Table 1.

Drug sensitivity of gemtuzumab ozogamicin (GO)‐resistant HL‐60 variant human leukemia cell lines

Drug	IC50
	HL‐60	HL/GO	HL/GO‐CSA
GO (μg/mL)	0.02	4.00	4.50
GO + MK571 (μg/mL)	0.02	3.95	2.90
MK571 (μM)	58.90	91.00	ND
DNR (μM)	0.03	0.13	0.15

Cells were incubated with various concentrations of GO with or without a minimally toxic concentration of MK571 (10 μM) for 72 h. The cells were also treated with daunorubicin (DNR) in the same manner. IC₅₀ values were then determined by using the XTT assay. CSA, cyclosporine A; ND, not determined because the IC₅₀ value was beyond the assay range due to the high degree of resistance.