Drugs Acting on the Cardiovascular System I. Agents Used to Treat Congestive Heart Failure (CHF)
Drugs for CCF • Heart failure is the progressive inability of the heart to supply adequate blood flow to vital organs. • It is classically accompanied by significant fluid retention. • It is a leading cause of mortality and morbidity.
Drugs for CCF • The most common symptoms of CCF includes: • shortness of breath • edema • fatigue • Causes of heart failure includes: • Coronary artery disease (CAD) • Hypertension • Diabetes • Mitral valve disease • Chronic alcohol use
Overview • CHF results when the output of the heart is insufficient to supply adequate levels of oxygen for the body. • Components of failure: • Impaired contractility • circulatory congestion • Compensatory elevation in angiotensin II production results in sodium retention and vasoconstriction and increases both matrix formation and remodeling.
Therapeutic agents • Increase cardiac contractility • Reduce preload (left ventricular filling pressure) and aortic impedance (systemic vascular resistance) • Normalize heart rate and rhythm
Drugs That Inhibit the Activity of the Renin—Angiotensin System Drugs that either: • interfere with the biosynthesis of angiotensin II (angiotensin converting enzyme, [ACE] inhibitors) or • act as antagonists of angiotensin receptors (angiotensin receptor blockers, [ARBs])
ACE inhibitors are indicated in all patients with LV dysfunction, whether symptomatic or asymptomatic. • ACE inhibitors are becoming increasingly important in the treatment of CHF and have been shown to prevent or slow the progression of heart failure in patients with ventricular dysfunction.
Principles of the renin—angiotensin system decapeptide octapeptide
Several parameters regulate the release of renin from the kidney cortex. • Reduced arterial pressure • decreased sodium delivery to the cortex • increased sodium at the distal tubule • stimulation of sympathetic activity • All increase renin release.
Renin cleaves the protein angiotensinogen and releases the decapeptide angiotensin I. • Angiotensin I is converted enzymatically (mostly in the lung) to an octapeptide, angiotensin II, or the heptapeptide angiotensin III. • Both angiotensin II and angiotensin III stimulate the release of aldosterone.
Angiotensin II: • is a vasoconstricting agent • causes sodium retention via release of aldosterone. • In the adrenal gland, angiotensin II is converted to angiotensin III, which is less active as a vasoconstricting agent than angiotensin II.
The actions of angiotensin II are mediated by: • AT1 • AT2 • AT4 receptors located in most tissues. • The pressor actions of angiotensin II are mediated by AT1 receptors. • Angiotensin II can be produced • ACE pathways • locally (e.g. in the myocardium, kidney, adrenals or in vessel walls by the action of non-ACE pathways) by the action of chymases and cathepsins
Because angiotensin II is produced via pathways other than the ACE pathway, angiotensin II receptor antagonists may be more effective and specific in reducing angiotensin II actions.
ACE inhibitors • Mechanism • ACE inhibitors inhibit the production of angiotensin II from angiotensin I. • These agents counteract: • elevated peripheral vascular resistance • sodium and water retention • resulting from angiotensin II and aldosterone.
ACE inhibitors • increase cardiac output • induce systemic arteriolar dilatation (reduce afterload). • ACE inhibitors cause venodilation • induce natriuresis, thereby reducing preload. • These drugs are especially useful for long-term therapy.
Therapeutic uses • Treatment of CHF • Reducing risk of recurrent post-myocardial infarction (MI) • Treating hypertension
Selected drugs • Enalapril is a prodrug that is deesterified in the liver to produce enalaprilat, which inhibits ACE. • Therapeutic uses. • Enalapril is a first-line drug in the treatment of CHF • used to treat mild-to-severe hypertension. • ACE inhibitors are also used to slow the progression of renal disease, especially in diabetic patients. • Adverse effects and contraindications. • Blood dyscrasias and aplastic anemia are rare but serious • Renal function may be impaired.
Captopril • the first ACE inhibitor • the only sulfur-containing ACE inhibitor • is absorbed from the gastrointestinal (GI) tract • adverse effects: rash, taste disturbance, pruritus, weight loss, and anorexia. • Lisinopril • is an ACE inhibitor that permits once-a-day dosing. • The bioavailability of lisinopril is not affected by food.
Adverse effects common to all ACE inhibitors • a dry cough • rarely angioedema • hypotension • Hyperkalemia • Fetal toxicity
Angioedema: allergic disorder in which large, localized, painless swellings similar to hives appear under the skin.
