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Poisonous priority – will the Enlarged Board of Appeal allow an antidote?


Poisonous priority has been a cause for concern amongst patent attorneys since the concept was first raised in the February 2012 Issue of the CIPA Journal. The concern has arisen from the suggestion that, in some circumstances, a European (or UK) divisional application could destroy the novelty of a claim in a parent patent or application that is not entitled to priority, or vice versa.

The reality of poisonous priority, and its potential to pose a serious threat to patent validity, was confirmed in EPO Board of Appeal decision T1496/11. However, conflicting case law in this area has led to a referral to the Enlarged Board of Appeal and we can expect some clarification in due course.

A Recap Of Poisonous Priority

The following example briefly summarises a situation in which poisonous priority may arise:

  • Application 1 – This priority patent application discloses a relatively narrow chemical formula (a) supported by representative examples.
  • Application 2 – This European patent application discloses and claims a broader chemical formula (A) which, within its scope, includes a, and which is supported by additional examples justifying A. This application claims priority to application 1.
  • Application 3 – A divisional application is filed based on application 2, which contains the narrow chemical formula (a).

It is suggested that application 3 is novelty destroying for the claim of application 2 because formula a is entitled to the priority date (i.e. the filing date of application A) but formula A is not entitled to the priority date. Application 3 is novelty-only prior art against the claim of application 2.

An Antidote?

An antidote to this situation has been suggested which involves splitting the claim of application 2 into two theoretical alternatives:

  1. Formula A, excluding a;
  2. Formula a.

The first alternative is not entitled to priority but it excludes a and so is not anticipated by a in application 3. The second alternative is entitled to priority and so is not anticipated by a in application 3.

It is suggested that by considering the claim in this way, any issue of poisonous priority is overcome.

Conflicting Case Law

Acceptance of this ‘antidote’ is subject to the interpretation of Enlarged Board of Appeal decision G2/98, which concerns the situation in which a claim may have different priority dates. G2/98 held that ‘the use of a generic term or formula in a claim for which multiple priorities are claimed… is perfectly acceptable… provided it gives rise to claiming of a limited number of clearly defined alternative subject-matters’.

On the one hand, some Board of Appeal decisions (such as T1127/00, T1443/05, T1877/08, T0476/09) have interpreted this requirement in a narrow sense, to require that the claim itself defines this limited number of clearly defined alternative subject-matters, or that the clearly defined alternative subject-matters are explicitly identified as such in the application. Also, in the UK, a High Court decision of 2013 (Nestec v Dualit) appears to have followed this restrictive view. By interpreting G2/98 narrowly, it may not be possible to use the antidote as suggested above, because 1) formula A, excluding a; and 2) formula a are not explicitly presented as alternatives in the claim, or in the application as filed. Rather they are only theoretical alternatives.

On the other hand, some Board of Appeal decisions (notably T1222/11 and T571/10) have interpreted the requirement of ‘a limited number of clearly defined alternative subject-matters’ in a broad sense, such that a claim can be split theoretically without requiring the clearly defined alternative subject-matters to be spelt out as such in the application.

As a result of this divergent case law, in the Appeal proceedings for T557/13, the Board of Appeal recently announced its intention to refer one or more questions regarding partial priority to the Enlarged Board of Appeal. The questions to be referred have yet to be finalised, although this is likely to occur in the next few months. The questions will attempt to establish the extent to which partial priority can be claimed, and the resulting effect on poisonous priority.

We will keep you informed of developments on this matter. In the meantime, as poisonous priority remains a real risk, we recommend careful consideration when filing a later application that claims priority to an earlier application, especially if the later application provides for claims of broader scope than in the earlier application. There may be some measures that the applicant can take in a later application to reduce the risk of encountering poisonous priority issues.

If you are considering including new subject matter in an application, and that application is to claim priority from an earlier application, contact us.

This article is for general information only. Its content is not a statement of the law on any subject and does not constitute advice. Please contact Reddie & Grose LLP for advice before taking any action in reliance on it.

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