Will functional antibody inventions find new life in the US with mean-plus-function claims? (Ex parte Chamberlain, Appeal No. 2022-001944)

The decision from the USPTO Appeals Review Panel (ARP) in Ex parte Chamberlain, Appeal No. 2022-001944 reaches a remarkable conclusion on written description for a broad functional antibody claim. The ARP found that claim language defining an antibody as means-plus-function, specifically "means for binding a target", was neither indefinite or lacking in written description. Could means-plus-function language therefore be a way of functionally claiming antibodies in the US? Caution is advised. Notably, the case in question fell down on written description for another feature of the claim (the method of treatment). Furthermore, the question of enablement was not reviewed. Nonetheless, with limited avenues available for broad antibody inventions in the US, Chamberlain suggests that means-plus-function language is worth a shot. 

Legal background: Enablement, written description and mean-plus-function

Compared to Europe, the USPTO deals far less favourably with functional claim language. In the biotechnology field, functional claim language will often be rejected by the USPTO for lack of one or both of enablement and written description. US patent law specifically requires that:

"The specification shall contain a written description of the invention, and of the manner and process of making and using it, in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains, or with which it is most nearly connected, to make and use the same" (35 U.S.C. § 112(a))

Enablement was considered by the US Supreme Court in Amgen v Sanofi, with respect to an antibody genus claim. The question before the Supreme Court was whether the specification of Amgen's anti-PCSK9 antibody patent contained enough information to enable a skilled person to make and use the broadly claimed invention. The Supreme Court confirmed the strict case law on enablement, finding that the broad antibody claim in question placed undue burden on the skilled person attempting to work the invention over the whole scope of the claim. 

Describing antibodies

However, even before the question of enablement is raised, an antibody claim must also satisfy the written description requirement. The written description requirement necessitates the invention to be adequately described in the specification. This is different to the enablement requirement, which requires only that the skilled person be able to perform the invention. In other words, even if the specification contains instructions sufficient to enable a skilled person to make and use the claimed invention (enablement), the applicant is still obligated to identify the invention (written description). 

A mean-plus-function claim in US patent law is a category of claim that recites means for performing a function without structurally defining the means. A mean-plus-function claim is construed as covering the corresponding structure described in the specification and "equivalents thereof". Means-plus-function claims may be rejected as indefinite under 35 U.S.C. § 112(f) if the specification fails to disclose a corresponding structure. The case law on means-plus-function claims is primarily focussed on the computer-implemented inventions. Mean-plus-function claims can also be found to lack written description and enablement if the description of the structure in the specification is inadequate. 

Case Background: Defining an antibody by function

Antibodies comprise highly variable antigen binding regions and a constant region (Fc) that determines the function of the antibody. The case in Chamberlain related to a patent application for anti-C5 antibodies with modified constant binding regions that improved the functionality of the antibodies (US 16/803,690). The claims specifically related to a method of treating a patient with an anti-C5 antibody having a modified Fc domain. Claim 9 recited: 

"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 substitutions M428L/N434S [...], wherein said anti-C5 antibody with said amino acid substitutions has increased in vivo half-life as compared to said antibody without said substitutions."

In claims, the antigen-binding activity of the anti-C5 antibodies was thus defined purely with mean-plus-function claim language. Even for a functional claim, the claim was thus very broad. The claim did not specify the antibody variable region sequence, mechanism of action, or epitope. The specification disclosed only one example of an anti-CR5 antibody, which was already known in the prior art. 

The claims were rejected in prosecution for lack of written description (35 USC § 112(a)). Enablement was not raised as an objection. The written description rejection was upheld on appeal and subsequent rehearing by the PTAB. The PTAB also found claim 9 indefinite as a mean-plus-function claim (35 U.S.C. § 112(f)). However, the PTAB noted the lack of case law on mean-plus-function antibody claims. 

