Summary of "Pharmacodynamics"
Big-picture definitions
Pharmacokinetics: what the body does to the drug (ADME — Absorption, Distribution, Metabolism, Excretion). Example pathway: oral → GI → hepatic portal → first-pass metabolism in liver → systemic circulation → distribution to tissues → effect → clearance by kidney/liver.
Pharmacodynamics: what the drug does to the body — cellular/tissue effects once the drug reaches its target. Focus is on receptor interactions and downstream signaling that produce cellular and clinical responses.
Receptor locations and drug properties
- Extracellular (membrane) receptors
- Targeted by large, hydrophilic/polar drugs that cannot cross the plasma membrane.
- Intracellular receptors
- Targeted by small, hydrophobic/lipophilic drugs that can cross the membrane.
- Often act via transcriptional changes — slower onset but longer duration.
Major extracellular receptor types (signal transduction)
a) Ligand-gated ion channels (ionotropic)
- Mechanism: ligand binds → channel opens/closes → ions (Na+, K+, Cl–, Ca2+) flow → immediate change in membrane potential and cellular activity.
- Example: GABA-A receptor (Cl– channel). GABA or a GABA-A agonist opens the channel → Cl– influx → hyperpolarization → decreased neuronal excitability. Benzodiazepines (e.g., lorazepam) enhance GABA-A effects (useful for anxiety, seizures).
b) G-protein-coupled receptors (GPCRs; seven-transmembrane)
- General mechanism:
- Ligand binds extracellular pocket → receptor conformational change.
- Intracellular G protein activated (GDP → GTP exchange).
- Downstream effectors activated/inhibited → second messenger production → kinases → target phosphorylation → cellular response.
- Major G-protein families:
- Gq → activates phospholipase C (PLC) → PIP2 → DAG + IP3
- DAG activates PKC.
- IP3 stimulates Ca2+ release from ER/SR → ↑ intracellular Ca2+.
- Result: phosphorylation/activation of channels/enzymes, altered ion flux (e.g., increased cardiac myocyte contraction via adrenergic signaling).
- Gs → stimulates adenylate cyclase → ATP → cAMP → activates PKA → phosphorylation of targets.
- Gi → inhibits adenylate cyclase → ↓ cAMP → ↓ PKA activity.
- Gq → activates phospholipase C (PLC) → PIP2 → DAG + IP3
c) Receptor tyrosine kinases (RTKs)
- Mechanism: ligand (e.g., insulin) binds → receptor tyrosine kinase domains autophosphorylate tyrosine residues → recruitment of adaptor proteins/second messengers → phosphorylation cascades → cellular effects (metabolic regulation, gene expression).
Intracellular receptor signaling
- Small, lipophilic drugs cross the membrane and bind cytosolic or nuclear receptors.
- Receptor–ligand complex often translocates to the nucleus → interacts with transcription factors → modulates gene transcription → changes protein synthesis → cellular responses.
- Characteristics: slower onset but long-lasting effects.
- Examples: corticosteroids, thyroid hormone–like effects; nitric oxide as a diffusible messenger in some contexts.
Receptor regulation: desensitization, tachyphylaxis vs tolerance
Tachyphylaxis (rapid desensitization)
- Onset: fast (minutes to hours) after rapid/high exposure.
- Mechanisms (rapid):
- Receptor phosphorylation → arrestin binding → receptor inactivation.
- Receptor internalization (endocytosis) → fewer receptors at membrane.
- Decreased receptor synthesis (short-term downregulation).
- Clinical note: increasing drug concentration often will not restore effect.
Tolerance (slower, chronic adaptation)
- Onset: develops over repeated/chronic exposure (hours → days → weeks).
- Mechanisms:
- Receptor downregulation/internalization and decreased synthesis (similar to tachyphylaxis but slower).
- Increased expression of metabolic enzymes that degrade the drug (pharmacokinetic tolerance).
- Clinical note: tolerance can often be partly overcome by increasing dose (e.g., saturating enzymes or using higher concentrations), unlike some forms of tachyphylaxis.
