Uricosuric diuretics

Site and mechanism of the action of diuretics

The mechanism of action of diuretics can be established by studying the molecular mechanism of action, the site of action within the nephron, and the relationship between the pharmacokinetics of the diuretic and its effect. The molecular mechanism of action is known for diuretic agents such as acetazolamide (carbonic anhydrase), theophylline (phosphodiesterase), digitalis glucosides (Na-K-ATPase), spironolactone (aldosterone antagonism) and dopamine (specific receptors?). The "receptor" for the clinically most important diuretics, i.e. loop diuretics, thiazides, and other potassium-sparing diuretics is, however, unknown. It appears from recent studies of the ion transport in the diluting segment that there probably is a sodium-chloride co-transport in this segment and that loop diuretics specifically inhibit the active chloride transport. The main site of diuretic action is well established for the different groups of diuretics: carbonic anhydrase inhibitors act on the proximal tubulus, loop diuretics on the diluting segment, thiazides on the cortical diluting segment/distal tubulus, and potassium-sparing agents on distal tubulus/collecting ducts. Moreover, some diuretics have additional tubular sites of action. It is also important to realize that other effects of diuretics, e.g. inhibition of the tubuloglomerular feedback mechanism or renal and extra-renal hemodynamic effects, can modify the tubular diuretic effect. Finally, the renal handling of diuretics is of importance to the diuretic effect by determining the concentration of the drug at the "receptor" sit (s). It is emphasized that knowledge of the different aspects of the mechanisms of action of diuretics is a prerequisite for rational use of diuretics, clinically as well as experimentally.

Aldose reductase inhibitory and uricosuric activities of FK366 in healthy volunteers

The pharmacokinetics, and aldose reductase (AR) inhibitory and uricosuric activities of FK366 were studied in healthy volunteers given a single oral dose of 150, 300, or 600 mg after fasting, 600 mg after a meal, or 300 mg twice a day for 8 days after meals. The AR inhibition was assessed by the percent reduction from the predrug dulcitol values in red blood cells converted from exogenous galactose by AR. Aldose reductase inhibition paralleled the plasma concentrations of FK366, with maximum inhibitions of 31.6, 48.0, and 56.9% at doses of 150, 300, and 600 mg, respectively. With multiple dosing, the inhibition scarcely differed between the first (41.8%) and last doses (41.5%). Serum uric acid decreased dose dependently, with a minimum concentration of 4.0 mg/dL (predrug: 5.5 mg/dL) 8 hours after receiving 600 mg. With multiple dosing, serum uric acid levels declined rapidly and remained at a concentration of 3.1 mg/dL beginning at day 3. Urinary excretion of uric acid was high on day 1 (879 mg/day), but decreased significantly to 654 mg/day on day 2 and then stabilized. The pharmacokinetics of FK366 were linear over the dose range studied, with an elimination half-life of 8.2 hours and urinary recovery of 27.2% as unchanged drug. FK366 was well tolerated by all subjects.

Lead nephropathy, gout, and hypertension

The EDTA (calcium disodium edetate) lead mobilization test revealed lead as the probable cause of renal disease in industrial lead workers and in patients with gout or essential hypertension. The data reviewed here demonstrate persistence of lead nephropathy in the contemporary scene despite the introduction of modern industrial and environmental exposure standards. Renal function and biopsy studies showed that lead nephropathy is a chronic tubulointerstitial renal disease with modest proteinuria which frequently presents with hyperuricemia, gout and hypertension. Only evaluation of body lead stores by either the EDTA lead mobilization test or by x-ray fluorescence is helpful in diagnosing lead nephropathy. While chelation therapy is safe and helpful in reversing early lead nephropathy, the best treatment is prevention. These studies further raise the possibility that chronic environmental lead poisoning and associated renal disease and hypertension may be a more widespread problem than suspected. Assessment of the true extent of chronic lead poisoning requires large scale epidemiological studies.

