McKenna's Pharmacology, 2e

19

C H A P T E R 2  Drugs and the body

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 .

KEY POINTS

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.

Tissue sites of action (therapeutic, toxic)

Tissue storage sites (fat, protein)

D i s t r i

P l a s m a

b u t i o n

Reabsorption

Absorption

Distribution

Excretion by: Kidney Bile Lungs etc.

Drug dose Administered into:

Drug

D i s t r i

GI tract Muscle Subcutaneous tissue etc.

Drug bond to plasma proteins (albumin)

Metabolites

b u t i o n

Reabsorption

Biotransformation in Liver

Kidneys Plasma etc.

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.