Pharmacokinetics of quinidine and three of its metabolites in man.

Disposition parameters of quinidine and three of its metabolism, 3-hydroxy quinidine, quinidine N-oxide, and quinidine 10,11-dihydrodiol, were determined in five normal healthy volunteers after prolonged intravenous infusion and multiple oral doses. The plasma concentrations of individual metabolites after 7 hr of constant quinidine infusion at a plasma quinidine level of 2.9 +/- (SD) 0.3 mg/L were: 3-hydroxy quinidine, 0.32 +/- 0.06 mg/L; quinidine N-oxide, 0.28 +/- 0.03 mg/L; and quinidine 10,11-dihydrodiol, 0.13 +/- 0.04 mg/L. Plasma trough levels after 12 oral doses of quinidine sulfate every 4 hr averaged: quinidine, 2.89 +/- 0.50 mg/L; 3-hydroxy quinidine, 0.83 +/- 0.36 mg/L; quinidine N-oxide, 0.40 +/- 0.13 mg/L; and quinidine 10,11-dihydrodiol, 0.38 +/- 0.08 mg/L. Relatively higher plasma concentrations of 3-hydroxy quinidine metabolite after oral dosing probably reflect first-pass formation of this quinidine metabolite. A two-compartment model for quinidine and a one-compartment model for each of the metabolites described the plasma concentration-time curves for both i.v. infusion and multiple oral doses. Mean (+/- SD) disposition parameters for quinidine from individual fits, after i.v. infusion were as follows: Vl, 0.37 +/- 0.09 L/kg; lambda 1, 0.094 +/- 0.009 min-1; lambda 2, 0.0015 +/- 0.0002 min-1; EX2, 0.013 +/- 0.002 min-1; clearance (ClQ), 3.86 +/- 0.83 ml/min/kg. Both plasma and urinary data were used to determine metabolic disposition parameters. Mean (+/- SD) values for the metabolites after i.v. quinidine infusion were as follows: 3-hydroxy quinidine: formation rate constant kmf, 0.0012 +/- 0.0005 min-1, volume of distribution, Vm, 0.99 +/- 0.47 L/kg; and elimination rate constant, kmu 0.0030 +/- 0.0002 min-1. Quinidine N-oxide: kmf, 0.00012 +/- 0.00003 min-1; Vm, 0.068 +/- 0.020 L/kg; and kmu, 0.0063 +/- 0.0008 min-1. Quinidine 10,11-dihydrodiol: kmf, 0.0003 +/- 0.0001 min-1; Vm, 0.43 +/- 0.29 L/kg; and kmu, 0.0059 +/- 0.0010 min-1. Oral absorption of quinidine was described by a zero order process with a bioavailability of 0.78. Concentration dependent renal elimination of 3-hydroxy quinidine was observed in two out of five subjects studied.