High Court
Court of Appeal


Following the leading Supreme Court judgment in Regeneron v Kymab, and the subsequent clarification by Lord Justice Birss in the decision of Illumina v MGI, the Court of Appeal has continued the recent trend of refining the principles of sufficiency in UK patent law. In its recent decision in FibroGen v Akebia(1) (FibroGen) (with the leading judgment also given by Lord Justice Birss) the Court of Appeal provided further guidance in relation to claims with structural and functional features.

The Court overturned Lord Justice Arnold's decision at first instance,(2) holding that he had applied an unnecessarily high threshold for measuring the sufficiency of claims with structural and functional features. Such claims are a feature of many pharmaceutical patents, and the importance of this decision was acknowledged by Sir Christopher Floyd (agreeing with Lord Justice Birss):

on the issues of sufficiency, we are differing from a patent judge of enormous experience and distinction, and the issues addressed in this case are of importance to the patenting of inventions in this important area of technology.

The FibroGen decision considers how excessive claim breadth, including undue burden, should be applied in relation to a claim with a mixture of structural and functional features, and the principle that a skilled person must be able to identify substantially all compounds covered by the claim without undue burden.

This case is in the life sciences sector, but demonstrates a development in the law of sufficiency in the United Kingdom and there is no reason why it will not apply equally to all patent subject matter.


The principle of "sufficiency" provides that a patent may be revoked if "the specification of the patent does not disclose the invention clearly enough and completely enough for it to be performed by a person skilled in the art",(3) to ensure that the extent of the patent's disclosure corresponds with its technical contribution to the art. This is colloquially referred to as "classical insufficiency", and the relevant date for assessing that sufficiency is the time the patent application was filed.

There are other forms of insufficiency too that have been established by case law. "Excessive claim breadth", also known as "Biogen" insufficiency, occurs when a claim extends beyond the technical contribution of the patent, and so does not support the invention as claimed. "Uncertainty insufficiency" is another category of insufficiency and applies where the language used in the patent claims is so ambiguous that it renders the patent invalid for not disclosing the invention clearly enough.

Case law has also established the concept of "plausibility" in respect of insufficiency, among other areas of patent law. In the context of insufficiency, this requires that the assertion that the invention will work across the scope of the claim must be plausible or credible.

It is established case law in England and Wales that the question of whether the requirements of sufficiency are met concerns whether the skilled person can readily perform the invention over the whole area claimed without undue burden and without inventive skill.


FibroGen owns a number of patents, split into "Family A" and "Family B" patents. All the relevant patents concerned the use of inhibitors of an enzyme called "hypoxia inducible factor-prolyl hydroxylase" (HIF-PH) for treating various types of anaemia associated with kidney disease.

Akebia was seeking to clear the way for its HIF-PH inhibitor, vadadustat, which is used to treat chronic kidney disease (CKD) anaemia and which FibroGen alleged infringed its patents.

A key feature of the patents is that the claims combined structural and functional features, being the use of a heterocyclic carboxamide compound (selected from a group) that inhibits HIF-PH enzyme activity in the prevention of anaemia associated with CKD.

High Court

In the High Court, Lord Justice Arnold found that the Family A patents were infringed and were not obvious over the prior art, but lacked plausibility and could not be performed across the breadth of the claims without undue burden. Therefore, the Family A patents were found to be infringed but invalid for insufficiency (and would be "AgrEvo obvious" – in other words, the invention would not be inventive because it did not make a technical contribution to the art). The Family B patents were found to be invalid on the grounds of obviousness and for the same insufficiency reasons as Family A, but not infringed as there was no imminent threat to infringe.

In respect of claims which combine structural and functional features, Lord Justice Arnold's key finding on the test for insufficiency was that "the skilled person or team must be able to identify substantially all compounds covered by the claim without undue burden".(4)

In holding that the claims were insufficient for want of plausibility, Lord Justice Arnold held that the patents (implicitly) promised that substantially all compounds which satisfy the structural definitions in the claims will have therapeutic efficacy, but that this was not plausible. The skilled person would have no reason to think that substantially all compounds would be effective, nor would they be able to make a reasonable prediction to this effect.

