Absolute bioavailability of chlorthalidone in man: a cross-over study after intravenous and oral administration

Seven normal human volunteers each received a constant-rate infusion of chlorthalidone for 2 h, and the same (commonly 50 mg) single oral dose on separate occasions. The concentration of unchanged chlorthalidone was analyzed over a 100 to 220 h period in plasma, red blood cells, urine and faeces after both dosage forms. A three compartment model was required to describe the intravenous plasma concentrations in five of the subjects. A two compartment model sufficed to account for the decay of the oral plasma concentrations in all seven subjects. The mean plasma t1/2 after i.v. dosing was 36.5 h (+/- 10.5 SD), and the mean plasma t1/2 after oral doses was 44.1 h (+/- 9.6 SD). The mean red blood cell concentration t1/2 after i.v. doses was 46.4 h (+/- 9.9 SD), and the mean red blood cell t1/2 after the oral doses was 52.7 h (+/- 9.0 SD). The shorter i.v. half-live was not equally manifest in all subjects, being mainly apparent in three of them. In all cases the urinary excretion rate plots were parallel to the plasma concentration curves. As the faster decay after i.v. administration was not accompanied by increased renal clearance, the difference must have been due to non-renal mechanism. The mean total of 65.4 (+/- 8.6 SD) % of the intranvenous dose was excreted in urine over infinite time, whereas the mean total excretion after the oral dose was 43.8 (+/- 8.5 SD) %. Faecal excretion ranged from 1.3--8.5% of dose in the i.v. study to 17.5--31.2% of dose in the oral study. The sum of the amounts present in urine plus faeces pointed strongly to an important metabolic route of elimination of chlorthalidone. Bioavailability estimates (F) from three sets of data were--a mean F of 0.61 from plasma concentrations, 0.67 from urinary excretion measurements and 0.72 from the erythrocyte concentrations. Simulations with a non-linear model indicated lesser validity of the estimate from erythrocyte concentrations. It was concluded that the average of plasma and urine data, F = 0.64, yielded the best estimate of the oral availability of chlorthalidone 50 mg in man.