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Synopsis: Enalapril, an angiotensin converting enzyme (ACE) inhibitor usually administered orally once daily, decreases blood pressure by lowering peripheral vascular resistance without increasing heart rate or output. It is effective in lowering blood pressure in all grades of essential and renovascular hypertension. Patients not responding adequately to enalapril monotherapy usually respond with the addition of a thiazide diuretic (or a calcium antagonist or [beta]-blocker), and rarely require a third antihypertensive agent. Enalapril is at least as effective as other established and newer ACE inhibitors, and members of other antihypertensive drug classes including diuretics, [beta]-blockers, calcium antagonists and [alpha]-blockers, but therapy with enalapril may be less frequently limited by serious adverse effects or treatment contraindications than with other drug classes. The most frequent adverse effect limiting all ACE inhibitor therapy in clinical practice is cough.

This favourable profile of efficacy and tolerability, and the substantial weight of clinical experience, explain the increasing acceptance of enalapril as a major antihypertensive treatment and supports its use as logical first-line therapeutic option.

Pharmacodynamic Profile: Enalapril is a prodrug which is hydrolysed after absorption to form the active angiotensin converting enzyme (ACE) inhibitor enalaprilat. ACE inhibition decreases plasma angiotensin II concentrations with counter-regulatory increases in renin and angiotensin I and a decrease in aldosterone level, although plasma aldosterone levels usually return to normal during long term administration. Increasing evidence suggests that the primary mechanism of action of ACE inhibition by enalapril, and its ensuing beneficial haemodynamic changes, is subsequent to direct reduction of angiotensin II levels in the vasculature, leading to dilatation of peripheral vessels and reduced vascular resistance which in turn lower blood pressure. Some studies have suggested a possible contributory role played by the sympathetic system and changes in vasoactive kinins or prostanoids.

Decreased total peripheral vascular resistance following administration of enalapril is not counteracted by any change in heart rate or cardiac output. The lack of reflex tachycardia is related to a resetting of the baroreflex rather than a reduction in its sensitivity. Blood pressure is reduced by enalapril in patients with essential or renovascular hypertension, with improved arterial compliance and regression of left ventricular hypertrophy during long term administration. A single dose of enalapril 5 to 40mg produces a maximal reduction of systolic and diastolic blood pressure of about 15 to 20% 6 to 8 hours after oral administration. The duration of effect persists for at least 24 hours, permitting once-daily administration; repeated daily administration confers an additional reduction in pressure and a steady-state antihypertensive response may take several weeks. No rebound phenomenon occurs on withdrawal of enalapril treatment.

The effects of enalapril on renal function are complex and somewhat variable depending on numerous factors. In general, there is an increase in renal plasma flow with no significant change in glomerular filtration rate or major changes in electrolyte balance during long term administration. A minor increase in serum potassium concentrations may occur. The majority of patients with pre-existing renal impairment show a stabilisation, and frequently an improvement of renal function. However, a marked deterioration in renal function can occur in some patients, particularly those with bilateral renal artery stenosis or unilateral stenosis to a single functioning kidney when volume depleted or receiving diuretic therapy, as these patients are highly dependent on intrarenal ACE regulation to maintain glomerular filtration.

Pharmacokinetic Profile: Administered as the maleate salt, enalapril was designed to improve the systemic bioavailability of the active ACE inhibitor enalaprilat, which is poorly absorbed in humans. Estimations from urinary recovery indicate that about 60% of an oral dose of enalapril is absorbed and peak serum enalapril concentrations are reached after about 1 hour, declining rapidly thereafter. Absorption is not affected by food. Enalapril is rapidly de-esterified, primarily in the liver, to form enalaprilat, which reaches peak plasma concentrations about 3 to 4 hours after enalapril administration. Plasma enalaprilat concentrations are linearly related to dose over the normal therapeutic range, and steady-state is reached after 3 or 4 oral doses. The absolute bioavailability of orally administered enalapril as enalaprilat is about 40%, and enalaprilat is about 50% protein bound in circulation. Enalaprilat appears to penetrate into most tissues, in particular the kidneys and vascular tissue, although central nervous system (CNS) penetration has not been conclusively shown after administration of therapeutic dosages. Minimal penetration occurs into breast milk but significant fetal transfer occurs after maternal administration.

