To deny inventive step for novel chemical compounds because of their structural similarity to known chemical compounds amounted to an allegation that a skilled person would have reasonably expected the same or similar usefulness of both the known and the novel compounds as the means for solving the technical problem underlying the application in question. Such an expectation would be justified, if the skilled person knew, be it from common general knowledge or from some specific disclosure, that the existing structural differences of the chemical compounds concerned were so small that they would have no essential bearing on those properties, which were important for solving the said technical problem and could be disregarded (T 852/91, see also T 358/04).

In T 643/96 the board held that the concept of bioisosterism did form part of the common general knowledge of those skilled in the art, but that it had to be applied with caution when deciding upon inventive step. In the field of drug design, any structural modification of a pharmacologically active compound was, in the absence of an established correlation between structural features and activity, expected a priori to disturb the pharmacological activity profile of the initial structure. This also held true for an alleged case of bioisosterism, which was one option of a structure-activity relationship, as long as it was not an established case of bioisosterism (see also T 548/91). In T 643/96 it was held that, when deciding upon inventive step in relation to pharmacologically active compounds, what was essential was not whether a particular substructure of a chemical compound was replaced by another known isosteric one, but whether information was available on the impact of such a replacement on the pharmacological activity profile of the specific (group of) compound(s) concerned (see also T 467/94, T 156/95).

In T 2402/10 the board stated that in the field of drug design any structural modification of a pharmacologically active compound is, in the absence of an established correlation between structural features and activity, a priori expected to disturb the pharmacological activity profile of the initial structure (see T 643/96, T 548/91).

In T 930/94 the board held that knowledge of the fact that one specific member of a class of chemical compounds did not lead to the effect achieved by several other members of this class, did not, without additional indications, mean that such an effect could be attributed to all the compounds in this group. In such circumstances, the effect in point did not lead to a recognition of the existence of a technical concept that could be generalised (T 641/97, T 209/98, T 853/03).

In T 989/93 the board stated that, in the absence of the appropriate common general knowledge, no conclusions are possible on the basis of the known properties of one group of chemical compounds (here: benzene derivatives) regarding the properties of a different group of chemical compounds (here: naphthalene derivatives).