Mizoribine requires individual dosing due to variation of bioavailability.

Background: Mizoribine (MZR) is an immunosuppressant used for the treatment of glomerular diseases, but there are few reports on the pharmacokinetics of MZR in children.

Methods: First, we performed a pharmacokinetic study on nine childhood-onset glomerular disease patients. The MZR dosages ranged from 1.8 to 14.5 mg/kg/dose. Pharmacokinetic parameters were analyzed using 38 MZR concentration-time curves. Second, nine patients who were newly treated with MZR were enrolled to validate the findings obtained from prior investigation.

Results: In the prior study, peak serum MZR concentration (C(max) ) was dose-dependent in each patient. Although proportionality between dosage and C(max) was observed in each patient, the regression coefficient was in a wide range from 0.075 to 1.04 and was specific to each patient. This variability was likely caused by individual variation of bioavailability. When the optimal time-point to monitor C(max) was investigated, the time-to-reach peak serum MZR concentration (T(max)) was similar among all the patients, which was from 2.5 to 3.5 h after administration of MZR. T(max) was most frequently observed at 3 h and the serum MZR concentration ratio relative to C(max) at 3 h was also highest (0.93 ± 0.07). In the following study, it was validated that monitoring C(3) is reproducible and reliable after adjusting the dosage of MZR to obtain target serum concentration.

Conclusion: Individual dosing is required to optimize C(max) in childhood-onset glomerular disease patients. The safe dosage of MZR for each patient could be predicted by evaluating the serum MZR concentration 3 h after administration.