Spironolactone Drug Classification
Spironolactone is a potassium-sparing diuretic that functions primarily as a competitive mineralocorticoid (aldosterone) receptor antagonist, with additional anti-androgenic and weak progestogenic activity. 1, 2
Primary Pharmacological Classification
Mineralocorticoid Receptor Antagonist (MRA)
- Spironolactone competitively antagonizes aldosterone at the mineralocorticoid receptor in the distal renal tubule 1
- It inhibits sodium reabsorption by blocking both Na/K ATPase directly and the epithelial sodium channel (ENaC) indirectly 1
- This mechanism results in potassium retention, classifying it as a potassium-sparing diuretic 3, 2
Secondary Receptor Activities
Anti-Androgenic Properties
- Spironolactone is a non-selective steroid receptor antagonist with moderate affinity for androgen receptors 4
- It competitively inhibits testosterone and dihydrotestosterone binding to androgen receptors in target tissues 5, 6
- It reduces testosterone production and enhances testosterone aromatization to estradiol 1
- This anti-androgenic activity explains its off-label use in acne, hirsutism, and female pattern hair loss 7, 8, 9
Progestogenic Activity
- Spironolactone acts as a weak agonist of the progesterone receptor 1
- It also has moderate affinity for progesterone receptors, contributing to menstrual irregularities as a side effect 4
Structural Classification
Spironolactone is a synthetic 17-lactone steroid (17-hydroxy-7α-mercapto-3-oxo-17α-pregn-4-ene-21-carboxylic acid γ-lactone acetate) 2, 9
FDA-Approved Indications
The FDA classifies spironolactone for use in:
- Heart failure with reduced ejection fraction (NYHA Class III-IV) 2
- Hypertension (as add-on therapy) 2
- Edema associated with hepatic cirrhosis or nephrotic syndrome 2
Important Clinical Distinction from Eplerenone
While both are MRAs, eplerenone is a more selective mineralocorticoid receptor antagonist with minimal affinity for androgen and progesterone receptors, achieved by replacing spironolactone's 17α-thioacetyl group with a carbomethoxy group 4, 10. This structural difference makes eplerenone less likely to cause endocrine side effects (gynecomastia, menstrual irregularities, sexual dysfunction) but also makes it less potent as an MRA 1.