The single-dose pharmacokinetics of midazolam and its primary metabolite in pediatric patients after oral and intravenous administration

The first-dose pharmacokinetics of midazolam and its primary alpha -hydroxy metabolite were studied after single-dose administration. Eligible study patients were enrolled into one of three study arms: Arm I (midazolam/metabolite pharmacokinetic evaluation after oral administration of a syrup formulation), Arm II (the absolute bioavailability of midazolam syrup), and Arm III (midazolam and metabolite pharmacokinetics after IV administration). Complete blood sampling for pharmacokinetic analysis was available in 87 subjects. Midazolam absorption after administration. of the oral syrup formulation was rapid, with adolescents absorbing the drug at approximately half the rate observed in younger children (ages 2 to < 12 years). Furthermore, midazolam t(1/2) was prolonged and CL/F reduced in adolescents as compared with younger children. Although the midazolam V-d/F appeared larger in the youngest age group after oral administration, this observation was not apparent after IV dosing, suggesting subject differences in bioavailability rather than distribution. Like midazolam, the disposition characteristics for alpha -hydroxymidazolam were also highly variable, with the greatest formation of metabolite (reflected by the AUC ratio) observed in children ages 2 to < 12 years. The AUC ratios of alpha -hydroxymidazolam to midazolom after IV dosing were similar across all age groups and were smaller than corresponding values following oral administration. The absolute bioavailability of midazolam averaged 36% with a very broad range (9%-71%). No relationship between midazolam bioavailability and age was observed. Overall, the disposition characteristics of midazolam and its alpha -hydroxy metabolite were highly variable, appeared independent of age and dose administered, and were linear over the dose range studied (0.25 to 1 mg/kg). These data suggest that an initial oral dose of 0.2 to 0.3 mg/kg should be adequate for successful sedation of most pediatric patients. The inherent variability in midazolom bioavailability and metabolism underscores the importance of titrating midazolam dose to desired effect. (C) 2001 the American College of Clinical Pharmacology.