ACE Inhibitors & ARBs — ACE inhibitors and ARBs: mechanism,...

SECTION 1

Historical Context & Motivation

The story of ACE inhibitors begins not in a pharmaceutical laboratory but in the Brazilian rainforest. In the 1960s, Sérgio Ferreira, a Brazilian pharmacologist, isolated peptides from the venom of Bothrops jararaca (the jararaca pit viper) that potentiated the hypotensive effects of bradykinin. These bradykinin-potentiating peptides (BPPs) were found to inhibit angiotensin-converting enzyme, providing proof-of-concept that pharmacological blockade of the renin–angiotensin–aldosterone system (RAAS) could lower blood pressure. This discovery catalyzed decades of drug development that would fundamentally change cardiovascular medicine.

1898

Discovery of Renin

Robert Tigerstedt and Per Bergman identify renin in rabbit renal cortex extracts, demonstrating that the kidney produces a pressor substance—laying the groundwork for understanding the RAAS.

1965

Venom-Derived Peptides

Sérgio Ferreira isolates bradykinin-potentiating peptides from pit viper venom, revealing that ACE inhibition can enhance vasodilatory bradykinin and reduce vasoconstrictor angiotensin II.

1975

Captopril Synthesized

David Cushman and Miguel Ondetti at Squibb develop captopril, the first orally active ACE inhibitor, using a rational drug design approach based on the venom peptides.

1981

FDA Approval of Captopril

Captopril receives FDA approval for hypertension, marking the first commercially available ACE inhibitor. Enalapril follows in 1985, offering improved tolerability and once-daily dosing.

1995

Losartan — First ARB

Losartan becomes the first FDA-approved angiotensin II receptor blocker (ARB), offering RAAS blockade without the bradykinin-related side effects of ACE inhibitors.

The central question driving these developments was clear: could physicians selectively interrupt the RAAS cascade to reduce blood pressure, protect organs, and improve survival—without unacceptable adverse effects? Understanding how ACE inhibitors and ARBs answer this question requires a thorough grasp of RAAS physiology, the pharmacodynamic distinctions between the two drug classes, and the clinical evidence guiding their use.

SECTION 2

Core Principles & Definitions

Before exploring individual drugs, it is essential to establish the foundational concepts that govern RAAS pharmacology. Both ACE inhibitors and ARBs target the same hormonal axis, yet they intervene at different points—producing overlapping but distinct pharmacological profiles. The following principles define the conceptual framework for this lesson.

The RAAS Cascade

Renin (from juxtaglomerular cells) cleaves hepatic angiotensinogen to form angiotensin I, which ACE then converts to the potent vasoconstrictor angiotensin II. Angiotensin II acts on AT₁ receptors to raise blood pressure, stimulate aldosterone secretion, and promote cardiac remodeling.

ACE Inhibitor Mechanism

ACE inhibitors competitively block angiotensin-converting enzyme (ACE), a zinc-dependent peptidase (also called kininase II). This reduces angiotensin II formation and simultaneously prevents the degradation of bradykinin, a vasodilatory peptide.

ARB Mechanism

ARBs selectively block the angiotensin II type 1 (AT₁) receptor, preventing angiotensin II from exerting its vasoconstrictor, aldosterone-secreting, and pro-fibrotic effects. Unlike ACE inhibitors, ARBs do not affect bradykinin metabolism.

Dual Benefit: Hemodynamic + Organ-Protective

Both classes lower blood pressure through afterload reduction, but their value extends to organ protection—reducing left ventricular hypertrophy, slowing progression of diabetic nephropathy, and decreasing proteinuria via efferent arteriolar dilation.

Aldosterone Escape

With chronic ACE inhibitor or ARB therapy, aldosterone levels may rise again through non-ACE pathways (e.g., chymase). This aldosterone escape phenomenon sometimes necessitates addition of a mineralocorticoid receptor antagonist such as spironolactone.

✦ KEY TAKEAWAY

Think of the RAAS as a relay race: renin hands the baton to angiotensin I, ACE passes it to angiotensin II, and angiotensin II crosses the finish line by binding the AT₁ receptor. An ACE inhibitor tackles the runner mid-handoff, preventing the baton from reaching angiotensin II—and simultaneously preventing another enzyme (kininase II) from degrading protective bradykinin. An ARB lets the race proceed but blocks the finish line—angiotensin II is present but cannot activate the AT₁ receptor. Both strategies prevent the downstream effects of the RAAS, but the ACE inhibitor's dual action on bradykinin explains its unique side-effect profile.

