6.13 : Adrenergic Antagonists: Chemistry and Classification of β-Receptor Blockers

β-adrenergic antagonists, or β-blockers, modulate the sympathetic nervous system by targeting β-adrenoceptors and inhibiting catecholamine-mediated sympathetic responses. β-blockers differ in their adrenoceptor subtype affinity, lipophilicity, and α-blocking capabilities. The history of β-blocker development began with the prototype, dichloroisoprenaline, which exhibited partial agonist activity. As a result, propranolol was developed as a pure antagonist but nonselective agent, paving the way for more targeted therapies. The introduction of β₁-selective drugs, such as practolol, atenolol, and later metoprolol, led to the development of cardioselective β-blockers.

Categorizing β-blockers generation-wise helps differentiate between the older, nonselective molecules of the first generation and the cardioselective molecules of the second generation. First-generation β-blockers include nonselective agents like propranolol, while second-generation drugs are characterized by their cardioselectivity, as seen in drugs like atenolol and metoprolol. The clinical significance of β-blockers extends to those with additional α-blocking properties, which offer a unique combination of effects. These dual-action drugs, such as carvedilol and labetalol, offer therapeutic benefits in managing hypertension and heart failure by simultaneously targeting both α- and β-adrenergic receptors.

To summarize, β-blockers play a vital role in modulating the sympathetic nervous system through their diverse characteristics and modes of action. From their early beginnings with dichloroisoprenaline to the development of cardioselective agents, β-blockers have evolved to provide tailored therapeutic benefits across various medical conditions.