TOXICITY OF IDARUBICIN AND DOXORUBICIN TOWARDS NORMAL AND LEUKEMIC HUMAN BONE-MARROW PROGENITORS IN RELATION TO THEIR PROLIFERATIVE STATE

The effect of growth stimulation on the sensitivity of normal and leuk emic human bone marrow progenitors to idarubicin and doxorubicin was s tudied. Clonogenic assays from colony-forming units of the granulocyte -macrophage lineage (CFU-GM) and leukemic clonogenic cells (CFU-L) wer e applied using human placenta conditioned medium (HPCM) as source of growth factors. Before seeding cells inclonogenic assay they were expo sed to the anthracyclines for 2 h without preincubation, or following a 48 h preincubation period in the presence of HPCM. Drug concentratio ns used ranged from 0.001-0.1 mu g/ml for idarubicin and from 0.1-1.5 mu g/ml for doxorubicin. In addition, a limited number of bone marrow samples were exposed to the cytostatically active metabolite of idarub icin; idarubicinol (Idol; range 0.001-0.1 mu g/ml). Proliferation of C FU-GM and CFU-L during 48 h was measured by iododeoxyuridine (IdUrd) i ncorporation. Spontaneous proliferation of CFU-GM increased from 38 to 88% after 48 h stimulation by HPCM. The mean number of proliferating CFU-L increased from 40 to 77% when stimulated with HPCM. Doxorubicin inhibited colony formation of CFU-GM and CFU-L to 50% (IC50(CFU-GM), I C50(CFU-I)) at mean concentrations of 0.355 mu g/ml and 0.103 mu g/ml when applied before preincubation with HPCM, and 0.108 mu g/ml and 0.0 55 mu g/ml when applied after preincubation. Idarubicin appeared the m ost potent drug in all experiments regardless of the preincubation pro cedure or sample origin, with average IC50(CFU-GM) and IC50(CFU-L) of 0.008 mu g/ml and 0.006 mu g/ml, respectively, when applied before pre incubation with HPCM, and 0.006 mu g/ml and 0.005 mu g/ml when applied after preincubation with HPCM. Idarubicinol showed intermediate poten cy with average IC50(CFU-GM) of 0.022 mu g/ml and 0.023 mu g/ml when a pplied before and after preincubation with HPCM, respectively. In orde r to assess the effect of growth stimulation on drug sensitivity, samp les were evaluated pair-wise (sensitivity of the same sample before vs . after preincubation with HPCM) and were submitted to the Wilcoxon te st for matched pairs. Statistically significant enhancement of cytotox icity was demonstrated for Doxorubicin vs. CFU-GM (p less than or equa l to 0.021) and a strong trend versus CFU-L (p=0.06).The data further demonstrate that proliferation-dependency of doxorubicin toxicity is m ore pronounced for CFU-GM than for CFU-L. These data also show that id arubicin and idarubicinol toxicity is proliferation-independent. Idaru bicin is relatively more potent than doxorubicin in suppressing the gr owth potentialof low or nonproliferating progenitors. The in vitro IC5 0 of idarubicin appeared a factor 10 lower than the in vivo reached pl asma levels achieved after standard dose bolus injections of idarubici n. In addition, the data show that doxorubicin exerts a higher differe ntial toxicity towards leukemic progenitors compared to idarubicin and idarubicinol.