SECTION 3

The RAAS Pathway & Drug Targets

The RAAS cascade proceeds from angiotensinogen through angiotensin I (via renin) to angiotensin II (via ACE). ACE inhibitors block the conversion step (dashed cyan box), simultaneously elevating bradykinin. ARBs block angiotensin II at the AT₁ receptor level (dashed pink box), without affecting bradykinin degradation. The green panel illustrates the bradykinin pathway that explains the differential side-effect profiles.

The diagram above illustrates the critical distinction between these two drug classes. Notice that ACE sits at a crossroads: it is both the enzyme that generates angiotensin II and the enzyme (kininase II) that degrades bradykinin. When an ACE inhibitor blocks this enzyme, it produces a dual pharmacological effect: decreased angiotensin II production and increased bradykinin availability. The accumulation of bradykinin mediates vasodilation through nitric oxide (NO) and prostacyclin release but is also responsible for the well-known dry cough that affects up to 15% of patients on ACE inhibitors. ARBs bypass this issue entirely because they do not interfere with bradykinin metabolism—they simply block the downstream AT₁ receptor, allowing angiotensin II to circulate but preventing it from triggering vasoconstriction, aldosterone release, and pro-fibrotic signaling.

SECTION 4

Detailed Mechanism of Action

ACE Inhibitors: Dual Enzyme Blockade

ACE is a zinc-dependent dipeptidyl carboxypeptidase expressed predominantly on the luminal surface of pulmonary vascular endothelial cells, though it is also found in renal, cardiac, and vascular tissue. ACE cleaves the C-terminal dipeptide His-Leu from the decapeptide angiotensin I to yield the octapeptide angiotensin II. ACE inhibitors contain functional groups—sulfhydryl (captopril), dicarboxylate (enalaprilat, lisinopril), or phosphonate (fosinoprilat)—that chelate the zinc ion at the enzyme's active site, rendering it catalytically inactive.

Because the same enzyme degrades bradykinin (a nonapeptide vasodilator), ACE inhibition simultaneously elevates bradykinin concentrations. Bradykinin stimulates the B₂ receptor on endothelial cells, promoting release of nitric oxide (NO) and prostacyclin (PGI₂), both of which contribute to vasodilation, antiproliferative effects, and anti-thrombotic actions. This dual action—reducing angiotensin II and augmenting bradykinin—gives ACE inhibitors a broader pharmacological profile than ARBs, but it also accounts for the bradykinin-mediated adverse effects (dry cough, angioedema).

ARBs: Selective AT₁ Receptor Antagonism

ARBs selectively and competitively (or in some cases insurmountably) block the AT₁ receptor, a G_q-protein-coupled receptor responsible for virtually all of the known deleterious cardiovascular effects of angiotensin II, including vasoconstriction, aldosterone release, sympathetic activation, cellular hypertrophy, and fibrosis. Because ARBs block at the receptor rather than the enzyme level, they do not inhibit bradykinin degradation, which eliminates the cough and substantially reduces (though does not completely abolish) the risk of angioedema. An important nuance is that when AT₁ receptors are blocked, circulating angiotensin II levels rise reflexively and may stimulate the unblocked AT₂ receptor. The AT₂ receptor is thought to mediate counter-regulatory effects—vasodilation, anti-proliferation, and anti-inflammatory actions—which may provide additional cardiovascular benefit, although this remains an area of active investigation.

Hemodynamic & Renal Effects

Both drug classes reduce systemic vascular resistance (afterload) without significant reflex tachycardia, because they also attenuate sympathetic nervous system activation mediated by angiotensin II. In the kidney, angiotensin II preferentially constricts the efferent arteriole of the glomerulus to maintain intraglomerular pressure. When ACE inhibitors or ARBs dilate the efferent arteriole, intraglomerular pressure falls, reducing filtration pressure and consequently proteinuria. This is the mechanistic basis for their renoprotective effects in diabetic nephropathy and other proteinuric kidney diseases. However, in patients with bilateral renal artery stenosis, this efferent dilation can precipitate acute kidney injury because the stenosed kidneys depend on angiotensin II–mediated efferent constriction to maintain glomerular filtration.

⚕️ Clinical Pearl

A modest rise in serum creatinine (up to 30% from baseline) after initiating an ACE inhibitor or ARB is expected and acceptable—it reflects reduced intraglomerular pressure, not toxicity. However, a rise exceeding 30% or accompanied by hyperkalemia warrants investigation for bilateral renal artery stenosis or volume depletion.

SECTION 5

Indications & Drug Classification

ACE inhibitors and ARBs are among the most widely prescribed drug classes worldwide, with indications that extend well beyond simple blood pressure reduction. The following table summarizes major FDA-approved and guideline-supported indications, along with representative agents and their pharmacokinetic features.

