Section 2.3: General Pharmacology: Mechanisms of Drug Action

Section 2.3: General Pharmacology: Mechanisms of Drug Action

Introduction to Pharmacodynamics

Pharmacodynamics (PD) describes the biochemical and physiological effects of drugs and their mechanisms of action—essentially, "what the drug does to the body." The majority of drugs exert their effects by interacting with specific cellular macromolecules called receptors. In this section, we cover agonist and antagonist classifications, quantitative pharmacodynamics (including dose-response curves and the therapeutic index), and the four major superfamilies of receptors.

Agonists, Antagonists, and Receptor Binding

Agonists

An agonist binds to a receptor and stabilizes it in an active conformation to produce a biological response.

• Full Agonists: Bind to a receptor and elicit a maximal response (100% intrinsic activity or efficacy). Examples include epinephrine at beta-1 receptors and morphine at mu-opioid receptors.
• Partial Agonists: Bind to the same receptor site but elicit a submaximal biological response even when all receptors are occupied (intrinsic activity between 0 and 100%). A partial agonist can act as a functional antagonist in the presence of a full agonist by competing for receptor binding and lowering the overall response. Examples include buprenorphine (mu-opioid receptor) and aripiprazole (dopamine D2 receptor).
• Inverse Agonists: Bind to the same receptor site as an agonist but suppress the basal (constitutive) activity of the receptor, producing an effect opposite to that of a full agonist (negative intrinsic activity). Famotidine (H2 histamine receptor) acts as an inverse agonist.

Antagonists

An antagonist binds to a receptor but possesses zero intrinsic activity; it does not activate the receptor but prevents the binding of endogenous agonists.

• Competitive (Reversible) Antagonists: Bind reversibly to the active (orthosteric) site of the receptor. Because the binding is reversible, increasing the concentration of the agonist can displace the antagonist. On a log dose-response curve, a competitive antagonist:
  • Shifts the curve parallelly to the right (increases the $EC_{50}$, the concentration required to produce a 50% response).
  • Does not change the $E_{\text{max}}$ (maximal efficacy remains the same).
  • Examples include atropine (muscarinic antagonist) and metoprolol (beta-blocker).
• Non-Competitive (Irreversible or Allosteric) Antagonists: Bind either irreversibly via covalent bonds to the active site, or reversibly to an allosteric site (a site distinct from the agonist binding site), which alters the receptor's conformation. In either case, the antagonism cannot be overcome by adding more agonist. On a log dose-response curve, a non-competitive antagonist:
  • Shifts the curve downward, decreasing the $E_{\text{max}}$.
  • Does not affect the $EC_{50}$ (potency of the remaining active receptors is unchanged).
  • Examples include phenoxybenzamine (irreversible alpha-1 blocker) and ketamine (NMDA receptor allosteric antagonist).

Quantitative Pharmacology and Therapeutic Index

Drug effects are quantified using dose-response curves.

• Potency: Refers to the concentration ($EC_{50}$) or dose ($ED_{50}$) of a drug required to produce 50% of its own maximal effect. A lower $EC_{50}$ indicates higher potency.
• Efficacy ($E_{\text{max}}$): The maximum biological effect a drug can achieve. Efficacy is a more clinically relevant parameter than potency; a drug that is highly potent but has low efficacy is often less useful than a less potent drug with high efficacy.
• Therapeutic Index (TI): A measure of drug safety, calculated from quantal dose-response curves (which plot the fraction of the population responding versus dose): $$\text{Therapeutic Index } (TI) = \frac{TD_{50}}{ED_{50}} \quad \left(\text{or } \frac{LD_{50}}{ED_{50}} \text{ in animal studies}\right)$$ where $TD_{50}$ is the median toxic dose, $ED_{50}$ is the median effective dose, and $LD_{50}$ is the median lethal dose.

