The Court of Appeals for the Federal Circuit’s In Re Xencor Decision: Jepson Claims Require Written Description for Their Preambles

On March 13, 2025, the US Court of Appeals for the Federal Circuit (Federal Circuit) issued a decision titled In Re: Xencor, Inc. (the Xencor decision). The Xencor decision affirms the decision of the Appeals Review Panel (ARP) of the US Patent and Trademark Office, which held both pending claims of US Patent Application No. 16/803,690 (the ’690 application) unpatentable for lacking written description for their preambles.

Claims at Issue

The two pending claims of the ’690 application relate to use of anti-C5 antibodies with a modified Fc domain. Xencor drafted the claims in Jepson and means-plus-function claim formats (claims 8 and 9, respectively, reproduced below):

8. 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 substitution 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 substitution has increased in vivo half-life as compared to said antibody without said substitutions.

9. 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 substitution 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 substitution has increased in vivo half-life as compared to said antibody without said substitutions.

The procedural history of the case and the ARP’s decision were the subject of a prior Goodwin Client Alert.

In the present case, the Federal Circuit focused on whether the preamble of a claim is limiting on the scope of the claim and thus must satisfy the written-description requirement. The preamble is the introductory portion of the claim preceding the transitional phrase — e.g., “comprising” or “consisting of.” The preamble typically sets the context or specifies the general category of the invention. While not always essential to the claim itself, the preamble can help clarify the scope of the invention and can provide important background information such as describing the purpose or intended use of the invention.

Claim 9 (Means-Plus-Function Claim)

The Federal Circuit first addressed claim 9. The preamble of claim 9 reads, “a method of treating a patient by administering an anti-C5 antibody,” which can be divided into two separate phrases or parts: “treating a patient” and “administering an anti-C5 antibody.”

The Xencor decision affirms the decision of the ARP finding that the full preamble limited the scope of the claim and that the full preamble also lacked written description. In particular, the Xencor decision reiterates that a preamble is limiting if when the claim is read as a whole, the preamble “recites limitations of the claim, or, if the claim preamble is ‘necessary to give life, meaning, and vitality’ to the claim.”¹

Xencor argued on appeal that the claim’s preamble phrase, “treating a patient,” is not limiting on the scope of the claim and that written description is satisfied. Xencor did not dispute that the preamble’s recitation of “administering an anti-C5 antibody” is limiting, because the body of the claim referred back to the preamble for antecedent basis: “wherein said anti-C5 antibody.” However, the Federal Circuit did not find that the preamble could be split as suggested by Xencor.

The Xencor decision first notes that the phrase “treating a patient” is connected to “administering an anti-C5 antibody” with the word “by,” disincentivizing the splitting of these two parts of the preamble. Further, the “treating of the patient is done by administering the anti-C5 antibody.” Each gives life and meaning to the other. In addition, the language in the claim preamble provides a “raison d’etre for the claim.” The limitation of an increased in vivo half-life makes sense only with respect to a living being where the preamble informs of the meaning of the claim. Thus, the Xencor decision finds that the entire preamble, including “treating a patient,” to be limiting of the claim.

The Xencor decision next turned to whether the ’690 application satisfied the written-description requirement, and the decision found that it did not. The Xencor decision essentially maintains the ARP’s conclusion that the specification as filed does not provide adequate written description to support at least the genus of “treating a patient,” which is interpreted to mean “treating all patients and all diseases.”

Claim 8 (Jepson Claim)

Here, Xencor argued that a preamble of a Jepson claim cannot be considered part of the claimed invention because it describes what is already “well known” in the prior art, upon which an improvement is made. According to Xencor, it is only the improvement that is the invention, not what already is “well known,” and so only the improvement requires written description.

The Federal Circuit’s decision disagrees with Xencor, agreeing with the ARP decision that the preamble of a Jepson claim requires written description. The Federal Circuit stated that the “claim preamble is used ‘to define the claimed invention, thereby limiting claim scope.’”² The Federal Circuit further stated that the invention is not only the improvement but also “the claimed improvement as applied to the prior art, so that inventor must provide written description sufficient to show possession of the claimed improvement to what was known in the prior art.”

The Xencor decision reasons that if an inventor were permitted to “assert, without showing, that she possessed what is claimed to be in the prior art, she might be able, improperly, to obtain a patent on something she does not actually possess.” As contrasting examples, the Xencor decision compares an automobile and a time machine. By now, an automobile is “well known” and so a full written description of an automobile may not be necessary to satisfy the written-description requirement. On the other hand, a patentee cannot merely recite a time machine in the preamble of a Jepson claim and assert that it is “well known” in the art. Rather, the patentee would either need evidence that a time machine is, in fact, “well known,” or the time machine would need to be described in sufficient detail to indicate possession of it.

Holding that the preamble of Jepson-style claim 8 requires written description, the Xencor decision finds for the same reasons used to reject claim 9: that there was insufficient written description for at least the genus of treating all patients and all diseases and that Xencor failed to demonstrate that anti-C5 antibodies are “well known.”

Before the Xencor decision, the Jepson claim format had become rarely used for its limiting statements in the preamble as to what constituted the prior art, with the inventive improvement being laid out thereafter. Now with the Xencor decision, which requires an applicant to describe and enable the prior art in its application, or prove that it is well known in the art, the Jepson claim format is likely to become more of a rarity.

 

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^[1] Pitney Bowes, Inc. v. Hewlett-Packard Co., 182 F.3d 1298, 1305 (Fed. Cir. 1999) (citations omitted).
^[2] Catalina Mktg. Int’l, Inc. v. Coolsavings.com, Inc., 289 F.3d 801, 808 (Fed. Cir. 2002).

 

This informational piece, which may be considered advertising under the ethical rules of certain jurisdictions, is provided on the understanding that it does not constitute the rendering of legal advice or other professional advice by Goodwin or its lawyers. Prior results do not guarantee similar outcomes.