Major indications for ACE inhibitors and ARBs with landmark trial evidence.

Indication	ACE Inhibitors	ARBs	Key Evidence
Hypertension	First-line therapy; all agents effective (lisinopril, enalapril, ramipril, benazepril)	First-line; especially for ACE inhibitor–intolerant patients (losartan, valsartan, irbesartan, olmesartan)	JNC 8 / AHA 2017 guidelines
Heart Failure (HFrEF)	Cornerstone therapy; reduce mortality (enalapril, lisinopril, ramipril)	Alternative if ACE inhibitor intolerant (valsartan, candesartan, losartan)	CONSENSUS, SOLVD, Val-HeFT, CHARM
Post-MI with LV Dysfunction	Reduce remodeling and mortality (captopril, ramipril, trandolapril)	Valsartan non-inferior to captopril (VALIANT)	SAVE, AIRE, TRACE, VALIANT
Diabetic Nephropathy	Preferred for type 1 DM with microalbuminuria (captopril)	Preferred for type 2 DM with nephropathy (losartan, irbesartan)	Lewis trial, RENAAL, IDNT
Chronic Kidney Disease (proteinuric)	Reduce proteinuria and slow progression	Reduce proteinuria and slow progression	KDIGO 2021 guidelines
Stroke Prevention	Ramipril + diuretic reduces recurrence	Telmisartan as alternative	HOPE, ONTARGET, PRoFESS

Side-by-side comparison of ACE inhibitor and ARB subclasses. ACE inhibitors are classified by their zinc-binding moiety (sulfhydryl, dicarboxylate, or phosphonate), while ARBs are grouped by their biphenyl tetrazole structure. Note shared contraindications (teratogenicity) and the key differentiator: bradykinin effects.

A useful mnemonic for remembering drug suffixes is straightforward: all ACE inhibitors end in -pril (captopril, enalapril, lisinopril, ramipril), and all ARBs end in -sartan (losartan, valsartan, candesartan, telmisartan). Among the ACE inhibitors, lisinopril is notable because it is the only one that is not a prodrug—it requires no hepatic activation and can therefore be used more reliably in patients with significant hepatic impairment. Fosinopril has dual hepatic and renal elimination, making it uniquely dose-adjustment-friendly in renal insufficiency. Among ARBs, losartan has a uricosuric effect that can be beneficial in patients with concomitant gout, and telmisartan has the longest half-life among the sartans and exhibits partial PPARγ agonist activity, which may confer modest metabolic benefits.

SECTION 6

Clinical Scenario: Selecting RAAS Blockade

The following worked example walks through a common clinical decision: choosing between an ACE inhibitor and an ARB for a patient with multiple comorbidities, and then monitoring for adverse effects.

Case: 58-Year-Old Male with Hypertension, Type 2 Diabetes, and Microalbuminuria

Step 1 — Identify the Clinical ProblemA 58-year-old male presents with blood pressure 152/94 mmHg, HbA₁c 7.8%, and urine albumin-to-creatinine ratio (UACR) of 180 mg/g (normal < 30 mg/g). His serum creatinine is 1.2 mg/dL (eGFR 68 mL/min/1.73 m²) and serum potassium is 4.3 mEq/L. He has no history of cough or angioedema.

Diagnosis: Hypertension + Type 2 DM + Diabetic Nephropathy (microalbuminuria)

Step 2 — Select the Drug ClassGuidelines (AHA/ACC, KDIGO) recommend RAAS blockade with an ACE inhibitor or ARB as first-line therapy for hypertensive patients with diabetic kidney disease. Both classes reduce proteinuria and slow nephropathy progression. For type 2 diabetes with nephropathy, ARBs (losartan, irbesartan) have the strongest trial evidence (RENAAL, IDNT). However, ACE inhibitors (ramipril—HOPE trial) also have robust cardiovascular outcome data. Since this patient has no contraindication to either class (no cough history, potassium is normal, no bilateral renal artery stenosis), either is appropriate.

Decision: Start losartan 50 mg daily (strong RENAAL evidence for T2DM nephropathy)

Step 3 — Anticipate Hemodynamic EffectsLosartan will block AT₁ receptors, reducing systemic vascular resistance and blood pressure. In the kidney, efferent arteriolar dilation will lower intraglomerular pressure, reducing proteinuria. We expect a modest decrease in eGFR (up to 30% is acceptable) and a potential mild rise in serum potassium due to reduced aldosterone-mediated K⁺ excretion.