Narrow Therapeutic Index (NTI) Drugs: NTI drugs have a TI value close to 1, meaning there is little margin between the effective dose and the toxic dose. These drugs require Therapeutic Drug Monitoring (TDM) to measure blood levels and prevent toxicity. Examples include:

• Warfarin (risk of hemorrhage/subtherapeutic clot).
• Digoxin (cardiac arrhythmias/hypokalemia risk).
• Lithium (neurological toxicity, renal impairment).
• Phenytoin (nystagmus, ataxia, seizures).

Major Receptor Superfamilies

Receptors are classified into four main superfamilies based on their structure and signal transduction mechanisms:

1. G-Protein Coupled Receptors (GPCRs / Metabotropic)

GPCRs are transmembrane proteins that span the cell membrane seven times. They are coupled to intracellular heterotrimeric G-proteins ($\alpha, \beta, \gamma$ subunits). Upon ligand binding, GDP is exchanged for GTP on the $\alpha$-subunit, causing it to dissociate and activate or inhibit downstream effector proteins.

• $G_s$ Pathway: Stimulates adenylyl cyclase $\rightarrow$ increases cyclic AMP (cAMP) $\rightarrow$ activates Protein Kinase A (PKA). Examples: Beta-1 and Beta-2 adrenergic receptors.
• $G_i$ Pathway: Inhibits adenylyl cyclase $\rightarrow$ decreases cAMP $\rightarrow$ opens $K^+$ channels and closes $Ca^{2+}$ channels. Examples: Alpha-2 adrenergic, $M_2$ muscarinic, and mu-opioid receptors.
• $G_q$ Pathway: Activates Phospholipase C (PLC) $\rightarrow$ cleaves $PIP_2$ into Inositol 1,4,5-trisphosphate ($IP_3$) and Diacylglycerol (DAG). $IP_3$ binds to receptors on the sarcoplasmic reticulum, releasing $Ca^{2+}$ into the cytoplasm. DAG, along with calcium, activates Protein Kinase C (PKC). Examples: Alpha-1 adrenergic, $M_1$, and $M_3$ muscarinic receptors.

2. Ligand-Gated Ion Channels (Ionotropic Receptors)

These receptors consist of multiple subunits forming a central membrane-spanning pore. Binding of a ligand directly opens or closes the channel, allowing specific ions ($Na^+, K^+, Ca^{2+}, Cl^-$) to flow down their electrochemical gradient. Response time is extremely rapid (milliseconds).

• GABA-A Receptor: Permeable to Chloride ($Cl^-$) ions. Binding of GABA opens the channel, causing hyperpolarization of the membrane and neuronal inhibition. Benzodiazepines and barbiturates act as positive allosteric modulators.
• Nicotinic Acetylcholine Receptor (nAChR): Permeable to Sodium ($Na^+$). Binding of acetylcholine depolarizes skeletal muscle cells, triggering contraction.

3. Enzyme-Linked Receptors (Receptor Tyrosine Kinases)

Single transmembrane proteins with an extracellular ligand-binding domain and an intracellular domain possessing intrinsic enzymatic activity.

• Receptor Tyrosine Kinases (RTKs): Binding of ligands (e.g., insulin, growth factors) induces receptor dimerization. This activates the kinase domain, leading to autophosphorylation of tyrosine residues and recruitment of signaling complexes (e.g., the Ras-MAPK pathway). Response takes minutes to hours.

4. Intracellular/Nuclear Receptors

Soluble proteins located in the cytoplasm or nucleus. Their ligands must be highly lipophilic to passively cross the cell membrane. Upon binding, the ligand-receptor complex acts as a ligand-activated transcription factor. It binds to specific DNA sequences (hormone response elements) to regulate expression of target genes. Because they alter transcription, their onset is slow (hours to days), but their effects are long-lasting. Examples: glucocorticoid receptors, thyroid hormone receptors, sex hormones (estrogen, progesterone), and peroxisome proliferator-activated receptors (PPARs).