Sensitivity of fresh acute myeloid leukemia cells to etoposide: relationship with cell growth characteristics and DNA single-strand breaks

In this study, we evaluated the individual in vitro sensitivity of fresh acute myeloid leukemia (AML) cells to VP-16, and attempted to correlate VP-16 cytotoxicity with AML cell growth characteristics and drug-induced DNA single-strand breaks (SSB). Primary (PE1) colony inhibition assays allowed us to characterize two distinct groups of AML: group I (patients 1 through 6), which displayed sensitivity to VP- 16 similar to that of normal CFU-GM (IC90 of 20.52 +/- 2.44 micrograms/mL v 20.48 +/- 2.23 micrograms/mL after 1 hour drug exposure, respectively); and group II (patients 7 through 11), which was more sensitive to VP-16 (IC90 of 7.26 +/- 2.93 micrograms/mL, P = .004). Subsequently, groups I and II were termed normosensitive and hypersensitive, respectively. This objective VP-16 sensitivity classification, as determined by PE1, remained unaltered when assessed by secondary (PE2) colony inhibition assay (evaluating the self-renewal fraction of AML progenitors), or by cytofluorometric viability assay (evaluating the ultimately differentiated blast cell population). These findings would suggest that individual sensitivity to VP-16 of a particular cell population is maintained throughout CFU-AML differentiation. Finally, we report that sensitivity of AML cells to VP- 16 did not correlate either with cell growth characteristics or with SSB generation. Indeed, AML cell sensitivity to VP-16 appeared more closely related to DNA repair kinetics after drug removal, ie, hypersensitivity being essentially characterized by a prolonged retention of SSB during the posttreatment period. Interestingly, the established HL-60 cell line, which presented greater sensitivity to VP- 16 cytotoxicity than KG1, HEL, and K562, was also found to exhibit delayed DNA SSB repair kinetics, as compared with the other AML cell lines. These results suggest that hypersensitivity to VP-16 of some AML cells may be related to a deficient DNA-repair mechanism.