BIOCHEMICAL CONSEQUENCES OF RESISTANCE TO TIAZOFURIN IN HUMAN MYELOGENOUS LEUKEMIC K562 CELLS

Tiazofurin exhibits antitumor activity in murine and human tumor cells . In a recent phase I/II trial in patients with end-stage leukemia, ti azofurin showed good response; however, repeated treatment resulted in clinical resistance to the drug. To elucidate the mechanisms of resis tance in human leukemic cells, two variants of human myelogenous leuke mia K652 cells resistant to tiazofurin were developed by drug-selectio n pressure. Compared to a concentration producing 50% cell proliferati on reduction that was 9.1 muM in sensitive cells. the resistant varian ts displayed concentrations producing 50% cell proliferation reduction s of 12 and 16 mM. The activity of the target enzyme, IMP dehydrogenas e, was not altered in the resistant cells. Studies on tiazofurin metab olism revealed that resistant variants formed <10% of the active metab olite, thiazole-4-carboxamide adenine dinucleotide. This correlated wi th the activity of NAD pyrophospliorylase, the enzyme that synthesizes thiazole-4-carboxamide adenine dinucleotide, which was reduced to 10% in the resistant lines. Concurrently, the activity of thiazole-4-carb oxamide adenine dinucleotide phosphodiesterase was elevated in the ref ractory cells. Compared to the sensitive counterpart, the levels of GM P and NAD were lower in the resistant lines. Guanine salvage activity was decreased in the resistant cells. Basal dGTP and dATP concentratio ns were elevated in the resistant line; nevertheless, tiazofurin incub ation decreased dGTP levels in only the sensitive cells. Although ther e was no difference in the K(m) of tiazofurin transport or efflux, the V(max) of uptake of the drug was reduced in the resistant lines. Sens itive and resistant cells exhibit similar cytotoxicity to agents which do not share the mechanism of action of tiazofurin, suggesting that r efractory cells are still sensitive to other standard antileukemic dru gs.