Angiotensin receptor blockers (ARBs) are selective competitive antagonists at the AT1 receptor, blocking all angiotensin II-mediated effects at that receptor while leaving the AT2 receptor accessible to circulating angiotensin II. This pharmacological distinction from ACE (angiotensin-converting enzyme) inhibitors (ACEi) has two major clinical consequences: ARBs do not cause bradykinin accumulation and therefore do not cause ACEi-characteristic cough or angioedema, and ARBs preserve AT2 receptor signaling, which mediates vasodilatory, natriuretic, and anti-proliferative effects that may contribute to their cardiovascular and renal protective profile.
All ARBs share the same fundamental mechanism: high-affinity, competitive, reversible binding to the AT1 receptor, a Gq-coupled seven-transmembrane receptor. AT1 blockade prevents angiotensin II from activating vasoconstriction, aldosterone secretion, sodium retention, sympathetic facilitation, and the hypertrophic signaling pathways that drive cardiac and vascular remodeling. Because ARBs do not inhibit ACE (kininase II), bradykinin metabolism proceeds normally, and plasma bradykinin levels do not rise. This explains the absence of cough with ARBs, confirming that cough is a bradykinin-mediated class effect of ACEi, not of AT1 blockade per se. Because AT2 receptors are not blocked by ARBs, the rising angiotensin II levels that result from loss of AT1-mediated negative feedback on renin secretion (AT1 blockade raises plasma renin activity and angiotensin II, just as ACEi do) can continue to stimulate AT2 receptors, potentially augmenting the vasodilatory and anti-proliferative counter-regulatory effects that AT2 signaling provides.1
Losartan is the prototype ARB and illustrates the class pharmacokinetics most fully. After oral absorption (bioavailability approximately 33%), losartan undergoes first-pass hepatic metabolism primarily by cytochrome P450 2C9 (CYP2C9) and, to a lesser extent, CYP3A4, generating the active carboxylic acid metabolite EXP3174. EXP3174 has approximately 10–40 times the AT1 receptor affinity of the parent losartan and accounts for the majority of the pharmacological effect; its plasma half-life of approximately 6–9 hours is longer than that of losartan itself (approximately 2 hours), supporting once-daily dosing. The clinical implication of CYP2C9 involvement is that CYP2C9 poor metabolizers (individuals carrying two loss-of-function alleles such as CYP2C9*2 or *3, with a prevalence of approximately 1–3% in European populations) generate significantly less EXP3174 and may have a blunted antihypertensive response to standard losartan doses; switching to an ARB that does not require CYP activation (such as valsartan or irbesartan) may be preferable in this population.2
Candesartan cilexetil is an ester prodrug that is hydrolyzed during gastrointestinal absorption to active candesartan, with oral bioavailability of approximately 40%. Candesartan is eliminated by dual renal and biliary pathways. Valsartan, irbesartan, and olmesartan are orally active without prodrug conversion. Valsartan has a bioavailability of approximately 23%, undergoes minimal hepatic metabolism, and is eliminated primarily by biliary/fecal excretion (approximately 70%) with renal excretion of unchanged drug accounting for the remainder; protein binding exceeds 94%, limiting dialyzability. Irbesartan is metabolized by CYP2C9 to an inactive glucuronide conjugate (no active metabolite), with a half-life of 11–15 hours supporting once-daily dosing. Olmesartan medoxomil is a prodrug hydrolyzed to olmesartan during gut absorption; olmesartan is eliminated by dual renal and biliary routes. As a class, ARBs are highly protein-bound (greater than 90%), have low to moderate oral bioavailability, and are generally not significantly dialyzable, with dose adjustment in renal impairment required for some agents at advanced CKD (chronic kidney disease) stages but not uniformly across the class.2
Losartan is unique among ARBs in that its pharmacological activity depends on CYP2C9-mediated conversion to EXP3174. Drugs that inhibit CYP2C9 (fluconazole, amiodarone, fluvastatin) reduce EXP3174 generation and may attenuate the antihypertensive effect; drugs that induce CYP2C9 (rifampin, carbamazepine) increase EXP3174 generation but also accelerate elimination of both compounds. CYP2C9 poor metabolizers may have markedly reduced antihypertensive efficacy at standard doses. By contrast, valsartan, olmesartan, irbesartan, and candesartan do not depend on CYP2C9 for activation and are not affected by CYP2C9 polymorphisms. This is a clinically meaningful pharmacogenomic distinction when selecting an ARB in patients known to be on strong CYP2C9 inhibitors or inducers.
