Purpose: Through enhancement of 6-mercaptopurine (6MP) bioavailability and inhibition of purine de novo synthesis, high-dose methotrexate (HD-MTX) may increase incorporation into DNA of 6-thioguanine nucleotides, the cytotoxic metabolites of 6MP. Patients with intermediate activity of thiopurine methyltransferase (TPMT^IA) have higher cytosol 6-thioguanine nucleotide levels. We investigated toxicity following HD-MTX during MTX/6MP maintenance therapy in relation to 6MP and TPMT.

Methods: Using linear mixed models, we explored myelo- and hepatotoxicity in relation to 6MP dosage and TPMT phenotype following 1,749 HD-MTX courses to 411 children with acute lymphoblastic leukemia on maintenance therapy.

Results: The degree of myelosuppression following HD-MTX was similar for patients with TPMT^IA and patients with high TPMT activity (TPMT^HA), when HD-MTX started with same blood counts and 6MP doses. However, since TPMT^IA had lower blood counts at initiation of HD-MTX compared with TPMT^HA patients (median WBC 2.8 vs. 3.3 × 10⁹/L, P = 0.01; median ANC 1.4 vs. 1.7 × 10⁹/L, P = 0.02), TPMT^IA continued to have lower WBC and ANC levels compared with TPMT^HA during all 28 days after HD-MTX [relative difference 9 % (95 % CI 2–17), P = 0.02 and 21 % (95 % CI 6–39), P = 0.005]. Still, the fractional decrease in WBC and ANC levels after HD-MTX did not differ between TPMT^IA and TPMT^HA patients (P = 0.47; P = 0.38). The degree of leukopenia, neutropenia, thrombocytopenia and rise in aminotransferases were all significantly related to 6MP dose (P < 0.001 for all analyses).

Conclusion: For both TPMT^IA and TPMT^HA patients, dose of 6MP prior to HD-MTX should be guided by pre-HD-MTX blood counts, but not by TPMT activity.