Effect and mechanism of allopurinol in ALL maintenance therapy: Reducing hepatotoxicity and improving myelosuppression effect via TPMT inhibition and increased DNA-thioguanine levels

Abstract Introduction: 6-Mercaptopurine (6-MP) based-maintenance therapy is essential to cure acute lymphoblastic leukemia (ALL). However, insufficient myelosuppression and 6-MP-related hepatotoxicity occasionally occur. The investigation of the strategies to improve the efficacy and toxicities is warranted. The role of allopurinol combination therapy has been reported, but its effects need further verification, and the underlying mechanism remains unclear. Methods: Two hundred-eight pediatric patients from the First Affiliated Hospital of Sun Yat-sen University and Sun Yat-sen Memorial Hospital were enrolled during ALL maintenance therapy (the SCCLG-ALL-2016 protocol). Among them, an allopurinol intervention subgroup (n = 12, low dose of 6-MP combined with allopurinol) and a control group (n = 8, standard dose of 6-MP along with glutathione tablets) were included to evaluate the therapeutic impact of allopurinol co-administration and to explore the potential underlying pharmacological mechanisms. Blood samples were collected for all the enrolled patients. Metabolite levels [DNA-thioguanine (DNA-TG), 6-thioguanine nucleotides(6-TGN), methyl mercaptopurine (6-MMP)], thiopurine methyltransferase (TPMT) activity and NUDT15/TPMT genotypes. Hematological values and hepatic function were measured as well. Results: Multivariable regression analysis indicated that 6-MP dose ≥ 32.64 mg/m2 per day (OR: 2.452, 1.360-4.415) and 6-MMP concentration >5555.83 pmol/8×108RBC (OR: 2.042, 1.147-3.636) were predictive characteristics for the development of hepatotoxicity in patients with wildtype TPMT/NUDT15 genotypes. Interestingly, TPMT/NUDT15 wild-type patients with inadequate myelosuppression exhibited significantly higher doses of 6-MP (mean±SD: 34.49±18.59 mg/m2vs. 40.07±20.65 mg/m2), increased 6-MMP concentrations (7895 vs. 4830 pmol/8×108RBC, p=0.022) and elevated 6-MMP:6-TGN/6-MMP: DNA-TG ratios (p<0.001), indicating that 6-MP-methylated pathway was overactivated and intensifying 6-MP dosing could not lead to adequate myelosuppression effect, but instead primarily lead to higher risk of hepatotoxicity. In the subgroups analysis, allopurinol combination with dose reduction in 6-MP, TPMT activity decreased significantly leading to a significant reduction in the 6-MMP:6-TGN and 6-MMP: DNA-TG ratios and improvement in ALT and AST levels. Meanwhile, DNA-TG levels increased by an average of 257.5 fmol/μg DNA [baseline vs. post-administration, DNA-TG: 418.8 vs. 676.3 fmol/μg DNA (p=0.034)], resulting in better control of WBC and ANC, with 83.3% of the patients meeting the therapeutic target. Conclusions: In conclusion, intensifying 6-MP dosing does not achieve adequate myelosuppression effect, but primarily increases the risk of hepatotoxicity by experiencing skewed metabolism. Low-dose 6-MP combined with allopurinol, rather than intensifying 6-MP dose, could be an alternative strategy for better efficacy and lower risk of hepatotoxicity.