Dose–response relationship: potency and efficacy
- Typical plot: sigmoidal dose–response curve (x-axis = log dose/concentration; y-axis = response or % response).
- Key parameters:
- Emax: maximum effect achievable (efficacy).
- EC50: concentration (dose) producing 50% of Emax (potency). Lower EC50 → greater potency.
- Potency vs efficacy:
- Potency: amount of drug needed to produce an effect (reflected by EC50; leftward shift = ↑ potency).
- Efficacy: maximal effect a drug can produce (reflected by Emax; higher or lower top of curve).
- Intrinsic activity:
- Full agonist: produces maximal response (Emax).
- Partial agonist: produces sub-maximal response even when occupying all receptors; can act as an antagonist in presence of a full agonist.
- Inverse agonist: decreases receptor activity below basal level.
- Neutral antagonist: blocks receptor without changing basal activity.
Antagonists: competitive vs non-competitive
Competitive antagonist
- Binds reversibly to the active site and competes with the agonist.
- Effect on curve: rightward shift (↑ EC50), decreased apparent potency; Emax unchanged.
- Clinical: increasing agonist concentration can overcome the antagonist (higher dose required).
- Examples: phentolamine (alpha blocker), propranolol (beta blocker; competitive at many beta receptors).
Non-competitive antagonist
- Binds irreversibly or to an allosteric site and alters receptor function.
- Effect on curve: decreases Emax (reduced efficacy); potency often unchanged. Increasing agonist cannot fully overcome the antagonist.
- Example: phenoxybenzamine (irreversible alpha blocker).
Therapeutic index (safety window)
- Definition: TI = TD50 / ED50
- ED50: dose producing desired effect in 50% of the population.
- TD50: dose producing toxic effect in 50% of the population.
- Interpretation:
- Narrow TI = higher risk; small margin between therapeutic and toxic doses → requires monitoring (serum levels, labs).
- Wide TI = safer; larger margin.
- Examples:
- Narrow TI drugs: gentamicin, warfarin, theophylline, digoxin, phenytoin, lithium — require monitoring (e.g., drug levels, INR for warfarin).
- Wide TI drugs: penicillin G, many corticosteroids.
Clinical examples & practice takeaways
- GABA-A receptor agonists open Cl– channels → hyperpolarization → decreased neuronal firing (lorazepam enhances GABA-A → anxiolytic/antiepileptic).
- Lorazepam 1 mg vs diazepam 10 mg: lorazepam is more potent (less dose required) but not necessarily more efficacious.
- Oxycodone produces greater analgesia than aspirin at any dose → oxycodone has higher efficacy.
- Propranolol (competitive beta-blocker) shifts the epinephrine dose–response curve rightward (higher epinephrine needed for same effect).
- Picrotoxin is a non-competitive antagonist at the benzodiazepine/GABA-A sedation pathway → reduces diazepam efficacy regardless of dose (lowers Emax).
- Chronic receptor blockade (e.g., daily prazosin, an α1 antagonist) can cause receptor upregulation (cells increase receptor number to compensate).
- Therapeutic index example: methylphenidate ED50 = 10 mg, TD50 = 30 mg → TI = 3 (narrower safety window).
- Warfarin: narrow TI — small changes in bioavailability or drug interactions can produce dangerous effects (monitor INR).
Practical clinical/exam tips
- Predict receptor type by drug size/polarity: hydrophilic/large → membrane receptors; hydrophobic/small → intracellular receptors.
- Curve interpretation:
- Potency changes shift curve left (↑ potency) or right (↓ potency) — EC50 changes.
- Efficacy changes move the maximum response (Emax) up or down.
- Identify agonist types and remember partial agonists can act as antagonists when mixed with full agonists.
- Distinguish competitive vs non-competitive antagonism by whether increasing agonist concentration restores maximal effect.
- Use TI = TD50 / ED50 to assess safety margin and need for monitoring.
Sources / Speaker
- Primary speaker: Zach (Ninja Nerds channel). The lecture was delivered by the Ninja Nerds educator (“Zach”). No other speakers were featured.
Category
Educational
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