Quantitative gas chromatographic determination of two oxidized metabolites of the diuretic mefruside in human urine, plasma and red blood cells.

A gas chromatographic method is reported for the quantitative analysis of two metabolites of mefruside, viz., 5-oxo-mefruside (mefruside lactone) and its hydroxy-carboxylic acid analogue in human body fluids. Use was made of extractive methylation as the derivatization technique, and quantitation was achieved, with a suitable internal standard, by means of a nitrogen-sensitive detector. Because the two metabolites are linked chemically through a lactone-open acid equilibrium, interconversion prior to their separation had to be avoided. A pH partitioning study was performed to find optimal separation conditions. The lactone could be extracted quantitatively at pH 7.4, without any trace of co-extracted hydroxy acid. The latter was extracted either at pH 2 directly (in the case of plasma and urine), or after conversion to the lactone at pH 7.4 (in the case of red cells or whole blood). Concentrations down to 25 ng per sample of both compounds could be analysed with a standard deviation of 5%. The two metabolites of mefruside equilibrated instantaneously between red cells and plasma in vitro. At 37 degrees, the red cell/plasma concentration ratio was 20 for the lactone, but only 0.1 for the open acid compound. 5-Oxo-mefruside was able to displace mefruside from its red blood cell binding sites in vitro.

Biliary excretion of chlorthalidone in humans.

A single oral dose of the diuretic chlorthalidone (100 or 200 mg) was given to six cholecystectomized patients with T-tube drainage of the common bile duct, and the 24 h bile and urine were collected during 3--7 days. Urinary recovery of chlorthalidone was 23--27 per cent of the dose, which is in the range of that in healthy volunteers. Chlorthalidone concentration in bile was 11--44 times lower than urine concentration in corresponding periods, and biliary recovery was only 0.6--1.4 per cent of the dose. When compared from equal periods of sampling of bile and urine, the same relative amount of drug was found in bile, whether the 100 or 200 mg dose had been given (viz., a fraction of 2.5--4.7 per cent and 2.5--5.7 per cent of corresponding urinary amounts respectively). It was concluded that excretion into bile constitutes only a minor route of elimination for unchanged chlorthalidone. Bile samples treated with glucuronidase and sulphatase showed no increase of chlorthalidone concentration. The open acid analogue of chlorthalidone, 3-(4-chloro-3-sulphamoylbenzoyl)-benzoic acid, was apparently not formed as a human metabolite, as evidenced by gas chromatographic analysis of both urine and bile.

Differential potentiometric method for determining dissociation constants of very slightly water-soluble drugs applied to the sulfonamide diuretic chlorthalidone.

A renewed application of potentiometric acid-base titrations is described, by which dissociation constants of practically water-insoluble drugs can be measured accurately. The method uses the difference in the amount of titrant between a suitable aqueous solvent and a solution of the drug in that solvent. Such potentiometric difference titrations were conducted on a 3.7 X 10(-4) M solution of chlorthalidone in 0.1 M aqueous KCl in the pH 3.5--10.6 range at 25 degrees. Nonlinear least-squares regression analysis was applied to the data. From four determinations, a value of 9.24 +/- 0.02 (mean +/- SEM) resulted for the apparent dissociation constant of the first chlorthalidone acid group. The thermodynamic dissociation constant was calculated at pKa1 = 9.35 (25 degrees) by using a correction for activity.

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.

Determination of chlorthalidone in plasma, urine and red blood cells by gas chromatography with nitrogen detection.

A sensitive and selective gas chromatographic method is described for determining the diuretic and antihypertensive drug chlorthalidone in plasma, urine and erythrocytes. Use is made of an alkali flame ionization detector (nitrogen detector), and the chlorthalidone and internal standard are chromatographed as methyl derivatives. Down to 10 ng of drugs in the biological sample can be measured accurately, with a standard deviation of 5%. Because the concentration of chlorthalidone found in erythrocytes is 50-100 times higher than that in plasma, the influence of haemolysis on the plasma concentration has been investigated. In addition, a pharmacokinetic study with human volunteers revealed that the apparent concentration of the drug found in plasma can be much too low (by more than 50%), if the plasma is not separated from the erythrocytes immediately after venipuncture. Precautions to be observed to ensure correct handling of blood samples (so that results for plasma concentrations will be reliable) are stressed. The findings have application in kinetic studies on chlorthalidone.