Amlodipine and Nifedipine

Mechanism

Pharmacological Class and Group

Antihypertensives, antianginals and arterial vasodilators belonging to the class of Type L Calcium Channel Blockers of the Dihydropyridine group.

Mechanism of Action

Vascular smooth muscle tone and peripheral resistance depend directly on the cytoplasmic influx of calcium of extracellular origin. L-type voltage-gated calcium channels (Cav1.2) are the main mediators of this cationic transport in the smooth muscle cells of the arterial tunica media.

Molecular Mechanism of Calcium Blockage in Vessels

1. Allosteric binding to the α₁ subunit of the L-type calcium channel: Amlodipine and nifedipine bind selectively and reversibly to the catalytic pore-forming subunit of the Cav1.2 channel dominantly expressed in the smooth muscle of peripheral resistance arteries. Binding occurs preferentially when the channel is in its inactive or depolarized state.

2. Blockade of Calcium influx and Inactivation of MLCK: Blockade of the channel prevents the influx of free calcium into the cell cytosol during the membrane depolarization phase:

↓ Ca²⁺ influx ↓ Calcium-Calmodulin Complex ↓ Myosin Light Chain Kinase (MLCK) Activity

Inhibition of MLCK prevents the phosphorylation of myosin heads and prevents their coupling to actin filaments, inducing sustained relaxation of vascular smooth muscle cells. This translates into systemic arterial vasodilation marked with a drop in PVR and afterload.

3. Vascular vs. Myocardial Selectivity: At therapeutic concentrations, dihydropyridines exhibit an affinity profile up to 100 times higher for the Cav1.2 channel of vascular smooth muscle compared to the cardiac myocyte, which explains their potent systemic vasodilatory effect without direct negative inotropic or dromotropic effects of clinical relevance.

Pharmacokinetics

Comparative Pharmacokinetics

Indicators and dose

Clinical Indications and Uses

• Systemic Arterial Hypertension: First line, highly effective in monotherapy, especially indicated in elderly patients or with isolated systolic hypertension.
• Chronic Stable Angina Pectoris: They reduce the double product of tension and promote vasodilation of the main epicardial coronary arteries.
• Prinzmetal's Angina (Vasospastic): Priority choice to prevent spontaneous coronary artery spasm induced by smooth muscle hyperreactivity.
• Raynaud's phenomenon: Effective prophylactic management of intermittent digital ischemia.

Dosage and Clinical Adjustment

Amlodipine:

• Hypertension/Angina: Initial dose of 2.5 mg to 5 mg once daily orally. Usual maintenance dose of 5 mg to 10 mg once daily.
• Hepatic Adjustment: Mandatory starting dose of 2.5 mg daily in patients with cirrhosis or moderate degree of liver failure.

Nifedipine (OROS / Extended Release):

• Hypertension / Angina: Initial dose of 30 mg once a day. Increase to usual maintenance dose of 60 mg to 90 mg once daily. The tablet should be swallowed whole without breaking or chewing.

Security

Absolute and Relative Contraindications

Absolute Contraindications

• Severe systemic arterial hypotension (Systolic blood pressure < 90 mmHg).
• Cardiogenic shock or acute decompensated heart failure.
• Severe symptomatic aortic stenosis.
• Short-acting dihydropyridines (immediate-release Nifedipine): Absolutely contraindicated in emergency hypertension, because the abrupt drop in pressure triggers severe reflex adrenergic discharges that cause myocardial ischemia and stroke.

Relative Contraindications

• Severe hepatic impairment (risk of accumulation due to prolonged clearance).
• History of marked bilateral malleolar edema unrelated to heart failure.

Adverse Effects and Toxicity

• Bilateral Malleolar Edema (10% - 15%): Of non-hydrostatic origin. It is due to selective precapillary arteriolar dilation that is not accompanied by similar postcapillary venodilation, raising the transcapillary hydrostatic filtration pressure towards the malleolar interstitial fluid. It is resistant to therapy with loop diuretics and usually responds to dose reduction or the association of an ACEI or ARB (which induce return post-capillary venodilation).
• Headache, facial flushing and dizziness: Secondary to rapid cerebral and peripheral vasodilation.
• Sympathetic Reflex Tachycardia: Common with immediate-release formulations (nifedipine) due to a sudden drop in blood pressure detected by baroreceptors.
• Gingival Hyperplasia: Idiopathic in nature due to local alteration in the metabolism of gingival fibroblasts exposed to calcium blockade.

Relevant Drug Interactions

• Potent CYP3A4 inhibitors (Clarithromycin, Grapefruit Juice in large quantities, Itraconazole): They netly double the levels of amlodipine or nifedipine in the blood, inducing profound arterial hypotension.
• CYP3A4 inducers (Rifampin, Phenytoin, Carbamazepine): They dramatically accelerate the hepatic metabolism of dihydropyridines, nullifying their therapeutic effect.

Clinical

Clinical Pearl: Pathophysiological Differentiation of Edema due to Calcium Antagonists

Amlodipine-induced edema does not represent intravascular volume overload or progressive myocardial failure, but rather an alteration in local capillary hydrostatic forces:

Amlodipine Precapillary arteriolar vasodilation ≫ Postcapillary venular tone ↑ Capillary hydrostatic pressure

The combination of amlodipine with a drug that modulates the RAAS (such as an ACEI or ARB) effectively reduces malleolar edema, because Angiotensin II blockade induces compensatory postcapillary venodilation, balancing pressures and reducing fluid extravasation.