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The Antibody Series #5 | Epitope-defined antibody claims: when “binds to this epitope” becomes a risk of insufficiency
February 2026
The Boards of Appeal of the EPO (BoA) are the appeal body that reviews decisions made at the EPO; here, they reviewed an appeal in opposition proceedings after the revocation of a patent relating to an antibody.
The real case: you have a promising anti-IL 23 antibody. Structural data indicates that it contacts two areas of IL 23p19. You draft a claim to cover not only your antibody, but also other antibodies that would target the same epitope, in order to leave room for optimizing sequences and developability.
Claim 1:
“1. An antibody, or antigen binding fragment thereof, that binds to human IL-23p19 at an epitope comprising residues 82-95 and residues 133-140 of SEQ ID NO: 29.”
Beginning of the story: the opposition division revoked the patent for insufficient description, under Article 83 EPC, via Article 100(b) EPC. This was the central point of the case.
The BoA’s teaching: defining an antibody solely by its binding to a discontinuous epitope does not amount to protecting a single antibody. It amounts to claiming a family of antibodies that share the same epitopic target. The patent must therefore give a skilled team the practical means to obtain other antibodies falling within the scope of the claims without excessive effort, i.e., without having to multiply exploratory trials and errors.
Here, the reasoning is practical: mapping the epitope of an antibody that has already been found is not enough. The application must also explain how to generate and select antibodies that bind to this discontinuous epitope. The Board considered that the teaching of the patent did not allow, beyond the exemplified antibody and very close variants, to reliably arrive at other antibodies covered by the claim.
Practical drafting tip: if you are aiming for “epitope” protection, describe the complete pathway. Provide a relevant immunogen, a selection and screening strategy that actually allows antibodies directed against this discontinuous epitope to be isolated, and ideally several representative antibodies, or common structural elements that clearly link the claimed scope to what you have actually made reproducible.
Disclaimer : This is not legal advice; it is merely practical guidance to be incorporated early on in the filing strategy.
Source: ECLI:EP:BA:2022:T043520.20220714.
