Thiopurine methyltransferase genotype-phenotype discordance and thiopurine active metabolite formation in childhood acute lymphoblastic leukaemia.

In children with acute lymphoblastic leukaemia (ALL) bone marrow activity can influence red blood cell (RBC) kinetics, the surrogate tissue for thiopurine methyltransferase (TPMT) measurements. The aim of this study was to investigate TPMT phenotype-genotype concordance in ALL, and the influence of TPMT on thiopurine metabolite formation. We measured TPMT (activity, as units ml(-1) packed RBCs and genotype) at diagnosis (n = 1150) and TPMT and thioguanine nucleotide (TGN) and methylmercaptopurine nucleotide (MeMPN) metabolites (pmol/8 × 10(8) RBCs) during chemotherapy (n = 1131) in children randomized to thioguanine or mercaptopurine on the United Kingdom trial ALL97. Median TPMT activity at diagnosis (8.5 units) was significantly lower than during chemotherapy (13.8 units, median difference 5.1 units, 95% confidence interval (CI) 4.8, 5.4, P < 0.0001). At diagnosis genotype-phenotype was discordant. During chemotherapy the overall concordance was 92%, but this fell to 55% in the intermediate activity cohort (45% had wild-type genotypes). For both thiopurines TGN concentrations differed by TPMT status. For mercaptopurine, median TGNs were higher in TPMT heterozygous genotype (754 pmol) than wild-type (360 pmol) patients (median difference 406 pmol, 95% CI 332, 478, P < 0.0001), whilst median MeMPNs, products of the TPMT reaction, were higher in wild-type (10 650 pmol) than heterozygous patients (3868 pmol) (P < 0.0001). In TPMT intermediate activity patients with a wild-type genotype, TGN (median 366 pmol) and MeMPN (median 8590 pmol) concentrations were similar to those in wild-type, high activity patients. In childhood ALL, TPMT activity should not be used to predict heterozygosity particularly in blood samples obtained at disease diagnosis. Genotype is a better predictor of TGN accumulation during chemotherapy.

Lennard L ，Cartwright CS ，Wade R ，Richards SM ，Vora A ... -  《-》

Thiopurine drugs in the treatment of childhood leukaemia: the influence of inherited thiopurine methyltransferase activity on drug metabolism and cytotoxicity.

Lennard L ，Welch JC ，Lilleyman JS 《-》

Mercaptopurine therapy intolerance and heterozygosity at the thiopurine S-methyltransferase gene locus.

Relling MV ，Hancock ML ，Rivera GK ，Sandlund JT ，Ribeiro RC ，Krynetski EY ，Pui CH ，Evans WE ... -  《JNCI-Journal of the National Cancer Institute》

Possible implication of thiopurine S-methyltransferase in occurrence of infectious episodes during maintenance therapy for childhood lymphoblastic leukemia with mercaptopurine.

Dervieux T ，Médard Y ，Verpillat P ，Guigonis V ，Duval M ，Lescoeur B ，Suciu S ，Vilmer E ，Jacqz-Aigrain E ... -  《LEUKEMIA》

Determinants of mercaptopurine toxicity in paediatric acute lymphoblastic leukemia maintenance therapy.

6-mercaptopurine (6-MP) is used in the treatment of childhood acute lymphoblastic leukaemia (ALL). Its red blood cell (RBC) metabolite concentrations (6-thioguanine [6-TGN] and 6-methylmercaptopurine nucleotides [6-MMPN]) are related to drug response. We investigated the impact of non-genetic covariates and pharmacogenetic polymorphisms affecting thiopurine methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA) on 6-MP metabolism and response. Sixty-six children with ALL treated according to EORTC 58951 protocol were included in this study. Six patients had a heterozygous genotype for the most common TPMT polymorphisms, nine for ITPA 94 C > A and 17 for ITPA IVS2+21 A > C. 6-MP metabolites concentrations were analyzed by mixed model analysis. During maintenance, steady-state RBC 6-TGN concentrations were lower in patients aged 6 years or younger (493 pmol/8 × 10(8) RBC) than in older children (600 pmol/8 × 10(8) RBC). 6-MMPN concentrations were low in patients with TPMT variant/wild-type ITPA (1862 pmol/8 × 10(8) RBC), intermediate in wild-type patients and high (16468 pmol/8 × 10(8) RBC) in patients wild-type TPMT/variant ITPA. A 6-MMPN threshold of 5000 pmol/8 × 10(8) RBC was associated with an increased risk of hepatotoxicity. In this study, age and both TPMT and ITPA genotypes influenced 6-MP metabolism. High 6-MMPN was associated with hepatotoxicity. These pharmacological tools should be used to monitor ALL treatment in children.

Adam de Beaumais T ，Fakhoury M ，Medard Y ，Azougagh S ，Zhang D ，Yakouben K ，Jacqz-Aigrain E ... -  《-》