Pharmacological Glossary

Definitions of commonly used pharmacological terms

Term	Description
Agonist	A drug that binds to and activates a receptor. Can be full, partial or inverse. A full agonist has high efficacy, producing a full response while occupying a relatively low proportion of receptors. A partial agonist has lower efficacy than a full agonist. It produces sub-maximal activation even when occupying the total receptor population, therefore cannot produce the maximal response, irrespective of the concentration applied. An inverse agonist produces an effect opposite to that of an agonist, yet binds to the same receptor binding-site as an agonist.
Allosteric Modulator	A drug that binds to a receptor at a site distinct from the active site. Induces a conformational change in the receptor, which alters the affinity of the receptor for the endogenous ligand. Positive allosteric modulators increase the affinity, whilst negative allosteric modulators decrease the affinity.
Antagonist	A drug that attenuates the effect of an agonist. Can be competitive or non-competitive, each of which can be reversible or irreversible. A competitive antagonist binds to the same site as the agonist but does not activate it, thus blocks the agonist’s action. A non-competitive antagonist binds to an allosteric (non-agonist) site on the receptor to prevent activation of the receptor. A reversible antagonist binds non-covalently to the receptor, therefore can be “washed out”. An irreversible antagonist binds covalently to the receptor and cannot be displaced by either competing ligands or washing.
B_max	The maximum amount of drug or radioligand, usually expressed as picomoles (pM) per mg protein, which can bind specifically to the receptors in a membrane preparation. Can be used to measure the density of the receptor site in a particular preparation.
Cheng-Prusoff Equation	Used to determine the K_i value from an IC₅₀ value measured in a competition radioligand binding assay: Where [L] is the concentration of free radioligand, and K_d is the dissociation constant of the radioligand for the receptor.
Competitive Antagonist	See Antagonist
DC₅₀	The molar concentration of a Degrader at which 50% of the target protein is degraded.
Desensitisation	A reduction in response to an agonist while it is continuously present at the receptor, or progressive decrease in response upon repeated exposure to an agonist.
D_max	The maximum level of degradation of target protein achievable by a Degrader, expressed as a percentage.
EC₅₀	The molar concentration of an agonist that produces 50% of the maximum possible response for that agonist.
ED₅₀	In vitro or in vivo dose of drug that produces 50% of its maximum response or effect.
Efficacy	Describes the way that agonists vary in the response they produce when they occupy the same number of receptors. High efficacy agonists produce their maximal response while occupying a relatively low proportion of the total receptor population. Lower efficacy agonists do not activate receptors to the same degree and may not be able to produce the maximal response (see Agonist, Partial).
Ex vivo	Taking place outside a living organism.
Half-life	Half-life (t½) is 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. Half-life refers to the duration of action of a drug and depends upon how quickly the drug is eliminated from the plasma. The clearance and distribution of a drug from the plasma are therefore important parameters for the determination of its half-life.
i.a.	Intra-arterial route of drug administration (see Useful Abbreviations).
IC₅₀	In a functional assay, the molar concentration of an agonist or antagonist which produces 50% of its maximum possible inhibition. In a radioligand binding assay, the molar concentration of competing ligand which reduces the specific binding of a radioligand by 50%.
i.c.	Intracerebral route of drug administration (see Useful Abbreviations).
i.c.v.	Intracerebroventricular route of drug administration (see Useful Abbreviations).
ID₅₀	In vitro or in vivo dose of a drug that causes 50% of the maximum possible inhibition for that drug.
i.d.	Intradermal route of drug administration (see Useful Abbreviations).
i.g.	Intragastric route of administration (see Useful Abbreviations).
i.m.	Intramuscular route of drug administration (see Useful Abbreviations).
Inverse Agonist	See Agonist
In vitro	Taking place in a test-tube, culture dish or elsewhere outside a living organism.
In vivo	Taking place in a living organism.
i.p.	Intraperitoneal route of drug administration (see Useful Abbreviations).
Irreversible Antagonist	See Antagonist
i.t.	Intrathecal route of drug administration (see Useful Abbreviations).
i.v.	Intravenous route of drug administration (see Useful Abbreviations).
K_B	The equilibrium dissociation constant for a competitive antagonist: the molar concentration that would occupy 50% of the receptors at equilibrium.
K_d	The dissociation constant for a radiolabeled drug determined by saturation analysis. It is the molar concentration of radioligand which, at equilibrium, occupies 50% of the receptors.
K_i	The inhibition constant for a ligand, which denotes the affinity of the ligand for a receptor. Measured using a radioligand competition binding assay, it is the molar concentration of the competing ligand that would occupy 50% of the receptors if no radioligand was present. It is calculated from the IC₅₀ value using the Cheng-Prusoff equation.
Negative Allosteric Modulator	See Allosteric Modulator
Neutral Antagonist	See Silent Antagonist
Non-Specific Binding	The proportion of radioligand that is not displaced by other competitive ligands specific for the receptor. It can be binding to other receptors or proteins, partitioning into lipids or other things.
pA₂	Measure of the potency of an antagonist. It is the negative logarithm of the molar concentration of an antagonist that would produce a 2-fold shift in the concentration response curve for an agonist.
pD₂	The negative logarithm of the EC₅₀ or IC₅₀ value.
pEC₅₀	The negative logarithm of the EC₅₀ value.
pIC₅₀	The negative logarithm of the IC₅₀ value.
pK_B	The negative logarithm of the K_B value.
pK_d	The negative logarithm of the K_d value.
pK_i	The negative logarithm of the K_i value.
p.o.	Oral (by mouth) route of drug administration (see Useful Abbreviations).
Positive Allosteric Modulator	See Allosteric Modulator
Potency	A measure of the concentrations of a drug at which it is effective.
s.c.	Subcutaneous route of drug administration (see Useful Abbreviations).
Specific Binding	The proportion of radioligand that can be displaced by competitive ligands specific for the receptor.
Silent Antagonist	A drug that attenuates the effects of agonists or inverse agonists, producing a functional reduction in signal transduction. Affects only ligand-dependent receptor activation and displays no intrinsic activity itself. Also known as a neutral antagonist.
Systemic	In the periphery of the body (not in the central nervous system – see Useful Abbreviations).
t_½	Biological half-life (see Half-life).