts. The contrast is instructive.

Teva v. Eli Lilly

Teva sued Lilly for infringement of three “headache patents” (U.S. Patent Nos. 8,586,045, 9,884,907, and 9,884,908) directed to reducing the incidence of, or treating, headache in a human by administering a humanized anti-CGRP antagonist antibody. Anti-CGRP antagonist antibodies bind CGRP and inhibit activity associated with headache, and murine anti-CGRP antagonist antibodies can be converted into humanized forms through known humanization methods.

The specification stated that anti-CGRP antagonist antibodies were “known in the art,” cited murine examples including antibody 4901, disclosed other murine anti-CGRP antagonist antibodies, stated that anti-CGRP antagonist antibodies “may be made by any method known in the art” (See U.S. Patent No. 8,586,045, at column 27, lines 41-42), disclosed one humanized antibody named G1, and identified prior-art humanization methods.

Representative Claim

Claim 30 of the ’045 patent is representative of the three patent claim sets. Claim 30 depends on claim 17, and rewritten with claim 17, recites: “A method for reducing incidence of or treating headache in a human, comprising administering to the human an effective amount of an anti-CGRP antagonist antibody, wherein said anti-CGRP antagonist antibody is a . . . humanized monoclonal antibody.” See U.S. Patent No. 8,586,045, at claims 17 and 30.

That language did much of the analytical work. The claim was not directed to the humanized antibody genus standing alone but was instead directed to a method of reducing the incidence of, or treating, headache by administering an effective amount of a humanized anti-CGRP antagonist monoclonal antibody. The Federal Circuit treated that distinction as substantive because the use of treating headache was different from the function that characterized the antibodies, namely antagonizing CGRP.

Procedural History

Teva brought suit in the District of Massachusetts in September of 2018, alleging Lilly indirectly infringed the headache patents via its Emgality product. The dispute then intersected with earlier IPR proceedings in which Lilly had argued that anti-CGRP antagonist antibodies were well known, prior art was replete with examples, methods of making them were extensively described, and humanization was routine. At trial, a jury found willful infringement, rejected Lilly’s written-description and enablement defenses, and awarded damages. The district court nevertheless granted JMOL for Lilly, holding the asserted claims invalid under § 112. On appeal, the Federal Circuit reviewed the JMOL de novo under First Circuit law, viewed the facts and inferences in favor of the verdict, accounted for Lilly’s clear-and-convincing burden on invalidity, and reversed because a reasonable jury could have rejected Lilly’s invalidity case.

How Procedural Posture Drove the Outcome

The posture mattered because the Federal Circuit was not writing on a blank factual record. The court treated the jury as having made all verdict-supporting factual findings that substantial evidence permitted, including findings that anti-CGRP antagonist antibodies were known, that humanization was routine, and that the specification taught the claimed therapeutic use. That standard made Lilly’s JMOL victory vulnerable: unless the record compelled invalidity despite the jury’s implicit findings and Lilly’s clear-and-convincing burden, the verdict had to be reinstated.

The Inter Partes Review (IPR) history also mattered in a practical evidentiary sense. Lilly’s prior statements about the well-known status of anti-CGRP antagonist antibodies, the extensive description of methods for making them, and the routine nature of humanization helped supply the record support that distinguished Teva from cases where a patentee merely asserted that a genus was known.

Holdings on Written Description

The Federal Circuit held that JMOL of no written description was improper. The court framed the relevant line of precedent as addressing circumstances “where a claim pertains to a well-known genus that is not, itself, the invention.” Teva, 172 F.4th at 1374. The headache patents made clear that the claimed invention was the use of anti-CGRP antagonist antibodies (or humanized versions thereof) to treat headache, not the antibodies themselves.

The court reasoned that a reasonable jury could have found the following. First, anti-CGRP antagonist antibodies and methods of making them were well known, replete, and extensively described in the prior art, based in part on Lilly’s own statements in earlier IPR proceedings. Second, humanization was a well-established and routine procedure by the November 2006 priority date. Third, a skilled artisan reading the specification would have understood that all humanized anti-CGRP antagonist antibodies treat headache.

Together with the specification’s disclosures, these findings were sufficient to support the jury’s rejection of Lilly’s written-description challenge.

The court rejected Lilly’s argument that the specification disclosed only one humanized version (G1). It reasoned that murine versions were well known, several were disclosed, and they were “just a routine (and likewise disclosed) procedure away from being humanized.” The court also rejected Lilly’s “semantic distinction without a difference” argument by distinguishing prior method-claim decisions in which the claims depended on undisclosed or unknown compounds that performed the claimed function. Here, by contrast, the claims used known anti-CGRP antagonist antibodies for the different therapeutic purpose of treating headache.

