S35-LABELED BENDROFLUMETHIAZIDE IN HUMAN BEINGS: CLINICAL STUDIES

Effect of food on the bioavailability of bendroflumethiazide

Bendroflumethiazide (BFT), 10 mg, was given orally to eight subjects after fasting overnight and together with a meal. Concentrations of the diuretic in plasma and urine were determined by GLC. As judged by AUC and urinary recovery of BFT, the bioavailability of the diuretic was not influenced by concomitant intake of a meal.

Aspects on pharmacokinetics of some diuretics

This review summarizes the present knowledge of some commonly used diuretics. Bendroflumethiazide and bumetanide are completely absorbed from the gut while the uptake of hydrochlorothiazide, chlorthalidone and furosemide averages about 65%. The degree of uptake of amiloride and spironolactone is unknown but exceeds 50%. Plasma t 1/2 of bumetanide and furosemide are approximately 1 h. The clinically important phase of the plasma concentration of bendroflumethiazide has a t 1/2 of 3 h, although a slower phase with a t 1/2 of 9 h has been described. Hydrochlorothiazide and amiloride, often used in combination, both have a t 1/2 of about 10 h. Canrenone, an active metabolite of spironolactone, has a t 1/2 of 15-20 h. Chlorthalidone is eliminated very slowly with a t 1/2 of about two days. This is partly caused by an extensive binding to carbonic anhydrase in the erythrocytes. The protein binding of bendroflumethiazide, bumetanide, canrenone and furosemide is approximately 95%. The binding of chlorthalidone and hydrochlorothiazide is about 75 and 40% respectively. All mentioned diuretics except spironolactone are in part eliminated renally, mainly via tubular secretion. This is the major elimination route for amiloride and hydrochlorothiazide, while it constitutes one third to two thirds for bendroflumethiazide, bumetanide and furosemide. Spironolactone is exclusively eliminated as metabolites.

Does cantharides blister fluid provide access to the peripheral compartment?

The pharmacokinetics of bendroflumethiazide (BFT) was investigated following the oral administration of 10 mg to 3 healthy volunteers. Each subject participated twice in the study. BFT was determined in plasma and cantharides blister fluid from 1/2 to 30 h post administration. Blister fluid was obtained from blisters 10-22 h old. Plasma levels were fitted to a tri-exponential equation and the concentration of the drug in the peripheral compartment was calculated from the microscopic rate constants. In 5 of 6 cases investigated, cantharides blister fluid levels paralleled the concentration of the drug in the peripheral compartment. The mean blister fluid levels exceeded the calculated concentration in Compartment 2 1.46 fold. In one case, the blister fluid level paralleled the plasma level. This subject clearly differed from the others as more than 10 h were required for blister formation in her. The results suggest that following the administration of BFT, cantharides blister fluid behaves as part of the peripheral compartment. The possible value of studying blister fluid levels in pharmacokinetic investigations is discussed.

Pharmacokinetics of bendroflumethiazide alone and in combination with propranolol and hydralazine

Bendroflumethiazide (Bft) was administered to 6 healthy subjects at 3 different dose levels (2.5, 5 and 10mg) in a cross-over design, either as capsules (2.5mg) or as tablets (5mg). Its pharmacokinetics were evaluated then and following administration of a fixed combination of Bft and propranolol and hydralazine to a further 7 volunteers. Plasma and urinary concentrations of Bft were determined by a new fluorimetric - thin-layer chromatography procedure. Peak plasma levels occurred after 2-3h and averaged 15, 27 and 45 microgram/l in the three dose groups. Areas under the plasma concentration - time curves (AUC0 leads to 12), which were 75, 147 and 250 microgram 1(-1)h respectively, and cumulative urinary recovery (20%) were independent of the dose administered and the type of formulation. Thus Bft kinetics proved to be linear within the dose range evaluated. The plasma clearance was calculated to be 505 ml/min, renal clearance 108 ml/min and nonrenal clearance 396 ml/min. Bioavailability of Bft was not altered following administration of the fixed combination. The amount of propranolol found in the circulation did not change, whereas that of hydralazine (determined as apparent hydralazine) increased by 59% when the fixed combination was administered.

Pharmacokinetics of bendroflumethiazide after low oral doses

The pharmacokinetics of bendroflumethiazide after oral administration of 1.25, 2.5, and 5.0 mg were studied in nine healthy male volunteers. Bendroflumethiazide was analyzed by GLC after extractive alkylation. After the lowest dose, the plasma concentration, could be followed to 14 hr, and the data were adequately fitted by a one-compartment model; the half-life was 3.1 hr. After the 2.5 and 5.0 mg doses, the plasma concentration was followed for 24 hr, and the data were fitted by a two-compartment model with half-lives of 8.9 hr. The urinary sodium concentration was doubled after bendroflumethiazide intake, but the urinary potassium concentration remained almost constant. The renal clearance of bendroflumethiazide was around 30 ml . min-1.

Pharmacokinetics of bendroflumenthiazide in hypertensive patients

After four weeks on placebo treatment, 8 hypertensive patients (WHO stage I) were treated for 2 weeks with bendroflumethiazide (bft) 2.5 mg and KCl 1.5 g daily. Subsequently they received bft 5 mg and KCl 1.5 g daily for a further fortnight. At the end of each period of treatment blood pressure was recorded and blood samples and urine were collected for analysis of bft by GLC. Before taking the daily dose of bft, no trace of the drug was found in plasma. Peak levels of bft were seen after 2.3 h and averaged 23 and 50 ng . ml-1 after 2.5 and 5 mg, respectively. After bft 2.5 mg the plasma level was too low for kinetic analysis. The plasma half-life after 5 mg averaged 4.1 h. The mean apparent volume of distribution was 1.18 1 . kg-1. Non-renal clearance averaged 200 ml . min-1. The renal clearance of bft was significantly lower (p less than 0.05) after 5 mg (48 ml . min-1) than after 2.5 mg bft (93 ml . min-1), although the creatinine clearance remained unchanged. No correlation was found between the plasma level of bft and its effect on blood pressure.