Chapter 2- Basic Principles of Pharmacology
♦In general, the definition of a drug is a chemical that affects one or more
♦Endogenous substances are chemicals that originate or are produced within
an organism for the use of normal biological processes. Exogenous substances
♦The most important determinants of a drug’s effects are its concentration at the
site of action as well as the rate of accumulation at that point.
♦Drug dose is expressed in terms of unit of drug per unit of body weight of the
organism. Drug dosage refers to the administrations of the drug per unit of
♦DRUGS AND RECEPTORS:
♦Pharmacodynamics refers to the biochemical and physiological effects of drugs
and their mechanisms of action.
♦Receptors, which are fairly large molecules, comprise of sites at which
biologically active chemicals (ligands) induce their effects.
♦Affinity refers to the relative capacity of a compound to maintain contact with
or be bound to a receptor.
♦Efficacy refers to the degree of biological activity or relative a capability of a
compound to activate the receptor after being bound to it.
♦Agonists are compounds with both an affinity for and capability if activating a
receptor (efficacy). There are 4 types of agonists:
Indirect- a drug that doesn’t bind to the receptor itself but enhances the
amount of the endogenous ligand to bind to a receptor.
Partial- drugs that display intermediate efficacy in receptor activation
between the efficacy of a full agonist and an antagonist. E.g. when these types of
drugs bind to a receptor they hinder the full agonists ability to bind completely
to the receptor. Hence, reducing the effect of that full agonist but not eliminating
it; can be useful in schizophrenia where there is too much receptor activity.
However, there is a disadvantage to these types of agonists; since they block the
full activity of the full agonist they may induce prolonged inactivity of the
receptor which can cause some side effects. Recently though, a drug known as
aripiprazole (abilify) partially blocks dopamine from binding to its receptors
but still stimulates the receptors to a certain degree do that the side effects of
prolonged inactivity don’t occur.
Inverse- a drug that appears to act through the same receptor as an agonist
but produces effects opposite to those of the agonist.
Mixed agonist-antagonist- when a drug acts as an agonist by itself, but
blocks the activity of another agonist in the same system.