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

 Noncompetitive enzyme inhibition cannot be reversed by increasing the drug

concentration, because the inhibitor will interact strongly with the enzyme and will

not be displaced by the drug.
 Cimetidine, a widely used anti-ulcer drug, is an important, potent inhibitor of

microsomal drug metabolism which retards the metabolism of many other drugs,
including warfarin and similar anticoagulants, theophylline and caffeine,

phenobarbital, phenytoin, carbamazepine, propranolol, diazepam, and
chlordiazepoxide.

Presystemic Metabolism

 All drugs that are absorbed from the stomach, small intestine and upper colon pass

into the hepatic portal system and are presented to the liver before reaching the

systemic circulation.

 Hence, an oral dose of drug could be completely absorbed but incompletely

available to the systemic circulation because of first-pass or presystemic

metabolism by the gut wall and/or liver.

First pass metabolism

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