Pharmacokinetics of midazolam in Mexicans - Evidence for interethnic variability

Objective: To determine midazolam pharmacokinetics in healthy Mexican volun teers and to compare them with the pharmacokinetics reported for other popu lations. Patients and Methods: Eleven healthy Mexican (mestizo) males received a sin gle midazolam dose intravenously (7.5mg) and orally (15mg) with a washout p eriod of at least 7 days. Midazolam plasma concentration was determined by high performance liquid chromatography. Pharmacokinetic parameters were obt ained and compared with historical controls reported for other populations. Results: Midazolam pharmacokinetic parameters in Mexicans determined after intravenous administration were (mean +/- SEM): clearance 2.54 +/- 0.43 ml/ min/kg, volume of distribution 0.52 +/- 0.1 L/kg and half-life 2.61 +/- 0.4 2 hours. After oral administration the area under the plasma concentration- time curve (AUC) was 941 mu g/L.h and the maximum concentration was 271 mu g/L. There was a significant correlation between AUC values observed after oral midazolam and clearance (r = -0.71, p < 0.02). Systemic midazolam clea rance observed in Mexicans was lower, while oral bioavailability was higher , compared with the values reported in the literature for healthy Caucasian volunteers under similar conditions. Conclusion: Midazolam bioavailability in Mexicans is higher than that repor ted for Caucasians. This appears to be because of a reduced systemic cleara nce. Since midazolam is mainly eliminated via biotransformation by CYP3A4, our data suggest the existence of interethnic variations in the activity of this enzymatic system. Increased midazolam bioavailability may lead to an increased incidence of adverse effects. Therefore, administration of midazo lam should not be blindly extrapolated from one population to another. Mida zolam doses in Mexicans should be reduced in relation to those used in Cauc asians.