Activity cliffs and structural cliffs for categorical responses

In the last ten years, the concept of activity cliff gained a lot of interest by medicinal chemists and QSAR modellers. The reason of this interest is its capacity to highlight problematic as well as interesting situations in analysing the activity/property of pairs of compounds, thus focusing the research on the reasons why two similar compounds show different activities. In the literature, activity cliffs as well as their dual aspect, i.e. structural cliffs, are calculated from activities represented by a continuous response. In this paper, the proposal is to estimate the activity cliffs of categorical responses, i.e. for compounds partitioned into different classes.