CA-2025-001040 - [2025] EWCA Civ 903

The law concerning plausibility

Article 52(1) of the European Patent Convention (“EPC”) provides that European patents “shall be granted for any inventions” provided that (among other things) they “involve an inventive step”. Article 56 provides that an invention “shall be considered as involving an inventive step if, having regard to the state of the art, it is not obvious to a person skilled in the art”. Article 83 requires that an application for a European patent “shall disclose the invention in a manner sufficiently clear and complete for it to be carried out by a person skilled in the art”. Article 100 provides the grant of a European patent may be opposed on grounds that include lack of patentability under Article 56 and failure to comply with Article 83. Article 138(1) provides that a European patent may be revoked with effect for a Contracting State by the courts of that State on grounds that again include lack of patentability under Article 56 and failure to comply with Article 83. Sections 1(1)(a), 3, 14(3) and 72(1) of the Patents Act 1977 give effect in the United Kingdom to Articles 52(1), 56, 83 and 138(1) EPC. None of those provisions mentions the criterion of plausibility. It has been developed through the case law initially of the Boards of Appeal of the European Patent Office and latterly of the courts of the Contracting States including the UK.

I traced the development of the law in my judgment in Sandoz v BMS at [7]-[42]. It is pointless to repeat the exercise, and so I will take that exposition as read and only recapitulate the key points. Given the arguments on the present appeal, however, it is worth putting the law into context by first explaining a fundamental aspect of patent law.

First to file

So far as I am aware, all patent laws in the world now operate on the “first to file” basis. (The USA used to be an exception, but that has ceased to be the case since the America Invents Act 2011). Without descending into detail, this essentially means that the first person to file an application to patent a particular invention is entitled to any resulting patent to the exclusion of any later applicant. Thus the date on which the application is filed, or the priority date if earlier, is critical. (The priority date is the date of an application by the same applicant in respect of the same invention filed up to one year earlier. The priority system enables an applicant to file an initial application in one country followed by multiple subsequent applications in other countries.) It is by reference to that date that issues of patentability are assessed. Thus it is by reference to the state of the art at that date that it is determined whether the claimed invention meets the core requirements of novelty, inventive step and industrial application. And it is by reference to the common general knowledge of the person skilled in the art at that date that the patent is interpreted, and that inventive step and sufficiency of disclosure are assessed. It is a cardinal principle of patent law that hindsight should, so far as is humanly possible, be excluded from these assessments. As has often been pointed out, it would be unfair to inventors if inventive step were to be assessed with hindsight, since many inventions appear obvious after the event.

It can readily be seen that it would undermine this system if applicants for patents were permitted to add information (what patent lawyers call “subject-matter”) to the information that is contained in their applications after the application date or, where applicable, the priority date. For this reason, patent laws typically contain substantive rules which are designed to prevent this. In the case of the EPC, the key two rules are Article 87(1), which provides that priority can only be claimed in respect of “the same invention”, and Article 123(2), which provides that “[a] European patent application or a European patent may not be amended in such a way that it contains subject-matter which extends beyond the content of the application as filed”. These provisions are implemented in the UK by sections 5(2) and 76 of the 1977 Act. The jurisprudence both of the Boards of Appeal and of national courts establishes that these rules are strict ones.

It would also undermine this system if applicants were permitted to rely upon information (other than common general knowledge) that is not present in the application (or priority document) when filed as establishing the patentability of the invention. That would also be objectionable for another reason, namely that it would be contrary to the “patent bargain” discussed by Lord Sumption in Generics (UK) Ltd v Warner-Lambert Co LLC [2018] UKSC 56, [2019] Bus LR 360 (“Warner-Lambert”) at [17]. In short, this is the principle that the reward of a 20 year monopoly is bestowed on the proprietor of a patent in return for the disclosure of the invention to the public in the patent application.

It does not follow from the principles discussed in the preceding paragraphs that, in the event of dispute, evidence dating from after the relevant date cannot be relied upon as establishing the position as at that date. In this jurisdiction the court receives evidence from suitably qualified expert witnesses whose primary function is to educate the court as to the common general knowledge of the skilled person or team at the relevant date, and hence to inform the court of any technical considerations which are material to the issues it has to consider. Such evidence is necessarily given after the event, and frequently long after the relevant date, but the experts are instructed to try to avoid hindsight. Furthermore, evidence as to subsequent events can cast light on the position as at the relevant date. It is sufficient to give three examples of this. First, the reaction of the scientific community when an invention is first made public may help to show that it was not obvious. Secondly, in rare cases, evidence that the invention has achieved commercial success by fulfilling a “long-felt want” may also help to show the same thing. Thirdly, evidence of subsequent attempts to perform the invention (or evidence that can be treated as a proxy for such evidence) may be relied upon to show that the disclosure in the application was insufficient.

