Effect of ZOL on P-gp–overexpressing MDR K562/D1-9 cells. Growth inhibitory effects of ZOL were determined by the trypan blue dye exclusion method. The MDR K562/D1-9 cell line (○) was 75 times more resistant to DNR than its parental K562 (•) cell line (A). The K562/D1-9 cell line is as sensitive to ZOL as the parental cell line (B).