Mercaptopurine (6MP) has been the standard drug for maintenance therapy of acute lymphoblastic leukemia. In a multicenter study we investigated whether thioguanine (6TG), which is converted more directly to the cytotoxic thioguanine nucleotides (TGN), offers a therapeutic advantage over 6MP. In this study (COALL-92), 6TG was randomized versus 6MP in maintenance therapy, whereby the doses of both drugs had to be adjusted to a white blood cell (WBC) count of between 2 and 3/nl. In 19 children the plasma levels of both drugs and/or the accumulation of their metabolites in red blood cells (RBC) were measured during intensive treatment in two consecutive chemotherapy blocks, and in 54 children the metabolites in RBC were measured every 3 months during daily treatment in maintenance therapy. There was a marked interindividual difference in the plasma kinetics of the two drugs; after identical doses of 100 mg/m2 an about 4-fold higher peak concentration of the parent drug was reached with 6MP. The main metabolites of 6TG were thioguanine nucleotides (TGN), whereas during 6MP treatment, methylated thioinosine nucleotides (TIN) predominated in erythrocytes. In patients receiving 6TG during maintenance therapy (22 patients) the concentration of methylated TGN reached about 40% of that of unmethylated TGN; after 6MP administration (32 patients) the methylated TIN were concentrated about 26-fold higher in RBC than were TGN. In contrast to 6TG, for 6MP the pattern of metabolites shifted toward the methylated ones with increasing dose. The median TGN concentration was about 7-fold higher in the TG branch, although the median dose was only about 70% of that of 6MP. The WBC values were equivalent in the two treatment groups. Our results suggest that the cytotoxic effect of 6MP is not based solely on the formation of TGN.

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