ALTERED FORMATION OF TOPOTECAN-STABILIZED TOPOISOMERASE-I DNA-ADDUCTSIN HUMAN LEUKEMIA-CELLS

Topotecan (TPT) is a topoisomerase I (topo I) poison that has shown pr omising antineoplastic activity in solid tumors and acute leukemia. In the present study, a band depletion assay was used to evaluate the ab ility of TPT to stabilize topo I-DNA adducts in human leukemia cell li nes and in clinical leukemia samples ex vivo, This assay showed that 5 0% of the cellular topo I in HL-60 human myelomonocytic leukemia cells became covalently bound to DNA at an extracellular TPT concentration of 4 mu mol/L. In contrast, in 13 clinical specimens of human leukemia harvested before treatment of patients with TPT, the TPT concentratio n required to stabilize 50% of the cellular topo I in topo I-DNA compl exes ranged from 3 to greater than 100 mu mol/L (median, 30 mu mol/L). Flow microfluorimetry showed that cellular TPT accumulation varied ov er only a twofold range and failed to provide evidence for transport-m ediated resistance in the clinical samples, These observations raise t he possibility that formation of topo I-DNA adducts is diminished in m any specimens of refractory/relapsed acute leukemia by a mechanism tha t might alter topo I sensitivity to TPT. (C) 1997 by The American Soci ety of Hematology.