Disturbance of 6-mercaptopurine metabolism by cotrimoxazole in childhood lymphoblastic leukaemia.

The effect of cotrimoxazole on the utilization of 6-mercaptopurine (6MP) was studied in a group of children receiving remission maintenance treatment for lymphoblastic leukaemia (ALL). This was done by measuring the level of an active metabolite of 6MP, 6-thioguanine nucleotide (6TGN), and comparing it both with the drug dose and with subsequent neutropenia in the presence or absence of concurrent cotrimoxazole. In children who were not taken cotrimoxazole, the concentration of 6TGN showed a significant positive correlation with the dose and a significant negative correlation with the absolute neutrophil count 2 weeks later. In those who were taking the antibiotic both these relationships were lost. This suggests that cotrimoxazole can interfere with both the absorption and the cytotoxicity of 6MP and may, in turn, alter its antileukaemic effect.