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 DNA following conversion to the thioguanylate form. In murine leukemic cell lines exposed to a range of thiopurine concentrations in vitro, cell survival curves have displayed a phenomenon termed paradoxical cytotoxicity, defined as a decrease in cytotoxicity with increasing drug concentration. The paradoxical cytotoxicity of thiopurines has usually been attributed to concentration-dependent perturbations in the cell cycle. The present study assessed whether the paradoxical cytotoxicity of 6-MP occurred in cultured human leukemic cells, and investigated the biochemical and cell-cycle alterations occurring in these lines at thiopurine concentrations associated with the reverse of cytotoxicity. Paradoxical cytotoxicity was observed in the two human leukemic cell lines examined, but only when 6-MP concentrations exceeded 100 microM. The extent of incorporation of 6-MP metabolites into DNA as thiol-versus non-thiol-containing metabolites was analyzed by performing parallel experiments with 14C- and 35S-radiolabeled drug. With 5 microM 6-MP, approximately 50% of drug was incorporated into DNA as a thionucleotide; however, with increasing drug concentrations, the degree of thionucleotide incorporation remained unchanged or decreased, and the amount incorporated as the desulfurated metabolite (presumably adenylate or guanylate) increased. With 500 microM 6-MP, less than 10% of the drug was incorporated as the thionucleotide. Perturbations in cell cycle reflected the relative amounts of thiol- and non-thiol-containing nucleotide formed at various concentrations of 6-MP. These results suggest that thiopurines may be vulnerable to a unique mechanism of detoxification, in which a human cell can metabolize a cytotoxic drug to a comparatively potent "self-rescue" agent.
Adamson
P C
PC
Pharmacology and Experimental Therapeutics Section, National Cancer Institute, Bethesda, MD 20892.
Balis
F M
FM
Hawkins
M E
ME
Murphy
R F
RF
Poplack
D G
DG
D002250:Carbon Radioisotopes; D002453:Cell Cycle / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D004247:DNA / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D006801:Humans; D019271:Hypoxanthine; D007041:Hypoxanthine Phosphoribosyltransferase / Q000378:metabolism; D007042:Hypoxanthines / Q000494:pharmacology; D008658:Inactivation, Metabolic; D015122:Mercaptopurine / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D013462:Sulfur Radioisotopes; D013866:Thioguanine / Q000494:pharmacology; D013873:Thionucleotides / Q000378:metabolism; D014407:Tumor Cells, Cultured / Q000187:drug effects
D002250:Carbon Radioisotopes; D007042:Hypoxanthines; D013462:Sulfur Radioisotopes; D013873:Thionucleotides; D019271:Hypoxanthine; D004247:DNA; D015122:Mercaptopurine; D007041:Hypoxanthine Phosphoribosyltransferase; D013866:Thioguanine