Acute lymphoblastic leukemia (ALL) accounts for 30 % of all childhood malignancies. The patients undergo four phases of treatment, finishing with a late maintenance phase in which 6-mercaptopurine and Methotrexate are essential components. Insufficient treatment intensity in this phase is associated with increased risk of relapse. Excessive variation in the bioavailability of 6-mercaptopurine has been observed which can cause both risks of undertreatment/relapse as well as overtreatment with severe side effects.

In the attempt to achieve individualized 6-mercaptopurine dosing different approaches have been pursued. Nonetheless variation in bioavailability remains a problem.

Earlier, oral tablets of 50 mg (Purinethol) were the only administration form of 6-mercaptopurine and it was primarily designed for adult patients. Challenges with accurate dosing and getting the children to swallow the tablets have been a widespread problem, forcing the caregivers to divide or crush the tablets as well as having to administer different dosages over 2-3 days. Due to these problems, an oral liquid formulation of 6-mercaptopurine (Xaluprine) has been developed. However this oral liquid has only been tested on healthy adult volunteers, and not on the target group, childhood patients. This project will assess the bioavailability and plasma kinetics of oral liquid and tablet formulation of 6-mercaptopurine in children with acute lymphoblastic leukemia.

The investigators hypothesize to observe comparable plasma kinetics, in children with acute lymphoblastic leukemia when treated with 6-mercaptopurine in the form of a tablet and oral liquid formulation, as previously observed in healthy adults.

Inclusion Criteria:

• Childhood acute lymphoblastic leukemia patients, age 0-18 years at diagnosis, treated at the department of pediatrics and adolescent medicine, Rigshospitalet.
• Informed consent

Exclusion Criteria:

• None