Quantitative Pharmacology
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Bioavailability: percentage of administered drug that reach the systemic circulation; often <>
Drugs are not bioavailable when:
drugs that are completely dissolved & excreted.
If drug is ionized in blood not absorbed across gut wall (cannot pass through cell membrane,may be pH dependent).
When oral drugs are absorbed through gut into the blood, drug goes to liver first i.e. “enterohepatic circulation” or portal circulation. Liver metabolizes & may degrade drug before systemic circulation. This is known as “first pass metabolism”. Example: lidocaine taken orally – only 1% bioavailable.
However, liver may not necessarily inactive drug, may create metabolite that is active. I.e. pro-drugs: liver metabolize it & drug becomes active. Example: Cortisone becomes hydrocortisone in the liver.
Protein bound drugs are inactive but since it’s in the systemic circulation, therefore, it’s still considered as bioavailable. Protein-bound drugs can be made available via the Donnan Effect & act as a reservoir for sustained release.
Once the unbound fraction binds to receptor, protein will release the bound drugs via mass action &become active. Ionized drugs are also considered bioavailable because they can be reverted to non-ionized form (similar to protein-bound drugs).
Category: Pharmacology Notes
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