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e-learning III - Drug-Receptor Interactions
Potency - It is the concentration or dose of a substance required to give a specified effect. Often, we use the concentration or dose required to give 50% of the maximal effect. These terms have special names in pharmacology - EC 50 and ED50.
EC50 - The concentration of an agonist that gives 50% of the maximum response.
Kd - The equilibrium dissociation constant used to measure drug affinity. It is the concentration of a ligand that occupies 50% of a receptor population and thus as Kd increases, so affinity decreases.
pEC50 - The negative log of the EC50. For example, an EC50 of 1E-9 M gives a pEC50 of 9.
pKd - The negative log of the Kd. A drug with a Kd of 1E-9 M has a pKd of 9.
Sigmoid curve - An S shaped curve. Seen with log concentration response relationships.
Rectangular Hyperbola - A curve that rises and then smoothly reaches a plateau. Characteristic of concentration-response or concentration-binding relationships. Predicted by the Hill-Langmuir equation.
Bmax - The maximum binding obtained in a saturation assay. It corresponds to the number of receptors in the system.
Emax - The maximal effect that can be obtained from a functional assay. It may depend on both receptor and system factors.
Efficacy - The degree to which an agonist can activate a receptor. A full agonist has the maximal possible efficacy. An antagonist has zero efficacy. A partial agonist has some efficacy, but less than that of a full agonist.
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