## Confirming First-Pass Metabolism via Absolute Bioavailability ### Definition of Absolute Bioavailability **Key Point:** Absolute bioavailability (F) is the fraction of an orally administered dose that reaches the systemic circulation unchanged, expressed as: $$F = \frac{AUC_{oral} \times Dose_{IV}}{AUC_{IV} \times Dose_{oral}}$$ When doses are equal, this simplifies to: $$F = \frac{AUC_{oral}}{AUC_{IV}}$$ ### Why This Is the Gold Standard Investigation **High-Yield:** - F directly quantifies the impact of first-pass metabolism, poor absorption, and degradation in the GI tract - An F value <30% indicates severe first-pass metabolism or poor absorption - An F value >70% indicates minimal first-pass loss - This is the **only in vivo investigation** that definitively answers whether systemic exposure is adequate ### Clinical Interpretation | F Value | First-Pass Impact | Implication | |---------|-------------------|-------------| | >80% | Minimal | Oral formulation likely adequate | | 30–80% | Moderate | Oral formulation possible; may need higher doses | | <30% | Severe | IV formulation may be necessary; oral unreliable | **Clinical Pearl:** Drugs with very low oral bioavailability (e.g., nitroglycerin, propranolol, morphine) are often given sublingually, transdermally, or intravenously to bypass first-pass metabolism. ### Why This Is Superior to Alternatives Absolute bioavailability is a **whole-organism, in vivo measurement** that captures all factors affecting systemic exposure: first-pass hepatic metabolism, intestinal wall metabolism, poor absorption, and GI degradation. It is the only investigation that directly answers the clinical question: "Will oral dosing achieve adequate plasma levels?" [cite:KD Tripathi 8e Ch 5; Goodman & Gilman 14e Ch 2]
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