Clinical pharmacokinetics of diuretics

Thiazide and the Thiazide-Like Diuretics: Review of Hydrochlorothiazide, Chlorthalidone, and Indapamide

The term thiazide is universally understood to refer to diuretics that exert their principal action in the distal tubule. The thiazide class is heterogenous and can be further subdivided into compounds containing the benzothiadiazine ring structure-the thiazide-type (e.g., hydrochlorothiazide)-and those lacking the benzothiadiazine ring-the thiazide-like (e.g., chlorthalidone and indapamide) drugs. Thiazide-like agents are longer acting and constitute the diuretics used in most of the cardiovascular outcome trials that established benefits of treatment with diuretics, but pragmatic aspects, such as lack of availability in convenient formulations, limit their use. Regardless of class heterogeneity, thiazides have retained importance in the management of hypertension for over 60 years. They are reliably effective as monotherapy in a majority of hypertensive patients, and augment the efficacy of other classes of antihypertensives when used in combination. Importantly, a thiazide-based treatment regimen lowers cardiovascular events, and their sturdy effect reinforces their place among the recommended first-line agents to treat hypertension in major domestic and international hypertension guidelines. There are few head-to-head comparisons within the class, but potential differences have been explored indirectly as well as in non-blood pressure mechanisms and potential pleiotropic properties. Until proven otherwise, the importance of these differences remains speculative, and clinicians should assume that cardiovascular events will be lowered similarly across agents when equivalent blood pressure reduction occurs. Thiazides remain underutilized, with only about one-third of hypertensive patients receiving them. For many patients, however, a thiazide is an indispensable component of their regimen to achieve adequate blood pressure control.

Tailoring Chlorthalidone Aqueous Solubility by Cocrystallization: Stability and Dissolution Behavior of a Novel Chlorthalidone-Caffeine Cocrystal

A cocrystal of the antihypertensive drug chlorthalidone (CTD) with caffeine (CAF) was obtained (CTD-CAF) by the slurry method, for which a 2:1 stoichiometric ratio was found by powder and single-crystal X-ray diffraction analysis. Cocrystal CTD-CAF showed a supramolecular organization in which CAF molecules are embedded in channels of a 3D network of CTD molecules. The advantage of the cocrystal in comparison to CTD is reflected in a threefold solubility increase and in the dose/solubility ratios, which diminished from near-unit values for D_0D to 0.29 for D_0CC. Furthermore, dissolution experiments under non-sink conditions showed improved performance of CTD-CAF compared with pure CTD. Subsequent studies showed that CTD-CAF cocrystals transform to CTD form I where CTD precipitation inhibition could be achieved in the presence of pre-dissolved polymer HPMC 80–120 cPs, maintaining supersaturation drug concentrations for at least 180 min. Finally, dissolution experiments under sink conditions unveiled that the CTD-CAF cocrystal induced, in pH-independent manner, faster and more complete CTD dissolution when compared to commercial tablets of CTD. Due to the stability and dissolution behavior of the novel CTD-CAF cocrystal, it could be used to develop solid dosage forms using a lower CTD dose to obtain the same therapeutic response and fewer adverse effects.

An Investigation into the Factors Governing Drug Absorption and Food Effect Prediction Based on Data Mining Methodology

Drug absorption is a complex process governed by a number of interrelated physicochemical, biopharmaceutical, and pharmacokinetic factors. In order to explore complex relationships among these factors, multivariate exploratory analysis was performed on the dataset of drugs with diverse bioperformance. The investigated dataset included subset of drugs for which bioequivalence between solid dosage form and oral solution has been reported, and subset of drugs described in the literature as low solubility/low permeability compounds. Discriminatory power of hierarchical clustering on principal components was somewhat higher when applied on the data subsets of drugs with similar bioperformance, while analysis of the integrated dataset indicated existence of two groups of drugs with the boundaries reflected in P_eff value of approximately 2 × 10^-4 cm/s and Fa and Fm values higher than 85% and 50%, respectively. Majority of the investigated drugs within the integrated dataset were grouped within their initial subset indicating that overall drug bioperformance is closely related to its physicochemical, biopharmaceutical and pharmacokinetic properties. Classification models constructed using the random forest (RF) and support vector machine with polynomial kernel function were able to predict food effect based on drug dose/solubility ratio (D/S), effective permeability (P_eff), percent of dose metabolized (Fm), and elimination half-life (τ_1/2). Although both models performed well during training and testing, only RF kept satisfying performance when applied on the external dataset (kappa value > 0.4). The results obtained indicate that data mining can be employed as useful tool in biopharmaceutical drug characterization which merits further investigation.

Pharmacokinetic and pharmacodynamic considerations in the treatment of the elderly patient with hypertension

INTRODUCTION

Hypertension is an important risk factor for developing cardiovascular diseases. It is more prevalent in the elderly population. Recently updated American and European guidelines recommend treating every elderly patient with hypertension independent of age, starting with a low dose of antihypertensive drugs. However, little information is available on the optimal dosages of antihypertensive drugs to treat the elderly safely. Areas covered: Comorbidities, co-medication and frailty status can alter the clinical outcome of drug treatment and can cause adverse events in the elderly. Also, due to pharmacokinetic and pharmacodynamic changes the interpatient variability when using antihypertensive drugs is considerable. In this review, an overview is given on the extent to which the previously mentioned parameters are changed in elderly patients and what this means for the exposure to antihypertensive medication. Also, recommendations on the starting dose of the most frequently used antihypertensive drugs are given based on literature data. Expert opinion: We believe that recommendations on starting dosages followed by a stepwise increase of dosages will lead to improved blood pressure control and less adverse drug reactions in the elderly patient. This may improve adherence to antihypertensive therapy.

