At the European Patent Office (EPO), one of the first steps for assessing inventive step of a claim is to determine the closest prior art. For medical use claims, the closest prior art is typically held to be a document relating to the same medical use as that claimed – when selecting the closest prior art, the first consideration is that it should be directed to a similar purpose or effect as the invention (Guidelines for Examination at the EPO, G-VII, 5.1). Thus, where a claim is directed to 'X for use in treating disease Y', the closest prior art would normally be a document directed to the treatment of disease Y, which position bears out in the existing case law (e.g. T 2571/12). However, a recent decision – T 2443/18 – issued by a technical Board of Appeal of the EPO, did not follow this approach.

What happened?

In T 2443/18, the claim in question was directed to the known drug tapentadol for use in the treatment of irritable bowel syndrome (IBS). Surprisingly, the Board of Appeal found that a document (D4) relating to tapentadol and confirming its efficacy in the treatment of visceral pain in animal models, was a suitable starting point giving a 'workable route' to the claimed invention. This was despite there being another prior art document (D3) on file directed to the use of a compound of the same functional class (another μ-opioid receptor agonist) in the treatment of IBS specifically (i.e. a document relating to the same purpose or effect as the invention). The Board noted that "if inventive step is to be denied, the choice of starting point needs no specific justification", before going on to find that the claimed subject-matter lacked inventive step starting from D4. It also seems that additional weight was given to the relevance of D4 based on experimental data and comments in the patent in suit which showed the benefits of tapentadol in the treatment of 'visceral hypersensitivity and referred pain' as 'major symptoms' of IBS.

Why did the Board diverge from the normal approach?

When trying to convince the Board that a document directed specifically to the treatment of IBS should be considered the closest prior art, the patentee argued that a drug developer would typically seek to provide a suitable drug for a given medical indication, rather than investigate new medical indications for a known drug. The latter approach was also alleged by the patentee to risk obscuring the proper assessment of inventive step by impermissible hindsight knowledge of the invention. However, the Board of Appeal did not find these arguments convincing. The Board instead found that:

"either approach may be taken by a person skilled in the art, depending on the stages reached in the development and life of a drug compound and the rationale for its development."

The Board also noted the similarity between IBS and visceral pain and found that:

"Document D4 relates to tapentadol and its confirmed efficacy in animal models of visceral pain... While D4 does not specifically refer to IBS, visceral pain is a typical manifestation of IBS, the therapeutic indication named in claim 1 of the patent in suit. In view of this considerable overlap in technical features and purpose... D4 cannot be ruled out as a suitable starting point for the assessment of inventive step"

Accordingly, the Board appears to have diverged from the usual approach to the selection of the closest prior art in view of the stage reached in the development of tapentadol and the overlap in technical features and purpose of using tapentadol to treat visceral pain.

Outlook – will this have a wider impact on normal practice?

Certainly, T 2443/18 was decided based on the specific facts of that case, and it is clear that the Board gave significant weight to the efficacy of tapentadol in the treatment of visceral pain shown in the prior art and indications in the patent in suit that visceral pain was a symptom of IBS (i.e. the indication claimed), when considering whether D4 was a suitable starting point for the inventive step assessment.

However, this decision may reflect more of a willingness of the Boards of Appeal to consider different possible starting points in considering the inventiveness of medical use claims, including those which may be less relevant to the purpose or effect of the invention than other cited prior art. Indeed, in another similar decision – T 0641/18 – issued by the same technical Board of Appeal, the closest prior art document was again found to be a document directed to the claimed compound for use in a similar (but not identical) medical indication to that claimed.  

In T 0641/18, the Board highlighted the similarity by which the claimed compound exerts its effect in the two medical indications. However, no mention was made of the stage reached in the development of the drug in question. Importantly, the claim in T 0641/18 was still found to comprise an inventive step because the closest prior art document specifically taught that the similar medical indication and claimed medical indication are not expected to be treated using the same compounds.

It remains to be seen whether different EPO technical Boards of Appeal will follow the approach set out in T 2443/18 and whether it will become more difficult in the future to demonstrate an inventive step for a claim directed to a new medical use of a known medicinal compound, particularly where there is any overlap in symptoms of the new indication versus those the compound is already known to treat.

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