Tiazofurin exhibits antitumor activity in murine and human
tumor cells. In a recent phase I/II trial in patients with end-stage
leukemia,
tiazofurin showed good response; however, repeated treatment resulted in clinical resistance to the
drug. To elucidate the mechanisms of resistance in human leukemic cells, two variants of human
myelogenous leukemia K652 cells resistant to
tiazofurin were developed by
drug-selection pressure. Compared to a concentration producing 50% cell proliferation reduction that was 9.1 microM in sensitive cells, the resistant variants displayed concentrations producing 50% cell proliferation reductions of 12 and 16 mM. The activity of the target
enzyme,
IMP dehydrogenase, was not altered in the resistant cells. Studies on
tiazofurin metabolism revealed that resistant variants formed < 10% of the active metabolite,
thiazole-4-carboxamide adenine dinucleotide. This correlated with the activity of
NAD pyrophosphorylase, the
enzyme that synthesizes
thiazole-4-carboxamide adenine dinucleotide, which was reduced to 10% in the resistant lines. Concurrently, the activity of
thiazole-4-carboxamide adenine dinucleotide phosphodiesterase was elevated in the refractory cells. Compared to the sensitive counterpart, the levels of GMP and
NAD were lower in the resistant lines.
Guanine salvage activity was decreased in the resistant cells. Basal
dGTP and dATP concentrations were elevated in the resistant line; nevertheless,
tiazofurin incubation decreased
dGTP levels in only the sensitive cells. Although there was no difference in the Km of
tiazofurin transport or efflux, the Vmax of uptake of the
drug was reduced in the resistant lines. Sensitive and resistant cells exhibit similar cytotoxicity to agents which do not share the mechanism of action of
tiazofurin, suggesting that refractory cells are still sensitive to other standard antileukemic drugs.