Aldosterone receptor antagonists: current perspectives and therapies

Aldosterone is a downstream effector of angiotensin II in the renin-angiotensin-aldosterone system and binds to the mineralocorticoid receptor. The classical view of aldosterone primarily acting at the level of the kidneys to regulate plasma potassium and intravascular volume status is being supplemented by evidence of new "off-target" effects of aldosterone in other organ systems. The genomic effects of aldosterone are well known, but there is also evidence for non-genomic effects and these recently identified effects of aldosterone have required a revision in the traditional view of aldosterone's role in human health and disease. The aim of this article is to review the biological action of aldosterone and the mineralocorticoid receptor leading to subsequent physiologic and pathophysiologic effects involving the vasculature, central nervous system, heart, and kidneys. Furthermore, we outline current evidence evaluating the use of mineralocorticoid receptor antagonists in the treatment of primary aldosteronism, primary hypertension, resistant hypertension, obstructive sleep apnea, heart failure, and chronic kidney disease.

Does Smoking Act as a Friend or Enemy of Blood Pressure? Let Release Pandora's Box

In spite of the great number of observations which show the certainty of cardiovascular damage from smoking, the opinions on that are not yet unanimous. There is a discrepancy that could be attributed to the lack of reproducible data particularly in some epidemiological studies. On the contrary, experimental findings conducted on both animals and humans give evidence of exactly reproducible results of cardiovascular alterations and among these the course of Blood Pressure (BP). Findings identify an increase in BP of active smokers or non-smokers exposed to passive smoking, while a lot of others refer a lowering of BP due to smoking. This discrepancy could be explained as follows. Initially, a vasoconstriction mediated by nicotine causes acute but transient increase in systolic BP. This phase is followed by a decrease in BP as a consequence of depressant effects played chronically by nicotine itself. Simultaneously, carbon monoxide is acting directly on the arterial wall causing, in the long run, structurally irreversible alterations. At this time, there is a change in BP that increases again, and often constantly, its levels following chronic exposure. Changes in response to antihypertensive drugs have been observed in hypertensive smokers since smoking influences metabolic steps of the drugs.

Thiazides and loop-diuretics therapeutic aspects

Thiazides and compounds with similar models of action exert their most important renal effects on the cortical-diluting segment of the nephron, most likely from the peritubular side. In contrast, the most important site of action of loop-diuretics is the luminal side of the ascending part of the diluting segment. The different sites of action explain the clinically proven efficacious combination of thiazides and loop-diuretics in severe cardiac failure. Most thiazides and loop-diuretics are eliminated via renal tubular secretion, which leads to decreased renal clearance in patients with chronic heart failure (CHF) as their renal blood flow is decreased even if glomerular filtration rate (GFR) is maintained. A rational approach to enhance the effects of loop-diuretics is to combine them with drugs that increase renal blood flow, thereby increasing the rate of delivery of the drug to its site of action. Dilutional hyponatremia is an important complication of treatment with diuretics. An efficacious treatment of that condition seems to be a combination of loop-diuretics and ACE-inhibitors. Thiazides decrease the urinary excretion of calcium, while loop-diuretics have the opposite effect. The possibility of loop-diuretic induced osteopenia cannot be ruled out, which should be considered when choosing between thiazides and loop-diuretics for the treatment of mild to moderate CHF.

Presentation of a structurally diverse and commercially available drug data set for correlation and benchmarking studies

A multivariate analysis of drugs on the Swedish market was the basis for the selection of a small, physicochemically diverse set of 24 drug compounds. Factors such as structural diversity, commercial availability, price, and a suitable analytical technique for quantification were considered in the selection. Lipophilicity, pKa, solubility, and permeability across human Caco-2 cell monolayers were measured for the compiled data set. The results show that, by use of a physicochemically diverse data set, experimental responses over a wide range were obtained. The paper also shows how experimental difficulties due to the diversity of the data set can be overcome. We anticipate that this data set can serve as a benchmark set for validation of new experimental techniques or in silico models. It can also be used as a diverse starting data set for the development of new computational models.

Hydrochlorothiazide efficacy and polymorphisms in ACE, ADD1 and GNB3 in healthy, male volunteers

OBJECTIVE

The antihypertensive effect of thiazide diuretics has recently been associated with genetic variation in the angiotensin I-converting enzyme (ACE), alpha-adducin (ADD1) and the G protein subunit beta3 (GNB3). Analysis of short-term diuretic effects may provide insight into the mechanisms behind these findings.

METHODS

A total of 103 male volunteers took 25 and 100 mg hydrochlorothiazide (HCT) after a placebo day, each. We measured volume, sodium, chloride, potassium, calcium excretion, blood pressure and heart rate.

