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The Antibody Series #2 | Definition via binding strength in antibody claims: when “binds strongly… but only minimally…” becomes a trap of lack of clarity
January 2026
Defining an antibody by its binding strength is common practice in patent claims, but it can quickly become a pitfall under Article 84 EPC on clarity. In this second installment of our series, we analyze a decision by the EPO Boards of Appeal that illustrates why relative terms such as “binds strongly” or “only minimally” can compromise the strength of a claim. Discover practical lessons to secure your IP protection strategy.
The EPO Boards of Appeal (BoA) are the appeal body that reviews decisions made by the European Patent Office. In this case, they examined an antibody claim on appeal.
The real case: You are developing an anti-PSMA antibody. In the laboratory, it gives a strong signal on LNCaP cells and little or no signal on cells that do not express PSMA. You want to protect this selectivity and write it into the claim using common laboratory terms.
Claim 1:
“1. An isolated monoclonal antibody or an antigen binding portion thereof which
- a) binds to prostate specific membrane antigen in its native form occurring on the surface of tumor cells
- b) can be internalized by a tumor cell,
- c) binds strongly to LNCAP cells but not or only minimally to cells which lack expression of prostate specific membrane antigen and
- d) is linked to a label or a cytotoxic agent, characterized in that
e1) it comprises at least three of the CDR sequences selected from the group consisting of the CDRs designated as CDR H1, H2, H3, L1, L2, and L3 as shown in Fig. 21 or
e2) it comprises at least three of the CDR sequences selected from the group consisting of the CDRs designated as CDR H1, H2, H3, L1, L2 and L3 as shown in Figure 20.”
Beginning of the story: This is a point of clarity under Article 84 EPC. This “strongly/minimally” issue was not the main subject of the decision. It was raised by the Board of Appeal.
The BoA’s teaching: “Strongly” and “minimally” are relative terms. Here, the claim gives no operational benchmark for distinguishing between what is ‘strong’ and what is “minimal.” The Board notes in particular:
- The absence of reference antibodies.
- The absence of affinity values or ranges determined by a specified method.
It also emphasizes that “minimally” may include cross-reactivity. The argument that a pathologist would understand these terms was not convincing, as the person skilled in the art is not limited to a pathologist and assessments may vary.
Practical writing tip: If you claim binding selectivity, replace relative terms with elements that a third party can reproduce and measure in a comparable way. Concrete example:
- A defined test (FACS, ELISA, SPR, BLI).
- A measurement condition (antibody concentration, incubation time, temperature, buffer, number of washes) .
- A numerical threshold (e.g., EC50 ≤ X, KD ≤ Y, MFI ≥ Z, LNCaP/negative signal ratio ≥ N).
- A control or reference standard.
If you cannot set these criteria, favor a structural definition of the antibody and leave the binding as a property demonstrated in the description.
Disclaimer: This is not legal advice; only a practical outline to be discussed early on, before filing.
Source: ECLI:EP:BA:2021:T094116.20210216.
