Thiopurine Methyltransferase Genetics Is Associated with Treatment Outcome and Hepatic Toxicity in Pediatric Patients with Acute Lymphoblastic Leukemia: a Report From the ALL-BFM Study Group.

Abstract 2625

Poster Board II-601

The thiopurines 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG) play an essential role in treatment protocols for acute lymphoblastic leukemia (ALL). Thiopurine methyltransferase (TPMT) is a key enzyme in the metabolism of thiopurines and underlies phenotypically relevant genetic variation. Heterozygotes or homozygotes for TPMT genotypes conferring lower enzyme activity demonstrate thiopurine drug metabolic patterns distinct from those of TPMT wild-type individuals. Underlining its clinical importance, several studies have demonstrated a relationship between low TPMT enzyme activity and thiopurine-associated toxicity as well as decreased relapse risk. Here we report on a prospective evaluation of the role of TPMT genetics for survival and treatment-related toxicity in a cohort of 814 pediatric ALL patients. These 814 patients were initially selected based on availability of DNA and represent 85.1% of the entire patient population (n=956) enrolled in the German-Austrian-Swiss multi-center trial ALL-BFM 2000 from October 1999 to September 2002. Genotyping for TPMT was performed by a denaturing HPLC method and subsequent sequencing of variant alleles using DNA prepared from either leukemic or remission bone marrows. This analysis revealed 755 (92.8%) patients with TPMT wild-type, 55 (6.8%) with a heterozygous, and 4 (0.5%) with a homozygous variant genotype (*2/*3A, *3A/*3A [n=2], *3A/*11), respectively. Genotype frequencies were in Hardy-Weinberg equilibrium. Allele frequencies were as follows: TPMT*1 = 96.12%, TPMT*2 = 0.25%, TPMT*3A = 2.95%, TPMT*3C = 0.56%, TPMT*9 = 0.06%, and TPMT*11 = 0.06%. Patients (n=55) heterozygous for allelic variants of TPMT conferring lower enzyme activity demonstrated significantly better event-free survival (EFS) and a lower relapse rate compared to homozygous wild-type patients (n=755) (six-years pEFS; heterozygotes vs. wild-type, 95% (SE 3%) vs. 84% (SE 1%), p(log-rank) = 0.04; p(point estimate difference) = <0.001, relapse incidence at six years, 4% (SE 3%) vs. 12% (SE 3%), p = 0.07). In a Cox regression analysis, adjusting for sex, age, presenting leukocyte count, immunophenotype and minimal residual disease the effect of TPMT genotype was still detectable, but lost statistical significance (hazard ratio for TPMT heterozygosity = 0.38, p = 0.10). An analysis stratified by minimal residual disease-defined risk groups will be presented. While TPMT heterozygotes did not demonstrate statistically significant differences when their toxicity data collected according to the National Cancer Institute's Common Toxicity Criteria were compared with wild-type patients for 6-MP-containing treatment phases, they had an increased risk of developing hepatic veno-occlusive disease associated with a two-week exposure towards 6-TG given during re-intensification. In conclusion, TPMT genotyping may contribute important information for clinical decision making in childhood ALL that goes beyond the prevention of toxicity in TPMT deficient patients.