[Hemolysis after combined azathioprine and allopurinol therapy]

In a 37 year old renal transplant woman haemolytic anaemia was observed in the course of a combined azathioprine-allopurinol therapy. Azathioprine is converted to 6-mercaptopurine, which then catalyzed by xanthine oxidase is oxidized to 6-thiouric acid. When allopurinol and azathioprine are administered simultaneously, high 6-mercaptopurine concentrations in plasma may be expected because of inhibition of xanthine oxidase activity by allopurinol. We assume that elevated plasma 6-mercaptopurine concentration could possibly be responsible for haemolytic anaemia in this renal transplant patient.