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PharmD clinical pharmacy program Level 3, Semester 2 Biopharmaceutics & Pharmacokinetics (PT608(

Drug Metabolism

The decline in plasma concentrations after drug administration results from drug

elimination or removal by the body.

The elimination of most drugs from the body involves the processes of both metabolism

(biotransformation) and renal excretion.

 The general goal of drug metabolism is to transform compounds into more polar

( more readily excretable) water soluble products.

 Almost all tissues can metabolize drugs, but the liver, GI tract, and lungs are the

major sites of drug metabolism of most drugs in humans.

 The liver plays a predominant role in drug metabolism.

♦ In general, metabolites are less active and more water soluble (which favors excretion in
the urine) than the parent substance.

In some instances, active metabolites are formed from inactive parent drugs, in which case

the parent is termed a prodrug.

Some examples of drugs with therapeutically active metabolites are:

• Procainamide (Procan; Pronestyl) used as anti-dysrhythmic agent: active metabolite

is N-acetyl procainamide.

• Propranolol HCl (Inderal) used as a non-selective b-antagonist: active metabolite is

4-hydroxypropranolol.

• Diazepam (Valium) used for symptomatic relief of tension and anxiety: active

metabolite is desmethyldiazepam.

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