Enalaprilat does not undergo further metabolism. Unchanged enalapril and enalaprilat are recovered in urine (61%) and faeces (33%). Faecal recovery represents unabsorbed drug and biliary excretion. The major route of elimination is renal: renal clearance is 18 L/h for enalapril and 8 to 9.5 L/h for enalaprilat. Enalaprilat undergoes polyphasic elimination with an initial elimination phase half-life of about 5 hours but a prolonged terminal phase of 30 to 35 hours, reflecting the strong binding of enalaprilat to plasma ACE.

The systemic clearance of enalaprilat is reduced by renal impairment. Excessive accumulation may occur in patients with moderate to severe renal dysfunction (creatinine clearance < 30 ml/min; 1.8 L/h) and dosage reduction is necessary in such patients. Enalapril and enalaprilat are removed during haemodialysis. No clinically significant change in enalaprilat disposition occurs in patients with hepatic impairment which would require routine dosage reduction.

Therapeutic Use: Enalapril in titrated dosages of 5 to 40 mg/day as monotherapy reduces systolic and diastolic blood pressure by about 15 to 25%, with diastolic pressure normalisation occurring in about 50 to 75% of patients with essential or renovascular hypertension; requirement for higher dosages or failure to achieve goal blood pressure is more likely in patients with more severe hypertension. Addition of another antihypertensive drug, usually a thiazide diuretic (or a calcium antagonist or [beta]-blocker) will frequently provide an adequate antihypertensive response in the remainder of patients nonresponsive to enalapril monotherapy. Similar 24-hour blood pressure control is achieved with once and twice daily administration, and once daily administration can therefore be considered appropriate.

Increased attention is currently being paid to individualising antihypertensive therapy, as it is clear from crossover studies that many patients unresponsive to a particular antihypertensive drug class usually respond to a different class. With enalapril there is a very close correlation between the acute antihypertensive response to the first dose and the likelihood of an adequate long term response. Enalapril is effective in the young and elderly, but Black patients respond less well to enalapril and other ACE inhibitor monotherapy than White patients. This racial difference is no longer apparent once a thiazide diuretic is added.

At usual therapeutic dosages in patients with mild to severe essential hypertension, enalapril is as effective as other ACE inhibitors (captopril, lisinopril, quinapril, benazepril, ramipril, delapril), diuretics [hydrochlorothiazide, bendroflumethiazide (bendrofluazide), altizide/spironolactone], [beta]-blockers (atenolol, metoprolol, mepindolol, propranolol, timolol), calcium antagonists (amlodipine, isradipine, diltiazem, nifedipine, nitrendipine, verapamil), [alpha]-blockers (doxazosin, prazosin), and other drugs (labetalol, cicletanine and clonidine). The only apparent difference in efficacy was that for a comparable decrease in diastolic blood pressure, enalapril produced a more significant reduction in systolic blood pressure than [beta]-blockers.

Stepped-care therapy starting with enalapril (plus hydrochlorothiazide/[alpha]-methyldopa) was at least as effective as standard 'triple therapy' (hydrochlorothiazide/propranolol/hydralazine) in moderate to severe essential hypertension, while enalapril/hydrochlorothiazide/oxprenolol was more effective than hydrochlorothiazide/oxprenolol/dihydralazine in mild to moderate essential hypertension. Enalapril/hydrochlorothiazide was also more effective than hydrochlorothiazide/timolol/hydralazine in patients with renovascular hypertension. In addition, the enalapril regimens presented certain advantages: improved compliance because of simplified dosage regimens requiring fewer tablets; fewer adverse effects necessitating withdrawal of treatment; less frequent hypokalaemia and, in renovascular hypertension, less likelihood of progressive deterioration in renal function.

Indeed, in patients with pre-existing renal impairment, renal function usually remains stable and often improves during long term enalapril therapy. However, some patients show an acute deterioration of renal function soon after the onset of therapy and careful monitoring of renal function is therefore necessary.