Pilot study of the uricosuric effect of DuP-753, a new angiotensin II receptor antagonist, in healthy subjects

The uricosuric effect of DuP-753, a novel, specific angiotensin II receptor antagonist, has been explored in a healthy male Japanese volunteers, given single oral doses of 25, 50, 100 or 200 mg (n = 6), or 100 mg (n = 6) or placebo (n = 3) once daily for 7 consecutive days. In the single-dose study, serum uric acid measured at 4 h after dosing showed a dose dependent decrease; the reductions from the corresponding pre-dose values were: 0.32 (25 mg), 0.77 (50 mg), 1.25 (100 mg) and 1.33 mg.dl-1 (200 mg). The urinary excretion of uric acid within the first 4 h after treatment was also increased in a dose-dependent manner, whereas the urinary excretion of creatinine remained unchanged. In the multiple-dose study, DuP-753 significantly decreased the serum uric acid concentration measured 4 h both after the first (pre-dose value: 5.68 vs 4 h after: 4.48 mg.dl-1) and last administrations (4.42 mg.dl-1). Simultaneously, the ratio of urinary uric acid to creatinine excretion was significantly increased within the first 4 h both after the first (DuP-753: 1.190 vs placebo: 0.576) and last administrations (1.02 vs 0.576). The findings suggest that DuP-753 possesses a uricosuric effect both after single and multiple doses in healthy subjects. The effect should be further examined in hypertensive patients.

Drug-induced rheumatic syndromes. Diagnosis, clinical features and management

In order to avoid inappropriate therapy and prolonged morbidity, it is important to recognise when a patient's rheumatic complaints are due to drugs. However, this is often difficult because of the large number of drugs that have been implicated and the diversity of clinical presentations. Arthropathy may be seen with several different syndromes, including drug-induced lupus erythematosus (DILE), serum sickness and gout. The most widely reported of these is DILE, which usually develops after some months or even years of drug therapy. While many authors do not specifically require their presence for the diagnosis of DILE, antinuclear antibodies have been detected in the great majority of reported patients with DILE, whatever the causative drug. In contrast, patients who develop arthropathy soon after commencing a drug rarely have antinuclear antibodies and appear to be distinct from patients with DILE. Apart from arthropathy, a number of other syndromes that appear to have an immunological basis may be induced by drugs. Cutaneous vasculitis is not uncommon and drugs are frequently considered to be the aetiological factor. Whether drugs may cause larger vessel systemic vasculitis is less certain. Rarely, polymyositis and scleroderma-like syndromes have been associated with drug therapy. Corticosteroid-induced osteoporosis is a complication of all the corticosteroid preparations that are widely used at present. However, the development of deflazacort, a so-called 'bone-sparing' steroid, has raised the possibility that the effect of corticosteroids on bone may be separable, at least in part, from the other actions of these drugs. Data have been conflicting with regard to whether there is a 'safe' dose of corticosteroid. Similarly, it is unclear whether prophylactic therapy with agents such as calcium, fluoride and vitamin D is beneficial. Nonetheless, recent findings suggest that approaches will be developed to minimise the risk of osteoporosis in patients who require corticosteroids. There are a number of other ways in which drugs may affect bones. Osteomalacia is a well-known but uncommon complication of treatment with anticonvulsants and occasionally other drugs. The mechanism probably relates to the induction of hepatic enzymes and the consequent increased metabolism of vitamin D in patients with borderline levels initially. Osteosclerosis may also result from drug therapy; usually with fluoride or retinol (vitamin A) and its analogues. With continued research, the true spectrum of drug-induced rheumatic syndromes should become more clearly defined.(ABSTRACT TRUNCATED AT 400 WORDS)

Angiotensin I converting enzyme inhibitors and the renal excretion of urate

Hyperuricaemia carries with it a high risk of tophi development affecting connective tissue in kidney, skin and joints, its overt clinical expression being gout. Diuretics, which are invariably prescribed in congestive heart failure and widely used for the treatment of essential hypertension, may cause hyperuricaemia and predispose to gout by inducing renal retention of urate. The angiotensin I converting enzyme inhibitors captopril and enalapril have been found to augment renal urate excretion both in normal volunteers and in hypertensive patients. Current evidence appears to indicate that the uricosuric effect of captopril and enalapril could be due to the rises in renin and angiotensin I these drugs elicit by angiotensin I converting enzyme inhibition, and/or to pharmacological actions not related, at least directly, to the renin-angiotensin-aldosterone system. Serum urate levels have been significantly reduced by monotherapy with captopril in hypertensive patients suffering from hyperuricaemia. Diuretic-induced hyperuricaemia in hypertensive patients can be prevented or counteracted by the administration of captopril and enalapril. Available clinical data support the argument that captopril and enalapril should be used as first choice drugs for the treatment of hyperuricaemic hypertensive patients. When diuretic-induced hyperuricaemia develops in patients suffering from congestive heart failure, captopril or enalapril should be added to the therapeutic regime in doses capable of countering the shift in plasma urate concentration, provided the clinical condition of the patients permits such additional pharmacological treatment. Therapy with captopril and enalapril should preferably be instituted in a gradual manner, especially in patients with hyperuricaemia, in order to prevent the precipitation of urate in the kidney and in the urinary tract.

