Introduction to Pharmacodynamics: Drug Actions and Receptor Interactions

Efficacy vs. Potency

By analyzing dose-response curves, we can identify two fundamental properties of any drug: maximal efficacy and relative potency.

Maximal Efficacy

Maximal efficacy is the largest effect a drug can produce, regardless of the dose. On a graph, this is represented by the height of the curve on the Y-axis. For example, if comparing two different analgesics (painkillers), the drug that reaches a higher point on the Y-axis has greater maximal efficacy and will provide more significant pain relief.

Relative Potency

Relative potency refers to the amount of drug required to elicit a specific effect. On a graph, this is indicated by the position of the curve along the X-axis (dosage). A more potent drug requires a smaller dose to achieve the same effect as a less potent one. For instance, morphine is more potent than codeine because it produces pain relief at much lower doses.

Exam Tip: Do not confuse potency with efficacy. A drug can be highly potent (requiring a tiny dose) but have low efficacy (it can't produce a very strong effect). Conversely, a drug can have high efficacy but low potency.

Drug-Receptor Interactions

Most drugs achieve their effects through receptor-mediated action. Receptors are macromolecules—mostly proteins—located on the cell surface, in the cytoplasm, or within the nucleus. The interaction follows a specific sequence:

$\text{Drug (D)} + \text{Receptor} \rightarrow \text{Drug-Receptor Complex} \rightarrow \text{Response}$

To understand how well a drug works at a receptor, we look at three key concepts:

• Selectivity: The degree of complementary correlation between the drug and the receptor; essentially, how well the drug "fits" its target.
• Affinity: The ability of a drug to bind to its receptor.
• Intrinsic Activity (IA) / Efficacy: The ability of the drug to initiate a pharmacological response after the drug-receptor complex has formed.

Pathways of Drug Action

Drugs generally act through one of two primary pathways: receptor-mediated or non-receptor mediated.

Receptor-Mediated Action

In this pathway, the drug interacts with specific chemical compartments (receptors). Receptors typically have two essential functions: recognizing a specific ligand (the drug) via a ligand-binding domain, and transducing that signal into a cellular response via an effector domain.

There are four major families of receptors of medical importance:

1. Ligand-gated ion channels: The ligand binds to an extracellular domain, opening a channel that allows ions (like $Na^+$) to flow through.
2. G-protein coupled receptors (GPCRs): These "seven-transmembrane" receptors pass through the membrane seven times. When a ligand binds, it activates a G-protein (such as $G_s$, $G_i$, or $G_q$), which then triggers secondary messengers like cAMP or $IP_3$ to amplify the signal.
3. Enzyme-linked receptors: These often involve tyrosine kinase activity. When a ligand binds, it causes receptors to aggregate (dimerize), leading to the phosphorylation of tyrosine residues, which then triggers a cellular response.
4. Intracellular receptors: These are located inside the cell. The ligand must be lipid-soluble to cross the plasma membrane. Once bound, the receptor-ligand complex often moves into the nucleus to regulate gene expression.

Non-Receptor Mediated Action

Not all drugs require a receptor to function. Some drugs act through direct chemical or physical interactions. For example, certain drugs may interfere with the metabolic processes of microorganisms, such as penicillin interfering with the peptidoglycan cell wall of bacteria.

Receptor Regulation

Receptors are not static; they exist in a dynamic state where their affinity and number can change based on drug exposure.

• Down-regulation: Prolonged use of an agonist (a drug that activates a receptor) can lead to a decrease in the number or sensitivity of receptors. An example is the reduced affinity of beta-adrenergic receptors caused by the prolonged use of salbutamol.
• Up-regulation: Prolonged use of an antagonist (a drug that blocks a receptor) can cause the cell to increase the number or sensitivity of receptors. This can lead to dangerous "rebound" effects; for instance, abrupt withdrawal of propranolol can cause rebound hypertension and tachycardia due to receptor up-regulation.