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Classify antihypertensive drugs. Discuss the detailed mechanism of action, uses, and adverse effects of AT1 receptor blockers (Sartans)

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"angiotensin receptor blockers" AND "hypertension"

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Antihypertensive Drugs: Classification, and AT1 Receptor Blockers (Sartans)

I. Classification of Antihypertensive Drugs

Antihypertensive drugs are broadly classified by their site and mechanism of action:

A. Diuretics

  1. Thiazides - Hydrochlorothiazide, Chlorthalidone, Indapamide
  2. Loop diuretics - Furosemide, Torsemide (used in CKD/fluid overload states)
  3. Potassium-sparing - Spironolactone, Eplerenone, Amiloride, Triamterene

B. Sympatholytic (Adrenergic-Blocking) Agents

  1. Centrally acting - Clonidine, Methyldopa, Moxonidine (α2-agonists/imidazoline receptor agonists)
  2. Ganglionic blockers - Mecamylamine (largely obsolete)
  3. Peripheral adrenergic neuron blockers - Reserpine, Guanethidine
  4. α1-receptor blockers - Prazosin, Doxazosin, Terazosin
  5. β-receptor blockers
    • Non-selective: Propranolol, Nadolol, Pindolol
    • Cardioselective (β1): Metoprolol, Atenolol, Bisoprolol, Nebivolol
    • Mixed α+β: Labetalol, Carvedilol

C. Renin-Angiotensin-Aldosterone System (RAAS) Blockers

  1. ACE inhibitors - Captopril, Enalapril, Lisinopril, Ramipril, Perindopril
  2. AT1 receptor blockers (ARBs / Sartans) - Losartan, Valsartan, Candesartan, Irbesartan, Olmesartan, Telmisartan, Azilsartan, Eprosartan
  3. Renin inhibitors - Aliskiren
  4. Aldosterone antagonists - Spironolactone, Eplerenone

D. Calcium Channel Blockers (CCBs)

  1. Dihydropyridines (primarily vascular) - Amlodipine, Nifedipine, Felodipine, Nicardipine
  2. Non-dihydropyridines (vascular + cardiac) - Verapamil (phenylalkylamine), Diltiazem (benzothiazepine)

E. Direct Vasodilators

  1. Arteriolar - Hydralazine, Minoxidil, Diazoxide
  2. Mixed (arterial + venous) - Sodium nitroprusside (hypertensive emergencies)
  3. Dopamine receptor agonists - Fenoldopam (IV, hypertensive emergencies)

F. Miscellaneous / Newer Agents

  • ARB-neprilysin inhibitor (ARNI) - Sacubitril/Valsartan (heart failure with reduced EF)
  • Endothelin antagonists - Bosentan (pulmonary hypertension)

II. AT1 Receptor Blockers (ARBs / Sartans)

Available Drugs

DrugBrand Name
LosartanCozaar
ValsartanDiovan
CandesartanAtacand
IrbesartanAvapro
OlmesartanBenicar
TelmisartanMicardis
EprosartanTeveten
AzilsartanEdarbi

A. Historical Development

Development of ARBs began in the 1970s with peptide analogues like saralasin, which had poor oral bioavailability and partial agonist activity. A breakthrough came in the 1980s with synthesis of imidazole-5-acetic acid derivatives (S-8307 and S-8308), which were specific but weak nonpeptide AT1 antagonists devoid of partial agonist activity. Stepwise modifications led to losartan, approved in the US in 1995 as the first orally active, potent, selective nonpeptide AT1 receptor antagonist. Seven additional agents have since been approved. (Goodman & Gilman's, Chapter 30)

B. Mechanism of Action

The RAAS Axis (Background)

  • Renin (released by kidneys) cleaves angiotensinogen → Angiotensin I (Ang I)
  • ACE converts Ang I → Angiotensin II (Ang II)
  • Ang II acts on two receptor subtypes:
    • AT1 receptors - mediates all classic (and harmful) cardiovascular effects
    • AT2 receptors - mediates potentially protective, vasodilatory, antiproliferative effects

