Germline polymorphisms can be significant determinants of toxicity to anti-leukemic therapy. For mercaptopurine (6MP) it is established that variant alleles of the thiopurine-S-methyl-transferase (TPMT) gene, resulting in low enzymatic activity, are associated with an increase in the concentration of thioguanine-nucleotide metabolites (TGN) and in the risk of hematotoxicity. Polymorphisms of genes encoding other enzymes involved in 6MP metabolism could also influence its pharmacokinetics and consequently its efficacy and toxicity. Among possible candidate genes, inosine-triphosphate-pyrophosphatase (ITPA) has been related to adverse events to thiopurine treatment of inflammatory bowel disease, but it has not been fully investigated for leukemia therapy. The aim of this study was to assess the association between severe life-threatening toxicities (Grade 3–4) during the continuation treatment of acute lymphoblastic leukemia (ALL) and the variant alleles of TPMT and ITPA genes, and the influence of these variant alleles on the concentration of 6MP metabolites, TGN and methylated nucleotides (MMPN). Patients with ALL on the St. Jude Total 13B protocol were assessed for toxicity according to NCI criteria. Relevant variant alleles of TPMT (SNPs rs1142345, rs1800462, rs1800660) and ITPA (SNP rs41320251) were determined using PCR assays. The association between the variant alleles and the development of adverse events during the continuation phase of treatment was assessed using weighted logistic regression. Concentrations of the two main metabolites of 6MP were measured in erythrocytes by HPLC; the association between genotypes and the concentrations of 6MP metabolites was evaluated using mixed linear effects models. TPMT and ITPA genotypes were determined for 233 patients; 13 (5.2%) were heterozygous for variant alleles of the TPMT gene and 31 (13.3%) for the ITPA gene; no patient had two variant alleles of either gene and 1 patient (0.4%) had a variant allele for both TPMT and ITPA. Since 6MP dose was individualized based on the TPMT variant allele among patients treated in the protocol, genetic polymorphisms of TPMT, not surprisingly, were not associated with toxicity. On the other hand, the presence of an ITPA variant allele was significantly associated with the incidence of Grade 3–4 fever with neutropenia in the univariate and in the multiple weighted logistic regression (odds ratio 3.0, 95% C.I. 1.2–7.8, p = 0.021). 6MP metabolites were measured in 257 samples from 108 patients, and the results revealed that variant alleles of both candidate genes were associated with changes in the concentration of the MMPN metabolites: TPMT with a reduction (p = 0.048) and ITPA with an increase (p = 0.047). Genetic polymorphism of ITPA rs41230251 is a significant determinant of severe and life-threatening fever with neutropenia and of 6MP metabolism in patients with ALL who are treated with 6MP doses individualized based on TPMT genotype.
Disclosure: No relevant conflicts of interest to declare.