Single R-Group Polymorphisms (SRPs) and R-cliffs: an intuitive framework for analyzing and visualizing activity cliffs in a single analog series.

We introduce Single R-Group Polymorphisms (SRPs, pronounced 'sharps'), an intuitive framework for analyzing substituent effects and activity cliffs in a single congeneric series. A SRP is a pair of compounds that differ only in a single R-group position. Because the same substituent pair may occur in multiple SRPs in the series (i.e., with different combinations of substituents at the other R-group positions), SRP analysis makes it easy to identify systematic substituent effects and activity cliffs at each point of variation (R-cliffs). SRPs can be visualized as a symmetric heatmap where each cell represents a particular pair of substituents color-coded by the average difference in activity between the compounds that contain that particular SRP. SRP maps offer several advantages over existing techniques for visualizing activity cliffs: 1) the chemical structures of all the substituents are displayed simultaneously on a single map, thus directly engaging the pattern recognition abilities of the medicinal chemist; 2) it is based on R-group decomposition, a natural paradigm for generating and rationalizing SAR; 3) it uses a heatmap representation that makes it easy to identify systematic trends in the data; 4) it generalizes the concept of activity cliffs beyond similarity by allowing the analyst to sort the substituents according to any property of interest or place them manually in any desired order.