Thiopurine methyltransferase and treatment outcome in the UK acute lymphoblastic leukaemia trial ALL2003

The influence of thiopurine methyltransferase (TPMT) genotype on treatment outcome was investigated in the United Kingdom childhood acute lymphoblastic leukaemia trial ALL2003, a trial in which treatment intensity was adjusted based on minimal residual disease (MRD). TPMT genotype was measured in 2387 patients (76% of trial entrants): 2190 were homozygous wild-type, 189 were heterozygous for low activity TPMT alleles (166 TPMT*1/*3A, 19 TPMT*1/*3C, 3 TPMT*1/*2 and 1 TPMT*1/*9) and 8 were TPMT deficient. In contrast to the preceding trial ALL97, there was no difference in event-free survival (EFS) between the TPMT genotypes. The 5-year EFS for heterozygous TPMT*1/*3A patients was the same in both trials (88%), but for the homozygous wild-type TPMT*1/*1 patients, EFS improved from 80% in ALL97% to 88% in ALL2003. Importantly, the unexplained worse outcome for heterozygous TPMT*1/*3C patients observed in ALL97 (5-year EFS 53%) was not seen in ALL2003 (5-year EFS 94%). In a multivariate Cox regression analysis the only significant factor affecting EFS was MRD status (hazard ratio for high-risk MRD patients 4·22, 95% confidence interval 2·97–5·99, P < 0·0001). In conclusion, refinements in risk stratification and treatment have reduced the influence of TPMT genotype on treatment outcome in a contemporary protocol.