Decision T 0709/23 reminds patentees that broad claims demand broad enablement. Functional biotech claims without robust disclosure may run into lack of sufficiency when post-published data undermine the link between the functional features and the claimed therapeutic effect.
The decision T 0709/23 dated 17 June 2025 concerns an appeal before the European Patent Office’s Boards of Appeal (Board 3.3.04). European patent application 17 168 574.6 in the name of Zoetis Services LLC, published as EP3219729 and entitled “Interleukin-31 monoclonal antibody”, functionally claimed antibodies for use in treating a pruritic condition or an allergic condition in cats.
Claim 1 of the Main request reads:
“An isolated antibody that specifically binds to feline IL-31, wherein said antibody reduces, inhibits or neutralizes feline IL-31-mediated pSTAT signalling in a cell-based assay, for use in treating a pruritic condition or an allergic condition in cats.”
The Opposition Division revoked the patent for lack of sufficiency of disclosure (Art. 83 EPC). The patent proprietor/appellant appealed the decision.
In the decision, the board interpreted claim 1 as a purpose-limited product claim under Art 54(5) EPC and took the view that the therapeutic indication “for use in treating a pruritic or allergic condition in cats” is a functional technical feature.
Two issues were raised under Article 83 EPC:
• whether the therapeutic effect demonstrated in dogs could be credibly extrapolated to cats, and • whether the skilled person would be able to carry out the invention over the entire scope of claim 1 without undue burden.
Regarding the extrapolation of therapeutic effect from dogs to cats, the board based its assessment on the second objection – namely, that the claimed invention cannot be carried out across the entire scope claimed – and accordingly, the board deemed that it was not necessary to decide whether the skilled person would have credibly extrapolated the data on IL-31 in dogs to cats. Nevertheless, the therapeutic effect observed in dogs was assumed to be credibly achieved also in cats.
With respect to the effect over the entire scope of claim 1, the board reiterated that the disclosure of one way of performing an invention is only sufficient if it allows the invention to be performed in the whole range claimed rather than only for some members of the claimed class to be obtained. Where the person skilled in the art has to resort to trial-and-error experimentation to identify compounds, if any, which meet the functional definition set out in the claim, this constitutes an undue burden, even if it involves routine experiments.
In the case at issue, the patent disclosed in vitro binding and signalling inhibition assays and in vivo efficacy data in dogs for anti-IL-31 antibodies. For cats, only binding/cross-reactivity and assay data of IL-31-induced signal transducer and activator of transcription protein (STAT) phosphorylation in dog cells using feline IL-31 were shown; no in vivo data were shown in cats.
In post-published document D51, the data generated using one of the disclosed antibodies (11E12) which bind to the conserved epitope described in the patent contradict the central teaching of the patent application that inhibition of feline IL-31-mediated pSTAT signalling correlated with therapeutic efficacy in vivo. Instead, the data in D51 show that an antibody targeting the conserved antigenic region on IL-31 according to the patent application failed to achieve significant therapeutic effects in cats despite showing pSTAT inhibition.
This demonstrates that there is no clear correlation between the functional features of inhibiting feline IL-31-mediated pSTAT signalling in a cell-based assay and the in vivo efficacy in treating a pruritic or allergic condition in cats. The board thus concluded that additional research was necessary to arrive at an anti-IL-31 antibody capable of treating a pruritic or allergic condition in cats. This was deemed to constitute an undue burden.
In view thereof, the appeal was dismissed.
This decision emphasises that broad functional definitions must be matched by commensurate disclosure. Reliance on data from other species or on in vitro assays not validated as representative of the technical effect can be risky to substantiate sufficiency. Accordingly, when drafting biotechnology claims defined by functional features, especially therapeutic uses, it is good practice to include specific structural embodiments (e.g., antibodies, binding epitopes) in the specification or at least a roadmap enabling the skilled person to derive such embodiments. Also, when relying on in vitro functional assays (e.g., signalling inhibition) careful drafting of the description is necessary. If the therapeutic effect is claimed, it is recommended to address in the specification why such assays are reliable predictors of the therapeutic effect or to provide further corroboration backed-up by experimental data.
This decision shows that post-filed data may be relevant in assessing sufficiency. In opposition/appeal proceedings at the EPO, sufficiency objections remain a potent ground, especially when there are post-filed data countering the data in the patent. While you cannot amend a patent post-grant to add support, post-published data may be used by opponents to challenge the sufficiency of the original disclosure. Therefore, when filing an application, we recommend ensuring that the teaching of the patent application is robust and covers the full scope of the claims.
Our experienced team of European patent attorneys at De Clercq & Partners is happy to assist you with protecting your inventions including those in the biotech field. Please reach out to us at info@dcp-ip.com.