RESULTS

Excretion and cardiovascular parameters were highly constant between the 2 placebo days. The resting heart rate was 2-3 beats/minute (bpm) higher per ACE insertion allele on all 4 study days. The HCT-induced excretion of sodium, chloride and volume was independent of the genotypes. The additional potassium excretion induced by 100 mg HCT was 44+/-21, 33+/-27 and 16+/-26 mmol (mean+/-SD, p<0.001) in ACE II, ID and DD carriers and the same trend was observed after 25 mg HCT. As a second finding, the 100 mg HCT-induced calcium retention was 0.2+/-1.2, 0.7+/-0.8 and 1.7+/-2.1 mmol in ADD1 Gly/Gly, Gly/Trp and Trp/Trp carriers (p=0.002) and the same trend existed after 25 mg HCT.

CONCLUSION

The effects of genetic polymorphisms were stronger with the higher diuretic dose. ACE insertion allele carriers appeared to be more prone to hypokalaemia than deletion allele carriers. ADD1 Trp460 carriers may especially benefit from the calcium-sparing effect of thiazides. Both associations should be further studied in long-term treatment with thiazide diuretics.

Pharmacogenomics of diuretic drugs: data on rare monogenic disorders and on polymorphisms and requirements for further research

This review summarizes the current status of our knowledge about the role of pharmacogenetic variation in response to diuretics and suggests future research topics for the field. Genes with a role in the pharmacokinetics of most diuretics are renal drug transporters, especially OAT1, OAT3 and OCT2 (genes SLC22A6, SLC22A8 and SLC22A2) whereas variants in carbonic anhydrase (CA), cytochrome P450 enzymes and sulfotransferases are relevant only for specific substances. Genes on the pharmacodynamic side include the primary targets of thiazide, loop, K+-sparing and aldosterone antagonistic diuretics: NCC, NKCC2, ENaC and the mineralocorticoid receptor (genes SLC12A3, SLC12A1, SCNN1A, B, G and NR3C2). Rare variants of these proteins cause Gitelman’s syndrome, Bartter’s syndrome, Liddle’s syndrome or pregnancy-induced hypertension. Polymorphisms in these and in associated proteins such as GNB3, α-adducin and angiotensin-converting enzyme (ACE) seem to be clinically relevant. In conclusion, first knowledge has evolved that efficacy of diuretic drugs may be determined by genetic polymorphisms in genes determining pharmacokinetics and pharmacodynamics of this drug class. In the future, the selection of a diuretic drug or the dosing schedules may be individually chosen based on pharmacogenetic parameters, however, many questions remain to be answered before this fantasy becomes reality.

Determination and characterization of diuretics in human urine by liquid chromatography coupled to pneumatically assisted electrospray ionization mass spectrometry

The aim of this work was to develop a method for the characterization and determination of diuretics in human urine samples by liquid chromatography (LC) coupled to pneumatically assisted electrospray ionization (ES) mass spectrometry (MS). The diuretics studied were substances forbidden by the IOC such as trichlormethiazide, furosemide, canrenoic acid, benzthiazide, bendroflumethiazide, bumetanide, etacrynic acid and spironolactone. For this purpose, the operational parameters of electrospray, such as counter electrode voltage, capillary voltage, sample cone voltage and source temperature, were optimized in order to obtain the best signal stability and the highest sensitivity for the greatest number of diuretic agents. The optimized separation method was successfully coupled with the MS system to analyze the above-mentioned diuretics extracted from spiked urine samples by a liquid extraction and clean-up procedure at basic pH, using ethyl acetate as solvent and the salting-out effect (NaCl). The mass spectra obtained provide adequate information for identification purposes. Positive urine samples obtained from athletes were also analyzed. The presence of these substances in human urine was confirmed by this method, making LC/ES-MS an analytical tool to be considered in the area of antidoping control.

Pharmacology of diuretics

The diuretics in our therapeutic armamentarium have predictable effects based on their nephron sites of action. All but spironolactone must reach the lumen or urinary side of the nephron to exert their effects. Thus, in settings of decreased renal function, doses must be increased to deliver more diuretic into the urine. In other edematous disorders, such as congestive heart failure (CHF) and cirrhosis, adequate amounts of diuretic reach the site of action if renal function is satisfactory. Diminished response in these conditions is caused by a decrease in the sensitivity of the nephron to the diuretic, the mechanism of which is unknown. Rather than using large single doses of diuretic in CHF and cirrhosis, multiple doses and/or combinations of diuretics should be used. Therefore, thiazide diuretics coupled with loop diuretics are most logical because they affect different nephron sites and the thiazide counteracts distal nephron hypertrophy that may occur with loop diuretics alone. Ample studies have shown that such combinations can result in a truly synergistic response. Using pharmacokinetics and pharmacodynamics of diuretics, we can design therapeutic regimens in which satisfactory control of fluid and electrolyte homeostasis can be achieved in the vast majority of patients.

Androgenetic alopecia. New approved and unapproved treatments

Although there are new FDA-approved drug therapies to treat hair loss, there are many unapproved agents with claims of effectively treating hair loss. A variety of new over-the-counter agents are available for consumers to use; however, it is difficult to assess how these agents work, costs of these agents, and efficacy. This article covers the new approved and the large multitude of unapproved treatments that are used to treat hair loss.

What is the plasma cofactor required by diuretics for direct vascular relaxant effect in vitro?

