U01.05.002 Bioavailability

Bioavailability (F) refers to the fraction of an administered drug that reaches systemic circulation in an unchanged form. It is a critical pharmacokinetic parameter that helps in understanding how much of a drug effectively reaches its target site after administration.

Key Points

Intravenous (IV) Administration:
- For drugs administered intravenously, the Bioavailability is always 100% (F = 1) since the drug is delivered directly into the systemic circulation.
Oral Administration:
- For orally administered drugs, the Bioavailability is typically less than 100% (F < 1). This reduction is due to:
  - Incomplete absorption in the gastrointestinal tract.
  - First-pass metabolism by the liver, where a portion of the drug is metabolized before reaching systemic circulation.

Calculation of Bioavailability

Bioavailability can be calculated using the following method:

Area Under the Curve (AUC):
The Bioavailability of a drug can be determined by comparing the AUC of the drug after oral administration to that after IV administration.

Formula:

F=AUC_oral/AUC_IV×100%

Summary Table

Route of Administration	Bioavailability (F)	Factors Affecting F
Intravenous (IV)	100%	None (direct delivery to circulation)
Oral	< 100%	Incomplete absorption, first-pass metabolism

Summary Table

Points to Remember

IV administration guarantees full Bioavailability.
Oral bioavailability can vary significantly between drugs due to various physiological factors.
AUC is a crucial concept in determining the bioavailability of drugs.
Understanding bioavailability is essential for optimizing drug dosing and therapeutic efficacy.

Bibliography

Rang, H. P., Dale, M. M., & Ritter, J. M. (2016). Pharmacology (8th ed.). Elsevier.
Goodman, L. S., & Gilman, A. (2018). The Pharmacological Basis of Therapeutics (13th ed.). McGraw-Hill Education.
Johnson, D. E., & Raffa, R. B. (2015). Understanding Bioavailability: A Practical Approach to Drug Development. Journal of Clinical Pharmacology, 55(7), 740-747.

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