Clinical Pharmacokinetics of Cardiac Glycosides in Patients with Renal Dysfunction
The pharmacokinetics of different cardiac glycosides are altered by renal dysfunction in different ways, depending on their basic pharmacokinetic properties. Digoxin: The linearity of digoxin pharmacokinetics is unchanged by renal dysfunction, as is the bioavailability. Protein binding may be slightly reduced, but the change is of no clinical significance. The apparent volume of distribution is reduced by one-third to one-half, the change being roughly proportional to the degree of renal impairment. The significance of this change in terms of adjustment of the loading dose is controversial. I believe that the initial oral loading dose should be reduced from 15 μg/kg to 10 μg/kg in renal dysfunction, and supplemented only if there is evidence of a lack of response and no evidence of toxicity. The renal clearance of digoxin is reduced in renal dysfunction and becomes very closely related to the measured creatinine clearance at values of creatinine clearance below 30 ml/min. As a result, the renal elimination rate constant, and therefore the fraction of the total body load lost per day via the kidneys, falls in renal dysfunction. In contrast, the non-renal clearance of digoxin is probably unaffected. However, because of the fall in apparent volume of distribution the non-renal elimination rate constant rises slightly, accounting for the slight increase in faecally excreted digoxin in renal dysfunction. This rise in the non-renal fractional daily loss is not sufficient to counteract the fall in renal fractional daily loss and digoxin maintenance dosages at steady-state need to be reduced. For this purpose the creatinine clearance acts as an initial guide to the extent of the expected reduction in dose, but doses altered on this basis should be regarded as first approximations to the correct dose, and the dose subsequently readjusted according to the patient’s clinical response, using the plasma digoxin concentration as a guide, it must be remembered, however, that because of technical problems with digoxin radioimmunoassay in renal dysfunction, and because of the difficulty in interpretation of the result, the plasma digoxin concentration in renal dysfunction is of less value than it is in normal renal function. The overall half-life of digoxin is prolonged in renal dysfunction and it therefore takes longer for a steady-state to be reached during maintenance dose therapy without a loading dose, and longer for toxicity, when it occurs, to resolve. Negligible amounts of digoxin are removed from the body by dialysis procedures. A transplanted kidney retains its ability to handle digoxin, and after transplantation the pharmacokinetics of digoxin return towards normal, depending on the overall improvement in renal function achieved. Digitoxin: There are technical problems with the measurement of digitoxin because of the need to separa te digitoxin and its metabolites chromatographically before using the measurement techniques commonly applied. Such separation has not always been carried out, and this makes the interpretation of the available data more difficult. The bioavailability of digitoxin is unaffected by renal dysfunction. Protein binding is probably significantly reduced but the clinical significance of this effect is unclear since the apparent volume of distribution and total body clearance of digitoxin appear to be unchanged. In the nephrotic syndrome, which must be considered separately from the other forms of renal dysfunction, there is impaired protein binding but also probably loss of protein bound drug via the renal glomerulus. This leads to a proportionately large increase in total body and renal clearances, a shortening of the half-life and a fall in the steady-state plasma digitoxin concentrations. In other forms of renal dysfunction there is probably no change in half-life or in steady-state plasma digitoxin concentrations. There does seem to be a decrease in digitoxin renal clearance but this may be compensated for by increases in nonrenal clearance, both by non-renal excretion of unchanged digitoxin and by metabolic clearance, with increased formation of the active hydroxylated and hydrolysed metabolites, as well as of the relatively inactive reduced metabolites. Overall, the changes seem to contribute little of clinical importance. Little digitoxin is removed from the body by dialysis procedures. Lanatoside C, deslanoside, and the acylated digoxins: Since these glycosides are largely metabolised to digoxin, one would expect changes in their pharmacokinetics similar to those of digoxin. However, there is only enough information to conclude that this is probably so in the case of β-methyldigoxin. The protein binding of β-methyldigoxin is reduced, as is its apparent volume of distribution, and total body clearance. The reduction in total body clearance is mostly attributable to a reduction in renal clearance, which falls in parallel with creatinine clearance, although always remaining lower than creatinine clearance. Non-renal clearance falls little or not at all. As a result of these changes the overall half-life of β-methyldigoxin is prolonged and the fractional daily loss at steady-state is decreased. Dosages of β-methyldigoxin therefore need to be reduced in renal dysfunction. Little β-methyldigoxin is removed by haemodialysis. For α-acetyldigoxin, renal clearance is reduced in proportion to renal function, and the half-life is prolonged. Little is removed by haemodialysis. Other glycosides: Little information is available about other cardiac glycosides. What little information there is, however, suggests that, as one would expect, the half-life is prolonged and renal clearance reduced for those glycosides which are mostly eliminated via the urine, while little or no change occurs for those glycosides which are mostly metabolised. Thus, for ouabain (g-strophanthin), and k-strophanthin, the half-life is prolonged and renal excretion decreased, while for proscillaridin, methylproscillaridin, and peruvoside there is no change. None of these glycosides is much affected by haemodialysis. Despite the changes in pharmacokinetics of digoxin compared with digitoxin, there is little to choose between the two drugs for use in patients with renal dysfunction. Arguments in favour of one or other can be marshalled but there is no good evidence that one is preferable to the other. © 1983, ADIS Press Australasia Pty Ltd.. All rights reserved.