Receptor Antagonist - Reversibility

Reversibility

Many antagonists are reversible antagonists that, like most agonists, will bind and unbind a receptor at rates determined by receptor-ligand kinetics.

Irreversible antagonists covalently bind to the receptor target and, in general, cannot be removed; inactivating the receptor for the duration of the antagonist effects is determined by the rate of receptor turnover, the rate of synthesis of new receptors. Phenoxybenzamine is an example of an irreversible alpha blocker—it permanently binds to α adrenergic receptors, preventing adrenaline and noradrenaline from binding. Inactivation of receptors normally results in a depression of the maximal response of agonist dose-response curves and a right shift in the curve occurs where there is a receptor reserve similar to non-competitive antagonists. A washout step in the assay will usually distinguish between non-competitive and irreversible antagonist drugs, as effects of non-competitive antagonists are reversible and activity of agonist will be restored.

Irreversible competitive antagonists also involve competition between the agonist and antagonist of the receptor, but the rate of covalent bonding differs and depends on affinity and reactivity of the antagonist. For some antagonist, there may be a distinct period during which they behave competitively (regardless of basal efficiacy), and freely associate to and dissociate from the receptor, determined by receptor-ligand kinetics. But, once irreversible bonding has taken place, the receptor is deactivated and degraded. As for non-competitive antagonists and irreversible antagonists in functional assays with irreversible competitive antagonist drugs, there may be a shift in the log concentration–effect curve to the right, but, in general, both a decrease in slope and a reduced maximum are obtained.