Amiodarone prolongs the action potential duration (class III effect) by blocking rapidly activating delayed rectifier potassium channels (IKr). Its loading dose requirement is explained by which pharmacokinetic property?

• A Amiodarone has high water solubility and is rapidly cleared by the kidneys, requiring loading to maintain plasma concentrations
• B Amiodarone induces its own metabolism within 48 hours, requiring dose escalation to maintain therapeutic levels
• C Amiodarone binds irreversibly to plasma proteins, and loading doses saturate the binding sites to allow free drug to reach the myocardium
• D Amiodarone has extremely high lipophilicity and a very large volume of distribution (~60 L/kg), requiring prolonged loading to saturate tissue compartments before steady state is reached ✓

Explanation

Amiodarone is among the most lipophilic drugs in clinical use, with a volume of distribution of approximately 40–60 L/kg (vs. typical drugs at 0.1–1 L/kg). This means enormous amounts accumulate in fat, liver, lung, and skin before adequate plasma and myocardial concentrations are achieved. The elimination half-life is 40–55 days, and complete drug washout after stopping takes months to over a year. This pharmacokinetic profile explains loading doses (e.g., 200 mg TDS for 1 week, then BD for 1 week, then OD) and the slow onset/offset of its effects and toxicities (thyroid, pulmonary, hepatic, corneal microdeposits, photosensitivity).

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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