6-Mercaptopurine in cerebrospinal fluid during oral maintenance therapy of children with acute lymphoblastic leukemia.

In three children receiving oral remission maintenance therapy for acute lymphoblastic leukemia, the concentrations of 6-mercaptopurine (6-MP) in cerebrospinal fluid (CSF), plasma and red blood cells were compared. CSF samples were obtained from an Ommaya reservoir previously inserted for treatment of CNS relapse. At the time of the study, the children were all in remission and had been on oral 6-MP (42-63 mg m-2) once daily for at least 24 weeks. Immediately before dose intake on the day of study (about 24 h after last dose), the concentrations of 6-MP in CSF, plasma and red blood cells were rather similar and below 20 ng ml-1 in all patients. After dose intake, the concentrations in plasma and in red blood cells increased to 40-200 ng nl-1 within 0.5-4 h. In contrast, the concentration of 6-MP in the CSF remained fairly constant around 4-10 ng ml-1 throughout the time period studied (up to 4 h). It is concluded that 6-MP can be detected in CSF during oral maintenance therapy and that the drug has different pharmacokinetic profile in CSF to that in plasma and red blood cells. Further studies are necessary to evaluate the significance of the 6-MP concentrations obtained in CSF for the prevention of CNS relapse.