Second medical use patents have taken a blow with the Supreme Court’s recent ruling in Warner-Lambert v Mylan and Actavis. The ruling that Warner-Lambert’s patent for a second medical use of pregabalin for the treatment of pain is invalid and not infringed establishes legal principles of general application that will disappoint innovators.

The infringement suit arose when Warner-Lambert, a subsidiary of Pfizer, asserted its patent against Actavis, which had marketed a generic version of pregabalin in the UK. Although Actavis’ product was marketed with a “skinny label” which identified only the indications for which patent protection had expired, Pfizer’s fears, later confirmed, were that generic pregabalin might be dispensed against prescriptions for patients suffering from neuropathic pain, depriving Pfizer’s branded drug Lyrica of market share. Both the High Court and the Court of Appeal had held the patent to be invalid, in revocation proceedings initiated by Mylan. The Supreme Court, dismissing Warner-Lambert’s appeal, has confirmed the finding of invalidity, and found that had key patent claims been valid, they would not have been infringed by Actavis.

The Supreme Court’s decision approves “plausibility” as a non-statutory test for sufficiency of disclosure, and defines its role in establishing validity of a patent claiming a new medical use of a known medicament. The Court’s opinion on the correct test for infringement of Swiss-form second medical use claims reveals a divergence of views, with the majority emphasising the importance of the outward presentation of the marketed product.

Sufficiency of disclosure is essentially a requirement that the patent discloses the invention in a manner sufficiently clear and complete for it to be performed by a skilled practitioner. The concept of a “plausibility” test originally arose at the European Patent Office (EPO) in order to avoid speculative claiming, and the Supreme Court has now approved this principle as requiring disclosure in the patent of more than mere assertion that a known compound is efficacious for the claimed treatment.

Lord Sumption, who gave the lead judgment, identified in the EPO and UK case law the “fundamental principle” that the patentee must not only make but disclose a contribution to the art, that contribution consisting in the discovery that the compound can be expected to work. Subsequent data may supplement this, but only if the therapeutic effect is plausible in the light of the disclosure in the patent.

As for how high to set the bar for plausibility, the majority view of the Court was that disclosure of a mere possibility of efficacy would not be enough, but the disclosure of reasonable scientific grounds for expecting that a medication might work may suffice. If the condition identified in a second medical use claim embraces a number of different pathologies, the disclosure should make plausible the efficacy of the product to treat them all.

The Court was unanimous in finding that the experiments disclosed in Warner-Lambert’s patent did not render plausible the treatment of central neuropathic claim. By a majority, the Court held that efficacy in treatment of peripheral neuropathic pain was not plausible, but dissenting opinions considered the Court to have set the plausibility bar too high, and would have found in favour of the patentee.

The ruling highlights the importance of including in a patent application the technical rationale for why experimental data make an invention plausible in relation to all significant areas of a second medical use claim.

Warner-Lambert’s second medical use claims were of the Swiss-form, which protect the process of “manufacture” of a medicament containing the specified substance only insofar as that process is undertaken “for” treating the specified indication. The High Court and Court of Appeal found that the intention of the manufacturer of a generic medicine was relevant to infringement, either on a subjective basis or on the basis of reasonable foreseeability that the product will intentionally be used for the patented indication.

By a majority view, the Supreme Court disagreed, emphasising the importance of the “outward presentation” of the generic product, particularly whether the patient information leaflet carves out the patented indication. A test of intention on behalf of the manufacturer would be unfair to downstream parties including distributors and pharmacists who could unwittingly incur liability for dealing in infringing products. A test based on the outward presentation of the product would be less likely to deter downstream parties from legitimate dealings in generic medicines. However, greater legal certainty would come at a cost for innovators, and a minority view of the Court preferred a test of subjective intent for this reason.

Because Warner-Lambert’s patent is held to be invalid, the Court’s views on infringement are not binding. However, the majority view is likely to be persuasive, and it may be that, in most cases, a generic product which does not identify in its packaging, labelling or patient instruction leaflet any potential uses which are subject to patent protection would not infringe a Swiss-form second medical use claim.

It is worth noting that second medical use inventions must now be claimed at the EPO in the form of purpose-limited product claims, not Swiss-form claims, and the test for infringement will necessarily differ. There is the possibility that purpose-limited product claims may provide for better enforceability; however, that is a matter that can only be addressed by future rulings or legislation.

Sheena Linehan is a patent attorney at IP law firm Potter Clarkson.