With the cost of developing a new drug estimated at $2.5 billion, repurposing existing drug compounds for new indications is of increasing interest. However, patent and regulatory regimes arguably provide lesser incentives for second medical uses and an uncertain framework for patent enforcement.

Pfizer, through its subsidiary Warner-Lambert, hopes to begin to redress the balance in a landmark patent appeal which is likely to be heard by the UK Supreme Court in Q4 this year. The case concerns Warner-Lambert’s patent for a second medical use of the drug pregabalin for the treatment of pain, which the Court of Appeal held to be partially invalid, affirming the decision of the High Court in favour of generic drug companies Actavis and Mylan.

Warner-Lambert’s key patent claims were held to be invalid for lack of “sufficiency of disclosure”, essentially a requirement that the patent discloses the invention in a manner sufficiently clear and complete for it to be carried out by a skilled practitioner. The main issue now before the Supreme Court is how the test for sufficiency should be applied.

A patent claim for a “first medical use” of a new drug covers any medical use of the drug, yet the patent meets the requirement for sufficiency if it discloses one such use. The courts applied a more stringent test in relation to Warner-Lambert’s patent, finding that, at the time of its filing, the patent did not make it “plausible” that pregabalin would be effective for treating all types of pain encompassed by the claims. The patent lacked evidence or a reasonably credible theory as to why central neuropathic pain could be treated. Only after the filing of the patent was pregabalin proven to be effective for treatment of central neuropathic pain, an important pain indication for which Pfizer’s blockbuster pregabalin drug Lyrica is marketed.

There is judicial precedent for a “plausibility” test to prevent broad and speculative patent claims. However, there is no basis for it in UK statute. The Supreme Court will decide whether (and what) role plausibility should play in the test for sufficiency, and whether a patent should be held insufficient for lack of plausibility even though it is in fact workable across the full scope of the claim. The Supreme Court will also decide whether later evidence can be used to fill any gaps.

Ultimately, Warner-Lambert’s goal is to establish the validity of key patent claims which it has asserted against suppliers of generic pregabalin. If the Supreme Court finds in Warner-Lambert’s favour, it would then have the power to remit the infringement issue for determination by the Court of Appeal.

A shift in the application of the law on sufficiency might make it easier to obtain enforceable patent protection for second medical uses, and could be relevant for deciding at what stage in a R&D programme a patent application should be filed.

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