There is, however, a fundamental distinction between relying upon later evidence to cast light on the position as at the relevant date on the one hand, and on the other hand permitting patent applicants and proprietors to rely upon information that could not have been available to the skilled reader of the patent application at the relevant date, either because it did not yet exist or because it was secret, as establishing the patentability of an invention.

Warner-Lambert

It is common ground that the decision of the majority of the Supreme Court in Warner-Lambert is authority binding on this Court as to the standard to be applied when assessing the sufficiency of disclosure of a medical use invention. (The claimed invention in Warner-Lambert was a second medical use, but AstraZeneca does not suggest, at least in this Court, that a distinction can be made between first and second medical use claims.) The majority adopted a standard which has subsequently come to be referred to as “ab initio plausibility”, meaning in essence that the application when read together with the common general knowledge must positively make it plausible that the invention will achieve the claimed technical effect. The minority preferred a standard which has subsequently come to be referred to as “ab initio implausibility”, meaning in essence that the application when read together with the common general knowledge should not give rise to doubt as to whether the invention will achieve the claimed technical effect.

For present purposes it is sufficient to cite two passages in the judgment of Lord Sumption for the majority. The first passage sets out the principle (emphases and line breaks added in [37]):

The principle is that the specification must disclose some reason for supposing that the implied assertion of efficacy in the claim is true. Plausibility is not a distinct condition of validity with a life of its own, but a standard against which that must be demonstrated. Its adoption is a mitigation of the principle in favour of patentability. It reflects the practical difficulty of demonstrating therapeutic efficacy to any higher standard at the stage when the patent application must in practice be made. The test is relatively undemanding. But it cannot be deprived of all meaning or reduced … to little more than a test of good faith.

Plausibility is not a term of art, and its content is inevitably influenced by the legal context. In the present context, the following points should be made.

First, the proposition that a product is efficacious for the treatment of a given condition must be plausible.

Second, it is not made plausible by a bare assertion to that effect, and the disclosure of a mere possibility that it will work is no better than a bare assertion. ….

But, third, the claimed therapeutic effect may well be rendered plausible by a specification showing that something was worth trying for a reason, ie not just because there was an abstract possibility that it would work but because reasonable scientific grounds were disclosed for expecting that it might well work. The disclosure of those grounds marks the difference between a speculation and a contribution to the art. This is in substance what the Technical Board of Appeal has held in the context of article 56, when addressing the sufficiency of disclosure made in support of claims extending beyond the teaching of the patent. In my opinion, there is no reason to apply a lower standard of plausibility when the sufficiency of disclosure arises in the context of EPC articles 83 and 84 and their analogues in section 14 of the Patents Act. In both contexts, the test has the same purpose.

Fourth, although the disclosure need not definitively prove the assertion that the product works for the designated purpose, there must be something that would cause the skilled person to think that there was a reasonable prospect that the assertion would prove to be true.

Fifth, that reasonable prospect must be based on what the TBA in Salk (para 9) called ‘a direct effect on a metabolic mechanism specifically involved in the disease, this mechanism being either known from the prior art or demonstrated in the patent per se.’

Sixth, in Salk, this point was made in the context of experimental data. But the effect on the disease process need not necessarily be demonstrated by experimental data. It can be demonstrated by a priori reasoning. For example, and it is no more than an example, the specification may point to some property of the product which would lead the skilled person to expect that it might well produce the claimed therapeutic effect; or to some unifying principle that relates the product or the proposed use to something else which would suggest as much to the skilled person.

Seventh, sufficiency is a characteristic of the disclosure, and these matters must appear from the patent. The disclosure may be supplemented or explained by the common general knowledge of the skilled person. But it is not enough that the patentee can prove that the product can reasonably be expected to work in the designated use, if the skilled person would not derive this from the teaching of the patent.”