OBJECTIVE

To determine the plasma cofactor which is required by diuretics (hydrochlorothiazide, chlorthalidone, indapamide, and furosemide) for direct vasorelaxant effects in vitro.

DESIGN

A randomized, double-blind, vehicle-control design was used to avoid experimenter bias.

METHODS

We plotted concentration-response curves for responses to hydrochlorothiazide, chlorthalidone, indapamide, and furosemide of male Wistar rat aortic rings in the presence of different bath solutions containing various plasma factors.

RESULTS

Plasma was found both to make possible and facilitate the vasodilator action of the diuretics tested by an action on the membrane and to decrease the action by binding the diuretics. The diuretics retained their vasorelaxant properties in Krebs solution alone, 30 min after having been incubated in a 50:50 solution of Krebs solution and plasma for 1 h. All four diuretics exerted significant vasorelaxant actions in Krebs solution-plasma, Krebs solution-serum, and Krebs solution plus human or bovine albumin (40 g/l) media. No vasorelaxant action was found in Krebs solution alone, Krebs solution plus denatured plasma, Krebs solution plus egg albumin, and Krebs solution plus insulin.

CONCLUSION

Albumin is the main cofactor required by the diuretics tested for direct relaxant action in vitro, and these findings may explain some of the contradictory evidence concerning this action in the literature.

Species differences in size discrimination in the paracellular pathway reflected by oral bioavailability of poly(ethylene glycol) and D-peptides

Animal models are frequently used to aid prediction of intestinal absorption in humans. However, there is little comparative quantitative information on species differences in paracellular permeation, which is an important route for oral absorption of small to medium-sized hydrophilic drug molecules. This study addresses this issue by comparing the molecular mass (MM) dependency in oral bioavailability between rat and dog of poly(ethylene glycol) (PEG), a polydispersed model mixture commonly used to characterize paracellular absorption, and of a series of eight D-peptides (based on D-phenylalanine). Fasted rats and dogs received PEG (400/900) and the D-peptides (MM 236-406 Da), orally and intravenously, with total 24-48 h urine collection to estimate oral bioavailability. After HPLC separation, the individual PEG oligomers and D-peptides were determined using radiometric detection, for radiolabeled material, and LC-MS, for unlabeled (PEG) material. All compounds were predominantly excreted unchanged following intravenous administration. After oral administration, the predominant peak in the radiochromatogram was unchanged material, indicating stability of the compounds in the gastrointestinal tract. A clear molecular mass dependency in oral bioavailability was seen with both series, but with absorption much greater in dog than rat. Thus, for PEG in rat, bioavailability decreased sharply from 79 to around 2% with increasing MM between 282 and 591 Da, and then tapered to around 1. 5% up to 1295 Da. Whereas in dog, bioavailability remained around 100% for oligomers up to 600 Da and then decreased quite sharply with increasing MM, tending to plateau around 13% beyond 900 Da. Likewise, for the d-peptides in rat, bioavailability decreased from 30 to 1% with increasing MM between 236 and 406 Da, whereas in dog it was 100%, declining to 16% over the same molecular range. This species difference appears to be due to a larger pore size and greater frequency of pores in the paracellular pathway of dog compared to rat. Furthermore, on the basis of comparison with literature data for PEG and selected drugs, rat would appear to be a better predictor than dog of absorption of hydrophilic compounds in human.

Role of duration of diuretic effect in preventing sodium retention

OBJECTIVE

To determine whether the duration of diuretic effect at the active nephron site enhances ability to excrete an exogenous salt load.

METHODS

We conducted a study that involved eight patients with New York Heart Association class II to III congestive heart failure. In a randomized, crossover manner, each patient received 3.25 mg intravenous bumetanide at 0 hours and again at 6 hours or a loading dose of 0.5 mg bumetanide at 0 hours followed by a continuous infusion of 0.5 mg/hr for 6 hours. Response was followed for 12 hours; a total of 6.5 mg of bumetanide was administered in each arm of the study. Eight hours after dosing began, we administered approximately 80 mEq sodium intravenously and examined its excretion over 4 hours.

RESULTS

The percentage of the load excreted was 86% +/- 15% versus 29% +/- 30% for the infusion and bolus regimens, respectively (p = 0.0005). More bumetanide was excreted during the infusion (667 +/- 133 micrograms versus 240 +/- 121 micrograms; p = 0.0002). During the infusion, however, more sodium was excreted relative to amounts of bumetanide, indicating that the efficiency of response was greater during the infusion, 0.10 +/- 0.02 mEq sodium per microgram bumetanide versus 0.07 +/- 0.05 mEq for the bolus (p = 0.0145).

CONCLUSIONS

These data support the notions that a long-acting loop diuretic maintains its efficacy and that a longer duration of action facilitates excretion of a sodium load, such as that which might occur during dietary indiscretion.

