Metabolism of 6-mercaptopurine in the erythrocytes, liver, and kidney of rats during multiple-dose regimens

Purpose: To describe the metabolism of 6-mercaptopurine (6-MP) in erythrocy tes and tissues of rats after repeated administration of 6-MP at two dose l evels and to provide evidence that in vivo modulation of 6-MP anabolism can be obtained by simultaneous treatment with ribavirin or hydroxyurea, two i nhibitors of enzymes involved in the bioactivation of 6-MP to the active 6- thioguanine nucleotides (6-TGN). Methods: Rats were treated i.p. with 6-MP at 12.5 and 25 mg/kg daily for 12 days and erythrocyte, liver, and kidney l evels of G-mercaptopurine nucleotides (6-MPN) and 6-TGN were investigated d uring the accumulation phase and for 50 days after the end of treatment. In combination studies, ribavirin at 75 and 100 mg/kg per day (for 6-MP, 25 a nd 12.5 mg/kg per day) or hydroxyurea at 200 mg/kg per day were given i.p. for 12 days. The measurements of thionucleotide levels in rat samples were performed by high-pressure liquid chromatography (HPLC). Results: The maxim al concentration (C-max) and the area under the concentration versus time c urve (AUC) of 6-MPN and 6-TGN in erythrocytes and tissues increased signifi cantly after the administration of 6-MP at 25 mg/kg per day as compared wit h 12.5 mg/kg per day. In particular, the C-max and AUC of 6-TGN in erythroc ytes of rats treated with 6-MP at 25 mg/kg per day were approximately 5-fol d higher than the 6-TGN values observed following treatment at 12.5 mg/kg p er day. Moreover, 6-TGN levels in erythrocytes were significantly higher th an those of 6-MPN (910.9 +/- 53.1 and 286.8 +/- 23.4 pmol/8 x 10(8) cells f or 6-TGN and 6-MPN, respectively, P < 0.05) after treatment with 6-MP at 25 mg/kg per day. The administration of ribavirin, an inhibitor of inosine mo nophosphate dehydrogenase, in association with 6-MP increased the amount of 6-MPN detected in erythrocytes and tissues while reducing 6-TGN levels in samples. The production and accumulation of 6-MPN and 6-TGN were increased in erythrocytes and tissues by hydroxyurea, an inhibitor of ribonucleotide reductase. Finally, a significant correlation between thionucleotide concen trations and erythrocyte counts was observed. Conclusion: The overall resul ts demonstrate that 6-MP is actively metabolized in rats and that its biotr ansformation can be modulated by agents acting on enzymes of the purine met abolism, resulting in significant changes in erythrocyte and tissue levels of 6-MPN and 6-TGN. These findings provide evidence that the rat is a suita ble model for investigation of the metabolism of 6-MP and its possible phar macologic modulation.