C H A P T E R 2
Drugs and the body
19
KEY POINTS
■■
Pharmacodynamics is the process by which a drug
works or affects the body.
■■
Drugs may work by replacing a missing body
chemical, by stimulating or depressing cellular
activity or by interfering with the functioning of
foreign cells.
■■
There are four primary drug targets: enzymes, ion
channels, carrier molecules and receptors.
■■
Drugs are thought to work by interacting with
specific receptor sites or by interfering with enzyme
systems in the body.
PHARMACOKINETICS
Pharmacokinetics involves the study of absorption, dis
tribution, metabolism (biotransformation) and excretion
of drugs. In clinical practice, pharmacokinetic consid
erations include the onset of drug action, half-life of a
drug, timing of the peak effect, duration of drug effects,
metabolism or biotransformation of the drug and the
site of excretion. Figure 2.2 outlines these processes,
which are described in the following sections.
KEY POINTS
Effective concentration
After a drug is administered, its molecules must first
be absorbed into the body; then they make their way
to the site of action in the responding tissues. If a drug
is going to work properly on these tissues and thereby
have a therapeutic effect, it must attain a sufficiently
high concentration in the body. The amount of a drug
that is needed to cause a therapeutic effect has to be
high enough to result in an
effective
or a
therapeutic
concentration
.
Drug evaluation studies determine the effective
concentration required to cause a desired therapeutic
effect. The recommended dose of a drug is based on the
amount that must be given to eventually reach the effec
tive concentration. Too much of a drug will produce
toxic (poisonous) effects and too little will not produce
the desired therapeutic effects.
Therapeutic index
Most drugs produce multiple effects. The relationship
between a drug’s desired therapeutic effects and its
adverse effects is called the drug’s
therapeutic index
. It is
also referred to as its
margin of safety
.
P l a s m a
Tissue
sites of action
(therapeutic, toxic)
Tissue
storage sites
(fat, protein)
Drug dose
Administered into:
GI tract
Muscle
Subcutaneous tissue
etc.
Absorption
Distribution
Drug
Drug bond
to plasma
proteins
(albumin)
Metabolites
Reabsorption
Reabsorption
Excretion by:
Kidney
Bile
Lungs
etc.
Biotransformation in
Liver
Kidneys
Plasma
etc.
D
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FIGURE 2.2
The processes by which a drug is handled by the body. Dashed lines indicate that some portion of a drug and its metabolites may be
reabsorbed from the excretory organs. The dynamic equilibrium of pharmacokinetics is shown.
