Plausibility: the ground that has felled UK compound patents, explained for commercial readers

The idea in one paragraph

A patent is a bargain: the applicant discloses an invention that works, and gets twenty years of exclusivity for it. If the application, read on the day it was filed, gives no reason to believe the invention works, there is no contribution to disclose and the bargain fails. “Plausibility” is the name English law gives to that threshold. A patent that claims a compound as a medicine must make it at least plausible, from what it says and what the skilled reader already knew, that the compound has the claimed activity. Later evidence that it does work, however good, cannot repair an application that gave no reason to expect it.

Why it has mattered so much in the UK

Compound patents for blockbuster medicines are typically filed early, from a research programme, claiming thousands of compounds by a general formula and naming a few. Some of those applications contain data; some contain only a statement that the compounds are useful for a disease. When the one compound that became the medicine is later carved out into a narrower claim, the question is whether the original application made that compound’s activity plausible.

In apixaban, the Court of Appeal in May 2023 agreed with the Patents Court that the application, which listed apixaban among a vast number of compounds with no data distinguishing it, did not. The patent fell, the UK SPC fell with it, and UK generics launched in 2022 while the same product remained protected across most of Europe for four more years.

In dapagliflozin, the Patents Court in April 2025 and the Court of Appeal in July 2025 reached the same result on similar facts: the application did not make it plausible that dapagliflozin was an SGLT2 inhibitor or a treatment for diabetes, so the patent and both SPCs fell, three years before their register dates.

In enzalutamide, by contrast, the Court of Appeal in July 2025 upheld the patent: the application contained enough to make the compound’s activity plausible, and the SPC date stood. The ground is not a magic word; it depends on what the document says.

The test, as the courts state it

The UK Supreme Court set the standard in 2018 in the pregabalin case (a second-medical-use patent): the specification must disclose a reasonable scientific basis for the claimed effect, judged through the eyes of the skilled person with the common general knowledge, as at the filing date. A bare assertion is not enough; data, a credible mechanism, or a reasoned extrapolation from what was known can be. The Court of Appeal has since applied that standard to compound claims as such, and has twice declined to soften it.

Why the EPO reaches different answers

The EPO’s Enlarged Board of Appeal addressed the same territory in March 2023 in G 2/21. It avoided the word “plausibility” and asked instead whether the skilled person, reading the application with the common general knowledge, would see the technical effect as encompassed by the technical teaching and embodied by the same invention as originally disclosed; where that is so, evidence published after filing may be relied on to prove the effect. In practice the EPO’s threshold has proved lower than the English one for the same documents, and the English courts have said that G 2/21 does not require them to change course. The result is a European patent that survives opposition in Munich and falls in London, and a product that is generic in the UK years before it is generic in Germany.

Which patents are exposed

• Early compound patents with little or no data, especially where the medicine is one compound in a huge claimed class.
• Selection patents that pick a compound or a form out of an earlier genus on the strength of an asserted advantage.
• Second-medical-use patents that assert a new indication from a hypothesis.
• Not patents whose application reports the relevant experiments, however briefly, nor patents on products whose activity was predictable from known structure-activity relationships. Enzalutamide is the counter-example to apixaban.

A validity attack on plausibility can be mounted on the documents alone, which is why it has become the first thing a generic’s advisers read when a UK launch is being planned.

What it means for dates

For a generics or launch planner. The UK is the jurisdiction where a plausibility case changes the date; the English courts will list a validity trial in time for a launch, and a win removes the compound patent and the SPC together. The same case run at the EPO or in Germany may fail, so the plan has to be country by country.

For a parallel trader. A UK revocation moves the UK date only; the German, French and Dutch SPCs stay on the register and in force. Importing a product that is generic in the UK into a country where the SPC stands is still infringement there. Equally, a UK generic launch collapses the UK price, which changes the economics of importing originator stock into the UK.

For a buyer. A product with a plausibility case pending in the UK can lose exclusivity on a judgment date rather than a register date; the case notes list the ones in play. The price mechanics after that follow the usual Category M path.

MPA Search carries the register dates; the case notes carry the judgments that move them.