Thiopurine S-methyltransferase (TPMT) is a cytosolic enzyme that catalyzes the S-methylation of thiopurines. The TPMT locus is subject to genetic polymorphism, with heterozygous individuals (about 10% of Caucasians) having intermediate, and homozygous individuals (about 0.33% of Caucasians) having low TPMT activity. More than 95% of defective TPMT activity can be explained by the most frequent variant alleles TPMT*2, TPMT*3A, TPMT*3B, TPMT*3C and TPMT*3D. In the present study, we analyzed the association TPMT of genotype with minimal residual disease (MRD), before and after application of a four week cycle of 6-mercaptopurine (6-MP; 60 mg/m2/d) during induction consolidation treatment. Samples were derived from patients enrolled in our ongoing Berlin-Frankfurt-Muenster (BFM) trial on treatment of childhood ALL (ALL-BFM 2000) from 10/1999 to 03/2002. Routine bone-marrow samples were taken at diagnosis and after completion of induction (treatment day 33) and induction consolidation (treatment day 78). Semiquantitative estimation of MRD was performed by standardized PCR analysis of leukemia clone-specific immunoglobulin and T-cell receptor gene rearrangements and TAL1 deletions. In order to be included in the study presented here, patients had to have at least one MRD marker with a sensitivity of 10−4 and successful MRD monitoring at both follow-up time points (day 33 and day 78); MRD loads smaller than 10−4 were defined as negative. Genotyping was performed by a DHPLC method. We were able to analyze 814 patients with childhood ALL. 755 (92.8%) patients were homozygous for the TPMT wild-type allele, 55 (6.8%) were heterozygous, and 4 (0.5%) homozygous for a variant TPMT allele. Except for immunophenotype, no major differences with regard to characteristics known to be associated with treatment response were observed between patients homozygous for the wild-type allele and those being heterozygous. The four patients homozygous for a variant TPMT allele were already MRD negative after completion of remission induction on treatment day 33. Therefore, they were not included in further analyses. In patients homozygous for the wild-type allele or heterozygous, MRD levels on day 33 were equally distributed between the groups. However, when MRD levels were assessed on day 78, after administration of induction consolidation treatment, including a 4-week cycle of 6-MP, significant differences with regard to clearance of MRD were observed between patients homozygous for the TPMT wild-type allele and heterozygotes. For heterozygous patients, this distribution translated into a 2.4-fold reduction in risk of having measurable MRD after induction consolidation treatment (relative risk = 0.42; 95% confidence interval = 0.16–0.99; P = 0.047). This effect remained significant in multivariate analysis including immunophenotype (relative risk = 0.36; 95% confidence interval = 0.15–0.87; P = 0.023). Our results indicate that TPMT genotype has a substantial influence on MRD after administration of 6-MP during induction consolidation treatment of childhood ALL, most likely through modulation of 6-MP dose intensity. In addition, our data support a role for MRD in the assessment of treatment response to specific drugs.

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