In considering whether the invention could be performed across the scope of the claim without undue burden, Lord Justice Arnold held that it would require a substantial research project to identify compounds other than those specifically identified in the patents which met the criteria for efficacy. It was probable that, with sufficient resources, some compounds over and above the identified compounds would be identified, but this would only ever scratch the surface of what is required given the scope of the claims.

Court of Appeal

On appeal, FibroGen challenged the insufficiency (and AgrEvo obviousness) findings in relation to both patent families, and the obviousness findings in respect of Family B. FibroGen argued that the trial judge had misconstrued the claims and misapplied the law in respect of plausibility, and that the finding on undue burden was wrong in principle. Akebia challenged the finding on infringement in respect of Family A. It is the findings on insufficiency that are of interest and the Court of Appeal's decision provides clarification on plausibility and undue burden.

Insufficiency and reasonable prediction/plausibility
Lord Justice Birss, in his leading judgment, undertook a thorough analysis of the law on excessive claim breadth, plausibility, undue burden and uncertainty in his findings on insufficiency.

Plausibility findings and excessive claim breadth

Lord Justice Birss referred to the principles identified by Lord Justice Kitchin in respect of insufficiency in Regeneron v Genentech(5) and in particular the fourth principle, which states that an invention can be defined using general terms, provided that it can reasonably be expected that the invention will work with anything falling within the scope of these terms. It must be possible to make a reasonable prediction that the invention will work with substantially everything falling within the scope of the claim, or in other words, the assertion that the invention will work across the scope of the claim must be plausible or credible.

Lord Justice Birss proposed a three-stage test for plausibility, or what he said may be more appropriately called "reasonable prediction":

  • identify what it is that falls within the scope of the claimed class;
  • determine what it means to say that "the invention works" (ie, what is it for?); and
  • determine whether it is possible to make a reasonable prediction that the invention will work with substantially everything falling within the scope of the claim.

In applying this three-stage test, he found that:

  • the trial judge had not correctly identified the compounds falling within the claimed class, which was an error of principle and approach;
  • the problem to be solved meant treating the CKD form of anaemia; and
  • the correct question to be asked was whether it was possible to make a reasonable prediction (or was it plausible) that compounds which satisfy the structural features and which also satisfy the functional features of the claim will be useful to treat CKD anaemia. He held that, on the facts, it would be plausible to make such a prediction.

The Court of Appeal therefore overturned the High Court's findings on reasonable prediction/plausibility.

Undue burden findings
Lord Justice Briss also considered the correctness of the principle that the skilled person must be able to identify substantially all compounds covered by the claim without undue burden (Lord Justice Kitchin's fifth principle in Regeneron v Genentech). This principle states that it must still be possible to perform the invention across the scope of the claim without undue effort.

In doing so, Lord Justice Birss reviewed the German Supreme Court decision in Dipeptidyl-Peptidase-Inhibitoren,(6) which he said provided a "clear and persuasive authority" that the proposition that a skilled person must be able to identify substantially all compounds covered by the claim without undue burden is incorrect in principle. Functional language may mean that a relevant claim can cover compounds which have not yet been found so it cannot be the law that the functional language of a claim requires the skilled person to be able to identify substantially all compounds covered by the claim.

The Court of Appeal ultimately held that Lord Justice Arnold's approach – namely, that the skilled person or team must be able to identify substantially all compounds covered by the claim without undue burden – was wrong. It held that for claims with a functional feature which defines the claimed compounds, or a mix of such structural and functional features, "it must be possible, without undue burden, both to identify compounds which satisfy the relevant test, and to find out whether any given compound satisfies the test". The Court also held that "it is not necessary as a matter of law, for sufficiency (or for AgrEvo obviousness) . . . to establish that the skilled person can identify all or substantially all the compounds which satisfy the test".

