Plasma and erythrocyte concentrations of mercaptopurine after oral administration in children.

Plasma and erythrocyte concentrations of 6-mercaptopurine (6-MP) were determined by gas chromatography-mass spectrometry. Eleven children (9 with acute lymphatic leukemia) were studied after oral intake of 6-MP doses ranging between 31 and 128 mg/m2 body surface area. The concentrations of 6-MP in plasma were found to vary considerably between patients even after dose normalization to 75 mg/m2. After dose normalization the mean peak plasma concentration was 0.68 microM (range 0.12-1.38) and the area under the plasma concentration-time curve (AUC) was 1.37 microM.h (range 0.12-3.04). The mean time taken to reach the peak concentration was 1.3 h (range 1-2), and the half-life of elimination was 1.8 h (range 0.6-2.5). No patient had detectable 6-MP concentrations 12 h after dose intake. The concentrations of 6-MP tended to be higher in erythrocytes than in plasma. The mean peak concentration in erythrocytes was 131% and the AUC 145% of that found in plasma. The mean half-life of elimination from erythrocytes was 2.0 h (range 0.7-2.8). These data indicate that 6-MP can pass through all membranes rapidly to reach intracellular concentrations equal to or even higher than in plasma. In summary, marked interindividual differences in pharmacokinetics were found, probably due to highly variable bioavailability of oral 6-MP. Further studies are needed to determine whether measurements of plasma concentrations of 6-MP can be used to optimize maintenance treatment of childhood leukemia.