A new oral formulation of a drug is tested in volunteers. Giving 100 mg orally yields an area under the plasma concentration-time curve of 40 mg.h/L, while giving 100 mg intravenously in the same subjects yields an AUC of 100 mg.h/L. The absolute oral bioavailability of this formulation is:

• A 140 percent
• B 60 percent
• C 40 percent ✓
• D 20 percent

Explanation

Absolute bioavailability equals the ratio of dose-normalized AUCs: (AUC oral divided by AUC intravenous) x 100, since the doses are identical here, giving 40 x 100 / 100 = 40 percent. The intravenous route defines 100 percent availability because the entire dose enters systemic circulation, and any shortfall after oral dosing reflects incomplete absorption plus presystemic first-pass extraction.

Reference: Katzung Basic and Clinical Pharmacology, 16th ed.

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