ARBs share the major cardiovascular and renoprotective indications of ACEi and are generally considered therapeutically interchangeable for most indications, with the critical practical distinction that ARBs are the agents of choice when ACEi are not tolerated due to cough or non-angioedema adverse effects. The evidence base for ARBs is extensive, and in some indication areas the trials were specifically designed to establish non-inferiority or equivalence to ACEi rather than superiority.
In heart failure with reduced ejection fraction (HFrEF), the CHARM-Alternative trial established that candesartan significantly reduces cardiovascular death and heart failure hospitalizations in patients intolerant of ACEi, providing the pivotal evidence base for ARBs as alternative first-line therapy in this population. The Val-HeFT and CHARM-Added trials evaluated ARB addition to background ACEi therapy in HFrEF: both showed modest additional benefit in specific endpoints, but in light of later ONTARGET and ALTITUDE data demonstrating harm from dual RAAS (renin-angiotensin-aldosterone system) blockade in high-risk populations, the current guideline recommendation is against routine ACEi-ARB combination and instead favors substitution of an ARB for ACEi in cough-intolerant patients. Sacubitril-valsartan (ARNI, angiotensin receptor-neprilysin inhibitor) has now superseded both ACEi and ARB monotherapy as the preferred neurohormonal agent for eligible patients with HFrEF, as discussed in Section 4.3
In post-myocardial infarction (MI) management, the VALIANT trial established valsartan as non-inferior to captopril for reducing all-cause mortality in patients with MI complicated by left ventricular systolic dysfunction, heart failure, or both; the combination arm of VALIANT (captopril plus valsartan) showed no additional mortality benefit over either agent alone and increased adverse effects, directly supporting the contraindication of routine ACEi-ARB combination.5 In hypertension, ARBs are guideline-recommended first-line agents with outcomes data broadly comparable to ACEi, and they are the preferred first-line RAAS-blocking agents in patients with a history of ACEi-induced cough. In diabetic nephropathy in type 2 diabetes, the IDNT (irbesartan) and RENAAL (losartan) trials established independent renoprotective effects of ARBs beyond blood pressure lowering, reducing the progression to end-stage renal disease (ESRD) and doubling of serum creatinine in patients with overt proteinuria. The renoprotective mechanism mirrors that of ACEi: efferent arteriolar dilation reduces intraglomerular hypertension and proteinuria.2
The absolute prohibition on ACEi-ARB combination therapy deserves explicit emphasis because it is frequently tested and clinically misapplied. Although early small studies suggested additive proteinuria reduction with combination therapy, the ONTARGET trial (telmisartan plus ramipril in over 25,000 high-risk patients) demonstrated that combination therapy produced more hypotension, more AKI (acute kidney injury), more hyperkalemia, and more requirement for dialysis than either agent alone, without additional reduction in cardiovascular mortality or the primary composite endpoint. Based on ONTARGET, all major cardiology and nephrology guidelines explicitly contraindicate routine combination ACEi-ARB therapy. The only remaining context where brief ACEi-ARB overlap occurs is during the transition to sacubitril-valsartan, where the 36-hour washout period specifically refers to ACEi discontinuation; ARBs within sacubitril-valsartan (valsartan) are part of the fixed combination and are not an added ARB.4
ONTARGET enrolled 25,620 high-risk patients with atherosclerotic disease or diabetes and organ damage. The telmisartan-plus-ramipril arm showed more hypotension (4.8% vs. 1.7%), more syncope, more renal impairment, and more dialysis requirement than ramipril alone, with no reduction in the primary composite of cardiovascular death, MI, stroke, or heart failure hospitalization (16.3% vs. 16.5%). The trial definitively established that dual RAAS blockade in this population causes additive toxicity without additive efficacy. This applies equally to ACEi-ARB combinations and to ACEi-aliskiren or ARB-aliskiren combinations in patients with diabetes or CKD (chronic kidney disease), based on the ALTITUDE data reviewed in Module PEP-01.
The natriuretic peptide system constitutes a hormonal counter-regulatory axis opposing the vasoconstrictor, sodium-retaining actions of the RAAS (renin-angiotensin-aldosterone system) and sympathetic nervous system. Three structurally related peptides, atrial natriuretic peptide (ANP), brain (B-type) natriuretic peptide (BNP), and C-type natriuretic peptide (CNP), are released in response to different stimuli and act through distinct receptor subtypes to produce natriuresis, vasodilation, suppression of renin and aldosterone, and anti-fibrotic effects. Neprilysin, a zinc metallopeptidase, is the primary enzyme responsible for their degradation, making it the pharmacological target of sacubitril.