Expected: BP ↓ 10–15 mmHg, UACR ↓ 30–50%, Cr may ↑ ≤ 30%, K⁺ may ↑ 0.3–0.5 mEq/L

Step 4 — Monitor for Adverse EffectsAt the 2-week follow-up, check serum creatinine and potassium. If creatinine rises from 1.2 to ≤ 1.56 mg/dL (30% increase), continue the drug—this reflects effective efferent arteriolar dilation. If potassium rises above 5.5 mEq/L, consider dose reduction, dietary counseling, or adding a potassium-lowering agent. Because an ARB was chosen, dry cough is not expected. If the patient were on an ACE inhibitor and developed a persistent dry cough, switching to an ARB would be appropriate.

At follow-up: Cr 1.3 mg/dL (8% ↑, acceptable), K⁺ 4.6 mEq/L (safe), BP 138/86 mmHg → continue therapy, titrate to target

Step 5 — Optimize & ReassessBlood pressure remains above the target of < 130/80 mmHg (per ACC/AHA 2017). Options include titrating losartan to 100 mg daily or adding a thiazide diuretic (chlorthalidone) or dihydropyridine calcium channel blocker (amlodipine). Importantly, do NOT add an ACE inhibitor to the ARB—dual RAAS blockade (as demonstrated in the ONTARGET trial) increases the risk of hyperkalemia, hypotension, and acute kidney injury without improving cardiovascular outcomes.

Plan: ↑ losartan to 100 mg daily + add amlodipine 5 mg. Avoid dual RAAS blockade.

SECTION 7

Adverse Effects & Contraindications

Both ACE inhibitors and ARBs share several adverse effects related to RAAS suppression, but their bradykinin-related profiles differ markedly. The following table provides a comprehensive comparison of adverse effects, their mechanisms, and their clinical significance.

Comparison of adverse effects between ACE inhibitors and ARBs with mechanistic explanations.

Adverse Effect	ACE Inhibitors	ARBs	Mechanism
Dry cough	5–15% incidence; more common in women and Asian patients	Rare (< 1%); equivalent to placebo	Bradykinin and substance P accumulation in bronchial mucosa stimulate C-fibers
Angioedema	0.1–0.7%; life-threatening; more common in Black patients	Lower risk but not zero; caution if prior ACE inhibitor angioedema	Bradykinin-mediated vascular permeability; may involve complement pathways
Hyperkalemia	Common; monitor K⁺ closely	Common; similar risk as ACE inhibitors	↓ Aldosterone → ↓ renal K⁺ secretion in collecting duct
Hypotension (first-dose)	Risk in volume-depleted patients; start low dose	Similar risk; start low dose	Acute removal of angiotensin II–mediated vascular tone in RAAS-activated states
Acute kidney injury	Risk in bilateral renal artery stenosis or severe volume depletion	Same risk	Efferent arteriolar dilation → ↓ GFR when perfusion is already compromised
Teratogenicity	Contraindicated in pregnancy (Category D/X)	Contraindicated in pregnancy (Category D/X)	Fetal renal dysgenesis, oligohydramnios, pulmonary hypoplasia, skull defects
Dysgeusia (taste disturbance)	Mainly captopril (sulfhydryl group); metallic taste	Not typically reported	Zinc chelation by sulfhydryl moiety

⚠️ CONTRAINDICATIONS MNEMONIC

Remember the major contraindications with the mnemonic PARK: Pregnancy, Angioedema (history of), Renal artery stenosis (bilateral), and hyperKalemia (severe, > 5.5 mEq/L). Additionally, avoid combining an ACE inhibitor with an ARB (dual RAAS blockade) or with a direct renin inhibitor like aliskiren—these combinations worsen renal outcomes and hyperkalemia without mortality benefit.

SECTION 8

Connection to ARNI & Emerging RAAS Therapies

While ACE inhibitors and ARBs have been the backbone of RAAS-targeted therapy for decades, evolving understanding of neurohormonal pathways has driven the development of novel agents. The most significant recent advance is sacubitril/valsartan, an angiotensin receptor–neprilysin inhibitor (ARNI). Sacubitril inhibits neprilysin, the enzyme responsible for degrading natriuretic peptides (ANP, BNP, CNP), bradykinin, and adrenomedullin. Combined with the ARB valsartan, the ARNI simultaneously enhances the beneficial natriuretic peptide system while blocking the deleterious RAAS.

Comparison of ACE inhibitors, ARBs, and the ARNI sacubitril/valsartan.

