Chapter 3  ·  Module 2  ·  Pharmacodynamics

Quantitative Pharmacodynamics

Emax, EC50, potency vs. efficacy, therapeutic index, and receptor reserve at a glance

Emax and EC50 — The Two Curve Parameters

Parameter 1

Emax — Efficacy Ceiling

  • Maximum effect a drug can produce
  • Determined by intrinsic efficacy and effector pathway capacity
  • Full agonist reaches system Emax
  • Partial agonist reaches a lower Emax regardless of dose
  • No amount of additional drug exceeds a drug’s own Emax

Parameter 2

EC50 — Potency Measure

  • Concentration producing 50% of Emax
  • Lower EC50 = more potent = leftward curve shift
  • Steep curve = small dose change causes large effect change
  • Steep curves characterize narrow therapeutic index drugs
  • EC50 from functional assay; Kd from binding assay (not identical when receptor reserve is present)

Graded vs. Quantal

Graded curve: continuous response in one preparation; parameters = Emax and EC50; answers “how much effect?” Quantal curve: all-or-none endpoint across a population; parameters = ED50, TD50, LD50; answers “what fraction of patients respond?” EC50 and ED50 are different measurements.

Potency vs. Efficacy — Independent Properties

Property Measured by Graph appearance Clinical relevance
Potency EC50 (lower = more potent) Curve shifts left (more potent) or right (less potent) Matters when dose size must be minimized
Efficacy Emax (higher = more efficacious) Curve plateau reaches higher (more efficacious) or lower (less efficacious) Matters when maximum effect is required

Clinical Examples

Morphine vs. codeine: same Emax (both full mu-opioid agonists), morphine ~10x more potent. Buprenorphine vs. morphine: buprenorphine more potent (higher affinity, lower EC50) but lower Emax as partial agonist — inadequate for severe pain requiring maximum opioid effect. Furosemide vs. hydrochlorothiazide: furosemide has higher Emax for diuresis — the essential difference for patients with severe heart failure or renal impairment.

The Therapeutic Index

Definition

Therapeutic index = LD50 / ED50 (or TD50 / ED50). A larger therapeutic index means a wider margin between effective and toxic doses. In clinical practice: therapeutic window = range of plasma concentrations producing therapeutic effect without unacceptable toxicity.

Wide therapeutic index

Safe Margin — No Monitoring Required

  • Toxic dose much higher than effective dose
  • Standard population-based dosing is safe
  • Examples: penicillins, cephalosporins, most statins

Narrow therapeutic index

Tight Margin — Monitoring Required

  • Toxic dose close to effective dose
  • Individual plasma concentration monitoring required
  • Warfarin (monitor international normalized ratio)
  • Lithium (monitor plasma levels; toxicity with dehydration)
  • Digoxin (monitor levels and electrocardiogram)
  • Phenytoin (nonlinear kinetics; monitor free levels)
  • Aminoglycosides (nephrotoxicity, ototoxicity; monitor peaks and troughs)

Receptor Reserve (Spare Receptors)

Core Concept

Maximum tissue response is achieved when only a fraction of receptors are occupied, because downstream signal amplification saturates the effector pathway at low occupancy. Receptors beyond that threshold are “spare.” Consequence: EC50 (functional) is lower than Kd (binding) whenever reserve is present.

Implication 1

Potency is Tissue-Dependent

  • EC50 varies across tissues with different reserve sizes
  • EC50 cannot reliably estimate receptor Kd
  • Same drug appears more potent in tissues with larger reserve

Implication 2

Partial Agonists and Reserve

  • Large reserve: partial agonist may reach full Emax
  • Limited reserve: partial agonist shows ceiling effect
  • Buprenorphine ceiling on respiratory depression (limited reserve) vs. meaningful analgesia (greater reserve)

Implication 3

Irreversible Antagonists

  • Reserve acts as buffer against permanent receptor blockade
  • Emax preserved until reserve is exhausted by blockade
  • Then Emax falls steeply
  • Aspirin: minimal platelet cyclooxygenase reserve — low doses fully suppress thromboxane A2 synthesis