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The question of whether restrictions conceded during the prosecution history of a patent could be assessed during post-grant proceedings in the UK was decisively answered in the recently published Court of Appeal decision in the case of Actavis v Eli Lilly (full judgment available here.).

In some jurisdictions, such as the USA, it is normal for courts hearing patent cases to refer to the prosecution history of the application, that is, the correspondence between the applicant and the patent office, when interpreting the claims of the patent (the so-called "file wrapper estoppel"). In European jurisdictions, however, the prosecution history is generally regarded as not being useful for interpretation of patent claims. Since the UK House of Lords addressed the question in Kirin-Amgen v Hoechst Marion Roussel (full judgment available here.), the question of whether British Courts would study the prosecution history of a patent in post-grant proceedings appeared to be settled. Specifically, the House of Lords took a view that the use of a patent office file should be discouraged, or even prohibited, when construing a claim, since the meaning of the patent as granted should not change according to whether or not the notional person skilled in the art studied the patent office file, and that “life is too short” for the limited assistance that studying this file could provide. This approach was broadly in line with the approach of the Courts in the Netherlands, Germany and the EPO.

However, last year the High Court decision in the case of Actavis v Eli Lilly (full judgment available here) gave rise to an interesting suggestion - that some form of file wrapper estoppel could indeed be employed in post-grant proceedings before the UK courts.

In this High Court case, the parties advanced different interpretations as to how the term "pemetrexed disodium" (used as a cancer treatment) would be interpreted by the person skilled in the art. The alleged infringer advanced a narrow interpretation that pemetrexed disodium would be interpreted as pemetrexed disodium and nothing else, whilst the patent proprietor advanced a broader interpretation that pemetrexed disodium should be interpreted to cover equivalents to this compound: namely, any salt of pemetrexed that was pharmaceutically acceptable and sufficiently soluble. A statement was found in the prosecution history of the patent that the claims were limited to pemetrexed disodium as such. The alleged infringer argued that the existence of this statement prevented the proprietor from advancing any other interpretation at trial. The patent proprietor argued that, as a general rule, the prosecution history was rarely useful as an aid to interpretation and that the present case was no exception to this general rule.

The High Court, in judgment, stated that inspection of the patent office file may be of use in resolving cases where patent proprietors "abuse the system" by accepting a narrow interpretation of claims during prosecution and then arguing for a broad construction of those same claims for the purpose of infringement. The High Court then decided that, in the light of the evidence on the patent office file, the claim had to be limited to pemetrexed disodium as such, and could not embrace equivalents. Hence, there was no infringing activity of the claims.

However, this line of reasoning was rejected by the Court of Appeal. The Court of Appeal instead reasoned that, even if a statement in the prosecution history was discovered showing that the patent proprietor accepted a restriction to a claim, it was always open to the patent proprietor to say that he need not have done so, and that the apparent concession he made in prosecution was wrongly made. As such, the prosecution history was considered to provide negligible assistance in interpreting the claims, in line with the earlier consensus established by Kirin-Amgen. The general principle, that the prosecution history was rarely useful as an aid to interpretation of the claims of the patent, was held to stand.

It therefore seems that, when it comes to inspecting the prosecution history to assist in post-grant claim construction, life will remain too short for the British Courts.

Peter Lucas and Mark A G Jones