Clofibrate kinetics after single and multiple doses

The kinetics of chlorphenoxyisobutyric acid (CPIB) were studied in 5 healthy subjects after single 500-mg, 1,000-mg, and 2,000-mg doses of clofibrate, and in steady-state after 8 days' treatment with 1,000 mg twice daily. Maximum plasma concentrations of CPIB were observed 4 to 6 hr after dosing. A mean plasma half-life of 16.7 hr was recorded which was independent of dose and duration of treatment. Total plasma clearance (-Cl) calculated from area under the curve with the use of the total plasma concentration was 5.6 ml/min for the 500-and the 1,000-mg doses but increased to 6.8 ml/min for the 2,000-mg dose and was even higher (8.1 ml/min) in steady-state. This change in -Cl is a consequence of progressive reduction in the plasma protein binding of clofibrate at plasma concentrations above 50 microgram/ml, since -Cl rises in association with reduced protein binding at the high plasma concentrations measured after the 2,000-mg single dose and in steady-state. -Cl and apparent volume of distribution were identical for all doses tested when calculations were based on the nonprotein-bound CPIB concentrations only. Due to the inconsistant protein binding of CPIB, total steady-state concentrations could not be predicted from the single dose kinetic data.