The US Court of Appeals for the Federal Circuit found a Jepson claim unpatentable where the specification did not provide adequate written description for the portion of the claim purporting to recite what was already well known in the prior art. In re Xencor, Inc., Case No. 24-1870 (Fed. Cir. Mar. 13, 2025) (Hughes, Stark, Schroeder, JJ.)

Xencor filed a patent application claiming a modified anti-C5 antibody treatment with certain amino acid substitutions that provide for longer serum half-lives and reduce the need for more frequent treatment. The application included:

• A Jepson claim reciting, “[i]n a method of treating a patient by administering an anti-C5 antibody with an Fc domain, the improvement comprising” certain amino acid substitutions, wherein the modified antibody has “increased in vivo half-life.”
• A non-Jepson claim directed to “a method of treating a patient by administering an anti-C5 antibody comprising” certain amino acid substitutions, wherein the modified antibody “has increased in vivo half-life.”

The specification provided one example of an anti-C5 antibody, 5G1.1, and three high-level examples of potential uses for anti-C5 antibodies. The examiner rejected the claims for lack of written description. Xencor unsuccessfully appealed the rejection to the Patent Trial & Appeal Board. Xencor then unsuccessfully petitioned the Board for reconsideration. Xencor appealed to the Federal Circuit, which resulted in a remand to the Board’s Appeals Review Panel (ARP).

The ARP concluded that Jepson claim preambles require written description support and that the preamble language of “treating a patient” was limiting – even without the Jepson claim format – because it gave life and meaning to the claim recitations “increased in vivo half-life” and “administering.” Because the specification did not provide a representative number of species to support the broad genus of anti-C5 antibodies, a description of conditions that can successfully be treated with an anti-C5 antibody, or even a single working example describing treatment with an anti-C5 antibody with the claimed modifications, the ARP found that the claims lacked written description and that Xencor had not shown that anti-C5 antibodies were well known. Xencor again appealed, arguing that “treating a patient” was not limiting and that Jepson preambles do not require written description support.

With respect to the preamble of the method claim, the Federal Circuit noted that Xencor agreed that the “administering” portion was limiting but nonetheless argued that “treating a patient” was not. Although a preamble can be split into limiting and non-limiting parts, the Court reasoned that the preamble here could not be neatly packaged into separate portions because the phrase “treating a patient” was directly connected through the word “by” to the phrase “administering an anti-C5 antibody,” and each phrase gave meaning to the other. The Court further explained that the entire preamble provided the raison d’être of the claimed method: When a patient is treated with the modified anti-C5 antibody, the treatment lasts longer, reducing the frequency of treatments. Accordingly, the Court agreed with the ARP that the recitation “treating a patient” was limiting.

The Federal Circuit next concluded that substantial evidence supported the ARP’s determination that the specification did not provide written description support for “treating a patient.” Because the specification was not limited to treating a particular disease, “treating a patient” meant “treating all patients and all diseases.” While the specification provided three examples of classes of diseases that might benefit from the claimed treatment, the Court agreed with the ARP that this disclosure was inadequate to demonstrate possession of a method of treating any particular disease, let alone all diseases.

Finally, the Federal Circuit explained that a Jepson claim preamble requires written description support because it is used to define the claimed invention and the claim scope. The Court cautioned that a patentee cannot obviate the written description requirement by using a Jepson claim to avoid the requirement that the inventor be in possession of the claimed invention – otherwise, a patentee could obtain a Jepson claim with a preamble that recited a time machine as well known in the art without describing a time machine. To provide adequate written description for a Jepson claim, the applicant must establish that what is claimed to be well known in the art actually is well known in the art. The Court explained that the amount of disclosure necessary varies depending on the level of knowledge of the person skilled in the art, the unpredictability of the art, and the newness of the technology.

Given the large number of possible antibodies in the anti-C5 antibody genus and the limited disclosure in the specification, the Federal Circuit affirmed the ARP’s determination that the Jepson claim lacked adequate written description.