Increase in the basal tone of guinea pig thoracic aorta induced by ouabain is inhibited by spironolactone canrenone and potassium canrenoate

Guinea pig aorta rings repeatedly stimulated with phenylephrine (1 microM) in the presence of Krebs solution containing ouabain (0.8 microM) or low K+ (0.5 mM) concentration produced an increase in basal tone. This effect is due to an increase in intracellular Ca2+ as a consequence of Na(+)-K+ATPase pump inhibition induced by receptorial (ouabain) or ion imbalance (low K+) mechanism. We investigated the effect of spironolactone and its metabolites canrenone and potassium canrenoate on the increase in basal tone of guinea pig aorta rings. Spironolactone, canrenone, and potassium canrenoate, in a concentration-dependent manner (3-30 microM), inhibited the increase in basal tone induced by ouabain, most likely acting as antagonist for ouabain binding site on Na(+)-K+ATPase pump. Indeed, this effect appears to be a feature of these drugs since structurally related drugs, such as aldosterone and hydrocortisone, were ineffective. Conversely, all the drugs tested reduced, to a certain degree, the increase in basal tone produced by low K+ Krebs solution, implying that this could be a non-specific effect. Our results may indicate that spironolactone, canrenone, and potassium canrenoate act in hypertension by interfering with mechanisms in which an ouabain-like factor is involved.

Renal excretory responses to single and repeated administration of diuretics in healthy subjects: clinical connotations

Administration of an initial oral dose of hydrochlorothiazide 25 mg to healthy subjects is followed by increased 24-hour urinary outputs of sodium, chloride, and potassium. On the fourth day of once-daily dosing with hydrochlorothiazide 25 mg, 24-hour natriuresis and chloriuresis are no longer augmented, but the elevation in 24-hour kaliuresis that follows the first dose remains unchanged. Twenty-four-hour urinary calcium output is consistently reduced during repeated once-daily administration of hydrochlorothiazide 25 mg. The first oral dose of the loop diuretic torasemide augments the average natriuresis and kaliuresis in the 6 hours immediately after dosing in healthy subjects, in a dose-dependent fashion, within the 2.5 to 10-mg range. These increased urinary outputs are followed by rebounds below postplacebo values between 6 and 24 hours after dosing. As a result of this biphasic response, torasemide 2.5 mg qualifies as a nondiuretic formulation (it does not elevate 24-hour natriuresis), whereas torasemide 5 and 10 mg qualify as diuretic formulations. After the seventh dose of torasemide 5 or 10 mg during a regimen of once-daily therapy, 24-hour urinary sodium and chloride outputs no longer differ from their postplacebo counterparts. Twenty-four-hour kaliuresis tends to increase in a dose-dependent fashion after the first dose of torasemide (torasemide 2.5 and 5 mg do not augment it significantly), but this tendency is no longer present after the seventh once-daily dose, when torasemide (2.5, 5, or 10 mg) does not elevate the mean 24-hour kaliuresis. Twenty-four-hour calciuresis tends to increase in a dose-dependent manner (torasemide 2.5 mg does not elevate it significantly) after the first dose of torasemide; this calciuretic effect does not change in intensity after 7 days of once-daily treatment. The time course of natriuresis over the 24 hours following the administration of any given formulation of a loop or of an early distal tubular diuretic to healthy subjects is alike after the first and after the nth once-daily dose; therefore, it constitutes a definite characteristic of any given oral formulation. In the case of torasemide, lower doses have more protracted effects on natriuresis, to the extent that the time course of natriuresis over the 24 hours after administration of torasemide 2.5 mg to healthy subjects resembles the time course after administration of hydrochlorothiazide 25 mg, rather than the time course after administration of the overtly diuretic formulation torasemide 10 mg.

Effect of aldosterone antagonist on the DC potential in the endolymphatic sac

The effect of aldosterone and canrenoate, known as an aldosterone antagonist, on the DC potential in the endolymphatic sac (ESP) was examined in the guinea pig. Intravenous administration of aldosterone (1 mg/kg) induced no change in the ESP for 60 minutes after the injection. However, canrenoate, an aldosterone antagonist, produced a dose-dependent decrease in the ESP. Pretreatment with aldosterone attenuated the decrease in the ESP by canrenoate. The results suggest the possibility that aldosterone may play a role in the endolymph absorption of the endolymphatic sac.

The pharmacology and therapeutics of diuretics in the pediatric patient

Selection of appropriate diuretic therapy in children is hampered by a lack of age-specific pharmacokinetic and pharmacodynamic data, especially in premature neonates. Well-designed clinical trials in neonates, infants, and younger children are necessary prerequisites to safer and more efficacious diuretic therapy.

Concentrative uptake of digoxin by slices of chicken renal cortex

The purpose of this investigation was to define, under controlled in vitro conditions, the processes contributing to the uptake and accumulation of [3H]digoxin by incubated slices of chicken renal cortex. Progressive uptake was evident in time-course experiments with the slice-to-medium concentration ratio (S/M) reaching 5.25 after 120 min. No metabolism was evident. Increasing the ratio of unlabeled to labeled digoxin resulted in a concentration-dependent decrease in relative uptake of the label, suggesting saturability. Incubation under conditions of metabolic inhibition reduced digoxin S/M by about 50%, indicating that both energy-requiring and passive mechanisms contribute to the overall accumulation process. The structural nature of the uptake process was explored by incubating digoxin in the presence of potential inhibitors of transport. The organic cation quinine and the non-glycosidic steroids digoxigenin and spironolactone were without effect even at greater than 1000-fold excess compared to digoxin. Similarly, the sugar digitoxose had no inhibitory activity on digoxin accumulation by the slices. On the other hand, the glycosides digitoxin, digoxigenin-bis-digitoxoside and digoxigenin-mono-digitoxoside inhibited dogoxin uptake in a concentration-dependent manner. These results indicate a structural preference for an intact glycoside rather than for either the steroidal or sugar portion of the molecule alone. An inhibitory effect of ouabain and a stimulatory effect of reduced medium potassium concentration suggest a possible role for Na+,K(+)-ATPase in the uptake of digoxin by the renal cortex.