ANP is a 28-amino-acid peptide synthesized and stored as a prohormone in atrial cardiomyocytes. The primary stimulus for ANP release is atrial wall stretch from increased intracardiac filling pressure. ANP acts through natriuretic peptide receptor A (NPR-A), a transmembrane receptor with intrinsic guanylyl cyclase activity; ligand binding activates the cytosolic guanylyl cyclase domain, generating cyclic guanosine monophosphate (cGMP) as the intracellular second messenger. Downstream cGMP effects in the kidney include natriuresis and diuresis via reduced sodium reabsorption in the collecting duct and reduced glomerular afferent resistance; in the vasculature, cGMP activates protein kinase G, which phosphorylates myosin light chain kinase and reduces vascular smooth muscle calcium sensitivity, producing vasodilation; in the adrenal cortex, ANP suppresses aldosterone secretion independently of angiotensin II. ANP also suppresses renin secretion from JG cells through direct NPR-A activation, providing a negative feedback loop to the RAAS.6
BNP is a 32-amino-acid peptide synthesized predominantly in ventricular cardiomyocytes in response to increased ventricular wall stress, volume overload, and pressure overload. In heart failure, BNP is released in proportion to the degree of cardiac decompensation, making it the most clinically used natriuretic peptide biomarker. BNP also signals through NPR-A, producing the same cGMP-mediated vasodilatory, natriuretic, and anti-RAAS effects as ANP, but with a longer plasma half-life (approximately 20 minutes vs. 2–3 minutes for ANP) that reflects differences in receptor clearance and enzymatic degradation rates. The N-terminal fragment of the BNP prohormone, NT-proBNP (N-terminal pro-B-type natriuretic peptide), is co-secreted in equimolar amounts but is biologically inactive. NT-proBNP has a half-life of approximately 60–120 minutes and is eliminated by renal excretion; it is not a substrate for neprilysin. This distinction is of major practical importance: in patients receiving sacubitril-valsartan (which inhibits neprilysin), BNP levels rise because BNP is a neprilysin substrate and its degradation is impaired; NT-proBNP levels are not affected by neprilysin inhibition and remain a valid biomarker for monitoring heart failure status in sacubitril-treated patients.710
CNP is a 22-amino-acid peptide produced primarily by vascular endothelial cells and acts locally through natriuretic peptide receptor B (NPR-B), also a guanylyl cyclase receptor. CNP lacks the natriuretic and diuretic effects of ANP and BNP but produces vasodilation and inhibits vascular smooth muscle proliferation; it plays a role in long bone growth through NPR-B signaling in chondrocytes. Neprilysin (neutral endopeptidase 24.11, also designated CD10 or enkephalinase) is a zinc metallopeptidase expressed at high density on the surface of renal tubular cells, pulmonary endothelium, and multiple other tissues. It cleaves ANP and BNP at specific peptide bonds, inactivating them. It also degrades bradykinin, substance P, angiotensin I, enkephalins, and endothelin-1, making neprilysin a broad-spectrum vasoactive peptide-degrading enzyme. Inhibiting neprilysin therefore increases the half-lives of multiple vasoactive peptides simultaneously; this substrate breadth is both the therapeutic rationale and the source of the key adverse effect of neprilysin inhibition in clinical use.6
BNP as biomarker: Elevated in heart failure in proportion to wall stress and decompensation. Diagnostic cut-points: BNP >100 pg/mL supports HF diagnosis; BNP <35 pg/mL has high negative predictive value. Limitations: falsely elevated in obesity, renal failure, AF (atrial fibrillation); falsely low in acute flash pulmonary edema (early).
BNP on sacubitril-valsartan: Neprilysin inhibition reduces BNP degradation → BNP levels rise artifactually. BNP is NOT a valid monitoring biomarker in sacubitril-treated patients. Use NT-proBNP instead (not a neprilysin substrate; unaffected by sacubitril).
NT-proBNP cut-points: Age-stratified: <50 years: 450 pg/mL; 50–75 years: 900 pg/mL; >75 years: 1800 pg/mL. NT-proBNP is renally eliminated; levels rise in CKD (chronic kidney disease) independent of cardiac status — interpret accordingly.