Angiotensin II receptor blockers (ARBs) • Mechanism of action • The actions of angiotensin II are mediated by receptors that are 7-transmembrane proteins that couple to numerous signal transduction pathways. • AT1 receptors are responsible for: • the pressor actions • increased aldosterone biosynthesis • the proliferative and fibrotic actions of angiotensin II.
In general, AT2 receptors antagonize the action of AT1 receptors. • AT4 receptors bind angiotensin IV and seem to be involved in memory and learning.
ARBs: prototype drug: Valsartan Mechanism • with high affinity for AT1 receptors (about 20,000-fold higher than for AT2 receptors). • Oral doses are absorbed rapidly • Peak levels of the drug are obtained in about 3 hours • a half-life of about 6 hours. • Valsartan is excreted in the feces, probably via biliary excretion.
Therapeutic uses. • Left ventricular dysfunction following an MI. • reducing blood pressure • is available in combination with hydrochlorothiazide for patients refractory to monotherapy. (Diovan)
Adverse effects and contraindications. • Dizziness and hyperkalemia can occur with valsartan.
Since ARBs do not lead to accumulation of kinins, the incidence of both the nonproductive cough and angioedema associated with ACE inhibitors is reduced
Other ARBs all have the same mechanism of action and adverse effect profile but have subtle pharmacokinetic differences. • They vary markedly in their relative affinity for AT1 and AT2 receptors
Cardiac glycosides • Cardiac glycosides are used for treatment of: • CHF • Arrhythmias (atrial fibrillation and flutter and paroxysmal atrial tachycardias). • However, their use overall has diminished in the absence of data supporting a reduction in mortality.
The most common • digoxin • digitoxin • the major active ingredients found in digitalis plants, which are collectively referred to as digitalis.
Mechanism • Cardiac glycosides inhibit Na+/K+-ATPase, resulting • in increased • intracellular Na+ • decreased intracellular K+. • Increased Na+ reduces the normal exchange of • intracellular Ca2+ for extracellular Na+ and yields • somewhat elevated intracellular Ca2+.
There are multiple isoforms of Na+/K+-ATPase; • the cardiac isoform has the highest affinity for digitalis. • Following treatment, each action potential produces a greater release of Ca2+ to activate the contractile process. • The net result is a positive inotropic effect. (increase the strength of muscular contraction)
Effects of cardiac glycosides • Cardiac glycosides have both: • direct effects on the heart • indirect effects mediated by an increase in vagal tone.
Pharmacologic properties and selected drugs • Cardiac glycosides distribute to most body tissues and accumulate in cardiac tissue. • The concentration of these drugs in the heart is twice that in skeletal muscle and at least 15 times that in plasma. • The dose of cardiac glycosides must be individualized.
Digoxin • Digoxin has somewhat variable oral absorption; it can be given orally or intravenously. • The peak effect after an intravenous (IV) dose occurs in 1.5—2 hours • the half-life (t1/2) of digoxin is approximately 1.5 days. • The maintenance dose of digoxin is approximately 35% of the loading dose.
Digoxin produces a therapeutic effect (and its toxic effects disappear) more rapidly than digitoxin. • However, because of a relatively rapid clearance, lack of compliance may diminish the therapeutic effects. • Digoxin is eliminated by the renal route • the t1/2 is prolonged in individuals with impaired renal function. • Digoxin dosage can be adjusted on the basis of creatinine clearance.
Digitoxin • Digitoxin is completely absorbed from the gastrointestinal tract • its t1/2 is approximately 5—6 days. • Digitoxin is metabolized in the liver and excreted via the biliary route. • Impaired renal function does not alter the half-life of digitoxin. • Digitoxin should be administered cautiously in the presence of hepatic dysfunction.
Adverse effects and toxicity • Narrow therapeutic index • Cardiac glycosides can cause fatal adverse effects. • These drugs induce virtually every type of arrhythmia. • The most common site of action outside the heart is the GI tract (anorexia, nausea, vomiting, and diarrhea can occur) resulting either from direct action or through stimulation of the chemoreceptor trigger zone (CTZ). • Use of these drugs may result in disorientation and visual disturbances.
Toxicity is treated primarily by: • discontinuing the drug. • Potassium may help in alleviating arrhythmias. • Antidigoxin antibodies (digoxin immune Fab) or hemoperfusion are useful in acute toxicity. • Antiarrhythmic agents such as phenytoin and lidocaine may be helpful in treating acute digoxin-induced arrhythmias
Drug interactions • Drugs that bind digitalis compounds, such as cholestyramine and neomycin, may interfere with therapy. • Drugs that enhance hepatic metabolizing enzymes, such as phenobarbital, may lower concentrations of the active drug (especially digitoxin).