4.14:
Combined Effects of Drugs: Antagonism
Antagonism is an inhibitory mechanism where one drug blocks or reduces the effect of another drug. Commonly, an antagonist binds the receptor's active site and prevents agonist binding, blocking receptor activity.
Other forms of antagonism include allosteric, functional, chemical, and pharmacokinetic.
Allosteric antagonists bind receptors, such as ion channels, at a site distinct from the agonist-binding site to prevent activation.
A functional antagonist binds a different receptor to produce a physiological response opposite to the agonist-bound receptor. For example, histamine binds to histamine receptors, inducing bronchial smooth muscle contraction. In contrast, epinephrine binds β-adrenoceptors and relaxes muscle cells.
In chemical antagonism, a drug combines with another to form an inactive product. A positively charged protamine interacts with negatively charged heparin, forming an inactive salt aggregate. The aggregate prevents excessive bleeding by reversing the anticoagulant effects of heparin.
Lastly, a pharmacokinetic antagonist decreases another drug's concentration at the site of action. It does so by reducing its absorption, changing its distribution, or increasing its elimination.
4.14:
Combined Effects of Drugs: Antagonism
The combined effects of drugs can result in various interactions, of which an important type is antagonism. Antagonism is a mechanism where one drug inhibits or counteracts the effects of another drug. Antagonism can occur through various means, including receptor binding, allosteric modulation, functional interaction, chemical reactions, and pharmacokinetic processes.
The most common type is receptor antagonism, where one drug acts as an antagonist to block the effects of another drug by binding to the same receptor. This prevents the agonist drug from activating the receptor and producing its intended effect. For example, naloxone, a competitive antagonist, can block the effects of opioid drugs by binding to opioid receptors and reversing their activation.
In addition, functional or physiological antagonism occurs when a drug binds to a different receptor and produces a physiological response that opposes the effect of the agonist-bound receptor. For example, histamine stimulates acid secretion, while omeprazole prevents acid secretion by inhibiting the proton pump. These drugs can be considered functional antagonists.
Some drugs follow chemical antagonism, which involves the combination of two drugs to form an inactive product. Examples include chelating agents like dimercaprol, which bind to heavy metals and reduce their toxicity. The neutralizing antibody infliximab, which sequesters the inflammatory cytokine tumor necrosis factor (TNF), has an anti-inflammatory effect.
Antagonism can also occur through pharmacokinetic interactions, where one drug affects another drug's absorption, distribution, metabolism, or elimination. For instance, if a drug increases the metabolism or elimination of another drug, it can decrease the concentration and effectiveness of the second drug, leading to antagonistic effects.