Which pharmacological property best distinguishes propranolol from metoprolol in clinical practice?

Explanation

## Distinguishing Propranolol from Metoprolol ### Cardioselectivity: The Primary Clinical Discriminator **Key Point:** Metoprolol is a cardioselective (β1-selective) beta-blocker, whereas propranolol is non-selective, blocking both β1 and β2 adrenoceptors. This is the single most clinically important pharmacological distinction between the two agents. ### Receptor Selectivity Profile | Feature | Propranolol | Metoprolol | |---------|-------------|------------| | **Receptor Selectivity** | Non-selective (β1 + β2) | Cardioselective (β1 >> β2) | | **Safe in Asthma/COPD** | No (β2 blockade → bronchospasm) | Relatively safer (use with caution) | | **Safe in Peripheral Vascular Disease** | Less safe (β2 blockade → vasoconstriction) | Relatively safer | | **Safe in Diabetics** | Less safe (masks hypoglycaemia symptoms) | Relatively safer | | **Lipophilicity** | High | Moderate-High | | **BBB Penetration** | Significant | Also significant | | **Membrane Stabilization** | Present | Absent | | **ISA** | Absent | Absent | ### Why Option C is Incorrect The claim that "metoprolol does not cross the blood-brain barrier" is factually wrong. Metoprolol is itself moderately lipophilic and does cross the BBB to a clinically meaningful degree — it is well-known to cause CNS side effects such as fatigue, sleep disturbances, and depression. The distinction in CNS penetration between propranolol and metoprolol is one of degree, not an absolute difference, and is NOT the primary clinical discriminator. ### Why Option B is Correct Cardioselectivity is the textbook primary distinction (KD Tripathi, Harrison). It directly governs which patients can safely receive each drug: - Propranolol's β2 blockade makes it **contraindicated** in bronchospastic disease and relatively unsafe in diabetics and peripheral vascular disease. - Metoprolol's β1 selectivity allows its use in heart failure, post-MI, and hypertension even in patients with mild obstructive airway disease. ### Other Options Analysed - **Option A:** Propranolol does have membrane-stabilizing activity (local anaesthetic effect) and metoprolol does not — this is factually true but is clinically less discriminatory because this property is only relevant at supratherapeutic doses. - **Option D:** Both propranolol and metoprolol lack intrinsic sympathomimetic activity (ISA), making this option factually incorrect. **High-Yield:** The NEET PG/INI-CET favourite: metoprolol = cardioselective (β1), propranolol = non-selective (β1 + β2). Cardioselectivity is the primary clinical discriminator governing contraindications and patient selection. **Clinical Pearl:** A patient with hypertension and mild asthma should receive metoprolol (cardioselective) rather than propranolol (non-selective), as propranolol's β2 blockade risks precipitating bronchospasm. [cite: KD Tripathi Essentials of Medical Pharmacology 8e, Ch 12; Harrison's Principles of Internal Medicine 21e]