LANOXIN (Page 6 of 8)

10.2 Treatment

Chronic Overdose

If there is suspicion of toxicity, discontinue LANOXIN and place the patient on a cardiac monitor. Correct factors such as electrolyte abnormalities, thyroid dysfunction, and concomitant medications [see Dosage and Administration (2.5)]. Correct hypokalemia by administering potassium so that serum potassium is maintained between 4.0 and 5.5 mmol/L. Potassium is usually administered orally, but when correction of the arrhythmia is urgent and serum potassium concentration is low, potassium may be administered by the intravenous route. Monitor electrocardiogram for any evidence of potassium toxicity (e.g., peaking of T waves) and to observe the effect on the arrhythmia. Avoid potassium salts in patients with bradycardia or heart block. Symptomatic arrhythmias may be treated with Digoxin Immune Fab.

Acute Overdose

Patients who have intentionally or accidently ingested massive doses of digoxin should receive activated charcoal orally or by nasogastric tube regardless of the time since ingestion since digoxin recirculates to the intestine by enterohepatic circulation. In addition to cardiac monitoring, temporarily discontinue LANOXIN until the adverse reaction resolves. Correct factors that may be contributing to the adverse reactions [see Warnings and Precautions (5)]. In particular, correct hypokalemia and hypomagnesemia. Digoxin is not effectively removed from the body by dialysis because of its large extravascular volume of distribution. Life threatening arrhythmias (ventricular tachycardia, ventricular fibrillation, high degree A-V block, bradyarrhythma, sinus arrest) or hyperkalemia requires administration of Digoxin Immune Fab. Digoxin Immune Fab has been shown to be 80-90% effective in reversing signs and symptoms of digoxin toxicity. Bradycardia and heart block caused by digoxin are parasympathetically mediated and respond to atropine. A temporary cardiac pacemaker may also be used. Ventricular arrhythmias may respond to lidocaine or phenytoin. When a large amount of digoxin has been ingested, especially in patients with impaired renal function, hyperkalemia may be present due to release of potassium from skeletal muscle. In this case, treatment with Digoxin Immune Fab is indicated; an initial treatment with glucose and insulin may be needed if the hyperkalemia is life-threatening. Once the adverse reaction has resolved, therapy with LANOXIN may be reinstituted following a careful reassessment of dose.

11 DESCRIPTION

LANOXIN (digoxin) is one of the cardiac (or digitalis) glycosides, a closely related group of drugs having in common specific effects on the myocardium. These drugs are found in a number of plants. Digoxin is extracted from the leaves of Digitalis lanata. The term “digitalis” is used to designate the whole group of glycosides. The glycosides are composed of 2 portions: a sugar and a cardenolide (hence “glycosides”).

Digoxin is described chemically as (3β,5β,12β)-3-[(O -2,6-dideoxy-β-D-ribo -hexopyranosyl-(1→4)-O -2,6-dideoxy-β-D-ribo -hexopyranosyl-(1→4)-2,6-dideoxy-β-D-ribo -hexopyranosyl)oxy]-12,14-dihydroxy-card-20(22)-enolide. Its molecular formula is C41 H64 O14 , its molecular weight is 780.95, and its structural formula is:

Digoxin Chemical Structure
(click image for full-size original)

Digoxin exists as odorless white crystals that melt with decomposition above 230°C. The drug is practically insoluble in water and in ether; slightly soluble in diluted (50%) alcohol and in chloroform; and freely soluble in pyridine.

LANOXIN Injection and Injection Pediatric are sterile solutions of digoxin for intravenous or intramuscular injection. The vehicle contains 42.5% (W/V) propylene glycol and 10% alcohol (V/V). The injection is buffered to a pH of 6.8-7.2 with 0.17% dibasic sodium phosphate and 0.08% anhydrous citric acid. Each 2-mL ampule or vial of LANOXIN Injection contains 500 mcg (0.5 mg) digoxin (250 mcg [0.25 mg] per mL). Dilution is not required. Each 1-mL ampule or vial of LANOXIN Injection Pediatric contains 100 mcg (0.1 mg) digoxin. Dilution is not required.

12 CLINICAL PHARMACOLOGY

12.1 Mechanism of Action

All of digoxin’s actions are mediated through its effects on Na-K ATPase. This enzyme, the “sodium pump,” is responsible for maintaining the intracellular milieu throughout the body by moving sodium ions out of and potassium ions into cells. By inhibiting Na-K ATPase, digoxin

causes increased availability of intracellular calcium in the myocardium and conduction system, with consequent increased inotropy, increased automaticity, and reduced conduction velocity
indirectly causes parasympathetic stimulation of the autonomic nervous system, with consequent effects on the sino-atrial (SA) and atrioventricular (AV) nodes
reduces catecholamine reuptake at nerve terminals, rendering blood vessels more sensitive to endogenous or exogenous catecholamines
increases baroreceptor sensitization, with consequent increased carotid sinus nerve activity and enhanced sympathetic withdrawal for any given increment in mean arterial pressure
increases (at higher concentrations) sympathetic outflow from the central nervous system (CNS) to both cardiac and peripheral sympathetic nerves
allows (at higher concentrations) progressive efflux of intracellular potassium, with consequent increase in serum potassium levels.

The cardiologic consequences of these direct and indirect effects are an increase in the force and velocity of myocardial systolic contraction (positive inotropic action), a slowing of the heart rate (negative chronotropic effect), decreased conduction velocity through the AV node, and a decrease in the degree of activation of the sympathetic nervous system and renin-angiotensin system (neurohormonal deactivating effect).

12.2 Pharmacodynamics

The times to onset of pharmacologic effect and to peak effect of preparations of LANOXIN are shown in Table 7.

Table 7. Times to Onset of Pharmacologic Effect and to Peak Effect of Preparations of LANOXIN
a Documented for ventricular response rate in atrial fibrillation, inotropic effects and electrocardiographic changes.b Depending upon rate of infusion.

Product

Time to Onset of Effecta

Time to Peak Effecta

LANOXIN Tablets

0.5-2 hours

2-6 hours

LANOXIN Injection/IV

5-30 minutesb

1-4 hours

Hemodynamic Effects: Short- and long-term therapy with the drug increases cardiac output and lowers pulmonary artery pressure, pulmonary capillary wedge pressure, and systemic vascular resistance in patients with heart failure. These hemodynamic effects are accompanied by an increase in the left ventricular ejection fraction and a decrease in end-systolic and end-diastolic dimensions.

ECG Changes: The use of therapeutic doses of LANOXIN may cause prolongation of the PR interval and depression of the ST segment on the electrocardiogram. LANOXIN may produce false positive ST-T changes on the electrocardiogram during exercise testing. These electrophysiologic effects are not indicative of toxicity. LANOXIN does not significantly reduce heart rate during exercise.

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