In vitro drug resistance in acute myeloid and chronic B-lymphocytic leukaemic blasts and in normal blood and marrow populations

The sensitivities of AML and BCLL blasts to daunorubicin have been determined, using an in vitro (MTT) assay of resistance, and compared with the sensitivities of normal haemopoietic populations and cells of the multidrug-resistant, T-lymphoid line CEM VLB100. The role of the drug-efflux pump, P-glycoprotein, was determined by adding the ‘modifier’ cyclosporin and by measuring numbers of P-glycoprotein positive cells by immunofluorescence. id50s for 17 cases of de novo AML varied from 5 to 300 ng/ml giving a median of 105 ng/ml which was similar to the median of 11 normal marrow mononuclear cell preparations (80 ng/ml) but considerably less than the median id50 of eight blood lymphocyte samples (3500 ng/ml). id50s for five relapsed and two refractory AML samples ranged from 27 to 240 ng/ml, well within the de novo range: we had obtained presentation samples for two of these and, in both cases, id50s were lower at relapse. id50s, however, were raised in seven marrow mononuclear cell populations taken soon after remission induction (id50 for remission MNC and normal MNC = 200 and 80 ng/ml, respectively); this may reflect either a property of regenerating populations, or an activation of cellular resistance mechanisms following chemotherapy. id50s for 17 cases of BCLL ranged from 7 to 200 ng/ml with a median of 48 ng/ml which was significantly lower than the id50 of AML blasts or of blood lymphocytes. Cyclosporin induced less than two-fold reductions in id50s of blood lymphocytes, marrow mononuclear cells and de novo AML and BCLL blasts despite giving log reversals in resistance in the CEM VLB100 line. This reflected numbers of P-glycoprotein positive cells in our samples, which were high in CEM VLB100 but low in fresh normal or leukaemic cell suspensions. For both de novo AML and BCLL groups, however, the change in id50, on addition of cyclosporin, was significant. These data imply a minor role for P-glycoprotein in drug resistance of leukaemic blasts. Nevertheless, there was a positive correlation between daunorubicin id50s in de novo AML and time to remission which confirms that in vitro chemosensitivity assays can provide a useful measure of in vivo resistance.