Renin is a protease enzyme secreted from specialized cells in the kidney called juxtaglomerular cells, located in the wall of the afferent arteriole. Three signals trigger renin release: a fall in blood pressure sensed at the afferent arteriole, reduced sodium chloride delivery to the macula densa of the distal tubule, and stimulation of beta-1 adrenergic receptors on juxtaglomerular cells by the sympathetic nervous system. Once released, renin acts on angiotensinogen, a protein made continuously by the liver, to produce the inactive decapeptide angiotensin I.

Angiotensin-converting enzyme is a zinc-containing enzyme located on the surface of pulmonary vascular endothelium. As blood passes through the lungs, angiotensin-converting enzyme cleaves two amino acids from angiotensin I to produce angiotensin II, the active vasoactive peptide. This conversion is nearly complete in a single pulmonary pass. Angiotensin-converting enzyme has a second substrate: bradykinin. The same enzyme that generates angiotensin II also degrades bradykinin. This dual action is the mechanistic basis for the two most important adverse effects of angiotensin-converting enzyme inhibitors.

Angiotensin II acts primarily through type 1 (AT1) receptors on vascular smooth muscle and the adrenal cortex, producing two major effects: vasoconstriction, which raises blood pressure directly, and stimulation of aldosterone secretion from the adrenal zona glomerulosa.

Aldosterone acts on the mineralocorticoid receptor in the collecting duct of the kidney to increase sodium reabsorption and potassium excretion. The net result of the entire cascade is elevated blood pressure, expanded plasma volume, and reduced potassium. In heart failure and chronic kidney disease, this system is pathologically overactivated, which is why drugs that suppress the cascade produce survival benefit in these conditions.

All angiotensin-converting enzyme inhibitors present a zinc-chelating group that competitively occupies the enzyme's active site. This prevents the cleavage of angiotensin I to angiotensin II, reducing vasoconstriction and aldosterone secretion. At the same time, bradykinin — which would normally be degraded by angiotensin-converting enzyme — accumulates in tissues and plasma. The rise in bradykinin contributes to vasodilation and is responsible for the class's characteristic adverse effects.

A dry, persistent, non-productive cough occurs in 5 to 20 percent of patients taking angiotensin-converting enzyme inhibitors. The mechanism is bradykinin accumulation in the pulmonary interstitium, which sensitizes airway sensory fibers and lowers the cough threshold. The cough does not respond to antihistamines or antitussives. Switching to an angiotensin receptor blocker, which does not affect bradykinin metabolism, resolves the cough in virtually all patients. This is one of the most common reasons patients are transitioned from an angiotensin-converting enzyme inhibitor to an angiotensin receptor blocker in clinical practice.

Angioedema is a serious, potentially life-threatening adverse effect occurring in roughly 0.1 to 0.7 percent of patients. Like the cough, it is driven by bradykinin accumulation — not by histamine — which makes it mechanistically distinct from allergic angioedema. The swelling characteristically involves the lips, tongue, and periorbital region. Laryngeal involvement occurs in a significant minority of cases and can compromise the airway.

Because this is a bradykinin-mediated reaction rather than a histamine-mediated one, antihistamines and corticosteroids are ineffective. Epinephrine may provide temporary relief in laryngeal involvement through vasoconstriction but does not treat the underlying mechanism. The primary intervention is immediate discontinuation of the angiotensin-converting enzyme inhibitor. Once a patient has experienced angiotensin-converting enzyme inhibitor-induced angioedema, the drug is absolutely contraindicated permanently, and angiotensin receptor blockers should be used cautiously (they do not raise bradykinin but may have a small independent risk).

By suppressing aldosterone, angiotensin-converting enzyme inhibitors reduce potassium excretion in the renal collecting duct. This produces a tendency toward hyperkalemia, particularly in patients with chronic kidney disease, patients taking potassium-sparing diuretics, and patients on potassium supplements. Serum potassium monitoring is required at baseline and after initiation or dose changes.

Captopril and lisinopril are the two agents that are pharmacologically active as administered, requiring no biotransformation. All other angiotensin-converting enzyme inhibitors — including enalapril, ramipril, and perindopril — are ester prodrugs that undergo hepatic hydrolysis to release their active diacid forms. Active drug is eliminated primarily by the kidney across the entire class. This means dose reduction is required when kidney function is significantly impaired, typically when the glomerular filtration rate falls below 30 mL/min.

Fosinopril is the exception worth knowing: it has approximately equal hepatic and renal elimination of its active form, so it accumulates far less than other agents when kidney function declines. This makes fosinopril the preferred angiotensin-converting enzyme inhibitor in advanced chronic kidney disease.

Angiotensin-converting enzyme inhibitors have among the broadest evidence bases of any drug class in cardiovascular medicine. The four major indications are hypertension, heart failure with reduced ejection fraction, post-myocardial infarction left ventricular dysfunction, and diabetic nephropathy.