Sacubitril-valsartan (Entresto) represents the first approved angiotensin receptor-neprilysin inhibitor (ARNI), combining inhibition of neprilysin with AT1 receptor blockade in a single fixed-dose combination. The therapeutic rationale is synergistic: neprilysin inhibition amplifies the natriuretic peptide counter-regulatory axis while AT1 blockade simultaneously suppresses the angiotensin II-driven vasoconstrictor, sodium-retaining, and remodeling-promoting axis. The PARADIGM-HF trial demonstrated a magnitude of benefit exceeding that of prior HFrEF trials, establishing sacubitril-valsartan as the preferred neurohormonal agent when tolerated in patients with HFrEF.
Sacubitril (AHU377) is an ester prodrug that undergoes rapid hydrolysis by plasma and tissue esterases following oral absorption to generate LBQ657, the pharmacologically active neprilysin inhibitor. LBQ657 contains a zinc-coordinating carboxylate group that binds to the active site of neprilysin with high affinity, competitively and reversibly inhibiting the enzyme. The oral bioavailability of sacubitril is approximately 60%, and peak LBQ657 concentrations are achieved within 2–3 hours of dosing. LBQ657 has a plasma half-life of approximately 11–12 hours and is eliminated primarily by renal excretion of unchanged drug; dose reduction is required in patients with estimated glomerular filtration rate (eGFR) below 30 mL/min/1.73 m2. The valsartan component within sacubitril-valsartan has higher oral bioavailability (approximately 40–50%) than standalone valsartan formulations (approximately 23%) due to differences in formulation; it is eliminated predominantly by biliary/fecal excretion (approximately 70%). Both components are greater than 94% protein bound, and neither is significantly dialyzable.7
The PARADIGM-HF trial enrolled 8,442 patients with HFrEF (ejection fraction 40% or less, subsequently amended to 35% or less), New York Heart Association (NYHA) functional class II–IV (intravenous [IV notation here refers to class severity]) symptoms, and elevated natriuretic peptides, who were randomized to sacubitril-valsartan (200 mg twice daily) or enalapril (10 mg twice daily) in addition to optimal background therapy. The trial was stopped early at a median follow-up of 27 months because sacubitril-valsartan met prespecified criteria for overwhelming efficacy: compared to enalapril, sacubitril-valsartan reduced the primary composite endpoint of cardiovascular death or first hospitalization for heart failure by 20% (hazard ratio 0.80; 95% confidence interval 0.73–0.87; p less than 0.001), reduced all-cause mortality by 16%, reduced cardiovascular mortality by 20%, and reduced the rate of first and total heart failure hospitalizations by 21%. Secondary benefits included a slowing of the decline in kidney function and a greater improvement in NYHA class. Benefit was consistent across all prespecified subgroups, including patients previously on ACEi and those on ARBs (angiotensin receptor blockers), and the benefit extended across the range of ejection fractions studied.7
Based on PARADIGM-HF, the 2022 AHA/ACC/HFSA Heart Failure guidelines give sacubitril-valsartan a Class I, Level of Evidence A recommendation for patients with HFrEF who remain symptomatic on optimal medical therapy, with the strongest recommendation for substituting sacubitril-valsartan for ACEi or ARB in patients who can tolerate RAAS (renin-angiotensin-aldosterone system) blockade. In patients naive to RAAS therapy, initiation of sacubitril-valsartan as first-line neurohormonal therapy is acceptable when blood pressure permits. The starting dose is sacubitril 24 mg / valsartan 26 mg twice daily in patients previously on low-dose RAAS therapy or de novo, titrated over weeks to the target dose of sacubitril 97 mg / valsartan 103 mg twice daily as tolerated.8
PARADIGM-HF was the largest dedicated HFrEF outcomes trial at the time of publication. The number needed to treat (NNT) to prevent one primary endpoint event over 27 months was approximately 21. For all-cause mortality alone, the NNT was approximately 32. Both figures are clinically favorable and compare favorably with the NNTs for beta-blockers and MRAs in HFrEF. The trial population had already been optimized on ACEi or ARB before randomization (a run-in period confirmed tolerability), so the benefit of sacubitril-valsartan was demonstrated above and beyond prior standard of care. Patients with eGFR below 30, symptomatic hypotension, or prior angioedema on ACEi or ARNI were excluded from PARADIGM-HF.