Prior Method-Claim Decisions Distinguished by the Court

The main method-claim decisions the Teva court distinguished were Univ. of Rochester v. G.D. Searle & Co., 358 F.3d 916 (Fed. Cir. 2004), and Ariad Pharms., Inc. v. Eli Lilly & Co., 598 F.3d 1336 (Fed. Cir. 2010), and, in the enablement analysis, Idenix Pharms. LLC v. Gilead Scis., Inc., 941 F.3d 1149 (Fed. Cir. 2019). As characterized by Teva, University of Rochester involved method claims directed to selectively inhibiting PGHS-2 by administering a compound that selectively inhibits PGHS-2. The Teva court explained that the method-versus-compound distinction did not help the Rochester patentee because the specification disclosed no compounds usable in the claimed methods, and there was no evidence that such compounds were known.

As characterized by Teva, Ariad involved method claims directed to reducing NF-κB activity using substances that achieved that reduction. The Teva court described the problem as insufficient disclosure of the molecules capable of performing the claimed reduction, including hypothesized categories with no adequate examples or insufficient evidence of the required activity.

The distinction matters because Teva viewed those cases as claims to methods of doing X using something that does X. By contrast, the headache claims were not framed as methods of antagonizing CGRP using antibodies that antagonize CGRP. They were methods of treating headache using known anti-CGRP antagonist antibodies, which the court treated as a different invention from the function that characterized the antibodies.

In the enablement discussion, the court also distinguished Idenix, which Teva characterized as involving a method of treating hepatitis C with a large class of 2’-methyl-up nucleosides. The key difference was evidentiary: in Idenix, the record did not support a finding that all claimed nucleosides treated HCV, while in Teva a reasonable jury could find that all humanized anti-CGRP antagonist antibodies treated headache.

Holdings on Enablement

The Federal Circuit noted that “enablement and written description often rise and fall together” on facts like these. Ariad, 598 F.3d at 1352. The court held that JMOL of no enablement was also improper because the relevant enablement question followed the claim language. Lilly’s breadth argument assumed that the patents had to enable the full antibody genus as such, but the asserted claims covered “only the use of such antibodies for the different, limited purpose of treating headache.” Teva, 172 F.4th at 1381. Because the record supported a finding that all humanized anti-CGRP antagonist antibodies treat headache, the court treated the task of finding or making more antibodies as “more akin to extra credit than a necessary research assignment.” Id. That is a direct holding of Teva, but it is also a posture-sensitive holding because it depended on what the jury could reasonably find from the specification, prior art, and Lilly’s admissions.

In re Xencor

Facts and Technology

The application at issue related to modified antibodies with Fc substitutions M428L/N434S that increased in vivo half-life and were intended to reduce the need for frequent treatments. Anti-C5 antibodies bind the C5 complement protein, and the specification identified one anti-C5 antibody, 5G1.1. Eculizumab, a humanized version of 5G1.1 with a known sequence, had been used to treat PNH and studied for other indications. The prosecution problem was that this technical disclosure did not translate into written-description support for the full treatment language Xencor chose.

Claims at Issue

Claims 8 and 9 are representative of the claims at issue.

Claim 8, a Jepson Claim, recites:

“[i]n a method of treating a patient by administering an anti-C5 antibody with an Fc domain, the improvement comprising said Fc domain comprising amino acid substitutions M428L/N434S . . . wherein said anti-C5 antibody with said amino acid substitutions has increased in vivo half-life as compared to said antibody without said substitutions.” Xencor, 130 F.4th at 1354-55.

Claim 9 is a method claim with a means-plus-function element, and it recites:

“[a] method of treating a patient by administering an anti-C5 antibody comprising: a) means for binding human C5 protein; and b) an Fc domain comprising amino acid substitutions M428L/N434S . . . wherein said anti-C5 antibody with said amino acid substitutions has increased in vivo half-life as compared to said antibody without said substitutions.” Xencor, 130 F.4th at 1355.

The consequential words were not limited to the Fc substitutions. For claim 8, the Jepson preamble, which recited “[i]n a method of treating a patient by administering an anti-C5 antibody with an Fc domain,” became part of the claimed invention. For claim 9, the phrase “[a] method of treating a patient by administering an anti-C5 antibody” was limiting because it gave meaning to administering, to the claimed in vivo half-life improvement, and to the therapeutic context of the method. The court therefore analyzed written description against the full treatment language, not merely the Fc-engineering improvement.

