How is oral bioavailability defined, and how does it compare to intravenous exposure?

• A. F is the fraction of an orally administered dose that reaches systemic circulation; IV exposure corresponds to F = 1.
• B. F is the fraction of an orally administered dose that reaches systemic circulation.
• C. F equals the ratio of oral AUC to IV AUC without dose correction.
• D. F is always 100% for all routes.

Answer: (A)

Oral bioavailability is the fraction of an orally administered dose that reaches the systemic circulation intact after absorption and first-pass metabolism. This fraction is denoted F and ranges from 0 to 1 (0–100%). When a drug is given intravenously, the entire dose enters the bloodstream directly, so the IV exposure corresponds to F = 1 (100%). We quantify F by comparing systemic exposure after oral and IV dosing while accounting for dose: F = (AUC_po / Dose_po) / (AUC_iv / Dose_iv) = (AUC_po × Dose_iv) / (AUC_iv × Dose_po).

So the best statement is that F is the fraction of an orally administered dose that reaches systemic circulation, and IV exposure corresponds to F = 1. The other options are incomplete or misleading: one only mentions the oral fraction without the IV comparison; another uses AUC ratios without properly correcting for dose; and another claims F is always 100%, which is not true for most drugs.