Inhibition of DNA Topoisomerases by a Series of Benzoxazoles and their Possible Metabolites

Background: A series of 2-substituted benzoxazoles and their possible metabolites, N-(2-hydroxy-4(or 5)-nitrophenyl)benzamides and phenylacetamides, which were previously synthesized by our group, were investigated for their inhibitory activities on both eukaryotic DNA topoisomerase I (Topo I) and II (Topo II). Methods: DNA topoisomerase I and II inhibitory activity of compounds were determined by relaxation assay which could measure the conversion of supercoiled pBR322 plasmid DNA to its relaxed form. Results: According to DNA topoisomerase relaxation assay results, four derivatives (1c, 1f, 2b, 2e) among tested 21 compounds inhibited human Topo I, whereas three compounds (1c, 1e, 1f) inhibited human Topo IIα. 2-(4'-bromophenyl)-6-nitrobenzoxazole (1f) was observed to be the most effective Topo II inhibitor with the lowest IC50 value of 71 μM and 2-(4'-tert-butyl-phenyl)-6- nitrobenzoxazole (1c) with the IC50 value of 104 μM was considered the most effective Topo I inhibitor. However, it is noteworthy that these two compounds affected both Topo I and II enzymes. When the relationship between chemical structures and biological activities of the compounds were examined, the following two results were obtained: (i) CH2 bridge seems to decrease the Topo I and II inhibitions and (ii) bulky groups at R1 position increased both Topo I and II inhibitions for benzoxazole derivatives while small groups at R1 position increased the Topo I inhibition for benzamide derivatives. Conclusion: Biological activity and SAR results of tested derivatives may provide some predictions in order to design novel topoisomerase inhibitors.