Procedural History

The examiner rejected claims 8 and 9 for lack of written description in 2021. The Board affirmed in January 2023, Xencor sought rehearing, and Xencor appealed to the Federal Circuit. Before that appeal was heard, the United States Patent and Trademark Office requested remand for ARP review, and the ARP issued a superseding decision on May 21, 2024, maintaining the written-description rejections. The Federal Circuit then affirmed on March 13, 2025, reviewing legal issues de novo, reviewing Board fact findings for substantial evidence, and deferring to the Board’s expert-credibility determinations. That procedural posture was decisive: unlike Teva, where the appellate court protected jury-supported findings against JMOL, Xencor came to the court with adverse agency fact findings that were reviewed deferentially.

Holdings on Written Description

The Federal Circuit held that “treating a patient” was limiting because the phrase was directly connected to “administering an anti-C5 antibody” by the word “by,” the claim lacked dosage and rate, and “increased in vivo half-life” made practical sense only in a treatment context. The specification reinforced that reading by explaining that longer in vivo half-lives allow less frequent injections or lower dosing for therapeutic antibodies. The result was that “treating a patient” was not a decorative statement of purpose but rather supplied the raison d’être of the method and expanded the written-description burden.

The Federal Circuit also held that the preamble of a Jepson claim requires written description because Jepson form uses the preamble to define the claimed invention and limit claim scope. Claim 8 therefore could not be reduced to the improvement alone, namely “said Fc domain comprising amino acid substitutions M428L/N434S.” The invention was the improvement as applied to the recited prior-art method of treating a patient by administering an anti-C5 antibody, and Xencor could not avoid written description by simply asserting that the preamble subject matter was already known.

Because the specification did not limit treatment to a specific disease, the ARP and Federal Circuit treated “treating a patient” as extending to all patients and all diseases. The specification did not define “treating,” did not provide data associated with treating any patient with any disease or condition using any anti-C5 antibody with the claimed Fc modifications, and did not include a working example of treatment. Listing three general disease or condition classes and unmodified antibodies was therefore inadequate to show possession of the claimed treatment method across the full scope of the claim.

The ARP found the single disclosed anti-C5 antibody, 5G1.1, insufficient for the broader anti-C5 antibody genus because the genus involved different specificities and epitopes. It also adopted the Board’s findings that Xencor’s expert was not credible and that the prior-art examples did not establish that anti-C5 antibodies were so well known that the specification could omit further support. The Federal Circuit affirmed because those adverse factual findings were supported by substantial evidence.

Section 112(f) and MPEP Section 2181

Claim 9’s “means for binding human C5 protein” limitation invoked 35 U.S.C. § 112(f) (formerly § 112, ¶ 6). Under MPEP § 2181, a limitation using “means” or “step” or another nonce term, reciting function, and failing to recite sufficient structure is construed to cover the corresponding structure, material, or acts described in the specification and equivalents thereof. The ARP identified 5G1.1 as the sole corresponding structure and found that disclosure adequate for the means-plus-function limitation itself. A person of ordinary skill would understand 5G1.1 to refer to the original mouse monoclonal antibody and eculizumab, which is a humanized version. Equivalents of 5G1.1 need not be separately described.

However, claim 9 still lacked written description for the separate “treating a patient” limitation. The § 112(f) analysis narrowed the antibody element to 5G1.1 and equivalents, but that narrowing did not supply written description for the broad, unlimited treatment preamble. MPEP § 2181 is claim-interpretation guidance that anchors claim scope to disclosed structures and equivalents, but it does not resolve § 112(a) adequacy for other claim limitations.

Comparing Teva and Xencor

Factual Records

In Teva, the factual record was robust. Lilly’s own IPR statements admitted that anti-CGRP antagonist antibodies were “well known,” that prior art was “replete” with such antibodies, that methods of making them were “extensively described,” and that humanization was “a well-established and routine procedure.” Expert testimony and record evidence corroborated these points, and the Federal Circuit held a reasonable jury could credit that evidence.

In Xencor, no comparable record existed. The Board found Xencor’s expert not credible, the prior-art examples insufficient to establish that anti-C5 antibodies were well-known, and the specification limited to a single anti-C5 example, 5G1.1.

Scope of the Specification Relative to Claim Breadth

In Teva, the claims were directed to “reducing incidence of or treating headache” with humanized anti-CGRP antagonist antibodies, and the specification’s disclosures were evaluated in light of record evidence that the antibody genus and humanization methods were already known. In Xencor, the operative phrase was the much broader “treating a patient,” which the court treated as untethered to any particular disease, patient population, dose, or outcome. The specification provided no treatment data, no working example of treating any disease with a modified anti-C5 antibody, and no definition of “treating.” The mismatch between claim scope and specification content did not help Xencor’s case.

