Pharmacological properties : תכונות פרמקולוגיות

Pharmacodynamic Properties

5.1. Pharmacodynamic properties
Pharmacotherapeutic group:
Cardiac therapy, cardiac glycosides, digitalis glycosides.
ATC code:
C01AA05

Mechanism of action
Digoxin increases contractility of the myocardium by direct activity. This effect is proportional to dose in the lower range and some effect is achieved with quite low dosing; it occurs even in normal myocardium although it is then entirely without physiological benefit. The primary action of digoxin is specifically to inhibit adenosine triphosphatase, and thus sodium-potassium (Na+-K+) exchange activity, the altered ionic distribution across the membrane resulting in an augmented calcium ion influx and thus an increase in the availability of calcium at the time of excitation-contraction coupling. The potency of digoxin may therefore appear considerably enhanced when the extracellular potassium concentration is low, with hyperkalaemia having the opposite effect.

Digoxin exerts the same fundamental effect of inhibition of the Na+-K+ exchange mechanism on cells of the autonomic nervous system, stimulating them to exert indirect cardiac activity. Increases in efferent vagal impulses result in reduced sympathetic tone and diminished impulse conduction rate through the atria and atrio-ventricular node Thus, the major beneficial effect of digoxin is reduction of ventricular rate.
Intravenous administration of a loading dose produces an appreciable pharmacological effect within 5 to 30 mins, while using the oral route the onset of effect occurs in 0.5 to 2 hours.

Pharmacodynamic effects
The PROVED trial designed to determine the effectiveness of digoxin in 88 patients with chronic, stable mild to moderate heart failure. Withdrawal of digoxin or its continuation was performed in a prospective, randomised, double-blind, placebo-controlled multicentre trial of patients with chronic, stable mild to moderate heart failure secondary to left ventricular systolic dysfunction who had normal sinus rhythm and were receiving long-term treatment with diuretic drugs and digoxin.
Patients withdrawn from digoxin therapy showed worsened maximal exercise capacity (p = 0.003) an increased incidence of treatment failures (p = 0.039) and a decreased time to treatment failure (p = 0.037). Patients who continued to receive digoxin had a lower body weight (p = 0.044) and heart rate (p = 0.003) and a higher left ventricular ejection fraction (p = 0.016). The overall percentage of participants having one or more adverse event was similar in the two groups: 59 % in the placebo group and 69 % in the digoxin group. The types of adverse event were unspecified 
The RADIANCE trial examined the effects of discontinuation of digoxin in stable NYHA class II and III patients who were receiving diuretics and ACE inhibitors. The 178 patients were initially stabilised on a combination of captopril or enalapril, diuretics and digoxin, then randomised to continue digoxin therapy or change to placebo. The relative risk of worsening disease in the placebo group was 5.9 compared to the digoxin group. Withdrawal of digoxin was accompanied by worsening symptoms, reduced exercise tolerance, and a deteriorating quality of life, indicating that patients with CHF were at considerable risk from discontinuation of the drug in spite of the continuation of therapy with diuretics and ACE inhibitors. Approximately 56% in the placebo group and 49% in the digoxin group experienced unspecified side effects.

In the DIG trial, 6800 patients with heart failure were randomised to receive digoxin or placebo. No difference was found in all-cause mortality between patients who were treated with digoxin and those who were given placebo. In the digoxin group, there was a trend toward a decrease in the risk of death attributed to worsening heart failure (risk ratio, 0.88; 95% confidence interval, 0.77 to 1.01; p = 0.06). However, the patients who received digoxin had significantly (p<0.001) fewer hospital admissions when the drug was given in addition to diuretics and ACE inhibitors. Digoxin therapy was most beneficial in patients with ejection fractions of ≤25%, patients with enlarged hearts (cardiothoracic ratio of >0.55), and patients in NYHA functional class III or IV. In the DIG study, 11.9% of patients in the digoxin arm and 7.9 % of patients in the placebo arm were suspected of having digoxin toxicity, the most common symptoms being new episodes of ventricular fibrillation, supraventricular arrhythmia, tachycardia, or advanced atrioventricular block.

The AFFIRM study involved a total of 4060 patients recruited to a randomised, multicentre comparison of two treatment strategies in patients with atrial fibrillation and a high risk of stroke or death. The primary end point was overall mortality. There were 356 deaths among the patients assigned to rhythm-control therapy (amiodarone, disopyramide, flecainide, moricizine, procainamide, propafenone, quinidine, sotalol, and combinations of these drugs) and 310 deaths among those assigned to rate-control [β-blockers, calcium-channel blockers (verapamil and diltiazem), digoxin, and combinations of these drugs) therapy (mortality at five years, 23.8% and 21.3%, respectively; hazard ratio, 1.15 [95% confidence interval, 0.99 to 1.34]; p=0.08). More patients in the rhythm-control group than in the rate-control group were hospitalised, and there were more adverse drug effects in the rhythm-control group as well.

