The Edges of Mayne Pharma explored

An important aspect of the English common law system is the ability of courts to adapt the rules in order best to achieve justice.  Sometimes these new developments are tagged with the name of the case in which they first arose, or first arose to prominence.   Anton Piller orders (now enshrined in the rules as “search and seizure orders”), Arrow declarations and Norwich Pharmacal relief are just three examples.

Another tag used in patent litigation in the English Courts is “Mayne Pharma disclosure”.  The concept of Mayne Pharma disclosure can be traced back to the 2006 decision of Pumfrey J in a dispute relating to patents for oxaliplatin anti-cancer formulations.  The issue before the Court was whether if a party relied upon an experiment to prove a fact, it implicitly waived legal professional privilege regarding the underlying background to the experiment, preliminary trials, and "work-up" documents.  Pumfrey J held that in the interests of justice, it was appropriate for the court and the opposing party to be provided with the full picture of the testing rather than cherry-picked results, thus preventing the party relying on the experiment from concealing failed or preliminary trial runs.

Although the UK is somewhat of an outlier with respect to the need to disclose “work-up” documents, it is hard to disagree with Pumfrey J’s conclusions, particularly in relation to allegations of anticipation by inevitable result (as was the case in Mayne Pharma).  To take a hypothetical and deliberately simplified example, imagine a patent claiming a composition “Z” and defining it as having a melting point of “T” degrees Celsius.  A prior art document (“P”) describes a composition formed by combining “X” and “Y” with “heat”.  It does not name the composition as “Z” and nor does it provide a melting point, but a challenger suspects strongly that the prior art is in fact describing Z and that the composition will have the claimed melting point.  It therefore sets about reproducing P with a view to proving that the patent lacks novelty or is at least obvious.  It assigns a team of scientists to the task.  The team struggles to make progress given the lack of precision in P.  The first few attempts are complete failures.  However, after a dozen attempts, the team realises that if the combination of X and Y is stirred continuously for 3 hours under a gentle steam heat rather like a bain-marie, a composition is formed.  However, once it is taken off the heat, the composition does not solidify at all at room temperature.  It is only when by chance a container is left open for a weekend that the composition solidifies and when that is tested, the melting point is less than T degrees Celsius.  A few attempts later, the team discovers that if a little more X and a little less Y is added to the mixture, a composition is obtained with the characteristics of Z and including a melting point of T degrees Celsius.   The team is now able consistently to produce batches of Z with the right characteristics with ease.

Before 2006, the legal team for the challenger would have been able to annex the finalised results to their Notice of Experiments to support an allegation that P anticipated the patent on the basis that the inevitable result of reproducing the teaching of that document led to a product claimed by the patent.  The trouble it took the team to reproduce the teaching of P and to obtain a product with the appropriate characteristics could be swept under the carpet. 

Following the decision in Mayne Pharma, such a course of action would no longer be possible.  The essence of Mayne Pharma is that if a party wants to rely on an experiment, it must disclose documents relating to the work-up experiments leading to that experiment.

Any experienced English patent lawyer will indicate that there is one, and only one, golden rule to experiments in litigation – don’t rely on them unless you absolutely have to.  They will cost a fortune, run the risk of failure, especially when the other side’s legal team are witnessing a repetition and, often, be found to be irrelevant by the Court.  The Mayne Pharma principles only added to this long list of reasons why any plans to rely on experiments should be shelved in almost every case.    

It is therefore perhaps unsurprising that judgments on the scope of Mayne Pharma disclosure are few and far between.  In the two decades since, there have been but a small handful of judgments including Magnesium Elektron v Neo Chemicals in 2017 and Safestand v Weston in 2023.

In a decision dated 20 July 2026 in the litigation between Merck Sharp & Dohme (MSD) and Halozyme concerning Halozyme’s patents to modified human PH20 hyaluronidase, Mellor J had the opportunity to reconsider the scope of Mayne Pharma disclosure.  Although confirming that the principles remain part of English patent litigation, the Judge was clear that the rules do not provide a basis for broad disclosure of privileged materials that may relate in some way to the same case.  Rather, their purpose is to prevent selective deployment of privileged experimental material (as exemplified in the hypothetical example above).

On the basis of various explanations provided in the run-up to the hearing, Mellor J was satisfied that the information provided by Halozyme was adequate to discharge its obligations under Mayne Pharma.  MSD’s application was therefore rejected.

The case moves forward to a trial in late November 2026.  Given that Meade J is to be promoted to the Court of Appeal in the autumn, and given the technical complexity rating of the case (4 out of 5 on a scale of increasing complexity), it looks likely that Mellor J will hear the trial unless it is assigned to a newly appointed Patents Court judge expected to be appointed in the coming weeks.