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

Monotherapy treatment with chlorthalidone or amlodipine in the systolic blood pressure intervention trial (SPRINT)

This post hoc analysis of the Systolic Blood Pressure Intervention Trial (SPRINT) examined the performance of chlorthalidone (C) versus amlodipine (A) monotherapies. ANOVA was used to analyze the differences in systolic blood pressure (SBP) response between C and A. Logistic regression was used to examine monotherapy failure (adding a second antihypertensive agent or switching to a different antihypertensive agent) rates. Four hundred ninety-one participants were treated with C monotherapy (n = 210, mean dose = 22 mg/day) or A monotherapy (n = 281, mean dose = 7 mg/day). There was a significant difference in mean SBP reduction between the C and A monotherapies at the third visit (higher reduction with A, adjusted p = .018). Unadjusted analysis showed a higher failure with C in the standard treatment group. Although the average SBP at failure was higher and above the 140 mm Hg cutoff that indicated monotherapy failure with A (142.60) compared with C (138.40), more participants on C failed despite having SBP below the 140 cutoff. This was probably due to decisions made by the investigative teams to change the antihypertensive regimen, because, in their opinion, the clinical picture required it. After adjusting for baseline characteristics, C had higher failure than A only in the standard treatment group (1.64 odds ratio [OR], 95% CI 1.06-2.56, p = .028). A sub-analysis including participants who had never used antihypertensive treatment before randomization had similar results (2.57 OR, 95% CI 1.34-5.02, p = .004). Overall, in SPRINT chlorthalidone was associated with higher monotherapy failure than amlodipine in the standard treatment group because of decisions of the investigative teams.

Chlorthalidone: the forgotten diuretic

Chlorthalidone's safety and efficacy in the management of hypertension has been demonstrated in landmark trials. Despite understanding the effects of thiazides on urinary sodium excretion and intravascular volume, the exact mechanism of their antihypertensive effects is not clearly understood. Common compensatory mechanisms for decreases in circulating plasma volume include increased adrenergic tone and systemic vascular resistance, as well as increases in the renin-angiotensin-aldosterone system. Chlorthalidone has been shown to decrease platelet aggregation and vascular permeability and promote angiogenesis in vitro, which is thought to be, in part, the result of reductions in carbonic anhydrase-dependent pathways, including catecholamine-mediated platelet aggregation and downregulation of VEGF-C gene expression. This article reviews the comparative clinical data between chlorthalidone and hydrochlorothiazide, the pharmacologic properties that might explain some of their differences regarding half-life and efficacy, and what is known about the effect of chlorthalidone on intermediate endpoints.

Masking and manipulation

The list of prohibited substances in sports includes a group of masking agents that are forbidden in both in- and out-of-competition doping tests. This group consists of a series of compounds that are misused in sports to mask the administration of other doping agents, and includes: diuretics, used to reduce the concentration in urine of other doping agents either by increasing the urine volume or by reducing the excretion of basic doping agents by increasing the urinary pH; probenecid, used to reduce the concentration in urine of acidic compounds, such as glucuronoconjugates of some doping agents; 5alpha-reductase inhibitors, used to reduce the formation of 5alpha-reduced metabolites of anabolic androgenic steroids; plasma expanders, used to maintain the plasma volume after misuse of erythropoietin or red blood cells concentrates; and epitestosterone, used to mask the detection of the administration of testosterone. Diuretics may be also misused to achieve acute weight loss before competition in sports with weight categories. In this chapter, pharmacological modes of action, intended pharmacological effects for doping purposes, main routes of biotransformation and analytical procedures used for anti-doping controls to screen and confirm these substances will be reviewed and discussed.

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.

Mechanistic Approaches to Volume of Distribution Predictions: Understanding the Processes

PURPOSE

To use recently developed mechanistic equations to predict tissue-to-plasma water partition coefficients (Kpus), apply these predictions to whole body unbound volume of distribution at steady state (Vu(ss)) determinations, and explain the differences in the extent of drug distribution both within and across the various compound classes.

