On Scaffolds and Hopping in Medicinal Chemistry

The molecular scaffold is an oft-cited concept in medicinal chemistry suggesting that the definition of what makes a scaffold is rigorous and objective. However, this is far from the case with the definition of a scaffold being highly dependent on the particular viewpoint of a given scientist. It follows, therefore, that the definition of scaffold hopping and, more importantly, the detection of what constitutes a scaffold hop, is also ill-defined and highly subjective. Essentially, it is agreed that scaffolds should be substantially different from each other, although significantly similar to each other, to constitute a hop. In the latter, the scaffolds must permit a similar geometric arrangement of functional groups to permit the mode of action. However, this leaves the paradox of how to describe both scaffold similarity and dissimilarity simultaneously. In this paper, the current statuses of scaffolds and scaffold hopping are reviewed based on published examples of scaffold hopping from the literature. An investigation of the degree to which it is possible to formulate a more rigorous definition of scaffolds and hopping in the context of molecular topologies is considered. These techniques are adapted from chemoinformatics to be applied in the design of new medicinal compounds.