Childhood lymphoblastic leukaemia: sex difference in 6-mercaptopurine utilization

Twelve boys and 10 girls on similar long term remission maintenance treatment for lymphoblastic leukaemia had 79 random assays of their red cell 6 thioguanine nucleotide ( 6TGN ) concentrations performed as an index of cytotoxic activity generated by oral 6-mercaptopurine ( 6MP ). Correlation between the dose of 6MP and 6TGN was statistically significant in the girls (r = 0.58, P less than 0.001) but not in the boys (r = 0.15). Additionally, as a group the boys tolerated more 6MP (P less than 0.05), despite similar prescribing criteria, but this did not result in a higher mean 6TGN concentration or increased myelotoxicity. It appears that girls develop 6MP cytotoxicity at lower doses and more predictably than boys. If so, this may be relevant to the as yet unexplained but marked sex difference in prognosis apparent in some studies.