Sacubitril-valsartan carries the most consequential prescribing constraint in contemporary heart failure pharmacology: an absolute contraindication to concurrent use with ACEi and a mandatory 36-hour washout when transitioning between these drug classes in either direction. The mechanism is pharmacodynamically elegant and clinically unforgiving: both ACE and neprilysin inactivate bradykinin, so simultaneous inhibition of both enzymes removes two of the three principal bradykinin-clearing mechanisms, producing bradykinin accumulation far exceeding that seen with either agent alone.
Neprilysin inhibition by sacubitril raises bradykinin levels through a pathway entirely independent of ACE inhibition: neprilysin cleaves bradykinin at a distinct peptide bond (between Pro7 and Phe8), contributing to normal bradykinin clearance alongside ACE (kininase II) and carboxypeptidase N. When sacubitril is combined with an ACEi, both the neprilysin and ACE clearance pathways are simultaneously blocked, leaving only carboxypeptidase N for bradykinin inactivation. The resulting bradykinin accumulation is substantially greater than with either inhibitor alone and carries a clinically significant risk of angioedema, particularly laryngeal angioedema. The pharmacokinetic basis of the 36-hour washout is straightforward: enalaprilat (the active form of enalapril) has a half-life of approximately 11 hours, so 36 hours represents approximately 3.3 half-lives; for LBQ657 (the active sacubitril metabolite), the half-life is also approximately 11–12 hours, so the same 36-hour window applies in the reverse direction when transitioning from sacubitril-valsartan back to an ACEi.9
In PARADIGM-HF, angioedema occurred in 0.45% of sacubitril-valsartan patients versus 0.24% of enalapril patients, a statistically significant difference. Among African American patients, angioedema rates were approximately 2.4% with sacubitril-valsartan versus 0.5% with enalapril, reflecting the known racial disparity in bradykinin-mediated angioedema susceptibility discussed in Module PEP-01. The trial excluded patients with a history of prior ACEi-induced angioedema, so the rate in that higher-susceptibility population is not established from trial data. Current prescribing guidance treats prior ACEi-induced angioedema as a contraindication to sacubitril-valsartan, because the underlying susceptibility to bradykinin-mediated vascular permeability increase persists regardless of which enzyme is responsible for the bradykinin accumulation. Patients who experienced ACEi angioedema are also contraindicated from sacubitril-valsartan and should receive ARB (angiotensin receptor blocker) monotherapy for their HFrEF neurohormonal blockade requirement.9
Hypotension is the most common adverse effect of sacubitril-valsartan in clinical practice, occurring in approximately 18% of patients in PARADIGM-HF. The mechanism is the combined vasodilatory effect of neprilysin inhibition (increased ANP [atrial natriuretic peptide] and BNP [B-type natriuretic peptide]) and AT1 (angiotensin type 1) blockade. Initiation should be at the lowest available dose in patients with systolic blood pressure below 100 mmHg, those who are volume-depleted, or those on high-dose diuretics. Dose titration should be pursued gradually over a minimum of 2–4 weeks. Hyperkalemia occurs at a similar rate to ACEi/ARB in equivalent populations; serum potassium monitoring at baseline, 1–2 weeks after initiation, and with each dose titration is standard. Renal function monitoring follows the same protocol as for ACEi, and a creatinine rise of up to 30% is acceptable. Worsening renal function beyond this threshold should prompt dose reduction or temporary hold and evaluation for contributing factors including volume depletion and NSAID (non-steroidal anti-inflammatory drug) use.8
Absolute contraindications: Concurrent ACEi use (angioedema); within 36 hours of last ACEi dose; history of prior angioedema with ACEi or ARNI; severe hepatic impairment (Child-Pugh C); pregnancy.
Monitoring at initiation: Baseline BP (blood pressure), electrolytes, creatinine/eGFR (estimated glomerular filtration rate); recheck K+ and creatinine at 1–2 weeks after initiation or dose change. Hold if systolic BP <90 mmHg, K+ >5.5 mEq/L, or creatinine rise >30% above baseline.
Biomarker monitoring: Use NT-proBNP (N-terminal pro-B-type natriuretic peptide; not BNP) to monitor heart failure status; BNP is elevated artifactually due to neprilysin inhibition in treated patients.
Transitioning from ACEi: Stop ACEi; wait 36 hours minimum; then start sacubitril-valsartan at lowest dose. Transitioning back (if needed): stop sacubitril-valsartan; wait 36 hours; then start ACEi. No wash-out required when transitioning from ARB monotherapy to sacubitril-valsartan.
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