EPO decision T 0655/24: Striking the Right Balance in Antibody Claim Drafting

This is the second in a series of three posts on recent decision T 0655/24 and its implications for antibody practice at the EPO.

In the first post, Chris looked at the Board’s notably strict approach to post-filing evidence. Here, he turns to a more practical question: how the claims were structured, and why that illustrates a useful approach to antibody drafting.

The challenge of structure-only definitions for inventive step

A recurring theme in EPO practice is that a technical effect will only be taken into account for inventive step if it is credible across the scope of the claim. This creates a familiar tension in antibody drafting.

Claims defined only by partial structural features (e.g. CDR sequences), can be vulnerable to objection that changes elsewhere in the antibody (e.g. in the framework regions) could materially affect the relevant property, such as affinity or activity. In that case, the asserted effect will not considered credible across the full scope of the claim and so cannot support inventive step of the claim in that form, even where the exemplified antibody is acknowledged to be inventive.

A common response, which examiners often encourage, is to define the antibody by more complete structural features, for example by specifying full variable domain sequences. While often effective, this approach can come at the cost of unattractively narrow claims.

The challenge, then, is to limit the claim sufficiently that the technical effect is retained across the scope, without overshooting into an unnecessarily narrow claim.

Combining structural and functional features

T 0655/24 provides a good illustration of a useful alternative approach.

The claims included a limited structural definition — the specific Fc mutations L234F/L235E/D265A — combined with functional limitations reflecting the technical effects that those mutations exhibited in the exemplified antibodies. Specifically, the claims required that:

• the Fc region does not bind Fcγ receptors; and
• the plasma clearance rate does not deviate by more than 10% from a corresponding protein without the defined Fc mutations.

This combination proved effective. When the Opponent argued that additional mutations could negate the desirable properties of the FEA variant, the Board was not persuaded. Any such variants would simply fall outside the claim, because they would not meet the functional requirements.

The importance of aligned scope

The key point here is the inclusion of functional limitations that align the scope of the claim with the technical contribution on which the inventive step arguments are based.

By combining:

• a partial structural definition (the Fc mutations), with
• functional criteria that exclude non-working embodiments,

the claims avoid the need to define the entire Fc sequence, while still ensuring that the claimed scope is commensurate with the effect relied upon.

This general approach can be a useful alternative or addition to a claim that is limited to a more complete structural definition (typically, full VH/VL sequences, when the inventive step relates to the binding region). It is expressly envisaged by the EPO examination guidelines (G-II, 6.1.4):

Antibodies can also be defined by both functional properties and structural features. It is possible to claim an antibody characterised by the sequences of both variable domains or CDRs with less than 100% sequence identity when combined with a clear functional feature.

A neat drafting point

A further nice touch is that the plasma clearance limitation is defined relative to a reference antibody that differs only in the absence of the three Fc mutations. In other words, the claim does not impose any absolute level of stability defined by reference to a fixed antibody, only that the mutations do not materially worsen stability in the context of whichever Fc domain they are presented.

Practical takeaways

For those drafting antibody claims at the EPO, the decision reinforces some important practical points:

• Partial structural definitions alone may not be enough where the technical effect could be affected by variation elsewhere.
• Fully structural definitions can be unnecessarily narrow, particularly if further development of the antibody is likely.
• Combining structural and functional features can provide an effective alternative, capturing the inventive contribution while allowing variation in non-essential positions.
• Functional limitations should be carefully framed to reflect the technical contribution.

It is fundamental to EPO inventive step practice that the scope of the claim and the asserted technical effect are aligned. Patents are frequently revoked when this is not the case, especially on appeal. T 0655/24 is a helpful example of how to achieve it in practice.

Next in the series: Chris will look more closely at the decision and explain how it illustrates two approaches to arguing inventive step for antibody inventions, which can be very effective at the EPO.