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This guide is a quick reference guide to the important concepts and terms used in pharmacology.
Reviewed December 14 2020
The information is not intended to be an exhaustive list, but to provide assistance to those researchers who are new to this field of research.
A drug capable of binding and activating a receptor, leading to a pharmacological response that may mimic that of a naturally occurring substance. Can be classified as full, partial or inverse.
Full agonist - Is capable of eliciting a maximal response as it displays full efficacy at that receptor.
Does not produce a biological response on binding to a receptor but instead blocks or reduces the effect of an agonist. It may be competitive or non-competitive.
Physiological agonists and antagonists
Physiological agonists are drugs that mediate the same physiological parameters via alternative receptors or mechanisms. Physiological antagonists reduce or block the physiological effect of an agonist by causing an opposite physiological response without binding to the same receptor. Physiological antagonism is a non-competive form of antagonism.
A drug that binds to a receptor at a site distinct from the active site. A conformational change is induced in the receptor, altering the affinity of the receptor for the endogenous ligand.
Maximum amount of drug which can bind specifically to receptors in a membrane preparation. If one drug molecule binds to each receptor it acts as an indication of the concentration of receptors in the tissue.
Used to determine the Ki value from an IC50 value.
Concentration response curve:
An example concentration response curve in a typical in vitro preparation. Curve A represents a full agonist (EC50 = 3 x 10-8 M) whilst curve B is a partial agonist acting on the same receptor.
Desensitization - A loss of responsiveness which may be due to the continued presence of an agonist at a receptor or repeated presentation of the agonist.
EC50 - The molar concentration of an agonist that produces a 50% response of the maximum possible response for that agonist. Figures may also be stated as other percentages of the maximum response EC20 and EC80 representing a 20% and 80% response respectively.
ED50 - Dose of drug that produces 50% of its maximum response or effect. Can be a term used in vitro or in vivo (although it is more common in vivo).
Efficacy - Used to describe agonist responses in relation to receptor occupation. High efficacy agonists can produce a maximal response whilst occupying a relatively low proportion of receptors. Low efficacy agonists are unable to cause receptor activation to the same degree and a maximal response may not be achieved even at full occupation of the entire receptor population. Low efficacy agonists are often termed partial agonist.
Ex vivo - Experimentation using tissue in an artificial environment outside the living organism. An example may include short-term (up to 24 hour) culture of tissue, following its removal from the organism.
Half-life (t½) - An important pharmacokinetic measurement. The metabolic half-life of a drug in vivo is the time taken for its concentration in plasma to decline to half its original level. t½ refers to the duration of action of a drug and depends upon how quickly the drug is eliminated from the plasma.
Clearance and distribution of a drug from the plasma are important parameters for half-life determination.
IC50 - Molar concentration of an agonist or antagonist that causes 50% of the maximum possible inhibition. Figures may also be stated as other percentages of the inhibition.
In vitro - Studies carried out using components of an organism that have been isolated from their usual biological surroundings. The analysis is usually carried out in test-tubes or culture dishes.
In vivo - Experimentation using the whole living organism. Normal physiology will be involved in any response.
KA - Equilibrium dissociation constant for an agonist; the concentration at which 50% of receptors would be occupied at equilibrium.
KB - The equilibrium dissociation constant for a competitive antagonist; the concentration at which 50% of the receptors would be occupied at equilibrium.
Kd - The dissociation constant. Concentration of a drug which at equilibrium occupies 50% of receptors.
Ki - Inhibition constant for a drug where 50% of receptors will be occupied. Provides an absolute value and does not differ between experiments. Calculated from the IC50 value using the Cheng-Prusoff equation.
Proportion of radioligand that is not displaced by other competitive ligands that are specific for a particular receptor. It can be due to:
pA2 - Logarithmic measure of antagonist potency. It is the negative log of the molar concentration of an antagonist that would produce a 2-fold shift in the concentration response curve for an agonist.
pD2 - Negative logarithm of the EC50 or IC50 value.
pEC50 - Negative logarithm of the EC50 value.
pIC50 - Negative logarithm of the IC50 value.
pKA - Negative logarithmic measure of the potency of an antagonist.
pKB - Negative logarithm of KB value. For a competitive antagonist the pKB theoretically equals the pA2 value.
pKd - Negative logarithm of the Kd value.
pKi - Negative logarithm of the Ki value.
Measure of the effective concentration of a drug. It is a vague term and it is advisable to further categorise the measurement:
Occupancy - The proportion of receptors to which a drug is bound.
[D] = Drug concentration
K = Dissociation constant