Meta-analysis of randomised trials comparing thiopurines in childhood acute lymphoblastic leukaemia

Mercaptopurine has been used in continuing treatment for childhood acute lymphoblastic leukaemia since the mid 1950s. Recent advances in the understanding of thiopurine pharmacology indicated thioguanine might be more effective than mercaptopurine. The US and UK cooperative groups began randomized thiopurine trials and agreed prospectively to a meta-analysis. All randomized trials of thioguanine versus mercaptopurine were sought and data on individual patients were analysed by standard methods. Combining three trials (from US, UK and Germany), the overall event free survival (EFS) was not significantly improved with thioguanine (Odds ratio (OR) = 0.89; 95% confidence interval 0·78–1·03). Apparent differences in results between trials may be partly explained by the different types of patients studied. The larger treatment effect reported in males in the US trial was confirmed in the other trials. There was heterogeneity between sex/age subgroups (p=0·001), with significant EFS benefit of thioguanine only seen for males aged under 10 years old (OR=0·70; 0·58–0·84), although this did not result in a significant difference in overall survival (OR=0·83; 0·62–1·10). Additional toxicity occurs with thioguanine. Mercaptopurine remains the standard thiopurine of choice, but further study of thioguanine may be warranted to determine whether it may benefit particular subgroups.