This then begs the question: if not "substantially all compounds", how many is enough? This will depend on the facts of the case, but Lord Justice Birss commented as follows:

For claims of this type, it must be possible for the skilled person, without undue burden, to identify some compounds beyond those named in the patent, which are within the claimed class and therefore are likely to have therapeutic efficacy . . . . it must also be possible for the skilled person to work substantially anywhere within the whole claim . . . . So it must be possible for the skilled person, given any sensible compound within the structural class (or substantially any), to apply the tests without undue burden and work out if it is a claimed compound. (Emphasis added.)

In light of this, the Court of Appeal held that, on the evidence, although it would be a great deal of work, the skilled team would be able to find some compounds which were effective without undue burden. However, such work would be routine for the medicinal chemist and iterative in nature.

The Court of Appeal therefore overturned Arnold LJ's findings on undue burden and that the patents were insufficient on the ground of excessive claim breadth.

Sir Christopher Floyd also commented on undue burden, noting that he did not agree with Lord Justice Arnold's construction of the claim or approach that it would be necessary for the skilled person, as a cumulative task, to be able to identify substantially all of the compounds covered by the claim without undue burden. He stated that the test for undue burden is:

not the cumulative burden of identifying and testing for efficacy all the compounds covered by the structural formula, or even by the functional requirements of the claim. That is an objective which is almost bound to produce the response that the burden is excessive . . . . There are clear indications in the judgment that the judge approached the question of undue burden on a cumulative basis . . . if the judge had approached the question of undue burden correctly he would have found the patents met the test for sufficiency.

In respect of uncertainty, the Court of Appeal referred to the current status of the law as summarised in Anan Kasei v Neo Chemicals.(7) The issue was whether claims that included the term "a structural mimetic of 2-oxoglutarate" were conceptually uncertain and invalid.

The Court found that the lack of clarity in the patents as to the relevant potency of vadadustat could not on its own amount to conceptual uncertainty. This is a classic example of the difference between conceptual uncertainty and a fuzzy boundary. The remedy for a fuzzy boundary (where experts cannot agree on exactly where the claim boundary lies) would be that the patentee could not satisfy the court of infringement but it would be disproportionate in those circumstances to find the claim invalid.


This is an evolving area of patent law in the United Kingdom, and FibroGen provides a robust decision on sufficiency from the Court of Appeal. Notably, it is the second leading judgment from Lord Justice Birss within the past nine months to address sufficiency, and perhaps gives an indication that he sees a need to provide further clarity in respect of this principle. Certainly, this decision is likely to provide some certainty to those who draft and litigate patents, as well as to patent holders in the life sciences sector and beyond.

Specifically, the Court of Appeal's decision is positive from an innovator's point of view. Patents for chemical formulae when the therapeutic effect is included in the claims may be less vulnerable than Lord Justice Arnold's decision had previously implied. This may give pharmaceutical manufacturers increased confidence in the strength of such patents.

From a challenger's perspective, arguments that such patents claim inventions that are too broad, or the limits of which cannot be ascertained even through an extensive research project, will likely need to be more nuanced and different strategies to "clear the way" will need to be considered. Expert evidence will, of course, be key in these types of cases moving forward, and like with doctrine of equivalents analysis, this is another area of the law in which experts may need to be instructed at an early stage in proceedings.

This decision also brings the law in England and Wales into line with German jurisprudence, which the Court found highly relevant and influential. It is far too early to tell how UK case law will evolve post-Brexit, but this case may indicate that the UK judiciary does seem interested in aligning with wider European case law, and more convergence in decisions may be seen.

For further information on this topic please contact Nicole Jadeja, Sarah Taylor, Anna Harley or Josie Miller at Pinsent Masons by telephone (+44 20 7418 8250) or email ([email protected], [email protected], [email protected] or [email protected]). The Pinsent Masons website can be accessed at


(1) FibroGen Inc v Akebia Therapeutics Inc [2021] EWCA Civ 1279.

(2) Akebia Therapeutics Inc v FibroGen Inc [2020] EWHC 866 (Pat).

(3) Section 72(1)(c) of the Patents Act 1977.

(4) At paragraph [366] of the High Court decision.

(5) [2013] EWCA Civ 93.

(6) BGH X ZB 8/12.

(7) [2019] EWCA Civ 1646.