On March 13, 2025, the U.S. Court of Appeals for the Federal Circuit issued a decision in the case of In Re: Xencor, Inc.1 In this Appeal from the Appeals Review Panel of the Patent Trial and Appeal Board (ARP), with regard to U.S. Application No. 16/803,690 (’690 application), the court held that the preamble of the method of treatment claims at issue is limiting, and thus requires sufficient written description under 35 U.S.C. §112.
Key Takeaways
In re Xencor provides key takeaways for those pursuing method of treatment claims.
- First, the breadth of the method claims should be commensurate with the scope of the methods disclosed in the patent application or otherwise be well known to one in the art.
- Second, the patent application should specifically connect the claimed therapeutic agent to the patients to be treated, and the diseases claimed.
- Finally, when drafting claims, patent seekers should remove excess language from a claim where possible, with the understanding that every claim limitation may require written description support.
Claim Preambles
A claim generally has a preamble, a transition phrase, and a body. The preamble is the introductory clause which denotes the type of invention, its intended use, the claim’s relation to the prior art, or a result to be achieved. One of the claims at issue in the current case was drafted in the Jepson style, which is not generally used in the therapeutic context. A Jepson claim has a preamble that recites what is conventional or known in the art followed by a transition phrase reciting, “wherein the improvement comprises,” followed by a body that recites what the patent seeker considers the improvement.2
Preambles may or may not limit the subject matter of a claim, and such determinations are made on a case-by-case basis.3 A claim’s preamble will be limiting, for example, when the preamble provides antecedent basis for claim limitations in the body of the claim, or if needed to, “give life, meaning, and vitality’ to the claim.”4
Limiting Preambles of Method Claims
In its analysis of the ’690 application, the court first determined if the preambles of the two claims at issue (claims 8 and 9) were limiting. Claim 8 is a Jepson claim that recites:
In 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 as compared to a human Fc polypeptide, wherein numbering is according to the EU index in Kabat et al., wherein said anti-C5 antibody with said amino acid substitutions has increased in vivo half-life as compared to said antibody without said substitutions.5
Here, the court determined that “a method of treating a patient by administering an anti-C5 antibody with an Fc domain” was limiting because a Jepson claim is not just the improvement recited, “but the claimed improvement as applied to the prior art.”6 (Emphasis in original). As such, the preamble of a Jepson claim limits a claim’s scope, and, therefore, must satisfy the written description requirement.
Claim 9 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 as compared to a human Fc polypeptide, wherein numbering is according to the EU index of Kabat, wherein said anti-C5 antibody with said amino acid substitutions has increased in vivo half-life as compared to said antibody without said substitutions.7
Here also, the court found that the preamble “[a] method of treating a patient by administering an anti-C5 antibody” was limiting because the reference to an in vivo half-life in the body of the claim only made sense with respect to treating a living being, and, thus, provided, “a raison d’être for the claim.”8
Next, the court turned to the issue of whether the limiting preamble common to both claims was adequately supported by the specification of the patent application.
Written Description Assessment
To satisfy the written description requirement, a patent specification must describe the claimed invention in sufficient detail that one skilled in the art can reasonably conclude that the inventor had possession of the claimed invention.9 For both claims 8 and 9 the court found written description of the preamble lacking.
Regarding claim 9, the claim was construed to cover methods of treating a patient with the known anti-C5 antibody, eculizumab, and equivalents thereof. Even if the scope was limited to eculizumab itself, the court found that the specification of the patent application at issue did not provide sufficient written description for a “method of treating” any disease with eculizumab. In particular, the specification did not describe what diseases or conditions could be treated by eculizumab, nor was sufficient evidence put forth showing that methods of treatment with eculizumab were known at the time, such that one skilled in the art could reasonably conclude the inventors had possession of “treating all patients and all diseases.” Instead, the specification merely described three classes of diseases that could benefit from treatment with antibodies possessing Fc modifications.10 Additionally, the court highlighted the lack of description of a single working example of treatment with an anti-C5 antibody.11
Regarding claim 8, the claim scope encompasses treatment with a genus of anti-C5 antibodies, that is, any antibody that could bind C5. Here, the court concluded that the entirety of a Jepson claim, including the preamble, requires written description, stating that “[t]o provide adequate written description for a Jepson claim, the applicant must establish that what is claimed to be well-known in the prior art is, in fact, well-known in the prior art.”12 The court agreed with the ARP’s finding that the Applicant did not establish that treating a patient with a C5 antibody was well-known in the art.13 The court also concurred with the ARP’s finding that the claim at issue lacked written description. Echoing its own sentiment in Juno v. Kite,14 the court upheld the ARP’s finding that the examples of anti-C5 antibodies known in the prior art were insufficient to establish written description to a genus of anti-C5 antibodies.15
Conclusion
Recent cases decided by the courts have increased the stringency of the written description requirement. This trend places the burden on patent seekers to more fully describe and show possession of their claimed invention with a robust and detailed specification.
As of April 28, 2025, Xencor, Inc. has requested an en banc rehearing by the U.S. Court of Appeals for the Federal Circuit. Wilson Sonsini will continue to monitor developments.
For questions regarding patent strategies, please contact any member of Wilson Sonsini’s Patents and Innovations practice.
[1] In re Xencor Inc., No. 2024-1870, 2025 U.S. App. LEXIS 5832 (Fed. Cir. Mar. 13, 2025).
[4] Id. (quoting Pitney Bowes, Inc. v. Hewlett-Packard Co., 182 F.3d 1298, 1305, 51 USPQ2d 1161, 1165-66 (Fed. Cir. 1999)).
[5] U.S. Pat. App. Ser. No. 16/803,690, Claim 8.
[6] In re Xencor, Inc., U.S. App. LEXIS 5832 at 16-17.
[7] U.S. Pat. App. Ser. No. 16/803,690, Claim 9.
[8] In re Xencor, Inc., 2025 U.S. App. LEXIS 5832 at 11-12.
[9] MPEP 2163.I. Citing Moba, B.V. v. Diamond Automation, Inc., 325 F.3d 1306, 1319, 66 USPQ2d 1429, 1438 (Fed. Cir. 2003); Vas-Cath, Inc. v. Mahurkar, 935 F.2d at 1563, 19 USPQ2d at 1116.
[10] In re Xencor, Inc., 2025 U.S. App. LEXIS 5832 at 13-14.
[14] Juno Therapeutics, Inc. v. Kite Pharma, Inc., 10 F.4th 1330, 1335-36, 1340 (Fed. Cir. 2021).
[15] In re Xencor, Inc., 2025 U.S. App. LEXIS 5832 at 18-19.