The case was appealed to the Federal Circuit. After the applicant filed their opening brief, the USPTO director requested that the case be remanded back to the ARP in order to "clarify the USPTO’s position on the proper analysis of [...] means-plus-function claims in the field of biotechnology, and particularly in the antibody art". The Federal Circuit granted the motion to remand (In re Xencor Inc., No. 23-2048 (Fed. Cir. Jan. 23, 2024).

ARP decision - Means-plus-function does not require disclosure of equivalents

In Chamberlain, the ARP assessed two questions, first whether the preamble of the claim was limiting and lacked written description, and second whether the mean-plus-function language defining the genus of antibodies was definite and satisfied the written description requirement. On the first point, the ARP found that the method of treatment language in the preamble was limiting and lacked written description, given that the specification did not describe treating any disease with an anti-C5 antibody. However, on the second question, the means-plus-function antibody definition was found not only definite but also to satisfy the written description requirement. 

The ARP agreed that the language "means for binding human C5 protein" in the claim was a means-plus-function limitation. The ARP also found that the specific anti-CR5 antibody disclosed in the specification to be the corresponding structure for the claimed means. The ARP further found that "it is not necessary for the Specification to describe equivalents of [the antibody] to meet the definite requirement". The means-plus-function language was therefore found adequately described under 35 U.S.C. § 112, r I (written description) and definite under 35 U.S.C. § 112, r 2 (page 27, first paragraph).  Specifically, the ARP found that:

It is true that § 112, r 6 provides that a means-plus-function element "shall be construed to cover the corresponding structure, material, or act described in the specification and equivalents thereof" Id. ( emphasis added). That is, the claim is interpreted to cover both the corresponding structure, material, or act described in the Specification, as well as equivalents of that structure, material, or act. Notably, § 112, r 6 does not state that the Specification must also describe equivalents of that structure. If Congress had intended the statute to require a description of equivalents, it could have placed "and equivalents thereof' before "described in the specification," which it did not do. 

The decision by the ARP in Chamberlain therefore suggests that simply converting antibody functional language to means-plus-function may satisfy the written description requirement for the whole functional genus, even if only one or two example antibodies are provided. 

Final thoughts

Whilst Chamberlain has been hailed as a potential game-changer in the biotech field, a note of caution must be sounded. First, this is an APR decision. We await to see to the Federal Circuit will follow suit. Second, written description is only one of the barriers to functional antibody claims in the US. Claims must also satisfy the enablement requirement. Indeed, the ARP in Chamberlain noted that in further prosecution of the case "the Examiner may wish to consider whether the genus of 'an anti-C5 antibody' is adequately enabled". Whilst means-plus-function language could help to satisfy the written description requirement, the hurdle of enablement still remains. 

A more radical shift in what is possible to claim in the US may come less from claim-type selection, and more from technological advances in the field. As this Kat has previously opined (IPKat), the rapid acceleration of machine learning in the field may lessen the sufficiency burden for biotechnology invention. For enablement, the skilled person will have more tools at their disposal for working across the whole scope of the claim (IPKat). For written description, an inventor will be able to quickly produce more examples with which to describe the invention. In the meantime, however, there is no harm in also including a mean-plus-function claim in your claim set, just in case. 

Further reading

• EPO Board of Appeal maintains functional epitope antibody genus claim (T 1964/18) (Nov 2021)
• Strict US written description requirement applied to CAR-T-cell therapy (Juno v Kite) (Jan 2022)
• Sufficiency of broad functional genus and first medical use claims in Europe (T 0424/21) (Oct 2022)
• Patenting Antibodies: The epitope claim is dead, long live the epitope claim (Apr 2022)
• When generating antibodies for a target is more than routine (T 0435/20) (Apr 2023)
• US Supreme Court decision in Amgen v Sanofi: The European Perspective (May 2023)
• Broad functional claiming at the EPO (T 0835/21) (Sep 2023)
• USPTO call for comments: Impact of AI on patentability (May 2024)