In the second passage Lord Sumption rejected Warner-Lambert’s argument that the courts below were wrong to reject experimental data generated after the relevant date as establishing the plausibility of the claimed technical effect for the reasons he gave at [40]:

“This submission also is contrary to the legal basis of this particular head of insufficiency. We know that pregabalin works for the treatment of … neuropathic pain, because like any other medicament on the market, it underwent demanding clinical trials after the priority date, the results of which were made public. On that basis it received marketing authorisation for … neuropathic pain. This is always the case for a commercially valuable medicament, and no other kind will be worth litigating about. The question is not whether it works but whether the contribution to the art consisting in the discovery that it can be expected to work has been sufficiently disclosed in the patent. The inherent difficulty of demonstrating this before clinical trials is taken into account in the modest standard (ie plausibility) which is applied to test it. … This does not mean that subsequent data is never admissible in a dispute about sufficiency, but the purpose for which it is admitted is strictly limited. Where the asserted therapeutic effect is plausible in the light of the disclosure in the patent, subsequent data may sometimes be admissible either to confirm that or else to refute a challenger’s contention that it does not actually work… But it cannot be a substitute for sufficient disclosure in the specification.”

Sandoz v BMS

The principal legal issue in Sandoz v BMS was whether the same standard as that adopted in Warner-Lambert was applicable to the assessment of both sufficiency and inventive step in the case of a claim to a single chemical compound i.e. a pure product claim. This Court held that it was for the reasons I gave at [92]:

“… It is true that, as Lord Sumption noted at [23], the concept of plausibility originated as a response to over-broad claims, in particular claims to whole classes of compounds, as in Agrevo. Idenix is an example of its application in that context by the courts of this country. It is also true that, as Lord Sumption noted at [19]-[20], that the concept was also found to be of utility in addressing one of the problems with second medical use claims. Nevertheless the concept was applied by the Board of Appeal to a claim to single compound in BMS/Dasatinib, which was one of the cases relied upon by Lord Sumption (and one of the cases reviewed by the Enlarged Board in G 2/21). As the Claimants point out, the present case is strikingly similar to BMS/Dasatinib. Moreover, BMS/Dasatinib does not stand on its own, because the claim in Johns Hopkins, which was another of the cases relied upon by Lord Sumption and reviewed by the Enlarged Board, was effectively a claim to a specific molecule. The concept has also been applied by this Court in Generics v Yeda to a claim to what was in substance a single product, albeit a product comprising a mixture of polypeptides. Furthermore, the underlying principles are applicable as much to claims to single chemical compounds as to claims to classes of compounds and second medical use claims. The fundamental principle is that the scope of the patent monopoly must be justified by the patentee’s technical contribution to the art. This remains so whether the scope of the claim is broad or narrow. Thus when considering inventive step it is necessary to consider what technical problem the claimed invention solves. If it is not plausible that the invention solves any technical problem then the patentee has made no technical contribution and the invention does not involve an inventive step. Equally, when considering insufficiency it is necessary to consider whether the specification sufficiently discloses the claimed invention. If it is not plausible that the invention solves any technical problem then the patentee has made no technical contribution and the specification does not disclose any invention. It follows that, in order for a claim to a single chemical compound to be patentable, the application must make it plausible, when read in the light of the skilled person’s common general knowledge, that the compound has the utility asserted for it. Moreover, it makes no difference whether the claim incorporates the use of the compound as a technical feature or whether the claim is simply to the compound per se and the assertion of utility is only to be found in the specification. This is because, as explained above, there is no invention in merely identifying a new chemical compound; invention can only lie in identifying its utility.”

It should be noted that in that case it was common ground between the parties, as I explained at [4], that it made no difference whether the issue was viewed as one of inventive step or one of sufficiency. As explained in more detail below, this is not common ground in the present case.

BMS applied to the Supreme Court for permission to appeal on two grounds. Ground 1 was that Warner-Lambert was not of general application, but was confined to the special case of second medical use claims. Ground 2 was that the Court of Appeal had erred in its approach to the standard required by Warner-Lambert. By order dated 31 October 2023 the Supreme Court (Lord Reed, Lady Rose and Lord Stephens) refused permission to appeal “because the application does not raise an arguable point of law”. Paragraph 3.32 of Supreme Court Practice Direction 3 states that “[t]he reasons for refusing permission to appeal should not be regarded as having any value as a precedent”. Nevertheless, this Court cannot ignore the fact that the Supreme Court considered that the grounds were not sufficiently arguable to justify the grant of permission to appeal.