In heart failure with reduced ejection fraction, the CONSENSUS and SOLVD trials with enalapril established mortality benefit, making angiotensin-converting enzyme inhibitors foundational therapy. In patients following a myocardial infarction, the SAVE, AIRE, and TRACE trials demonstrated reduced mortality and reinfarction when angiotensin-converting enzyme inhibitors were started early and continued long-term, with greatest benefit in patients with anterior infarction or reduced ejection fraction.

In diabetic nephropathy, the Lewis trial showed that captopril reduces proteinuria and slows progression to end-stage renal disease in type 1 diabetes. The renal protective mechanism is reduction of glomerular hypertension through efferent arteriolar dilation, which lowers the filtration pressure driving protein loss across the glomerular membrane. For this reason, angiotensin-converting enzyme inhibitors are considered first-line antihypertensive therapy in diabetic patients with proteinuria regardless of the underlying diabetes type.

Bilateral renal artery stenosis is the most important mechanistic contraindication. When both renal arteries are narrowed, glomerular filtration depends on angiotensin II constricting the efferent arteriole to maintain filtration pressure despite reduced perfusion. Blocking angiotensin-converting enzyme removes this efferent arteriolar tone, causing the filtration pressure to collapse and precipitating acute kidney injury. The same physiology can occur in a patient with a single functioning kidney and unilateral renal artery stenosis. A rise in creatinine of up to 30 percent after starting an angiotensin-converting enzyme inhibitor is expected and acceptable; a rise beyond that, or a rapid rise, should prompt evaluation for renal artery stenosis or severe volume depletion.

Pregnancy is an absolute contraindication throughout all three trimesters. First-trimester exposure increases the risk of cardiovascular and central nervous system malformations. Second and third trimester exposure causes fetal renal tubular dysgenesis because the developing fetal kidney depends on its own renin-angiotensin-aldosterone system for normal development. The clinical consequence is oligohydramnios, fetal anuria, limb contractures, pulmonary hypoplasia, and potentially neonatal death. This fetal toxicity is shared by angiotensin receptor blockers.

History of angiotensin-converting enzyme inhibitor-induced angioedema is an absolute contraindication to further use of any drug in the class.

Nonsteroidal anti-inflammatory drugs, including ibuprofen, naproxen, and cyclooxygenase-2 inhibitors such as celecoxib, blunt the renal protective effects of angiotensin-converting enzyme inhibitors and increase the risk of acute kidney injury. The mechanism involves nonsteroidal anti-inflammatory drug inhibition of prostaglandin synthesis, which is required to maintain afferent arteriolar dilation and renal perfusion. When afferent dilation is lost at the same time that angiotensin-converting enzyme inhibition reduces efferent constriction, intraglomerular pressure can fall dramatically. This combination is particularly dangerous in elderly patients, patients with existing chronic kidney disease, and patients on diuretics.

Potassium-sparing diuretics — spironolactone, eplerenone, amiloride, and triamterene — combined with angiotensin-converting enzyme inhibitors produce additive suppression of potassium excretion, creating significant hyperkalemia risk. This combination is used intentionally in heart failure with reduced ejection fraction but requires close potassium monitoring.

Lithium levels rise when angiotensin-converting enzyme inhibitors are added because reduced aldosterone activity increases proximal tubular sodium reabsorption, and lithium is reabsorbed by the same mechanism. Lithium toxicity can result. Serum lithium levels must be monitored within one to two weeks of any angiotensin-converting enzyme inhibitor change in patients on lithium.

Aliskiren binds directly to the active site of renin, preventing renin from cleaving angiotensinogen to angiotensin I. With the first step blocked, all downstream products — angiotensin I, angiotensin II, and aldosterone — are reduced simultaneously. Unlike angiotensin-converting enzyme inhibitors, aliskiren does not affect bradykinin metabolism and therefore does not cause the cough or angioedema characteristic of that class.

Aliskiren has poor oral bioavailability (approximately 2 to 3 percent), largely due to limited gastrointestinal absorption. Despite this, once-daily dosing is sufficient because the drug binds renin with high affinity and has a long effective duration of action. Aliskiren is eliminated primarily via the hepatobiliary route as unchanged drug, with minimal renal excretion. This means it does not accumulate proportionally when kidney function declines — a pharmacokinetic advantage, though the pharmacodynamic risks of renin-angiotensin-aldosterone system blockade apply equally.

The ALTITUDE trial tested aliskiren added to an angiotensin-converting enzyme inhibitor or angiotensin receptor blocker in patients with type 2 diabetes and chronic kidney disease or cardiovascular disease. The combination increased rates of acute kidney injury, hyperkalemia, and hypotension without reducing cardiovascular events or slowing kidney disease progression. Based on this evidence, the combination of aliskiren with angiotensin-converting enzyme inhibitors or angiotensin receptor blockers is contraindicated in patients with diabetes, and strongly discouraged in chronic kidney disease. More generally, dual blockade of the renin-angiotensin-aldosterone system at two different levels produces additive harm without additive benefit.