Influence of phenyl ring disubstitution on bisbenzimidazole and terbenzimidazole cytotoxicity: synthesis and biological evaluation as radioprotectors.

DNA minor groove binders, Hoechst 33258 and Hoechst 33342, have been reported to protect against radiation-induced DNA-strand breakage, but their mutagenicity and cytotoxicity limit their use as protectors of normal tissue during radiotherapy and as biological radioprotectors during accidental radiation exposure. On the basis of these observations, two new nontoxic disubstituted benzimidazoles were synthesized, one having two methoxy groups (5-(4-methylpiperazin-1-yl)-2-[2'-(3,4-dimethoxyphenyl)-5'-benzimidazolyl]benzimidazole, 5) and another having a methoxy and a hydroxyl group (5-(4-methylpiperazin-1-yl)-2-[2'[2''-(4-hydroxy-3-methoxyphenyl)-5' '-benzimidazolyl]-5'-benzimidazolyl]benzimidazole, 6) ortho to each other on the phenyl ring. The radiomodifying effects of these nontoxic ligands were investigated with a human glioma cell line exposed to low linear energy transfer radiation by determining cell survival and cell proliferation compared with effects of the parent compound, Hoechst 33342. Cytotoxicity assayed by analyzing clonogenicity, cell growth, and metabolic viability showed that both 5 and 6 were nontoxic at 100 microM after 72 h of exposure, whereas Hoechst 33342 resulted in lysis of 77% of these cells in 24 h. Macrocolony assay (clonogenicity) showed that 73%, 92%, and 10% of the cells survived when treated with 100 microM 5, 6, and Hoechst 33342, respectively. Both 5 and 6 did not affect the growth of BMG-1 cells. At 10 microM, 5 and 6 showed 82% and 37% protection against radiation-induced cell death (macrocolony assay) while 100% protection was observed against growth inhibition. Disubstitution of the phenyl ring has not only reduced cytotoxicity but also enhanced DNA-ligand stability, conferring high degree of radioprotection.