Pharmacodynamics and Receptor Pharmacology

Receptor Structure and Classification

• G protein-coupled receptors (GPCRs): Seven transmembrane domains; activate intracellular G proteins (Gs, Gi/o, Gq/11, G12/13); account for ~30% of all FDA-approved drugs
• Ligand-gated ion channels: Receptor and ion channel are one unit; allow rapid signal transduction (e.g., nicotinic acetylcholine receptors, GABA receptors)
• Receptor tyrosine kinases (RTKs): Transmembrane with intracellular tyrosine kinase domain; phosphorylate downstream signaling proteins (e.g., EGF receptor)
• Nuclear receptors: Intracellular; bind ligand then translocate to nucleus to directly regulate gene transcription (e.g., steroid receptors, thyroid hormone receptor)
• Enzyme-linked receptors: Cytokine receptors, serine/threonine kinase receptors

Core Pharmacodynamic Concepts

• Agonists bind receptors and activate them, producing maximal or submaximal biologic response; full agonists produce maximum response (efficacy = 1.0), partial agonists produce submaximal response (efficacy 0–1.0)
• Antagonists bind receptors without activating them and block agonist binding; competitive antagonists bind reversibly at the agonist site (overcome by increasing agonist concentration), non-competitive antagonists bind irreversibly or at an allosteric site (cannot be overcome by increasing agonist)
• Potency (EC50 or ED50) measures the amount of drug needed to produce 50% of maximal effect; lower EC50 = higher potency
• Efficacy is the maximum response achievable regardless of dose; reflects intrinsic activity and is independent of potency
• Dose-response relationships follow sigmoidal curves; described mathematically by Hill equation; shift leftward with agonists, rightward with competitive antagonists

Signal Transduction Mechanisms

• Second messenger systems: Activation → G protein → adenylyl cyclase/phospholipase C → cAMP/IP3/DAG → protein kinase activation → gene transcription and cellular effects
• Desensitization/tachyphylaxis: Continued agonist exposure leads to decreased response via receptor phosphorylation, β-arrestin binding, internalization, and reduced sensitivity
• Supersensitivity/upregulation: Chronic antagonism or denervation increases receptor density and sensitivity to agonists
• Spare receptors: Maximum response occurs when <100% of receptors are occupied; provides physiologic reserve and explains why partial agonists can be full agonists in tissues with spare receptors

• Predictable therapeutic effects: Result from intended receptor activation (e.g., β-agonist bronchodilation, ACE inhibitor vasodilation); magnitude and onset depend on drug potency and tissue sensitivity
• Off-target effects and adverse effects: Occur when drugs bind unintended receptors; example—propranolol (β-blocker) can cause hypoglycemia unawareness by blocking β2-adrenergic receptors on pancreatic beta cells; example—anticholinergic drugs block M3 muscarinic receptors causing dry mouth, urinary retention, mydriasis
• Receptor selectivity variations with dose: At low doses, drugs may appear selective; at high doses, off-target binding becomes apparent (e.g., dopamine at low doses activates D1/D2 dopamine receptors; at high doses activates α1-adrenergic receptors causing vasoconstriction)
• Tolerance/tachyphylaxis: Chronic drug use leads to diminished response (e.g., nitrate tolerance from chronic nitroglycerin use due to superoxide scavenging; β-agonist tolerance in asthma with overuse); often managed by drug holidays or adding agents that prevent desensitization
• Withdrawal phenomena: Chronic antagonism followed by abrupt cessation causes unopposed endogenous agonist activity (e.g., clonidine withdrawal causing rebound hypertension; benzodiazepine withdrawal causing seizures)

• Dose-response assessment: Measure drug effect at multiple doses to determine EC50, Emax, and slope; construct dose-response curve; used in research but also guides clinical titration (e.g., finding minimally effective dose of antihypertensive)
• Receptor binding studies: In vitro assays (radioligand binding, ELISA-based assays) determine receptor affinity (Kd), selectivity profile, and distinguish agonist vs antagonist activity; used in drug development and in clinical research to confirm mechanism
• Functional assays: Measure downstream signaling (cAMP accumulation, IP3 generation, reporter gene activation, electrophysiology) to confirm pharmacodynamic activity; distinguish full vs partial agonists
• Clinical pharmacodynamic biomarkers: Changes in measurable endpoints that reflect receptor activation (e.g., pupil dilation from muscarinic antagonism, heart rate reduction from β-blockade, INR elevation from warfarin's inhibition of vitamin K-dependent clotting factors); used to confirm drug effect and guide dosing
• Genetic testing for receptor polymorphisms: Variations in receptor genes may predict response (e.g., β2-adrenergic receptor Arg16Gly polymorphism influences asthma control with β-agonist therapy)

First-line selection based on pharmacodynamics

• Choose drugs with high selectivity for target receptors to minimize off-target effects (e.g., β1-selective blocker like metoprolol over non-selective propranolol in patients with COPD to avoid bronchospasm)
• Select full agonists when maximum response is needed; consider partial agonists when partial effect is sufficient and fewer adverse effects are desired (e.g., aripiprazole as atypical antipsychotic with partial D2 agonism, lower EPS risk than full antagonists)
• Use competitive antagonists when agonist still available in system (can overcome blockade if needed); choose non-competitive antagonists for more sustained effect (e.g., aspirin irreversibly inhibits COX-1; difficult to reverse except with new platelet synthesis)

Dosing strategies based on pharmacodynamics

• Dose titration: Start low, titrate gradually to minimize adverse effects while achieving therapeutic benefit; respects principle of desensitization and variable receptor density among individuals (e.g., antidepressant SSRIs require weeks to achieve response due to desensitization mechanisms)
• PRN vs scheduled dosing: PRN dosing for agents causing tachyphylaxis may reduce tolerance; scheduled dosing maximizes sustained effect for others
• Drug holidays: Interrupt chronic therapy periodically to reduce desensitization (e.g., nitrate-free intervals in angina management)

Combination therapy

• Synergism: Combining drugs targeting different receptors in same pathway can enhance effect (e.g., combining β-blocker + ACE inhibitor in heart failure)
• Prevent antagonism: Avoid combining agonists and antagonists at same receptor (e.g., don't use β-agonist and β-blocker together for asthma + hypertension; choose selective β1-blocker instead)

Special populations

• Altered receptor density: Chronic disease or medications change receptor expression; example—chronic heart failure involves β-adrenergic receptor downregulation (desensitization), requiring higher doses of β-agonists or alternative inotropes