MATERIALS AND METHODS

Vu(ss) values were predicted for 92 structurally diverse compounds in rats and 140 in humans by two approaches. The first approach incorporated Kpu values predicted for 13 tissues whereas the second was restricted to muscle.

RESULTS

The prediction accuracy was good for both approaches in rats and humans, with 64-78% and 82-92% of the predicted Vu(ss) values agreeing with in vivo data to within factors of +/-2 and 3, respectively.

CONCLUSIONS

Generic distribution processes were identified as lipid partitioning and dissolution where the former is higher for lipophilic unionised drugs. In addition, electrostatic interactions with acidic phospholipids can predominate for ionised bases when affinities (reflected by binding to constituents within blood) are high. For acidic drugs albumin binding dominates when plasma protein binding is high. This ability to explain drug distribution and link it to physicochemical properties can help guide the compound selection process.

Hydrochlorothiazide versus chlorthalidone: evidence supporting their interchangeability

Thiazide diuretics are one of the preferred pharmacologic treatments for hypertension. Hydrochlorothiazide and chlorthalidone have been the 2 most commonly used diuretics in major clinical trials. Treatment guidelines and compendia often consider these 2 drugs interchangeable agents within the class of thiazide or thiazide-like diuretics. Many sources list them as equipotent. Despite these beliefs, there is some suggestion that cardiovascular outcomes are not necessarily the same with these 2 drugs. We conducted a literature search from 1960 to 2003 to identify studies that evaluated the pharmacokinetic and blood pressure-lowering effects of these 2 agents. There are significant pharmacokinetic and pharmacodynamic differences between these diuretics. Chlorthalidone is approximately 1.5 to 2.0 times as potent as hydrochlorothiazide, and the former has a much longer duration of action. Whether these pharmacokinetic and pharmacodynamic features cause differences in outcomes is not known.

Simultaneous determination of atenolol and chlorthalidone in plasma by high-performance liquid chromatography. Application to pharmacokinetic studies in man

An HPLC method developed to detect in a single run both atenolol and chlorthalidone, extracted from plasma, using two detectors (UV for chlorthalidone and fluorometric for atenolol) connected in series, is described. The drugs were separated on an ODS column at room temperature using a 0.05 M sodium dodecyl sulphate in phosphate buffer (pH 5.8)-n-propanol (95:5, v/v) solution, delivered at a flow-rate of 1.3 ml/min. Having ascertained the sensitivity (10 ng/ml of both drugs) and the intra-day reproducibility (pre-study validation), the reliability of the method was verified by inter-day assays (within-study validation) carried out during the analysis of plasma samples collected from healthy volunteers after single-dose treatment with atenolol+chlorthalidone tablets (pharmaceutical preparations containing 100+25 mg and 50+12.5 mg of the two drugs, respectively).

Detection of diuretic agents in doping control

Since the inclusion of diuretics in the list of banned substances in sports in 1988, a large number of screening and confirmation procedures to detect the presence of these substances in urine samples have been developed. In this paper, a review of the analytical methodology described to analyze diuretics is presented. The paper has been focused on the needs of doping control and mainly screening procedures including sample preparation and liquid or gas chromatographic separation have been considered. More relevant papers using capillary zone electrophoresis have been also considered. Mass spectrometry is mandatory in doping control for confirmation purposes, and finally, mass spectrometric techniques described for diuretics have been reviewed.

Bioequivalence: individual and population compartmental modeling compared to the noncompartmental approach

PURPOSE

The purpose of this study were to evaluate the use of individual compartmental and population compartmental methods for bioequivalence determination, and to determine their utility as adjuncts to the current methods used for bioequivalence assessment.

