Phenoxybenzamine is used in preoperative management of phaeochromocytoma. Compared to phentolamine, which statement correctly characterises its mechanism of action?
- A It is an irreversible non-competitive alkylating antagonist of alpha receptors ✓
- B It is a reversible competitive antagonist of both alpha-1 and alpha-2 receptors
- C It blocks only alpha-1 receptors and has a short half-life
- D It acts primarily by reducing catecholamine synthesis in the adrenal medulla
Correct answer: A. It is an irreversible non-competitive alkylating antagonist of alpha receptors
Explanation
Phenoxybenzamine forms a stable covalent bond with alpha adrenoceptors, rendering blockade irreversible (non-competitive). This is particularly useful in phaeochromocytoma as episodic catecholamine surges cannot overcome the blockade. New receptor synthesis over several days is required to restore function. Phentolamine, by contrast, is a reversible competitive antagonist. Phenoxybenzamine has no effect on catecholamine synthesis.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.