DIURETIC ACTION OF ETHACRYNIC ACID IN CONGESTIVE HEART FAILURE

Advances in the development of novel compounds targeting cation-chloride cotransporter physiology

For about half a century, the pharmacology of electroneutral cation-chloride cotransporters has been dominated by a few drugs that are widely used in clinical medicine. Because these diuretic drugs are so good at what they do, there has been little incentive in expanding their pharmacology. The increasing realization that cation-chloride cotransporters are involved in many other key physiological processes and the knowledge that different tissues express homologous proteins with matching transport functions have rekindled interest in drug discovery. This review summarizes the methods available to assess the function of these transporters and describe the multiple efforts that have made to identify new compounds. We describe multiple screens targeting KCC2 function and one screen designed to find compounds that discriminate between NKCC1 and NKCC2. Two of the KCC2 screens identified new inhibitors that are 3-4 orders of magnitude more potent than furosemide. Additional screens identified compounds that purportedly increase cell surface expression of the cotransporter, as well as several FDA-approved drugs that increase KCC2 transcription and expression. The technical details of each screen biased them toward specific processes in the life cycle of the transporter, making these efforts independent and complementary. In addition, each drug discovery effort contributes to our understanding of the biology of the cotransporters.

The treatment of heart failure. A methodological review of the literature

In this article literature concerning the major drugs used in the treatment of heart failure is reviewed. Because of major discrepancies in results from short term and uncontrolled studies versus long term randomised control trials, only the latter group of studies are addressed in detail. Of 3 randomised control trials of digoxin, 1 has been positive, and 2 negative. Digoxin is probably of benefit to a minority of heart failure patients. Four randomised control trials of oral nitrates have shown a reduction in left ventricular filling pressure, and trends favouring active treatment for the indices of exercise capacity and functional status. Of 2 randomised control trials of hydralazine one is totally negative, the other difficult to interpret because of major loss of patients to followup. Of 5 trials of quinazolone derivatives (prazosin and trimazosin), 2 have been positive, 2 showed non-statistically significant trends favouring active treatment, and 1 was completely negative. These results are consistent with a modest benefit of prazosin and trimazosin in some heart failure patients. Five trials of angiotensin-converting enzyme inhibitors (captopril and enalapril) have shown dramatic and consistent benefit in exercise capacity and functional status. These results support a clinical policy of initial treatment with diuretics and addition of either captopril or enalapril for patients who remain symptomatic on optimal diuretic therapy. A trial of digoxin is warranted in patients whose functional capacity remains reduced on this regimen.

Ethacrynic acid: two years' experience with a new diuretic

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Ethacrynic Acid

Ethacrynic acid, a chemically different diuretic, has been evaluated in 82 patients. The drug was remarkably potent when administered intravenously (0.5 to 1 mg./Kg.) or by mouth (doses to 50 mg. four times per day). It proved particularly effective in patients with refractory edema, and in edematous states accompanied by azotemia or electrolyte disturbances. Its natriuretic effects added to those of other diuretics and potentiation actually appeared to occur when it was combined with a carbonic-anhydrase inhibitor.

Renal clearance data indicated that ethacrynic acid has a unique mode of action. There was little effect on glomerular filtration or renal plasma flow and its major action was to block sodium and chloride reabsorption probably in both proximal and distal portions of the renal tubules.

Ion exchange mechanisms seemed little affected by the drug because it characteristically increased both hydrogen and potassium ion excretion. In general the degree of induced potassium loss correlated well with the rate of endogenous aldosterone secretion. Furthermore, ethacrynic acid did not cause potassium loss in one adrenalectomized subject nor in another subject pretreated with an aldosterone antagonist.

Diuresis induced by ethacrynic acid was often accompanied by the development of systemic metabolic alkalosis. Factors in the development of this alkalosis were (1) urinary K + loss with hypokalemia, (2) urinary H + loss, and (3) in edematous patients the rapid large loss of relatively HCO 3 - -free fluid from the extracellular fluid space. The systemic alkalosis was found to be accompanied by compensatory alveolar hypoventilation. These studies of the mechanisms of the alkalosis point to the occasional importance of the administration of chloride, of potassium, or of saline as corrective measures.

Diuresis with ethacrynic acid, like other diuretic agents, produced marked increases in the rate of aldosterone secretion of normal subjects. More variable effects were observed in patients with heart disease, possibly because improvement in their circulatory status operated to lower hormone secretion activity.

Ethacrynic acid had effects on urate excretion that resembled thiazide diuretics. Higher blood levels of intravenous administration were uricosuric. Lower blood levels of oral therapy caused renal urate retention and hyperuricemia.

Most of the problems associated with the use of ethacrynic acid appeared to be related more to its pharmacologic potency than to any truly toxic effect. Because of this it is suggested that the sensitivity and response pattern of each patient be determined in stepwise fashion. In most patients with edema an intermittent dosage schedule provides a more efficient diuretic response and allows time intervals for correction of any electrolyte imbalance. If used with a thorough understanding of its pharmacologic effects ethacrynic acid promises to be a most useful agent in the therapy of patients with difficult or refractory edema.