Individualized Use of 6‐Mercaptopurine in Chinese Children with ALL: A Multicenter Randomized Controlled Trial

Continuous 6‐mercaptopurine (6‐MP) dose titration is necessary because of its narrow therapeutic index and frequently encountered dose‐limiting hematopoietic toxicity. However, evidence‐based guidelines for gene‐based 6‐MP dosing have not been established for Chinese children with acute lymphoblastic leukemia (ALL). This multicenter, randomized, open‐label, active‐controlled clinical trial randomly assigned Chinese children with low‐ or intermediate‐risk ALL in a 1:1 ratio to receive TPMT–NUDT15 gene–based dosing of 6‐MP (N = 44, 10 to 50 mg/m2/day) or standard dosing (N = 44, 50 mg/m2/day) during maintenance therapy. The primary end point was the incidence of 6‐MP myelosuppression in both groups. Secondary end points included frequencies of 6‐MP hepatotoxicity, duration of myelosuppression and leukopenia, event‐free survival, and steady‐state concentrations of active metabolites (6‐thioguaninenucleotides and 6‐methylmercaptopurine nucleotides) in erythrocytes. A 2.2‐fold decrease in myelosuppression, the primary end point, was observed in the gene‐based–dose group using ~ 50% of the standard initial 6‐MP dose (odds ratio, 0.26, 95% confidence interval, 0.11 to 0.64, P = 0.003). Patients in the gene‐based–dose group had a significantly lower risk of developing thiopurine‐induced myelosuppression and leukopenia (P = 0.015 and P = 0.022, respectively). No significant differences were observed in the secondary end points of the incidence of hepatotoxicity and steady‐state concentrations of active metabolites in erythrocytes between the two groups. TPMT‐ and NUDT15‐based dosing of 6‐MP will significantly contribute toward further reducing the incidence of leukopenia in Chinese children with ALL. This trial is registered at www.clinicaltrial.gov as #NCT04228393. Dose reduction is strongly recommended for patients with loss‐of‐function TPMT and NUDT15 alleles to reduce the risk of thiopurine‐related toxicity.