Optimizing 6-mercaptopurine Therapy in Pediatric Acute Lymphoblastic Leukemia by Using Allopurinol

After one month of conventional maintenance therapy (MT) children and adolescents, treated for acute lymphoblastic leukemia on Nordic protocols and with wild type thiopurine methyltransferase (TPMT) are eligible for the study. They will first receive a 12 week phase with normal MT during which time repeated sampling of 6-mercaptopurine (6MP) metabolite levels and other laboratory parameters will be performed. After 12 weeks, allopurinol at a dose of 50 mg/sqm is added (simultaneously reducing the dose of 6MP by 50%) and during the next 12 weeks patients are monitored closely for toxicity and samples for determination of metabolite levels and hematological and liver toxicity are obtained regularly. If, after 4 weeks of allopurinol treatment, the levels of 6-thioguanine are below 200 nmol/mmol hemoglobin, the dose of allopurinol will be increased to 100 mg/sqm. Allopurinol treatment is continued for 12 weeks after which the patients switch to their original maintenance therapy.