Imexon-induced apoptosis in multiple myeloma tumor cells is caspase-8 dependent.

PURPOSE Imexon is a 2-cyanoaziridine agent that has been shown to inhibit growth of chemotherapy-sensitive myeloma cells through apoptosis with decreased cellular stores of glutathione and increased reactive oxygen species (ROS). We examined the mechanism of imexon cytotoxicity in a diverse panel of dexamethasone and chemotherapy-sensitive and -resistant myeloma cell lines. EXPERIMENTAL DESIGN We examined cellular cytotoxicity, apoptosis, and changes in redox state in dexamethasone-sensitive (C2E3), dexamethasone-resistant (1-310 and 1-414), chemotherapy-sensitive (RPMI-8226), and chemotherapy-resistant (DOX-1V and DOX-10V) myeloma cell lines. RESULTS We found significant cytotoxicity after 48-h incubation with imexon (80-160 microM) in dexamethasone and chemotherapy-sensitive and -resistant myeloma cell lines in a time- and dose-dependent manner. The mechanism of imexon cytotoxicity in all cell lines was related to induction of apoptosis with the presence of cleaved caspase-3. Moreover, after imexon exposure in C2E3 and 1-414 cell lines, we demonstrated caspase-8-dependent apoptosis. Bcl-2:bax was proapoptotic with imexon in C2E3, whereas bcl-2:bax was independent of steroid resistance, chemotherapy sensitivity, and chemotherapy resistance. Depletion of intracellular glutathione was documented in RPMI-8226 at high imexon concentrations (>or=225 microM) but not in other cell lines. Furthermore, ROS were found in C2E3, RPMI-8226, and 1-310 only at high imexon concentrations, whereas a sensitive marker of oxidative DNA damage, 8-hydroxydeoxyguanosine, was not increased in any cell line. CONCLUSIONS Our results demonstrate that imexon has significant broad antimyeloma activity that is mediated through apoptotic mechanisms that is not dependent on production of ROS. Moreover, we have identified a mechanism of cytotoxicity in dexamethasone-sensitive and -resistant myeloma cells induced by imexon that is caspase-8 dependent.