The legal standards for written description and enablement are well-established. To meet the written description requirement, the specification must convey with reasonable clarity that as of the filing date the inventor was in possession of the claimed invention. See, Vas-Cath, Inc. v. Mahurkar, 935 F.2d 1555, 1560 (Fed. Cir. 1991); Ariad Pharm., Inc. v. Eli Lilly & Co., 598 F.3d 1336, 1351 (Fed. Cir. 2010) (en banc) (specification must “clearly allow persons of ordinary skill in the art to recognize that [the inventor] invented what is claimed.”). The enablement requirement is met if the specification would allow a person of ordinary skill in the art to make and use the claimed invention without undue experimentation. See, Amgen, Inc. v. Sanofi, 598 U.S. 594, 611 (2023).

The Supreme Court observed in Amgen that “[d]espite recent advances, aspects of antibody science remain unpredictable … [and] scientists understand that changing even one amino acid in the sequence can alter an antibody’s structure and function” in unpredictable ways. Amgen, 598 U.S. at 612-13. In Ariad, the Federal Circuit found that the disclosure of three types of molecules supporting the claimed substance for interfering with a gene transcription factor was insufficient to meet the written description requirement. Ariad, 598 F.3d at 1355-58. In Amgen, the Supreme Court held that a claim to antibodies for treating high cholesterol was not adequately enabled even though the specification disclosed multiple embodiments including 26 different antibodies identified by their amino acid sequences. Amgen, 598 U.S. at 612.

Below are best practices for drafting antibody claims and specifications.

Antibody Sequence Claims

Consider these example claims:

1. An antibody comprising an amino acid sequence having at least a 90% sequence identity to SEQ ID NO:1.
2. The antibody of claim 1, wherein the amino acid sequence comprises the sequence of SEQ ID NO: 1.

Claims to novel sequences for each disclosed antibody, while narrow, can be fairly easy to obtain if the sequences are disclosed in a proper Sequence Listing and other procedural requirements are met. Amino acid claims are typically broader than the nucleic acid claims.

For percent-identity variations (e.g., 80% sequence identity of SEQ ID NO: 1), a patent applicant must show the sequences’ actual variation and that each performs the desired function. Coverage may be possible for sequences with physically and chemically similar amino acid substitutions (e.g., non-polar, polar, basic, or acidic); however, recent Supreme Court rulings have reduced the likelihood of success as discussed further below. But even novel antibody sequences may face obviousness rejections if complementarity-determining regions (CDRs) or their modifications are already known in the art.

For antibodies having variants that differ by one or more amino acids, written description and enablement rejections are common.  One strategy to consider may be to pursue an antibody as a composition without variants in a first patent application and then pursue the antibody with variants in a divisional or continuation application via a product-by-process claim.

Antibody Structure Function Claims

Consider this example claim:

1. An isolated human antibody, or antigen-binding portion thereof, that binds to human IL-12 and dissociates from human IL-12 with a K₄ of 1×10⁻¹⁰M or less and a k_off rate constant of 1×10⁻³ s⁻¹ or less, as determined by surface-plasmon resonance.

Structure-function claims generally cover a broader antibody set than sequence claims and thus require more written description and enablement support.  In such cases, the specification should disclose as many of the antibodies’ measurable properties as possible, such as the binding site (epitope), binding properties (K_d, K_off, etc.), binding function (inhibition/activation), and competitive binding. Courts have viewed the inclusion of antibodies’ 3D structures into the specification favorably. See, e.g., Amgen Inc. v. Sanofi, 872 F.3d 1367 (Fed. Cir. 2017).  If possible, including at least one 3D structure of an antibody of interest has been found to be a good idea.

In Abbvie Deutschland v. Janssen, the Federal Circuit held that an antibody description requires (1) a reasonable structure-function correlation or (2) a representative number of examples. See Abbvie Deutschland GmbH & Co.v. Janssen Biotech, Inc., 759 F.3d 1285 (Fed. Cir. 2014). The number of “representative number of examples” is fact-dependent, but more is always better. To be sure, the “newly characterized antigen test” is now dead. Amgen Inc. v. Sanofi, Aventisub LLC, 872 F.3d 1367, 1377 (Fed. Cir. 2017) (“We questioned the propriety of the ‘newly characterized antigen’ test and concluded that instead of ‘analogizing the antibody-antigen relationship to a ‘key in a lock,’’ it was more apt to analogize it to a lock and ‘a ring with a million keys on it.’”) (internal citations omitted).  In Amgen, the Court was unconvinced that conservative substitutions or single amino acid changes would result in a functionally equivalent antibody noting that after each amino acid substitution a scientist would be required to test the resulting functionality. Amgen v. Sanofi, 598 U.S. 594, 614 (2023).