Indirect cardiac contractility changes also result from changes in venous compliance brought about by the altered autonomic activity and by direct venous stimulation. The interplay between direct and indirect activity governs the total circulatory response, which is not identical for all subjects. In the presence of certain supraventricular arrhythmias, the neurogenically mediated slowing of AV conduction is paramount.

The degree of neurohormonal activation occurring in patients with heart failure is associated with clinical deterioration and an increased risk of death. Digoxin reduces activation of both the sympathetic nervous system and the (renin-angiotensin) system independently of its inotropic actions, and may thus favourably influence survival. Whether this is achieved via direct sympathoinhibitory effects or by re-sensitizing baroreflex mechanisms remains unclear.

Pharmacokinetic Properties

5.2.   Pharmacokinetic properties

Absorption
The Tmax following IV administration is approximately 1 to 5 hours while the Tmax for oral administration is 2 to 6 hours. Upon oral administration, digoxin is absorbed from the stomach and upper part of the small intestine. When digoxin is taken after meals, the rate of absorption is slowed, but the total amount of digoxin absorbed is usually unchanged. When taken with meals high in fiber, however, the amount absorbed from an oral dose may be reduced.

The bioavailability of orally administered digoxin is approximately 63 % in tablet form and 75 % as oral solution

Distribution
The initial distribution of digoxin from the central to the peripheral compartment generally lasts from 6 to 8 h. This is followed by a more gradual decline in serum digoxin concentration, which is dependent upon digoxin elimination from the body. The volume of distribution is large (Vdss = 510 litres in healthy volunteers), indicating digoxin to be extensively bound to body tissues. The highest digoxin concentrations are seen in the heart, liver and kidney, that in the heart averaging 30- fold that in the systemic circulation. Although the concentration in skeletal muscle is far lower, this store cannot be overlooked since skeletal muscle represents 40% of total body weight. Of the small proportion of digoxin circulating in plasma, approximately 25% is bound to protein.

Biotransformation
The majority of digoxin is excreted by the kidneys as an intact drug, although a small fraction of the dose is metabolised to pharmacologically active and inactive metabolites. The main metabolites of digoxin are dihydrodigoxin and digoxygenin.

Elimination
The major route of elimination is renal excretion of the unchanged drug.

Digoxin is a substrate for P-glycoprotein. As an efflux protein on the apical membrane of enterocytes, P-glycoprotein may limit the absorption of digoxin. P-glycoprotein in renal proximal tubules appears to be an important factor in the renal elimination of digoxin (see Section 4.5).

Following I.V. administration to healthy volunteers, between 60 and 75% of a digoxin dose is recovered unchanged in the urine over a six day follow-up period. Total body clearance of digoxin has been shown to be directly related to renal function, and percent daily loss is thus a function of creatinine clearance The total and renal clearances of digoxin have been found to be 193±25 ml/min and 152 ± 24 ml/min in a healthy control population.

In a small percentage of individuals, orally administered digoxin is converted to cardioinactive reduction products (digoxin reduction products or DRPs) by colonic bacteria in the gastrointestinal tract. In these subjects over 40% of the dose may be excreted as DRPs in the urine. Renal clearances of the two main metabolites, dihydrodigoxin and digoxygenin, have been found to be 79±13 ml/min and 100±26 ml/min respectively.

In the majority of cases however, the major route of digoxin elimination is renal excretion of the unchanged drug.

The terminal elimination half-life of digoxin in patients with normal renal function is 30 to 40 h.

Since most of the drug is bound to the tissues rather than circulating in the blood, digoxin is not effectively removed from the body during cardiopulmonary by-pass. Furthermore, only about 3 % of a digoxin dose is removed from the body during 5 h of haemodialysis.

Special patient populations

Paediatric population
In the newborn period, renal clearance of digoxin is diminished and suitable dosage adjustments must be observed. This is especially pronounced in the premature infant since renal clearance reflects maturation of renal function. Digoxin clearance has been found to be 65.6±30 ml/min/1.73m2 at three months, compared to only 32±7 ml/min/1.73m2 at one week. By 12 months digoxin clearance of 88 ± 43 ml / min / 1.73m2 has been reported. Beyond the immediate newborn period, children generally require proportionally larger doses than adults on the basis of body weight and body surface area.

Renal impairment
The terminal elimination half-life of digoxin is prolonged in patients with impaired renal function, and in anuric patients may be of the order of 100 h.

Hepatic impairment
Hepatic impairment has little effect on digoxin clearance.

Elderly
Age-related declines in renal function in elderly patients can result in a lower rates of digoxin clearance than in younger subjects, with reported digoxin clearance rates in the elderly of 53 ml/min/1.73m2.

Gender
Digoxin clearance is 12% – 14% less in females than males and may need to be considered in dosing calculations.