Core14: Bioavailability

📈 High-yield / 要点：Bioavailability F compares systemic exposure after extravascular dosing with IV dosing: F = AUC_EV / AUC_IV. IV = 100% bioavailability.

Definition / 定義

• Bioavailability / F = fraction of administered drug reaching systemic circulation.
• Practical formula:
  • F = AUC extravascular ÷ AUC intravenous
• AUC = area under the plasma concentration-time curve.
• EV / extravascular = PO, SC, IM, transdermal, etc.

IV vs extravascular / IVとEVの違い

Route	Phases	Cmax	Bioavailability
IV	Excretion phase only	Maximum at start of administration	100%
EV: PO, IM, SC	Absorption phase + excretion phase	Delayed after administration	Variable

Oral bioavailability / 経口バイオアベイラビリティ

• PO drugs are influenced by:
  • GI absorption.
  • GI metabolism.
  • Liver metabolism.
  • First-pass metabolism.
• Low oral bioavailability means poor systemic exposure after PO dosing.

Examples / 例

Drug	PO bioavailability	IV bioavailability	Clinical point
Vancomycin	<10%	100%	PO for C. difficile pseudomembranous colitis; IV for systemic use
Metronidazole	~98%	100%	Can be given PO or IV; good tissue penetration including CSF/abscesses

Remember / 覚え方

• AUC = total exposure
• IV = no absorption phase, F = 1
• PO bioavailability drops with poor absorption + first-pass metabolism
• Vancomycin PO stays in gut; metronidazole PO ≈ IV