DESULFURATION OF 6-MERCAPTOPURINE - THE BASIS FOR THE PARADOXICAL CYTOTOXICITY OF THIOPURINES IN CULTURED HUMAN LEUKEMIC-CELLS

The thiopurines have a wide array of effects on purine metabolism, but the primary mechanism of cytotoxicity for both 6-mercaptopurine (6-MP ) and 6-thioguanine (6-TG) appears to be incorporation of drug into DN A following conversion to the thioguanylate form. In murine leukemic c ell lines exposed to a range of thiopurine concentrations in vitro, ce ll survival curves have displayed a phenomenon termed paradoxical cyto toxicity, defined as a decrease in cytotoxicity with increasing drug c oncentration. The paradoxical cytotoxicity of thiopurines has usually been attributed to concentration-dependent perturbations in the cell c ycle. The present study assessed whether the paradoxical cytotoxicity of 6-MP occurred in cultured human leukemic cells, and investigated th e biochemical and cell-cycle alterations occurring in these lines at t hiopurine concentrations associated with the reverse of cytotoxicity. Paradoxical cytotoxicity was observed in the two human leukemic cell l ines examined, but only when 6-MP concentrations exceeded 100 muM. The extent of incorporation of 6-MP metabolites into DNA as thiol- versus non-thiol-containing metabolites was analyzed by performing parallel experiments with C-14- and S-35-radiolabeled drug. With 5 muM 6-MP, ap proximately 50% of drug was incorporated into DNA as a thionucleotide; however, with increasing drug concentrations, the degree of thionucle otide incorporation remained unchanged or decreased, and the amount in corporated as the desulfurated metabolite (presumably adenylate or gua nylate) increased. With 500 muM 6-MP, less than 10% of the drug was in corporated as the thionucleotide. Perturbations in cell cycle reflecte d the relative amounts of thiol- and non-thiol-containing nucleotide f ormed at various concentrations of 6-MP. These results suggest that th iopurines may be vulnerable to a unique mechanism of detoxification, i n which a human cell can metabolize a cytotoxic drug to a comparativel y potent ''self-rescue'' agent.