Genetic polymorphism of inosine triphosphate pyrophosphatase is a determinant of mercaptopurine metabolism and toxicity during treatment for acute lymphoblastic leukemia

Abstract

The influence of genetic polymorphism in inosine triphosphate pyrophosphatase (ITPA) on thiopurine-induced adverse events has not been investigated in the context of combination chemotherapy for acute lymphoblastic leukemia (ALL). This study investigated the effects of a common ITPA variant allele (rs41320251) on mercaptopurine metabolism and toxicity during treatment of children with ALL. Significantly higher concentrations of methyl mercaptopurine nucleotides were found in patients with the nonfunctional ITPA allele. Moreover, there was a significantly higher probability of severe febrile neutropenia in patients with a variant ITPA allele among patients whose dose of mercaptopurine had been adjusted for TPMT genotype. In a cohort of patients whose mercaptopurine dose was not adjusted for TPMT phenotype, the TPMT genotype had a greater effect than the ITPA genotype. In conclusion, genetic polymorphism of ITPA is a significant determinant of mercaptopurine metabolism and of severe febrile neutropenia, after combination chemotherapy for ALL in which mercaptopurine doses are individualized on the basis of TPMT genotype.

Figure 1

Schematic representation of the metabolism of mercaptopurine and the enzymes involved. GDP, guanosine diphosphate; GMP, guanosine monophosphate; GMPS, guanosine monophosphate synthase; GTP, guanosine triphosphate; HPRT, hypoxanthine phosphoribosyltransferase; IMPDH, inosine monophosphate dehdrogenase; IDP, inosine diphosphate; IMP, inosine monophosphate; ITP, inosine triphosphate; ITPA, inosine triphosphate pyrophosphatase; K, kinase; TPMT, thiopurine S-methyltransferase; XO, xanthine oxidase.

Figure 2

TPMT/ITPA multilocus genotype and concentrations of mercaptopurine thioguanine nucleotide metabolite (TGN) measured in bone marrow leukemia cells after initial treatment with mercaptopurine alone; concentrations did not differ significantly by TPMT/ITPA genotype. ITPA, inosine triphosphate pyrophosphatase; TPMT, thiopurine S-methyltransferase.

Figure 3

TPMT/ITPA multilocus genotype and concentrations of mercaptopurine metabolite methylmercaptopurine nucleotide (MMPN) measured in bone marrow leukemia cells after initial treatment with mercaptopurine alone; concentrations were higher in patients with a multilocus genotype TPMT wild type/ITPA variant (P = 0.0056, Wilcoxon test) and lower in patients with a multilocus genotype TPMT variant/ITPA wild type (P = 0.030, Wilcoxon test) as compared to patients with a wild-type genotype for both genes. Boxes include data between the 25th and 75th percentiles, and whiskers indicate the minimal and maximal values excluding the outliers. ITPA, inosine triphosphate pyrophosphatase; TPMT, thiopurine S-methyltransferase.

Figure 4

Multilocus TPMT/ITPA genotype and median concentrations of mercaptopurine metabolite thioguanine nucleotide (TGN) measured during chronic treatment with mercaptopurine in accordance with the St Jude Total 13B protocol. Mercaptopurine doses were adjusted for TPMT genotype so as to avoid toxic TGN concentrations. Red-blood-cell TGN concentrations differed by TPMT genotype, resulting in higher concentrations (P = 0.0095, pairwise Wilcoxon test) in patients with a multilocus genotype of TPMT variant/ITPA wild type than in patients with a wild-type genotype for both genes. ITPA, inosine triphosphate pyrophosphatase; TPMT, thiopurine S-methyltransferase.

Figure 5

Multilocus TPMT/ITPA genotype and median concentrations of mercaptopurine metabolite methylmercaptopurine nucleotide (MMPN) measured during chronic treatment with mercaptopurine in accordance with the St Jude Total 13B protocol. Mercaptopurine doses were adjusted for TPMT genotype so as to avoid toxic TGN concentrations. Red-blood-cell MMPN concentrations differed among patients according to TPMT/ITPA genotypes, showing higher concentrations in patients with a multilocus genotype TPMT wild type/ITPA variant (P = 0.0086, pairwise Wilcoxon test) and lower concentrations in patients with a multilocus genotype of TPMT variant/ITPA wild type (P = 0.048, pairwise Wilcoxon test) as compared to patients who were wild type for both loci. Boxes include data between the 25th and 75th percentiles, and whiskers indicate the minimal and maximal values excluding the outliers. ITPA, inosine triphosphate pyrophosphatase; TPMT, thiopurine S-methyltransferase.

Figure 6

ITPA genotype and cumulative incidence curves for the risk of grade 3/4 febrile neutropenia in patients with acute lymphoblastic leukemia during chronic continuation therapy that included mercaptopurine in accordance with the St Jude Total 13B protocol. At day 900 of maintenance therapy, the estimate of incidence of these adverse events was 10.7% ± 2.2 in patients with wild-type ITPA and 26.6% ± 7.8 in patients with variant ITPA. ITPA, inosine triphosphate pyrophosphatase; TPMT, thiopurine S-methyltransferase.

Figure 7

TPMT genotype, ITPA genotype, and the odds ratio of severe toxicity during continuation therapy of children with acute lymphoblastic leukemia for whom the dose of mercaptopurine was not adjusted on the basis of TPMT genotype (St Jude Total 12) and in those for whom the dose was adjusted on the basis of TPMT genotype (St Jude Total 13B). Toxicity measured prospectively according to the Total 12 protocol (i.e., no mercaptopurine dose adjustment on the basis of TPMT) was grade 3/4 infection, whereas the comparable toxicity measured according to the Total 13B (i.e., mercaptopurine dose adjusted for TPMT) was grade 3/4 febrile neutropenia. Odds ratios are from a weighted logistic regression model and are adjusted for treatment arm and patient’s age, race, and sex. ITPA, inosine triphosphate pyrophosphatase; TPMT, thiopurine S-methyltransferase. v, variant allele; wt, wild-type allele.