Plasma or serum in therapeutic drug monitoring and clinical toxicology

The relative merits of plasma and serum in blood analysis are reviewed. The expression 'plasma concentration' is often used in the literature, although serum samples have been taken. In most cases serum and plasma concentrations of analytes are the same. The choice depends mostly on the policy of the hospital or the availability of the test tubes in the ward. Some of the advantages of plasma over serum are large volume, no delayed clotting, less risk of haemolysis. In addition, the sample is often suitable for both whole blood and plasma monitoring. Some of the disadvantages of plasma over serum are the (unknown) influence of the anticoagulant on the assay, on the protein binding and on the stability of the sample, the (unknown) influence of additives or impurities in the anticoagulants on the assay and on the concentration, the risk of the formation of small clots and dilution of the sample.

Pharmacokinetics and biological effects of captopril and hydrochlorothiazide after acute and chronic administration either alone or in combination in hypertensive patients

The pharmacokinetics of free unchanged captopril, total captopril and hydrochlorothiazide (HCTZ) were investigated in three groups of patients with moderate essential hypertension and normal renal function on the first and on the 45th days of an oral treatment with either captopril (50 mg once daily, n = 7) or HCTZ (25 mg once daily, n = 10) or their combination captopril 50 mg + HCTZ 25 mg once daily, n = 8. Simultaneously, the effects of the three treatments on plasma converting enzyme activity (PCEA) and plasma renin activity (PRA) were measured. Elimination half-lives of total captopril after acute (7.8 +/- 2.3 h) or chronic (7.0 +/- 0.5 h) captopril dosing were similar to those of HCTZ after acute (6.5 +/- 1.0 h) or chronic (8.0 +/- 2.5 h) HCTZ dosing. Elimination half-life of free unchanged captopril was 0.81 +/- 0.09 h after acute and 0.96 +/- 0.03 h after chronic captopril dosing. Addition of HCTZ to captopril induced no major change in free and total captopril pharmacokinetic parameters, in PCEA inhibition and in PRA increase (as they were determined after captopril alone) on acute as well as on chronic treatment. Addition of captopril to HCTZ induced no major change in HCTZ pharmacokinetic parameters and in PRA increase compared with those determined after HCTZ alone. Chronic treatment with captopril + HCTZ resulted, like chronic captopril treatment, in no accumulation of captopril, in no significant modification of free and total captopril pharmacokinetic parameters and of PCEA inhibition (as determined after acute administration) but led to a significant enhancement of the acutely induced increase in PRA. Chronic treatment with captopril + HCTZ resulted, like chronic HCTZ treatment, in no accumulation of HCTZ, in no significant modification of HCTZ pharmacokinetic parameters (as determined after acute administration) but also led to a significant enhancement of the acutely induced increase in PRA. Thus, the longer duration of the antihypertensive action of captopril + HCTZ as compared to that of captopril cannot be ascribed to a pharmacokinetic or biological interaction between captopril and HCTZ.

Plasma protein binding of furosemide in the elderly

The protein binding of furosemide was investigated in plasma from 22 old and 11 young subjects by equilibrium dialysis. The unbound fraction of furosemide was 3.16% in plasma from the elderly and 1.7% in plasma from the young. A significant correlation was found between the unbound fraction of furosemide and the plasma concentration of albumin. The average number of binding sites was 3.8 (elderly) and 2.7 (young) 10(-6) mol/g albumin. The average association constant (K) was 4.3 (elderly) and 4.2 (young) 10(5) M-1. By increasing the concentration of furosemide up to 200 micrograms/ml buffer the unbound fraction of the drug rose to 5.2% (elderly) and 3.5% (young).

Furosemide ototoxicity: clinical and experimental aspects

Furosemide is an ototoxic diuretic. Furosemide injection is followed by a rapid, but reversible decrease of the endocochlear potential and eighth nerve action potential with a more gradual decrease of the endolymph potassium concentration. In contrast to the reversible effects of furosemide alone on the cochlea, the combination of kanamycin with furosemide resulted in irreversible changes in cochlear function which were associated with elevated levels of kanamycin in the blood and perilymph of the experimental animals. There was a striking similarity between the blood level measured by high pressure liquid chromatography at the time of recovery of auditory function in experimental animals and the ototoxic blood levels proposed by others in clinical literature. These findings help to provide a pharmacologic basis for the clinical observation of furosemide-induced hearing loss.