Enalapril appears to be an appropriate antihypertensive treatment in patients with concurrent diabetes, providing adequate blood pressure control without any loss of metabolic control. Furthermore, thermore, enalapril leads to a marked reduction in proteinuria, a marker of incipient or overt nephropathy. This antiproteinuric effect is also seen in normotensive diabetic patients, and it has been suggested that this is an ACE inhibitor-specific effect (i.e. on intraglomerular pressure) which may be independent of a general reduction in systemic blood pressure.

When quality of life was assessed, it was found to be improved or maintained during enalapril therapy; occasionally, enalapril tended to be superior to [beta]-blockers (atenolol, propranolol, oxprenolol) when cognitive function or psychometric performance were compared.

Clinical Tolerability: The most frequent adverse events during enalapril treatment occurring in less than 10% of patients, which are usually mild, transient and do not limit therapy, are in descending order of frequency: headache, dizziness, fatigue, musculoskeletal pain, nausea/vomiting, orthostatic effects, weakness, somnolence, abdominal pain and palpitations. It is probable that many of these events are not related to treatment, but rather to hypertension itself or are nonspecific effects, as they occur more frequently in untreated patients. Diarrhoea, cough and possibly rash tend to occur more frequently during enalapril therapy than in untreated patients and are therefore more likely to be drug related.

Clinically relevant adverse effects likely to lead to withdrawal of enalapril are dominated by dry, persistent cough (1 to 6% withdrawal rate during long term therapy) followed by infrequent cases of angioedema, hypotension, hyperkalaemia and acute renal failure. Other rare idiosyncratic reactions have been reported.

ACE inhibitor therapy may suppress renal erythropoietin production leading to a reduction in haemoglobin levels and, rarely, anaemia in patients with chronic renal disease or renal transplant recipients, but this does not appear as a clinical problem in patients with normal renal function. Acute renal failure may occur particularly in patients with high-grade bilateral renal artery stenosis (or unilateral stenosis to a single functioning kidney). Potentially serious fetal or neonatal adverse effects may occur if enalapril is administered during pregnancy: the drug should therefore be avoided in such patients.

Enalapril compares favourably with other antihypertensive drug classes with respect to clinical tolerability, particularly when comparing the incidence of serious adverse events, general lack of clinically relevant adverse drug interactions and particular lack of intercurrent conditions which may contraindicate treatment. Enalapril is not associated with a number of CNS, metabolic or vasodilatory effects which can often limit therapy with other antihypertensive drug classes.

Dosage and Administration: Enalapril is usually administered within the dosage range of 10 to 40mg given once daily in essential hypertension. To monitor for the rare occurrence of hypotension an initial dose of 2.5mg should be administered under supervision. Enalapril therapy can then be commenced at the usual starting dosage of 10mg once daily in mild disease or 20mg for more severe diseases, and the dosage doubled every 1 to 2 weeks to achieve maximum response. If necessary, a second agent (usually a thiazide diuretic, a calcium antagonist or a [beta]-blocker) may be added. In many patients the addition of low dosage diuretic treatment may permit reduction of the enalapril dosage. Patients with renovascular hypertension should be started at a dosage of <= 5mg.

The daily dosage of enalapril should be reduced in patients with renal impairment: upward titration should start from 5mg and 2.5mg daily in those with mild and moderate to severe impairment, respectively. Since enalapril and enalaprilat are removed by haemodialysis, 2.5mg daily should be administered postdialysis, with subsequent titration.

The rare risk of certain adverse effects should be recognised (hypotension, hyperkalaemia and acute renal failure), particularly at the onset of therapy. Potential precipitating factors include volume depletion by prior diuretic therapy, salt restriction, dialysis, diarrhoea or vomiting, pre-existing renal disease (notably high-grade bilateral renal artery stenosis) and use of potassiumsparing diuretics. Concomitant use of enalapril with potassium supplements or potassium-sparing diuretics should be avoided if possible, or at least carefully monitored. In patients already receiving any form of diuretic therapy, it is preferable to withdraw the diuretic for 2 to 3 days before starting enalapril; the diuretic can subsequently be reintroduced, if necessary, frequently at a lower dosage.

Copyright 1992 Adis International