Quantitative structure-activity relationship study of novel rhinacanthins and related naphthoquinone esters as anticancer agents

The anticancer activity of rhinacanthins and related naphthoquinone esters is quantitatively analyzed through Fujita-Ban and Hansch approaches. The analyses have helped to ascertain the role of different substituents in explaining the observed inhibitory actions of these compounds. From both approaches it appeared that naphthalene ring instead of benzene ring, dimethyl substitution at R-1 and R-2, and hydrogen-bond acceptor substituents at R-3 (Figure 1) are advantageous to improve the activity of a compound against KB cell lines. This in turn leads to the suggestion that the rhinacanthin-N scaffold is the structural entity that needs exploration for new potential compounds. Further, in the Fujita-Ban analysis, it is observed that the compounds bearing a OMe substitution, relative to H, at R-4 have a slight positive contribution to pIC(50) (KB) whereas the substituents H or OMe at R-5, relative to OH, have negative contributions. In conformity with these findings, the Hansch approach revealed that a more hydrophobic group at R-4 and a more hydrophilic group at R-5 positions are beneficial in raising the activity. The two quantitative structure-activity relationship (QSAR) analyses, differing in parametric approach, therefore, provided the grounds for rationalizing the substituent selection to design more potent compounds of the series.