Diuretics. Clinical pharmacology and therapeutic use (Part I)

25 years have elapsed since the introduction of the first effective oral diuretic, chlorothiazide. Diuretics are now amongst the most widely prescribed drugs in clinical practice worldwide. Availability of these drugs has not only brought therapeutic benefit to countless numbers of patients but it has at the same time provided valuable research tools with which to investigate the functional behaviour of the kidney and other electrolyte-transporting tissues. Despite many remaining gaps in our knowledge of the biochemical processes involved in diuretic drug action, available compounds can be divided into 5 groups on the basis of their preferential effects on different segments of the nephron involved in tubular reabsorption of sodium chloride and water. Firstly, there is heterogeneous group of chemicals that share the common property of powerful, short-lived diuretic effects that are complete within 4 to 6 hours. These agents act on the thick ascending limb of Henle's loop and are known as 'high ceiling' or 'loop' diuretics. The second group are the benzothiadiazines and their many related heterocyclic variants, all of which localise their effects to the early portion of the distal tubule. The third group comprises the potassium-sparing diuretics which act exclusively on the Na+-K+/H+ exchange mechanisms in the late distal tubule and cortical collecting duct. The action of drugs in groups 2 and 3 is prolonged to between 12 and 24 hours. The fourth group consists of diuretics that are chemically related to ethacrynic acid but have the unusual property of combining within the same molecule the property of saluresis and uricosuria. These compounds have actions, to different individual extents, in the proximal tubule, thick ascending limb, and early distal tubule and are known as 'polyvalent' diuretics. Finally, there is a mixed group of weak or adjunctive diuretics which includes the vasodilator xanthines such as aminophylline, and the osmotically active compounds such as mannitol. Available evidence on the molecular mechanisms of action of diuretics in each group is reviewed. The haemodynamic, humoral and physical factors involved in control of electrolyte and fluid handling by the kidney in normal conditions and pathological states are discussed in relation to rational choices of different diuretics in the treatment of various oedematous and non-oedematous conditions.

Absence of a significant pharmacokinetic interaction between hydrochlorothiazide and triamterene when coadministered

Hydrochlorothiazide, triamterene, and hydroxytriamterene sulfate were monitored in the plasma and urine of 24 healthy young men taking single doses of a liquid preparation containing both hydrochlorothiazide and triamterene, liquid preparations containing either of these drugs alone, and a combination tablet recently formulated with a dose ratio of hydrochlorothiazide : triamterene (1 : 1.5) found to give optimal potassium-sparing effect. In contradiction to a recent publication, no interaction between the drugs affecting the bioavailability or renal clearance of either could be demonstrated. The previous report of drug-drug interaction probably arose from formulation-related problems with bioavailability from the two capsule and two tablet products which had been studied. A well-formulated hydrochlorothiazide-triamterene combination tablet promotes plasma concentrations and urinary excretion of hydrochlorothiazide, triamterene, and hydroxytriamterene sulfate which are virtually identical to those seen after either a combination liquid dosage form or simple liquid forms containing only one of the two drugs.

A new antihypertensive agent: Maxzide (75 mg triamterene/50 mg hydrochlorothiazide)

The combination of potassium-sparing diuretics with hydrochlorothiazide is extensively used by hypertensive patients. In the United States, a capsule formulation containing 50 mg triamterene and 25 mg hydrochlorothiazide (Dyazide) is the most popular fixed-combination product. However, several investigators have confirmed that absorption of both the triamterene and hydrochlorothiazide components of Dyazide is markedly reduced. This reduced bioavailability may reduce its effectiveness in hypertensive patients who change from optimally bioavailable hydrochlorothiazide tablets to Dyazide capsules in an effort to correct hypokalemia. Scored tablets containing 75 mg triamterene/50 mg hydrochlorothiazide (Maxzide) have been developed using a patented parallel granulation manufacturing process and evaluated in a series of bioavailability-bioequivalence studies. The triamterene and hydrochlorothiazide components of Maxzide tablets were found to be as bioavailable as liquid preparations of the active ingredients given singly or in combination. Unlike Dyazide capsules, the hydrochlorothiazide component of Maxzide tablets was found to be absorbed to the same extent as single-entity hydrochlorothiazide tablets. These studies demonstrated that two Dyazide capsules (total of 100 mg triamterene/50 mg hydrochlorothiazide) deliver to the bloodstream approximately one-half the quantity of hydrochlorothiazide as one Maxzide (75 mg triamterene/50 mg hydrochlorothiazide) tablet or one 50 mg hydrochlorothiazide tablet. Similarly, two Dyazide capsules deliver approximately one-half the quantity of triamterene as one Maxzide tablet. The safety of Maxzide tablets (dose of one tablet a day) was then evaluated in three groups of hypertensive patients: those who previously had been given two Dyazide capsules a day, those who had received four Dyazide capsules a day, and newly diagnosed patients who had received no medication. The clinical condition of the patients who changed from two or four Dyazide capsules to one Maxzide tablet was not compromised. Indeed, significant reductions in blood pressures were observed, especially in some patients who had remained hypertensive while receiving Dyazide. As expected, the blood pressures of the previously untreated group also decreased significantly with Maxzide tablets. Clinically significant hyperkalemia or hypokalemia did not develop. There were no clinically significant changes in blood urea nitrogen, creatinine, or uric acid levels. In addition to enhanced bioavailability, Maxzide tablets offer the advantages of once-a-day dosing.

Drug prescribing in renal failure: dosing guidelines for adults

The data base for rational guidelines to safe, efficacious drug prescribing in adults with renal insufficiency are presented in tabular form. Current medical literature was extensively surveyed to provide as much specific information as possible. When information is lacking, however, recommendations are based on pharmacokinetic variables in normal subjects. Nephrotoxicity, important adverse effects, and special considerations in renal patients are noted. Adjustments are suggested for hemodialysis and peritoneal dialysis when appropriate.