METHODS

Data from three bioequivalence studies of chlorthalidone were analyzed with PCNONLIN using individual compartmental modeling and NONMEM for population analyses. These results were compared with results obtained from the traditional noncompartmental or SHAM (slopes, heights, areas, and moments) approach for bioequivalence assessment and the 90% confidence interval procedure.

RESULTS

Individual compartmental modeling and population compartmental modeling techniques performed well on this routine set of bioequivalence data which displayed simple pharmacokinetic properties. A direct assessment of the analysis methods was made by comparing the final estimates and 90% confidence intervals for the test to reference ratios (T/R) of AUC and CMAX. The final estimates and 90% confidence intervals for AUC T/R and CMAX T/R were similar and suggest consistency of results, independent of the method used.

CONCLUSIONS

These results demonstrate the utility of modeling techniques as adjuncts to the traditional noncompartmental approach for bioequivalence determination.

Influence of chlorthalidone on the pharmacokinetics and pharmacodynamics of Org 10172 (Lomoparan), a low molecular weight heparinoid, in healthy volunteers

The influence of chlorthalidone (100 mg PO) on the pharmacokinetics and pharmacodynamics of Org 10172 (IV bolus injection of 3250 anti-Xa units), a low molecular weight heparinoid, was studied in six healthy male volunteers using an open randomized two-way crossover design. Chlorthalidone produced a slight decrease in clearance of anti-Xa activity from 7.1 +/- 1.0 to 6.6 +/- 0.8 mL/min and a decrease of the volume of distribution from 0.20 +/- 0.05 to 0.16 +/- 0.04 L/kg, whereas the volume of distribution of antithrombin activity increased from 0.14 +/- 0.05 to 0.26 +/- 0.10 L/kg (all differences P less than .05). During the entire study period no adverse events occurred. In summary, chlorthalidone showed separate effects on different fractions of Org 10172. The clinical implication of the slight change observed in plasma anti-Xa activity is likely to be limited, whereas the 80% increase in distribution volume of plasma antithrombin activity can not be defined as yet in terms of clinical relevance.

A decade of developments in diuretic drug therapy

New diuretics introduced into clinical medicine during the past decade include potent new loop diuretics such as bumetanide and piretanide, the uricosuric indanyloxyacetic acid derivative indacrinone, and a new generation of sulfamoyl diuretics such as indapamide and xipamide, which are recommended primarily for the treatment of hypertension. Pharmacokinetic studies of individual diuretics have demonstrated that the diuretic and natriuretic responses to the newer agents generally follow the plasma drug concentration-time curves and urinary drug excretion rates. Therapeutic monitoring can therefore be achieved in most patients with edema or hypertension by close clinical observation and laboratory analysis of plasma electrolyte and creatinine concentrations and urinary electrolyte excretion rates. Interest in the mechanisms involved in the renal and extrarenal vascular actions of the newer diuretics has led to a better understanding of how changes in venous compliance, peripheral vascular resistance, and renal blood flow distribution may contribute to the overall therapeutic response to these agents, especially in patients with severe congestive heart failure, renal insufficiency with low glomerular filtration rates, and hypertension with cardiorenal complications. Adverse reactions to modern diuretics, which are mainly an extension of their renal pharmacodynamic effects, have proved to be minimal, provided that the dosage is adjusted to meet but not exceed individual patient requirements. However, the long-term consequences of prolonged periods of diuretic-induced alterations in plasma potassium levels, and metabolic effects that include elevated blood lipids, are still under investigation.

Degradation of chlorthalidone in methanol: kinetics and stabilization

The reaction of chlorthalidone with methanol to give the corresponding methyl ether was investigated. The kinetics are pseudo-first-order in chlorthalidone, but the observed pseudo-first-order rate constants show an unexpected dependence on the initial chlorthalidone concentration, attributable to the presence of trace catalytic impurities in commercial chlorthalidone. Evidence is presented to show that trace heavy metals are probably responsible for the primary catalytic effect. Trace quantities of acetic acid are also present and show a smaller secondary catalytic effect. Kinetics in the presence of added heavy metals and acetic acid were examined. EDTA and povidone reduce the degradation rate. Stabilization by EDTA is due to its ability to chelate heavy metals. Stabilization by povidone is also primarily due to its ability to complex heavy metals; complexation data of ferric and nickel ions with povidone and with 1-methyl-2-pyrrolidinone as a monomer model are presented. In addition, complexation constants were calculated for the interaction of povidone with chlorthalidone, which may also play a role in stabilization.