Pharmacological Effects of AT1 Blockade

ARBs bind to AT1 receptors with high affinity, showing >10,000-fold selectivity for AT1 over AT2 receptors. The inhibition is often functionally insurmountable (even at high Ang II levels), providing a theoretical advantage of sustained receptor blockade even with missed doses or increased endogenous ligand. By blocking AT1 receptors, ARBs inhibit all major biological effects of Ang II:
  1. Vascular smooth muscle contraction → vasodilation, reduced peripheral resistance
  2. Rapid and slow pressor responses → reduced blood pressure
  3. Aldosterone secretion → reduced Na+ retention and volume
  4. Vasopressin release → reduced water retention
  5. Thirst stimulation → decreased fluid intake drive
  6. Adrenal catecholamine release → sympathetic attenuation
  7. Enhancement of noradrenergic neurotransmission → reduced sympathetic tone
  8. Cellular hypertrophy and hyperplasia → regression of vascular and cardiac hypertrophy
  9. Changes in renal function → reduced intraglomerular pressure, antiproteinuric effect

Key Advantage Over ACE Inhibitors

FeatureACE InhibitorsARBs
Bradykinin accumulationYes (causes cough, angioedema)No
Block all Ang II pathways?No - chymase can still make Ang IIYes - block AT1 regardless of how Ang II is made
AT2 stimulationMinimalYes - increased Ang II stimulates unblocked AT2
Cough5-20%Rare (<2%)
The ability to block Ang II regardless of its synthetic pathway (ACE-dependent or chymase-dependent) is a major mechanistic advantage. Additionally, ARBs permit unopposed AT2 receptor activation due to the reactive rise in circulating Ang II, which may confer vasodilatory and antiproliferative effects (though clinical benefit over ACEIs is not yet proven). - Goodman & Gilman's The Pharmacological Basis of Therapeutics, Ch. 30

Reactive Renin Rise

Both ARBs and ACEIs increase plasma renin concentration (PRC) and plasma renin activity (PRA) due to loss of Ang II negative feedback on renin release. With ARBs, this results in a several-fold increase in circulating Ang II levels (unlike ACEIs where Ang I accumulates instead), which further stimulates the unblocked AT2 receptors.

C. Pharmacokinetics

PropertyLosartanValsartanNotes
Bioavailability~33%~25%Most are well absorbed orally
Active metaboliteEXP-3174 (more potent)NoneEXP-3174 is 10-40x more potent
Protein binding>99%>95%Highly protein-bound
EliminationRenal + hepaticPrimarily fecal/hepatic
Half-lifeLosartan 2h; EXP-3174 6-9h~6hTelmisartan has longest t½ ~24h
Uricosuric effectYes (losartan only)NoUnique to losartan
All ARBs are administered once daily except for some (e.g., losartan may be given twice daily in higher doses). Telmisartan has the longest half-life (~24 hours) and highest lipophilicity. - Katzung's Basic and Clinical Pharmacology, 16th Ed.

D. Uses / Therapeutic Indications

1. Hypertension (Primary)

First-line antihypertensive therapy. Efficacy is similar to ACE inhibitors and other first-line agents. Preferred when ACEIs cause cough or angioedema.

2. Heart Failure

Useful alternatives when ACEIs are not tolerated. Valsartan and candesartan have established evidence in heart failure with reduced ejection fraction (HFrEF). The combination ARB+ACEI is not recommended due to increased risk of hypotension, hyperkalemia, and renal dysfunction without added prognostic benefit. - Goodman & Gilman's

3. Diabetic Nephropathy

ARBs (irbesartan, losartan) slow progression of diabetic nephropathy. They reduce proteinuria and stabilize renal function through hemodynamic effects on the glomerulus (reducing efferent arteriolar resistance and intraglomerular pressure). Now recommended in diabetes even in the absence of overt hypertension.

4. Chronic Kidney Disease (non-diabetic)

ARBs and ACEIs are first-line antihypertensive therapy in CKD for their renoprotective effects, independent of blood pressure reduction alone. - Brenner & Rector's The Kidney

5. Post-Myocardial Infarction

Used in patients with MI complicated by LV dysfunction when ACEIs are not tolerated.

