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The potentially very large number of compounds covered by a Markush formula can present problems when determining:

• the exact scope of the claim;
• whether all the compounds within it are enabled (whether or not it is sufficient); and
• whether it anticipates later patents for smaller collections of compounds that fall within its scope (see 2.3 below).

When determining the scope of the claim, the English courts are required to "don the mantle of the skilled reader" of the claim, at the priority date, and interpret it according to how it would be understood by that skilled reader. To assist them, the skilled reader can use his/her "common general knowledge" as at the priority date. The courts are, however, also required by the Protocol on the Interpretation of Article 69 of the European Patent Convention (the Protocol) to adopt an approach to claim construction that balances the interests of fair protection for the patentee and certainty for third parties. According to the House of Lords in Kirin-Amgen Inc v Hoechst Marion Roussel Ltd & Ors,¹ the Protocol should be interpreted to mean that a claim can include equivalent embodiments of the invention that do not fall literally within the scope of the claim, in circumstances where this is how it would be understood by a skilled reader.

In Pharmacia Corp v Merck & Co Inc [2001] EWCA Civ 1610, the patent in suit claimed a class of compounds using the following Markush formula:

Importantly, for the purposes of this case, X could be a hydroxy group substitution, but not a keto group. The active ingredient of the allegedly infringing product, Vioxx, which was sold by Merck, was 4-(4-methylsulphonylphenyl)- 3-phenyl-5H-furan-2-one (known as MK-966). This is represented in the following formula:

As accepted by both parties in the case, a compound of the formula of MK-966 would not literally infringe the claim, because it contains a keto substitution rather than a hydroxy substitution. It was, however, established that in aqueous solution MK-966 undergoes tautomerisation in equilibrium between keto and hydroxy substituted tautomers (called the furanone and the hydroxyfuran, respectively):

The question for the court was therefore whether MK-966 infringed that claim because the compound covered by the claim would be understood as a tautomer of the furanone. The Court of Appeal held that the skilled reader would know from his/her common general knowledge that the furanone will tautomerise in aqueous solution into the hydroxyfuran, that the two compounds would be inseparable in solution, and therefore that the reference to one would include the other. Hence, the furanone would fall within the claim. A factor in the decision, which illustrates the importance of the patent specification as a whole in interpreting the claims, was that the court took into account the statement in the patent that the compounds of the claimed class are said to be active in the body. Thus, to give fair protection to the patentee, the court held, it is reasonable to take into account the existence of the compound in the body, and it is the composite of both forms that have effect.

The influential AgrEvo/Triazole sulphonamides (T939/92) [1996] EPOR 171 decision of the Technical Boards of Appeal (TBA) at the European Patent Office (EPO) illustrates another problem that can be encountered when a patent claims a large class of chemical compounds. The patent at issue in AgrEvo claimed a class of compounds useful as herbicides. On appeal, the patent was held sufficient by the TBA because it had been established that all of these compounds could be made. Making the compounds was not inventive, however; instead, such invention as there was must lie in the compounds' herbicidal properties. The description did not support the assertion that all the compounds claimed were herbicidal. The technical problem of the prior art was therefore not solved across the claim and there could be no invention.  The patent was therefore held to be invalid for obviousness.

In the UK, the AgrEvo decision has more typically been applied as identifying a problem of insufficiency:  while it would not be necessary to demonstrate that each and every compound claimed by a Markush formula has the properties disclosed in the patent, it must be plausible that the technical contribution is common to substantially all of the compounds in the claim or the claim will be insufficient.

This article is an extract from the forthcoming second edition of Intellectual Property in the Life Sciences, published by Globe Law and Business.

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