On March 13, 2025, the United States Court of Appeals for the Federal Circuit issued a decision in In re Xencor, Inc., affirming the rejection of Xencor’s patent application for failing to meet the written description requirement under 35 U.S.C. § 112. This case raises important considerations regarding patent claim construction, particularly the treatment of Jepson claims and the role of preamble language in determining the scope of an invention.

Background of the Case

Xencor, Inc. filed U.S. Patent Application No. 16/803,690, seeking protection for modified anti-C5 antibodies with increased in vivo half-life, which could potentially improve treatments for autoimmune diseases. The Patent Trial and Appeal Board (PTAB) rejected Xencor’s claims on the grounds that they lacked sufficient written description. Xencor appealed the decision, leading to a review by the Appeals Review Panel (ARP) and ultimately by the Federal Circuit.

Key Legal Issues

1. Written Description Requirement

Under 35 U.S.C. § 112, a patent’s specification must provide a clear and precise written description of the claimed invention. The court found that Xencor’s application failed to demonstrate that the company had possession of the full scope of its claimed invention at the time of filing. Specifically:

• The application disclosed only one anti-C5 antibody (5G1.1), which the court found insufficient to support a broad genus claim covering all anti-C5 antibodies.
• The specification lacked data or examples demonstrating the treatment of any disease using an anti-C5 antibody with the claimed modifications.

The court emphasized that for claims involving biological inventions, written description sufficiency depends on the predictability of the field. Since antibodies have varying specificities and epitopes, a broader disclosure was necessary.

2. Role of Preamble Language in Claim Construction

A major point of contention was whether the preamble in Xencor’s method claim was limiting. The claim preamble recited, “A method of treating a patient by administering an anti-C5 antibody…” Xencor argued that “treating a patient” was merely a statement of purpose and should not require written description. However, the court held that:

• The preamble was necessary to give meaning to the claim, particularly in relation to the claimed “increased in vivo half-life.”
• The phrase “treating a patient” could not be separated from “administering an anti-C5 antibody,” making it an essential part of the invention.
• Since the specification failed to provide examples or data supporting the therapeutic effect of the modified antibodies, the claim lacked sufficient written description.

This ruling underscores that patentees cannot rely on preamble language to define the scope of an invention while avoiding corresponding disclosure requirements.

3. Jepson Claims and Their Written Description Requirement

The case also addressed whether Jepson claims—which define an invention as an improvement over prior art—require written description for both the improvement and the prior art elements. The court ruled that:

• A Jepson claim’s preamble is part of the invention and, therefore, requires written description support.
• Xencor failed to establish that anti-C5 antibodies were well-known in the prior art, meaning additional written description was needed.
• Simply asserting that something is conventional does not relieve the applicant of the obligation to provide sufficient disclosure.

This clarification is significant for applicants using Jepson claims, as it reinforces that both the improvement and the prior art must be supported by adequate written description.

Implications for Patent Applicants

The In re Xencor decision serves as a cautionary tale for patent applicants in the biotechnology and pharmaceutical fields:

1. Broad Genus Claims Require Broad Support – Disclosing a single species within a broad genus may not be sufficient if the field is unpredictable.
2. Preambles Can Be Limiting – If preamble language gives life and meaning to a claim, it will be treated as a limitation requiring written description support.
3. Jepson Claims Require Complete Written Description – If using a Jepson claim, applicants must provide written support for both the claimed improvement and the elements stated to be prior art.

Conclusion

The Federal Circuit’s decision in In re Xencor, Inc. reaffirms the importance of the written description requirement, particularly for complex biological inventions. Patent applicants should ensure that their specifications provide sufficient disclosure to support the full scope of their claims, especially when using Jepson claim structures or asserting broad genus claims in unpredictable fields. This case highlights the necessity of careful patent drafting to avoid claim rejections based on insufficient written description.

By Charles Gideon Korrell