Amiodarone inhibits potassium channels as its primary antiarrhythmic mechanism. Based on the Vaughan Williams classification, it is a Class III drug but also exhibits properties of which other classes?

• A Class I (sodium channel block) and Class II (beta-blockade) properties only
• B Class I (sodium channel block), Class II (beta-blockade), and Class IV (calcium channel block) properties ✓
• C Class I and Class IV properties, but has no beta-blocking activity
• D Class II (beta-blockade) properties only, in addition to Class III

Explanation

Amiodarone is a 'multi-class' antiarrhythmic: it blocks voltage-gated sodium channels (Class I), non-competitively inhibits beta-adrenoceptors (Class II), blocks voltage-gated potassium channels to prolong repolarization (Class III, primary), and blocks L-type calcium channels (Class IV). This multi-channel blockade underlies its broad-spectrum efficacy and complex drug interaction/toxicity profile. It also contains iodine and can cause thyroid dysfunction.

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

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