Fludrocortisone Acetate

Mechanism

Pharmacological Class and Group

High potency synthetic mineralocorticoid (aldosterone analogue).

Mechanism of Action

Fludrocortisone is a synthetic halogenated corticosteroid with an extreme affinity for the mineralocorticoid receptor, presenting a mineralocorticoid potency 125 times greater than its glucocorticoid potency.

Pharmacokinetics

Mechanism of Sodium Reabsorption in the Distal Tubule

1. Mineralocorticoid Receptor (MR) Binding: Fludrocortisone selectively binds to the cytosolic MR in the principal cells of the distal convoluted tubule and cortical collecting duct of the kidney.

2. Up-regulation of ENaC and Na+/K+ ATPase: The activated ligand-receptor complex translocates to the nucleus and induces transcription of key transporter proteins:

• ENaC: Opens and inserts epithelial sodium channels into the apical cell membrane.
• Na+/K+ ATPase pump: Increases expression in the basolateral cellular membrane, providing the necessary energy gradient.

3. Sodium Retention and Potassium/Hydrogen Excretion:

Fludrocortisone ↑ Active reabsorption of Na⁺ and ↑ Urinary excretion of K⁺ and H⁺

As a consequence of the net reabsorption of sodium and associated free water, circulating plasma volume increases, cardiac filling pressure increases, and blood pressure rises in a sustained manner.

High Resolution Pharmacokinetics

• Absorption and Bioavailability: Rapid and complete absorption through the oral digestive route. Bioavailability greater than 95%. Maximum plasma concentration (T_max) achieved in 1.5 to 2 hours.
• Distribution: Large apparent volume of distribution. High nonspecific plasma protein binding (approximately 70% to albumin binding).
• Metabolism: Mainly rapid hepatic degradation to inactive conjugated metabolites eliminated by secondary urinary clearance.
• Half-life (t_1/2): Short plasma half-life of 3.5 hours, but exhibits a long biological tissue elimination half-life of 18 to 36 hours, allowing single daily intake.

Indicators and dose

Clinical Indications and Off-Label Uses

• Primary Adrenal Insufficiency (Addison's Disease): Mandatory lifelong mineralocorticoid replacement therapy, in combination with hydrocortisone.
• Cerebral Salt Wasting Syndrome / Congenital Salt-Losing Adrenal Hyperplasia: For the rapid correction of profound hyponatremia and cellular dehydration.
• Severe Neurogenic Orthostatic Hypotension (off-label / refractory): To expand total blood volume in patients with severe autonomic failure refractory to physical measures.

Dosage and Clinical Adjustment

Primary Adrenal Insufficiency / Salt-wasting CAH:

• Usual adult dose of 0.05 mg to 0.2 mg once daily orally (typically administered in the morning to mimic physiological mineralocorticoid secretion). Most patients stabilize on 0.1 mg daily.
• Pediatrics (Infants with CAH): May require higher starting doses of 0.1 mg to 0.2 mg per day, accompanied by active oral sodium chloride supplementation due to the functional immaturity of the renal collecting ducts.

Adjustment in Hot Climates: In extreme climates with profuse sweating and marked loss of sodium chloride through the skin, a temporary increase in the fludrocortisone dose or dietary salt intake may be required.

Security

Absolute and Relative Contraindications

Absolute Contraindications

• Hypersensitivity to the active ingredient.
• Severe uncontrolled or refractory arterial hypertension.
• Decompensated congestive heart failure of any NYHA functional class.
• Uncorrected pre-existing severe hypokalemia.

Relative Contraindications

• Advanced chronic kidney disease (eGFR < 30 mL/min/1.73 m²), due to the enhanced risk of extreme water retention and acute pulmonary edema.
• Decompensated liver cirrhosis with active ascites.

Adverse Effects (ADR) and Specific Toxicity

• Frequent (1%-10%): Arterial hypertension, water retention with formation of bilateral peripheral edema, profound hypokalemia (net decrease in serum potassium levels that can cause muscle weakness, generalized myalgias and arrhythmias), headaches.
• Uncommon (0.1%-1%): Hypertrophic cardiomegaly due to sustained volumetric overload, marked increase in intraocular pressure (secondary glaucoma).
• Rare: Hypokalemic metabolic alkalosis.

Clinical Monitoring of Mineralocorticoid Therapy

The optimal adjustment of fludrocortisone dosage in Addison's disease is based on the evaluation of objective clinical symptoms:

• Signs of Underdosing: Marked orthostatic arterial hypotension, strong desire to consume salt, persistent fatigue, hyponatremia and elevated levels of Plasma Renin Activity (PRA).
• Signs of Overdose: Arterial hypertension in recumbent position, prominent bilateral malleolar edema, profound hypokalemia and hypernatremia with completely suppressed ARP.