Background and aims: To compare the metabolic effects induced by the anticancer drugs, 6-mercaptopurine (6-MP), 6-thioguanine (6-TG) and 6-methylmercaptopurine riboside (MMPR), which may inhibit the de novo biosynthesis of purine nucleotides or be mis-incorporated into DNA or RNA. Methods: Leukaemia cells were grown in culture, exposed to a thiopurine and cell extracts were analyzed for NTPs, dNTPs, drug metabolites and P-Rib-PP. Results: In leukaemia cells, 6-MP was converted to MRP-MP, thio-XMP, thio-GMP, thio-GDP and thio-GTP. Metabolites of 6-TG included thio-XMP, thio-GMP, thio-GDP and thio-GTP, while MMPR-MP was the only major metabolite of MMPR. MMPR (25 μM, 4 h) induced a 16-fold increase in P-Rib-PP and 6-MP (25 μM, 4 h) induced a delayed 5.2-fold increase. MPR-MP, thio-GMP and MMPR-MP are inhibitors of amido phosphoribosyltransferase from leukaemia cells with K(i) values of 114 ± 7.10 μM, 6.20 ± 2.10 μM and 3.09 ± 0.30 μM, respectively. Conclusion: The nucleoside-5'-monophosphate derivatives of the 3 thiopurines inhibit amido phosphoribosyltransferase in growing leukaemia cells but there is also an initial inhibition of the further conversion of IMP in the pathway. In growing cells, MMPR acts solely as an inhibitor of de novo purine biosynthesis while 6-TG and to a lesser extent, 6-MP, are converted to significant concentrations of di- and tri-phosphate derivatives which may have other mechanisms of cytotoxicity.
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