EFFECTS OF DNA TOPOISOMERASE-II INHIBITORS ON HUMAN BONE-MARROW PROGENITOR CELLS

Topoisomerase II (topo II) is a target for many cytotoxic agents. Two observations, however, warrant caution in their therapeutic use: first , these agents can inhibit differentiation and second, perturbations i n function render the enzyme error-prone. Illegitimate recombination e vents occurring at sites where topo II acts in differentiation could b e particularly important in the development of secondary malignancies (relatively frequent after therapy with agents that target topo II). T opo II inhibitors are heterogeneous in mechanisms of action; in site-s pecificity of cleavable complex 'entrapment' (where present) and in th e relative potency against the two topo II isoforms, all potentially i nfluencing the site of maximum DNA damage. The object of this study wa s to examine the effect of topo II inhibitors on human haemopoietic pr ecursor cells, to determine which have most impact on differentiation. We selected two which act via cleavable complex entrapment, but with different site preferences (m-AMSA and VP-16), and two acting via othe r mechanisms (merbarone and fostriecin). VP-In and m-AMSA showed simil ar patterns with tow dose stimulation of granulocyte-macrophage colony formation and high dose inhibition of all colony types. The stimulati on was accompanied by an increase in colony size and blast content, co nsistent with a low dose inhibition of differentiation. Fostriecin, in contrast, stimulated predominantly mixed and erythroid colonies. Merb arone failed to increase colony formation. Neither produced substantia l inhibition of colony formation. The effects on granulocyte-macrophag e progenitors were confirmed using 7-day suspension cultures, using ni troblue tetrazolium (NBT) reduction and 3-4,5,dimethylthiazol 2,5-diph enyl tetrazolium bromide (MTT) assays for differentiated cells and tot al cell mass, respectively. These results demonstrate that the effects of topo II inhibitors on haemopoietic cell proliferation and differen tiation are agent-specific and can involve lineage-restricted partial inhibition of differentiation.