2005B02 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 intensity of 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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