Comparative pharmacokinetic profiles of metoprolol and chlorthalidone administered alone or in combination to healthy volunteers

A potential pharmacokinetic interaction between the beta-blocking drug, metoprolol, and the diuretic, chlorthalidone, has been investigated in three single or multiple dose studies in healthy volunteers. The pharmacokinetic profile of metoprolol 100 mg was not affected by pretreatment with or co-administration of chlorthalidone 25 mg twice daily. Similarly, the pre-dosing steady-state level of chlorthalidone during chronic treatment and its blood level profile after a single 25 mg dose were not affected by metoprolol. The bioavailabilities of the 2 drugs administered in combination were identical to those observed when each drug was administered alone. These studies demonstrate that there is no pharmacokinetic interaction between metoprolol and chlorthalidone when doses of 100 and 25 mg, respectively, are co-administered twice daily.

Relative bioavailability of chlorthalidone in humans: adverse influence of polyethylene glycol

The bioavailability of two commercial preparations of chlorthalidone was studied in healthy male subjects. Reference solutions/suspensions for the two products were chlorthalidone dissolved in a solution of water-polyethylene glycol and a solution/suspension of chlorthalidone. Bioavailability of the chlorthalidone in water-polyethylene glycol solution was significantly reduced in comparison to one of the commercial preparations, and trends in the data suggested that it was less well absorbed than either the chlorthalidone in water solution/suspension or the other commercial preparation of chlorthalidone. These data, together with previous reports indicating that polyethylene glycol may retard the absorption of some drugs in vitro, suggest that this compound should not be used to aid dissolution of drug in a reference standard for bioavailability investigations.

Long-acting and short-acting diuretics in mild essential hypertension

A randomised, placebo-controlled, double-blind crossover study was conducted in 14 patients with mild essential hypertension comparing different regimens of administration of the "short-acting" diuretic, chlorothiazide and the "long-acting" diuretic, chlorthalidone. There were 6 randomised treatment phases each of 5 weeks duration. For blood pressure recorded both at the Clinic and at home the most prominent effects were seen with standing blood pressure, for which significant reductions of mean blood pressure compared to the placebo phase were observed with chlorthalidone 25 mg once daily (-5 +/- 1 (SE) mm Hg - p less than 0.05), chlorthalidone 50 mg once daily (-7 +/- 1 mm Hg - p less than 0.01) and chlorothiazide 500 mg twice daily (-4 +/- 1 mm Hg - p less than 0.05). Blood pressure reductions with chlorothiazide 500 mg once daily and 1000 mg once daily were not significant. About half of the patients completing the trial could be described as "non-responders". The observed biochemical changes (reduced plasma potassium and chloride concentrations and increased plasma bicarbonate and urate concentrations) were most marked in the phases with the most prominent blood pressure effects. The result support the suggestion that a sustained diuretic effect is desirable when diuretics are used in the treatment of hypertension. This can be obtained by selection of the appropriate dosage regimen for a particular diuretic, such as once daily administration for "long-acting" and twice daily for "short-acting" thiazide-type diuretics.

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.

Pharmacokinetics of chlorthalidone. Dependence of biological half life on blood carbonic anhydrase levels

The relationship between the whole blood concentration of carbonic anhydrase and the biological half life of chlorthalidone has been investigated in six volunteers. A linear relationship was observed and on the basis of this, a pharmacokinetic model to explain the long and variable biological half life of chlorthalidone is proposed and discussed.

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.