EPO clarifies sufficiency at the priority date in T 0883/23

The recent decision T 0883/23 has served as a reminder that a priority filing must contain not only the features of an invention but also the technical information necessary to make the invention credible. The patent (EP 3337478, owned by Ipsen Biopharm Ltd.) related to a drug for use in a method of treating metastatic adenocarcinoma of the pancreas in a human patient who has not previously received chemotherapy, in combination with three other drugs. In this Decision, the Board of Appeal considered whether the claimed subject-matter was directly and unambiguously derivable from the priority application within the meaning of Article 87(1) EPC. The Board’s decision made clear that in the field of therapeutic inventions, the requirement of “the same invention” cannot be met by the simple presence of structural or dosage features alone, but also depends on whether the priority filing sufficiently discloses the therapeutic effect of the treatment.

The priority application in question disclosed an escalation/de-escalation scheme for a clinical dose finding study. It described several dosage regimens for liposomal irinotecan and oxaliplatin, but did not report the results of the study. It was only the later PCT application that disclosed data showing that the higher starting dose in the protocol (80/60 mg/m²) was not suitable and that the de‑escalated 60/60 mg/m² regimen was tolerable in human patients.

While the Enlarged Board of Appeal decision G 2/98 established that the claimed subject‑matter must be “directly and unambiguously derivable” from the earlier application, the T 0883/23 Board considered that under the established case law (T 2506/12), the tolerability of the dose was a “functional technical feature” of the claim and that this must therefore be disclosed in the priority document. The Board further held that the case law “confirms the need for sufficient disclosure of the claimed invention in the priority document” (r. 1.5), i.e. not just at the filing date. The later‑filed data demonstrated the technical effect of the claimed dosage combination, but because it was absent from the priority filing, the Board held that the priority application did not disclose the same invention in the sense required by Article 87(1) EPC. For therapeutic inventions, such as that under discussion, it was established that satisfying sufficiency at the priority date requires evidence of a technical effect and a study protocol is not sufficient. Accordingly, the priority claim was not held to be valid. This meant that the feature of the ultimately claimed invention (the tolerability of the 60 mg/m² liposomal irinotecan and 60 mg/m² oxaliplatin combination) was considered not plausibly established at the priority date.

This decision confirms the need for sufficient disclosure of the claimed invention in the priority document and highlights the need for a careful filing strategy. Applicants in the life‑sciences sector may wish to file priority applications early in the development process, sometimes before experimental results become available. While this strategy can secure an early priority date, there are risks of doing so without meaningful supporting data. If the therapeutic effect, tolerability profile or other functional aspects of the claim are only established in later filings, the patentee may find that the priority claim fails. This means that any disclosure published between the priority date and the actual filing date of the subsequent application becomes full prior art for both novelty and inventive step. As the Board in T 0883/23 confirmed, sufficiency and priority entitlement are closely linked for therapeutic claims.

At Venner Shipley, we help to consider whether there is sufficient data and when a first priority application should be filed. If you would like to discuss this and preparing priority filings to ensure they contain a sufficient technical disclosure, please contact our team of UK and European patent attorneys.