4.10
Dose-Response Relationship: Selectivity and Specificity
Drugs exert their therapeutic effects by interacting with receptors, enzymes, or ion channels that are present throughout the human body. The strength…
Different types of receptors are expressed throughout the body. The interaction of these receptors with drug molecules may lead to a pharmacologic response.
Drug–receptor interactions are characterized by selectivity and specificity.
Drug specificity is the ability of a drug to interact with only a specific target.
For example, in the parietal cells of the stomach, omeprazole inhibits the proton pump and decreases acid secretion into the stomach lumen.
In contrast, drugs like amiodarone have low specificity as they interact with multiple targets in the body, including receptors, enzymes and ion channels. Such off-target interactions can lead to several adverse effects beyond the drug's primary action.
Drug selectivity describes the strong preference of a drug for its intended receptor over other receptors.
For instance, consider two β-adrenergic agonists, isoprenaline and salbutamol.
As isoprenaline is a nonselective β agonist, it can stimulate β-1 receptors of the heart and β-2 receptors of the bronchioles. This increases the heart rate and relaxes the bronchial muscles.
In comparison, salbutamol is a selective β agonist with a higher affinity for the β-2 receptors, causing bronchodilation.
Q1: What is the difference between drug selectivity and drug specificity?
Drug selectivity describes a drug's strong preference for its intended receptor over other receptors in the body. Drug specificity refers to a drug's ability to interact with only a specific target, producing the desired therapeutic effect without causing unintended physiological changes. High specificity ensures targeted action with minimal side effects, while selectivity determines which receptor among many similar ones the drug preferentially binds.
Q2: Why is salbutamol preferred over isoprenaline for treating asthma?
Salbutamol is a selective beta-2 adrenergic agonist with high affinity for beta-2 receptors in bronchial smooth muscle, causing bronchodilation without affecting the heart. Isoprenaline, a nonselective beta agonist, stimulates both beta-1 receptors in the heart and beta-2 receptors in the bronchioles, increasing heart rate alongside bronchial relaxation. Salbutamol's selectivity makes it safer and more effective for asthma treatment.
Q3: How does omeprazole demonstrate high drug specificity?
Omeprazole, a proton-pump inhibitor, exhibits high specificity by selectively inhibiting the proton pump in parietal cells of the stomach, decreasing acid secretion into the stomach lumen. Its targeted interaction with this specific enzyme produces the desired therapeutic effect without impacting other physiological processes. This high specificity results in minimal off-target interactions and fewer adverse effects.
Q4: What are off-target interactions and why do they occur with low-specificity drugs?
Off-target interactions occur when drugs bind to unintended receptors, enzymes, or ion channels throughout the body. Low-specificity drugs like amiodarone interact with multiple targets beyond their primary action, leading to several adverse effects. These unintended interactions result from weak drug-receptor selectivity, causing the drug to bind to similar molecular structures across different tissues and systems.
Q5: How do different receptor types throughout the body affect drug selectivity?
Different types of receptors are expressed throughout the body, and drugs interact with these receptors based on their selectivity and affinity. A selective drug preferentially binds to its intended receptor type over structurally similar receptors in other tissues. For example, selective beta-2 agonists target bronchial receptors while avoiding cardiac beta-1 receptors, allowing drugs to produce localized therapeutic effects while minimizing systemic side effects.
Q6: What consequences result from amiodarone's low specificity as an antiarrhythmic drug?
Amiodarone demonstrates low specificity by affecting multiple ion channels throughout the body, not just those involved in cardiac rhythm regulation. This broad interaction profile causes various side effects beyond its intended antiarrhythmic action. The drug's weak selectivity for specific ion channels means it produces unintended physiological changes in multiple organ systems, limiting its therapeutic window and increasing adverse effect risk.
Q7: Why is receptor affinity important for determining drug selectivity?
Receptor affinity determines how strongly a drug binds to a specific receptor compared to other receptors. High affinity for the intended receptor combined with low affinity for off-target receptors creates selectivity. Salbutamol's high affinity for beta-2 receptors versus beta-1 receptors exemplifies this principle, allowing the drug to preferentially activate bronchial smooth muscle while minimizing cardiac effects.