Chapter 2- Basic Principles of Pharmacology
In general, the definition of a drug is a chemical that affects one or more biological
Endogenous substances are chemicals that originate or are produced within an
organism for the use of normal biological processes. Exogenous substances are
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 time.
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.
There are two broad classes of receptors: ionotropic and metabotropic