The silatecan 7-tert-butyldimethylsilyl-10-hydroxy-camptothecin (DB-67) represents a new generation of
camptothecin derivatives that exhibits a potent in vitro
DNA topoisomerase I (TOP1)-mediated
DNA-damaging activity, improved blood stability, and holds significant promise for the treatment of human
cancers. In this study, we characterize the role of TOP1 in mediating the radiosensitization activity of
DB-67. As examined by clonogenic survival assay,
DB-67 exhibited potent radiosensitization activity at a concentration 10-fold lower than
camptothecin in the human
glioma D54-MG and T-98G cells, which harbor wild-type and mutant p53, respectively. Analyzed by the single-hit multitarget model,
DB-67 induced radiosensitization by obliterating the "shoulder" of the radiation survival curve in the D54-MG cells. The in vivo targeting of TOP1 by
DB-67 was investigated by immunoblot analysis. In a dose-dependent manner,
DB-67 specifically stimulates covalent linking of TOP1 to chromosomal
DNA at concentrations 10-fold lower than
camptothecin in the D54-MG cells. The potency of in vivo targeting of TOP1 by
DB-67 correlates well with its cytotoxicity and radiosensitization activity. Furthermore,
DB-67 exhibited substantially less cytotoxicity and radiosensitization activity in the TOP1 mutant Chinese hamster lung fibroblast DC3F/C-10 cells than in their parental DC3F cells. Together, our data show that
DB-67 exhibits potent cytotoxicity and radiosensitization activity by targeting TOP1 in mammalian cells and has great potential for being developed to treat human
cancers.