A collection of therapeutic, toxic and fatal blood drug concentrations in man

In order to assess the significance of drug concentrations measured in clinical and toxicological investigations, it is essential that good collections of data are readily available. As a guide to interpreting findings, the present work provides a compilation of therapeutic, toxic and fatal blood concentration ranges of 298 drugs of interest to clinical pharmacologists, clinical toxicologists, and forensic toxicologists. Wherever possible, ranges are expressed concisely in terms of the maximum blood concentrations which account for 10, 50 and 90% of the data collected. They provide easy access to the most reliable information which relates the blood drug concentration to the biological response it produces. Where appropriate, the different toxic effects of a drug and/or the different degrees of severity of toxic symptoms associated with different drug levels are clearly defined. The original sources of all data used are provided to allow the analyst to obtain further analytical, pharmacokinetic and toxicological information should this be necessary. Those factors (e.g. age, capacity for drug metabolism, drug interactions, etc) which can modify the relationship between a drug concentration and the response it produces are briefly discussed.

Clinical pharmacology of diuretics

Diuretics belong to the drugs most frequently used. Thiazide diuretics, loop diuretics and potassium sparing diuretics are the agents with practical significance. Many data concerning the pharmacokinetics of these drugs have been reported. Nevertheless, the metabolism of some diuretics is not yet fully elucidated. There are numerous pharmacodynamic drug interactions with diuretics which in general can be predicted from the spectrum of pharmacodynamic actions of the drugs involved.

Bioavailability of two preparations of furosemide and their pharmacological activity in normal volunteers

The relative bioavailability and diuretic effect of 2 commercially available tablet preparations of furosemide 40 mg was examined in 10 healthy male volunteers. A close linear relationship between the urinary excretion rate of furosemide and the rate of sodium ion excretion in urine and/or flow rate of urine was demonstrated. There were no significant differences in the urinary excretion of furosemide, sodium and potassium ions or urinary volume following the oral doses. The difference in drug content affected the urinary recovery of furosemide over 24 h but had no effect on the pharmacological response. The analytical power of ANOVA using the various parameters of the responses to furosemide was no lower than when the parameters of urinary excretion of furosemide were used.

The pharmacokinetics and pharmacodynamics of bumetanide in normal subjects

The pharmacokinetics and pharmacodynamics of bumetanide (1 mg) administered either orally or intravenously were studied in a group of normal subjects using high-pressure liquid chromatography. A two-compartment model adequately fitted the intravenous data. Renal clearance (85 ml min-1) contributed 65% to the total elimination of bumetanide irrespective of whether a model-dependent or model-independent method was used. Oral administration of bumetanide elicited a greater and a more prolonged pharmacological response than did intravenous bumetanide. An attempt is made to relate the pharmacokinetics of the drug to its pharmacodynamics.

Serum binding of diltiazem in humans

Diltiazem binding was measured by equilibrium dialysis in the serum of five healthy male subjects, aged 22 to 34 years, at seven serum diltiazem concentrations ranging from 0.03 to 2.06 micrograms/ml. The percentage of unbound diltiazem ranged from 19.6 +/- 3.1 to 22.6 +/- 4.1 (mean +/- S.D.) and was independent of serum diltiazem concentration. Serum diltiazem binding was also examined in the presence of six drugs that might be used concurrently with diltiazem in patients being treated for cardiac disease. The percentage of unbound diltiazem was not influenced by digoxin, hydrochlorothiazide, phenylbutazone, propranolol, salicylic acid, or warfarin at therapeutic concentrations of these drugs over the therapeutic range of diltiazem.

Clinical evaluation of labetalol alone and combined with chlorthalidone in essential hypertension: a double-blind multicentre controlled study

In a multicentre, double-blind, crossover, placebo-controlled study, the antihypertensive effect of labetalol 100 mg and chlorthalidone 10 mg, given alone or in combination, has been assessed in 32 hypertensive patients. The combination had a greater effect in reducing blood pressure than did its separate components. This was particularly evident after exercise. Heart rate increased during chlorthalidone therapy, decreased during labetalol therapy, and a summation effect was observed during treatment with the combination. In most cases additivity was observed, as no interaction between the single components was observed, except for heart rate after exercise, and for diastolic blood pressure in the upright position. No interaction was observed either in the biochemical indices or in the clinical side-effects.

Kinetics of furosemide in children with chronic renal failure undergoing regular haemodialysis

Furosemide was measured by gas-liquid chromatography in blood and dialysis fluid from 7 children with chronic renal failure, undergoing regular haemodialysis. It was administered chronically, in two or three daily doses (4.2-9.4mg/kg). Two children received 1 mg/kg intravenously for determination of the pharmacokinetics. The half-life was longer than in adults and in anephric patients on haemodialysis. Systemic and renal clearance were also much lower. Plasma protein binding in 2 out of 6 cases was reduced as compared to normal adults. The data do not suggest any need to modify the present dosage schedule despite the 4-5 fold increase in the half-life of furosemide. The contribution of haemodialysis to drug clearance was minimal, and accounted for less than 10% of the total clearance.