Bioavailability of a drug means availability of biologically active drug. It is a determination of the amount or fraction of administered dose of the given dosage form that reaches the systemic circulation in the unchanged form.
In other words, it is the rate at which and extent to which an active concentration of the drug is available at the site of action. On intravenous administration, all the drug is available for biological activity, i.e. bioavailability is 100%.
However, it is lower after oral ingestion because it depends on the following factors:
- Rate of absorption.
- Gastrointestinal tract degradation.
- First pass metabolism by gut wall and hepatic enzymes.
- Enterohepatic circulation.
- Presence of food and other drugs.
Incomplete bioavailability after subcutaneous or intramuscular injection is less common but may occur due to local binding of the drug.
Oral pharmaceutical preparations from different manufacturers or different batches from the same manufacturer that satisfy the chemical and physical standards laid down in pharmacopoeia is called chemically equivalent.
However, they may not yield similar blood levels, so they are biologically inequivalent. Due to this they may not provide equal therapeutic benefit, so they are therapeutically in equivalent also.
Differences in bioavailability may be due to variations in disintegration and dissolution rates which may depend on manufacture process.
Clinical Significance of Bioavailability
Bioavailability variation is of practical significance under following circumstances:
- For drugs with low safety margin (digoxin).
- For precise control of doses of drugs (oral hypoglycemics, oral anticoagulants, etc.).
- For success or failure of antimicrobial regimen.
In general for a large number of drugs, bioavailability differences are negligible. So the risks of changing formulations have been often exaggerated.
[Source: Principles of Pharmacology for Dental Students]