6. Stroke Prevention

Losartan (LIFE trial) reduced stroke incidence in hypertensive patients with LVH, with superiority over atenolol.

7. Marfan Syndrome

Losartan may be as effective as atenolol by reducing TGF-β signaling, since Ang II upregulates TGF-β, which drives aortic pathology. - Katzung's Basic and Clinical Pharmacology

8. Prevention of Type 2 Diabetes

Valsartan has been reported to decrease incidence of diabetes in patients with impaired glucose tolerance.

9. Heart Failure - ARNI (Combination)

Sacubitril/valsartan (Entresto) combines valsartan with sacubitril (neprilysin inhibitor) and is the preferred therapy over ACEIs in HFrEF with ongoing symptoms.

E. Adverse Effects

ARBs are generally well tolerated with a favorable adverse effect profile compared to ACEIs.

1. Hypotension

  • "First-dose" hypotension, especially in volume-depleted patients (on diuretics, salt restriction)
  • Generally mild

2. Hyperkalemia

  • Due to reduced aldosterone secretion
  • Clinically significant in patients with renal insufficiency, diabetes, or those also taking potassium supplements/potassium-sparing diuretics

3. Acute Renal Failure

  • Can occur in patients with bilateral renal artery stenosis or stenosis of a solitary kidney (same as ACEIs)
  • Contraindicated in renal artery stenosis

4. Cough

  • Rare (<2%), unlike ACEIs (5-20%)
  • Occurs because ARBs do not cause bradykinin accumulation
  • This is the principal clinical advantage over ACEIs

5. Angioedema

  • Can occur but is uncommon; less frequent than with ACEIs
  • Mechanism is bradykinin-independent (possibly through AT2 or other pathways)

6. Fetopathic Syndrome (Teratogenicity)

  • Contraindicated in pregnancy (all trimesters, especially 2nd and 3rd)
  • Risk of fetal hypotension, anuria, oligohydramnios, renal failure, fetal malformations, or death
  • Mechanism: fetal kidneys are RAAS-dependent for development

7. Sprue-like Enteropathy

  • A syndrome resembling celiac disease with severe chronic diarrhea, weight loss, and villous atrophy
  • Associated particularly with olmesartan
  • Reversible on drug discontinuation

8. Hyperuricemia (Losartan - Protective)

  • Uniquely, losartan has a uricosuric effect, reducing serum uric acid (beneficial in gout-prone hypertensives)

9. Renal Impairment

  • Contraindicated in nondiabetic renal disease (per some guidelines) and in bilateral renal artery stenosis
  • Should be used with caution in significant renal impairment

F. Contraindications

ContraindicationReason
PregnancyFetal nephrotoxicity, fetopathic syndrome
Bilateral renal artery stenosisAcute renal failure
Hyperkalemia / Addison's diseaseRisk of life-threatening hyperkalemia
Combination with ACEIs + ARBsIncreased adverse effects, no benefit
Combination with aliskiren in DM/CKDContraindicated (increased renal/CV risk)

G. Drug Interactions

  • Potassium supplements / K+-sparing diuretics → Hyperkalemia
  • NSAIDs → Reduced antihypertensive effect, increased renal risk
  • Aliskiren - combined use in diabetic or CKD patients is contraindicated

Summary Comparison: ARBs vs. ACEIs

FeatureACEIsARBs
Bradykinin elevationYesNo
Cough5-20%<2%
Angioedema~0.1-0.5%Rare
AT2 receptor activationMinimalYes (via elevated Ang II)
Complete Ang II blockadeNo (chymase pathway remains)Yes
EfficacyEquivalentEquivalent
Uricosuric effectNoLosartan only
Sprue-like enteropathyNoOlmesartan
Sources:
  • Katzung's Basic and Clinical Pharmacology, 16th Edition, Ch. 11
  • Goodman & Gilman's The Pharmacological Basis of Therapeutics, Ch. 30
  • Brenner and Rector's The Kidney, Antihypertensive Drugs section
  • Zhang et al. (2024) - Network meta-analysis of 6 ARBs in hypertension - comparative efficacy/safety of sartans in hypertensive patients (PMID: 38861046)
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