As a result of these opinions, a single genus antibody sequence likely will not cover any antibody that binds to a target, given the potential variability in antibody generation. The specification should therefore include hybridoma-assay support for producing the antibodies of interest. Some practitioners argue that method of treatment claims are easier to obtain for structure–function-defined antibodies, but recent USPTO prosecution histories show that these claims also face section 112(a) difficulties.

To be safe, for structure-function and measurable-property claims, include as much data as possible to show that a specific structural element leads to the desired function. Disclose as many working antibody examples to represent the claimed genus, and show that a specific structural element performs the same function in different backbones or environments.

Means Plus Function Claims

Consider this example claim:

1. A method of treating a patient by administering an anti-C5 antibody comprising: a means for binding human C5 protein; and an Fc domain.

Recently, the USPTO’s Patent Trial and Appeal Board (PTAB) clarified its ruling that was recently under appeal before the Court of Appeals Federal Circuit (CAFC) (In re Xencor) to confirm that means-plus-function language can be used in antibody claims and the specification need not disclose all equivalents.  In its May 21, 2024 ruling, the Appeals Review Panel (ARP) of the PTAB determined that the phrase “means for binding human C5 protein,” was adequately described under pre-AIA 35 U.S.C. § 112 first paragraph and was definite under pre-AIA 35 U.S.C. § 112 second paragraph.[1]  Ex parte Chamberlain, No. 2022-001944, pp. 28, 29 (P.T.A.B May 21, 2024).  Specifically, the ARP found that the disclosure identified two specific antibodies known in the art to bind to human C5 protein. Further, the ARP concluded that it was not necessary for the specification to describe equivalents to the specifically described C5 antibody (e.g., 5G1.1). Id. The CAFC heard oral arguments in December 2024, and the court issued its opinion on March 13, 2025.  With respect to the means-plus-function claim exemplified above, the remaining issues, which are unrelated to the issues of concern in this article, are directed to (1) whether the preamble should be construed as a limitation where the language neither recites essential structure or steps nor provides antecedent basis for the remaining claim language, and (2) whether the PTAB erred in finding the phrase “treating a patient” to lack written description for failing to disclose “treating all patients and all diseases.”  In its opinion, the CAFC held that the phrase “treating a patient” recited in a preamble is limiting and must be supported by adequate written description. See In re Xencor, __F.4th__(Fed. Cir. 2025) 2025 WL 793963.

Briefly, means-plus-function language removes the structure from the claim, replacing it with a “means for” performing a function. This avoids written description rejections because the “means for” ties to the specification’s disclosure; however, the caveat is that the specification must list as much structure and as many alternatives as possible, linking such structure and alternatives to the “means for” performing the function. Historically, an applicant’s claims have been limited to the specification’s support and art-known equivalents up to the date of patent issuance.

Importantly, for means-plus-function language to have value, the specification must provide robust support in the form of examples and embodiments. The antibodies must prove useful and to function as claimed. Moreover, avoid indefiniteness rejections under section 112(b) by clearly linking the corresponding structure/material to the claimed function. In light of the PTAB’s clarifying ruling, means-plus-function claim language is still a viable option under some circumstances for obtaining antibody claims and avoiding written description and enablement rejections.

Concluding Thoughts

The burden of drafting patent applications with sufficient support for antibody claims is not likely to lessen any time soon.  The courts and the scientific community are in agreement that antibody design and function are unpredictable and thus require more disclosure.  To that end, specifications should be drafted to include as much data and support as possible because patent applicants will not be able to rely on an assumption that minor changes to an antibody’s structure will result in a similar function. Nor can patent applicants rely on the assumption that an antibody’s activity/function will necessarily limit its structure.  Finally, while means-plus function claims are a viable option for obtaining antibody claims that encompass those antibodies described and exemplified in the specification, it is presently unclear what art-known equivalents will cover given the high unpredictability in the art.

[1] The ARP analyzed the claims under pre-AIA but stated that the outcome would be same under a post-AIA analysis.