2020B11 Using opioids as examples describe and illustrate with graphs what you understand by the terms
potency, efficacy, partial agonist, competitive antagonist, therapeutic index.



·      Definitions

·      Graphs




·      Measure of dose (or concentration) of drug required to produce a given effect

·      Ratio of ED50’s gives relative potency (also ratio of TD50’s or LD50’s)

·      It’s the main determinant of dose


·      The max receptor response achievable by a drug; comprises

o    Affinity: how likely it is to bind

o    Intrinsic activity (IA): propensity to activate the receptor once bound

Full agonist

·      IA = 1

·      High affinity for active conformation

Partial agonist

·      0 < IA < 1

·      Usually comparable affinity for both active and inactive conformation

Competitive antagonist

·      IA = 0

·      Affinity for active/inactive at same ratio as natural state of flux

·      Prevents access of endogenous ligand to active site

·      No change to maximal response.

Inverse agonist

·      -1 < IA < 0

·      High affinity for inactive conformation, reduces constitutive receptor activity

Therapeutic index

·      LD50:ED50 (animals) or TD50:ED50 (humans), where

o    ED50 = median effective dose

o    TD50 = median toxic dose

o    LD50 = median lethal dose

·      Indicator of drug safety for clinical use.




(population quantal log dose-response curve)

Partial agonist / efficacy

(individual graded log dose-response curve)

Competitive reversible antagonist

(population quantal log dose-response curve)

Therapeutic index

(population quantal log dose-response curve)



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