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.

Renal actions of a uricosuric diuretic, racemic indacrinone, in man: comparison with ethacrynic acid and hydrochlorothiazide

The effects of indacrinone (IND) have been investigated in a two part study. First, a total of 36 clearance studies have been performed in 14 healthy volunteers, under conditions of either maximal hydration or hydropenia, to compare the renal sites of action of single oral doses of IND, 20 mg, ethacrynic acid (EA), 100 mg, and hydrochorothiazide (HCTZ), 100 mg. Under conditions of maximal water hydration, IND increased fractional Na+ excretion from a mean of 1.19 +/- 0.05 to 4.93 +/- 0.67% of GFR. This was similar to the response seen with HCTZ, which increased fractional Na+ clearance up to 3.16 +/- 0.17% of GFR; EA increased fractional Na+ excretion up to 14.5 +/- 2.5% of GFR. The mean reduction in fractional free-water clearance (CH2O/GFR X 100%) invoked by IND, (delta = -34.8% of control) was similar to that produced by EA, (delta = -27.2% of control), and by HCTZ, (delta = -26.6% of control). During hydropenia with superimposed mannitol diuresis, both IND and EA caused a fall in fractional free-water reabsorption (TcH2O/GFR X 100%), delta IND = -20.3% of control, delta EA = -70.1% of control. HCTZ produced a significant increase in fractional free-water reabsorption, delta HCTZ = -20.7% of control. In all studies, single doses of IND were both uricosuric and hypouricaemic. Fractional urate excretion increased from a mean 6.7 +/- 0.1 to 15.2 +/- 2.1% of GFR whilst plasma urate concentration fell from a mean of 0.36 +/- 0.03 to 0.34 +/- 0.03 mM (P less than 0.05) within 2-3 h post drug. HCTZ and EA, in single doses, had little effect on urate excretion. In the second part of the study, a total of 16 healthy volunteers received either IND, 10 mg, or HCTZ 50 mg, orally for 8 days, whilst on a diet of controlled electrolyte content. Both drugs were well tolerated by both sets of subjects with no adverse clinical or pathological findings. Both IND and HCTZ caused a significant reduction in weight and standing systolic blood-pressure during the first 48 h of therapy. At the doses administered, IND and HCTZ displayed similar diuretic responses with respect to water, Na+, Cl-, Ca2+ and PO4(3-) excretion. IND produced less kaliuresis than HCTZ during the first treatment day but cumulative K+ loss was similar for both drugs over the eight days of therapy. Fractional urate excretion after IND remained elevated throughout the 8 days of therapy and the subjects remained isouricaemic for 7 days.(ABSTRACT TRUNCATED AT 400 WORDS)

Gout and uric acid nephropathy: some new aspects in diagnosis and treatment

It has been recognized that primary disorders of uric acid metabolism result from impaired renal excretion or increased endogenous production of uric acid. It has also been found that these two mechanisms do not comprise two distinct syndromes, but may each constitute a group of syndromes. Contrary to earlier as well as currently published reports we conclude from our clinical and experimental experience that the fraction of so-called over-producers is less than 1% of all patients with primary hyperuricaemia and gout. A procedure for the diagnosis of uric acid overproduction is suggested. The manifestation of hyperuricaemia and gout mainly depends on renal uric acid clearance and is greatly influenced by dietary habits in most of the patients. An impaired renal uric acid excretion results in an increased intestinal excretion; this partly compensates for the defect. Normalization of serum uric acid should be achieved by dietary regimens with or without additional drug treatment, but not by drug treatment alone. With drug treatment xanthine oxidase inhibitors are preferable to uricosurics; no other xanthine oxidase inhibitor besides Allopurinol has been in clinical trial, however. Due to the enhancement of uric acid clearance with uricosurics, there are groups of patients who should not be treated with these drugs. Fixed combinations of Allopurinol and uricosurics should not be used. Drugs which have uricosuric as well as other pharmacologic properties are under investigation. So far they have not reached general clinical application.