Feature	ACE Inhibitor	ARB	ARNI (Sacubitril/Valsartan)
RAAS Blockade Site	ACE enzyme	AT₁ receptor	AT₁ receptor + neprilysin inhibition
Natriuretic Peptides	No direct effect	No direct effect	↑ Levels (via neprilysin inhibition)
HFrEF Mortality	↓ (CONSENSUS, SOLVD)	↓ in ACE-intolerant (CHARM)	↓ Superior to enalapril (PARADIGM-HF)
Cough Risk	5–15%	< 1%	< 1% (ARB component)
Key Caution	Angioedema	Hyperkalemia	Must NOT combine with ACE inhibitor; 36-hour washout required

The landmark PARADIGM-HF trial (2014) demonstrated that sacubitril/valsartan reduced cardiovascular death and heart failure hospitalization by 20% compared with enalapril in patients with HFrEF. This led to a paradigm shift in heart failure management: current guidelines now recommend ARNI over ACE inhibitors in patients with HFrEF who remain symptomatic despite optimized medical therapy. However, combining an ARNI with an ACE inhibitor is strictly contraindicated due to excessive bradykinin accumulation (neprilysin also degrades bradykinin), which markedly increases the risk of life-threatening angioedema. A minimum 36-hour washout period must elapse between discontinuing an ACE inhibitor and initiating an ARNI.

🔬 Looking Ahead

Other emerging approaches to RAAS modulation include finerenone (a nonsteroidal mineralocorticoid receptor antagonist with proven cardiorenal benefits in diabetic kidney disease, per FIDELIO-DKD and FIGARO-DKD trials) and investigational AT₂ receptor agonists that aim to exploit the counter-regulatory arm of the RAAS. Understanding ACE inhibitors and ARBs provides the conceptual foundation for appreciating how these newer agents build upon and extend the RAAS blockade strategy.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

Explain why ACE inhibitors cause a dry cough while ARBs generally do not, despite both classes blocking the effects of the RAAS. Your answer should reference the specific enzymatic function of ACE beyond angiotensin conversion.

PROBLEM 2 — BASIC CALCULATION

A patient's baseline serum creatinine is 1.4 mg/dL before starting lisinopril. Two weeks later, the creatinine is 1.7 mg/dL. Calculate the percentage increase in creatinine and determine whether the drug should be continued or discontinued based on the 30% threshold.

PROBLEM 3 — INTERMEDIATE

A 65-year-old woman with HFrEF (EF 30%) and chronic kidney disease (eGFR 45 mL/min) is on enalapril 10 mg BID, carvedilol, and spironolactone. Her potassium is 5.3 mEq/L. She develops a persistent dry cough. Outline your management approach, including which drug to suspect, what to switch to, and what monitoring is needed given her triple RAAS blockade risk.

PROBLEM 4 — APPLIED

A 50-year-old man with type 2 diabetes (HbA₁c 8.2%), hypertension (BP 148/92), and a UACR of 350 mg/g (macroalbuminuria) presents for medication management. His eGFR is 55 mL/min and K⁺ is 4.1 mEq/L. He takes amlodipine 10 mg and metformin 1000 mg BID. Using trial-based evidence, recommend a specific RAAS blocker, justify your choice, describe expected renal effects, and outline monitoring parameters.

PROBLEM 5 — CRITICAL THINKING

The ONTARGET trial showed that combining an ACE inhibitor (ramipril) with an ARB (telmisartan) did not improve cardiovascular outcomes compared with monotherapy but increased adverse events including hyperkalemia, hypotension, and renal dysfunction. Analyze why dual RAAS blockade might have failed despite the pharmacological rationale of more complete RAAS suppression, and discuss how the concept of 'aldosterone escape' and compensatory pathways inform this analysis.

SUMMARY

Lesson Summary

ACE inhibitors (suffix -pril) block angiotensin-converting enzyme, reducing angiotensin II formation and simultaneously increasing bradykinin levels—explaining both their vasodilatory benefit and their signature adverse effects of dry cough (5–15%) and angioedema (rare but life-threatening). ARBs (suffix -sartan) selectively block the AT₁ receptor without affecting bradykinin, making them the preferred alternative for patients intolerant of ACE inhibitors.

Both classes are first-line agents for hypertension, heart failure with reduced ejection fraction, post-MI LV dysfunction, and diabetic nephropathy. Their renoprotective effect stems from efferent arteriolar dilation that lowers intraglomerular pressure and reduces proteinuria. Shared adverse effects include hyperkalemia, hypotension, and teratogenicity (contraindicated in pregnancy). Contraindications to both are captured by the mnemonic PARK: Pregnancy, Angioedema history, Renal artery stenosis (bilateral), and hyperKalemia. Dual RAAS blockade (ACE inhibitor + ARB) should be avoided due to increased risk of adverse events without additional benefit, as demonstrated in the ONTARGET trial.