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

2. Hepatic metabolism

➢ All drugs that are absorbed from the stomach, small intestine and colon pass into

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

circulation

• The liver is the primary site of drug metabolism.

• This first pass may result in extensive metabolism of the drug.

• This phenomenon is known as the first pass metabolism and results in decrease

bioavailability of the drug.

➢ The bioavailability of a susceptible drug may be reduced to such an extent as to

▪ Render the GI route of administration ineffective, or to

▪ Necessitate an oral dose which is many times larger than the IV dose,

e.g. propranolol.

➢ Propranolol is well absorbed, but only about 30% of an oral dose is available to the

systemic circulation owing to the first-pass effect.

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