Results of a Prospective Phase II Study of Individualized 6-Mercaptopurine Dosing Based on Pharmacogenomics in Childhood Acute Lymphoblastic Leukemia in East Asia

Introduction Variations in the NUDT15 and TPMT genes, which serve as pharmacogenomic markers for 6-mercaptopurine (6MP), contribute to individual differences in the optimal dosage of this medication. Nevertheless, given the observed difference in the tolerated dose of 6MP between East Asian and Western patients, it is essential to conduct a study to determine whether the same dosage guidelines are applicable to both populations. This study aimed to reduce treatment discontinuation and neutropenia during maintenance by adjusting the initial dose of 6MP based on the results of the NUDT15 or TPMT variants.

Methods

This is a prospective phase II trial being conducted at three institutions in Korea. Patients who initiated 6MP-based maintenance treatment for acute lymphoblastic leukemia (ALL) were included in the study. During the maintenance phase, 6MP was administered at an individualized starting dose based on the NUDT15 and TPMT variants of each participant. Original dose (50 mg/m²/day) of 6MP was administered to patients with both NUDT15 and TPMT wild-type, 30 mg/m²/day for NUDT15 intermediate activity (NUDT15 variant allele heterozygous) or TPMT heterozygosity, 10 mg/m²/day for NUDT15 low activity (NUDT15 variant allele homozygous), and 10 mg/m²/day three times per week for homozygous TPMT variants. The maintenance cycle consists of 12 weeks of daily 6MP at an individualized dosage, weekly methotrexate (MTX) at a dosage of 20 mg/m², and monthly vincristine and steroid pulses. Lastly, intrathecally administered MTX is administered every three months. During treatment, the total white blood cell count was targeted to be between 2000/uL and 3000/uL, with an absolute neutrophil count greater than 500/uL and a platelet count greater than 50,000/uL. The whole exome was sequenced using genomic DNA extracted from peripheral blood at complete remission or hair follicles. As the primary and secondary endpoints, the duration of treatment discontinuation, the frequency of neutropenia, the presence of neutropenic fever during maintenance therapy, and the recurrence rate were compared to historical controls. A cohort of 254 patients who underwent maintenance therapy between the years 2001 and 2018 was chosen as the historical control group.

Results Total 72 patients were enrolled from July 2019 until Aril 2022. This analysis was conducted on 52 patients who finished maintenance therapy. There were 32 males, and 20 females. The median age of the patients was 5.6 years (range, 2.2~15.9 years). The immunophenotypes of ALL was B-ALL in 40 patients, T-ALL in 10, Mixed phenotype in 1, and unspecified ALL in 1 patient. No patient had known pharmacogenetic variants in TPMT. The NUDT15 phenotypes based on diplotypes included normal activity (n=41), intermediate activity (n=9), and low activity (n=2), occurring in 78.8%, 17.3%, and 3.8% respectively (Table 1). The number of days discontinued due to any toxicity during the 1^st cycle of maintenance therapy was average 7 days (max 59 days) in all patients, and that of the study group with intermediate enzyme activity phentypes was significantly shorter than in historical controls, with an average 6 days vs. 12 days (P=0.03, Figure 1). The incidence of neutropenia during the 1^st cycle between two groups was not statistically different, however, among patients with NUDT15 or TPMT intermediate activity, the frequency of neutropenia was significantly lower in the study group (P=0.03, Figure 2). The frequency of neutropenic fever (NF) and fever without neutropenia during the 1st cycle did not differ between the two groups. However, when patients with NUDT15 or TPMT intermediate activity were analyzed, no fever occurred in the study group of 9 patients, while NF and fever occurred in 11 and 8 patients in the control group (Figure 3).

Conclusions Individualized administration of 6MP based on NUDT15 and TPMT genotypes during maintenance therapy in Korean pediatric ALL patients significantly reduced treatment discontinuation and febrile toxicity. A decrease in the initial dosage from 50 mg/m²/day to 30 mg/m²/day has the potential to reduce febrile toxicity in the patient group exhibiting intermediate activity of the NUDT15 enzyme, which is observed in approximately 20% of individuals in East Asia. Through this research, we expect to provide the dosing guidelines for 6MP in patients with ALL in East Asia.