Role of POSA Knowledge

Both decisions acknowledge that POSA (person of ordinary skill in the art) knowledge supplements specification disclosures. Teva demonstrates that when POSA knowledge is established in the record through admissions, expert testimony, and documentary evidence, it can bridge gaps between what the specification explicitly discloses and what the claims cover. Xencor demonstrates that POSA knowledge cannot be invoked by bare assertion. The applicant must show that what is claimed as well-known is, in fact, supported by the record.

Application of Enablement and Written Description to Method Claims

In Teva, written description and enablement were addressed together, and the Federal Circuit noted that they “often rise and fall together.” The key was that the claimed use was the limited purpose of treating headache, not ownership of the entire antibody genus for all purposes. In Xencor, only written description was directly decided by the Federal Circuit. The breadth of the treatment limitation, combined with the absence of treatment data or examples, defeated written description without requiring the court to reach enablement.

Counterfactual: Xencor Under a Teva v. Eli Lilly Claim-Construction Lens

It is unlikely that Xencor would have fared materially better under a Teva-style focus on subsidiary factual determinations in claim construction. The Federal Circuit in Xencor already separated legal claim-construction questions from factual written-description findings: it reviewed legal determinations de novo, reviewed Board fact findings for substantial evidence, and deferred to the Board’s expert-credibility determinations. That posture left Xencor with two adverse fact-finding problems that a more fact-sensitive claim-construction approach would not cure. The Board did not credit Xencor’s expert on whether anti-C5 antibodies were well known, and the Board found the prior-art examples insufficient to establish the genus as well known.

The intrinsic record did help Xencor in one limited respect: it supported construing the claims as therapeutic methods. The specification linked antibody administration to treatment frequency, clearance, half-life, patient characteristics, and therapeutic use. But that same intrinsic evidence cut against Xencor once “treating a patient” was held limiting, because it confirmed that the claimed method needed written-description support as a treatment method, not merely as an Fc-engineering platform.

However, the specification was too sparse to survive that burden. It did not identify what patients with what diseases or conditions could be treated with an anti-C5 antibody carrying the claimed Fc substitutions, did not include a working example of treating any disease with such an antibody, and did not provide data associated with treating any patient using the claimed modified anti-C5 antibodies. Even claim 9’s § 112(f) narrowing of the antibody element to 5G1.1 and equivalents did not fill the separate gap in treatment disclosure.

A stronger Xencor claim might have looked more like the claim in Teva: a specific disease, a specific antibody or supported antibody class, and a specification explaining why the claimed class works for that disease. On the record actually before the court, however, Xencor had broad treatment language, a single anti-C5 example, no treatment example, and no credited showing that the anti-C5 genus was well known. Under any fact-sensitive construction, the problem was not simply how the preamble was read but rather that the specification did not support the breadth that the preamble created.

The Central Legal Issue: Method Claims Using Known Compounds

As such, the central question is: when a method-of-use claim covers the use of known compounds or compositions, and the invention is the new use rather than the molecule itself, does the specification satisfy enablement and written description requirements even if it does not exhaustively describe or reduce to practice every member of the claimed composition genus?

Teva supports the proposition that a method-of-use claim may survive written description and enablement challenges under these circumstances if certain conditions are met. The composition genus must be well-known in the art and must not itself be the invention. The specification and record must show that a person of ordinary skill in the art would know how to make or obtain the genus components. The specification must convey that the entire claimed genus works for the specific claimed method, such as the finding in Teva that all humanized anti-CGRP antagonist antibodies treat headache. Finally, the POSA knowledge must be established in the evidentiary record rather than merely asserted.

This is not a universal rule. The Teva court expressly noted that if the claims had covered the antibody genus itself, the analysis would differ.

Xencor limits this principle in at least four important ways. First, POSA knowledge cannot be invoked by assertion but must be established in the record through credible evidence. Second, broad treatment language such as “treating a patient” without disease limitation can create a treatment genus so large that no reasonable specification disclosure can support it. Third, a section 112(f) limitation may narrow a structural component to known or disclosed corresponding structure and equivalents, but it will not supply written description for a separate, broad treatment limitation. Fourth, Jepson claiming does not eliminate the written description requirement for the preamble and indeed makes the preamble affirmatively limiting.

The distinction between the two outcomes ultimately reduces to specificity, evidentiary support, and standard of review. The claims in Teva targeted a specific therapeutic purpose, namely reducing incidence of or treating headache, and the appellate posture required the Federal Circuit to credit jury-supported findings about POSA knowledge and therapeutic applicability. The claims in Xencor targeted undifferentiated treatment of a patient, and the appellate posture required deference to adverse agency fact findings that the anti-C5 genus was not shown to be well known and that the specification did not support the full treatment scope.