Pharmacodynamics Basics
Pharmacokinetics tells you where a drug goes; pharmacodynamics tells you what it does when it gets there. Understanding receptor types, dose-response relationships, potency, efficacy, and the therapeutic index is foundational to rational prescribing and dosing.
PK vs PD: complementary sciences
Pharmacokinetics (PK) — covered in our pharmacokinetics basics guide — describes drug concentration over time: how fast a drug is absorbed, how widely it distributes, how quickly it is metabolised and excreted.
Pharmacodynamics (PD) describes the relationship between drug concentration and pharmacological effect. The integrated PK/PD model links these two:
The standard PD equation relating concentration to effect is the Emax model (Hill equation):
Where Emax = maximum effect, EC50 = concentration at half-maximum effect, n = Hill coefficient (sigmoidicity). When n = 1, the equation simplifies to the standard hyperbolic curve.
The Emax model (also called the Emax pharmacology model or Hill equation) is the most widely used mathematical framework for describing concentration-response relationships in pharmacodynamics. The Emax pharmacology model characterises four critical parameters: E0 (baseline effect), Emax (maximum achievable effect above baseline), EC50 (the drug concentration producing 50% of Emax — a measure of potency), and the Hill coefficient n (which controls the steepness or sigmoidicity of the curve). When n = 1, the curve is a rectangular hyperbola (classic Michaelis-Menten shape); when n > 1, the curve becomes sigmoidal, reflecting cooperative binding or switch-like responses. The Emax model is used extensively in PK/PD modelling for dose optimisation, particularly in antimicrobial stewardship (linking AUC/MIC to bacterial kill), oncology (exposure-response modelling for targeted therapies), and anaesthesiology (concentration-effect relationships for propofol and opioids). The NCBI StatPearls pharmacodynamics chapter and the FDA exposure-response modelling resources provide detailed mathematical treatments. See also PubChem for compound-specific pharmacodynamic data.
Understanding pharmacodynamics principles is essential for rational drug therapy. The dose-response curve — plotting drug concentration (or dose) on the x-axis against pharmacological effect on the y-axis — reveals key properties of every drug: potency (EC50 position), efficacy (Emax plateau), and the therapeutic window between minimum effective concentration and minimum toxic concentration. Pharmacodynamics principles underpin therapeutic drug monitoring decisions (where target concentrations are derived from concentration-effect relationships), drug interaction predictions (pharmacodynamic interactions produce additive, synergistic, or antagonistic effects), and the design of clinical trials (dose-finding studies use Emax modelling to identify optimal dosing). The EMA clinical pharmacology guidelines describe regulatory expectations for PD characterisation in drug development.
Drug receptor superfamilies
Mechanism
G protein activation → cAMP, IP3/DAG, or ion channel modulation
Examples
β-adrenoceptors (salbutamol, metoprolol), μ-opioid (morphine, buprenorphine), H1 (cetirizine), dopamine D2 (haloperidol)
Mechanism
Direct ion flux on agonist binding (fast, ms timescale)
Examples
GABA-A (benzodiazepines, propofol), nAChR (neuromuscular blockers), NMDA (ketamine)
Mechanism
Autophosphorylation → intracellular signalling cascades (RAS-MAPK, PI3K-AKT)
Examples
EGFR (erlotinib, gefitinib), HER2 (trastuzumab, lapatinib), insulin receptor
Mechanism
Ligand binds intracellular receptor → translocates to nucleus → modulates transcription
Examples
GR (corticosteroids), ER (tamoxifen, fulvestrant), AR (enzalutamide), PPARγ (pioglitazone)
Mechanism
Competitive or irreversible enzyme inhibition
Examples
ACE (lisinopril), COX-1/2 (ibuprofen, aspirin), DPP-4 (sitagliptin), HMG-CoA reductase (atorvastatin)
Mechanism
State-dependent or frequency-dependent channel blockade
Examples
Na+ channels (lidocaine, phenytoin), Ca2+ channels (amlodipine, verapamil), K+ channels (amiodarone)
Full receptor database: IUPHAR/BPS Guide to Pharmacology ↗.
Agonists, antagonists, and partial agonists
| Drug type | Intrinsic efficacy | Effect | Example |
|---|---|---|---|
| Full agonist | High (1.0) | Produces maximum effect | Morphine (μ-opioid), salbutamol (β2), fentanyl (μ-opioid) |
| Partial agonist | Intermediate (0.1–0.9) | Sub-maximal effect even at full receptor occupancy | Buprenorphine (μ-opioid), buspirone (5-HT1A) |
| Competitive antagonist | 0 | Blocks agonist binding; effect reversible with high agonist | Naloxone (μ-opioid), metoprolol (β1), cetirizine (H1) |
| Irreversible antagonist | 0 | Permanent receptor inactivation until new receptor synthesis | Aspirin (COX-1/2; irreversible acetylation) |
| Inverse agonist | Negative | Reduces basal receptor activity below constitutive level | Some antihistamines at H1 (cetirizine has partial inverse agonism) |
Frequently asked questions
What is pharmacodynamics and how does it differ from pharmacokinetics?
What are the main types of drug receptors?
What is the difference between potency and efficacy?
What is the therapeutic index and how does it affect drug safety?
What is the Emax model in pharmacology?
E = E0 + (Emax × Cn) / (EC50n + Cn). E0 = baseline effect; Emax = maximum achievable effect; EC50 = concentration at 50% of Emax (potency); n = Hill coefficient (curve steepness). When n = 1: rectangular hyperbola; n > 1: sigmoidal (cooperative binding). Applications: antimicrobial PK/PD (AUC/MIC vs bacterial kill — see stewardship guide), oncology exposure-response, anaesthesiology concentration-effect, and cardiovascular dose-response. The Emax model distinguishes potency (EC50 — position on x-axis) from efficacy (Emax — plateau height): a partial agonist like buprenorphine has lower Emax than morphine despite similar binding affinity. Reference: NCBI StatPearls pharmacodynamics ↗ and EMA clinical pharmacology guidelines ↗.