Zonisamide is a sulfonamide-derived antiepileptic used as adjunctive therapy for focal seizures. Which statement about its pharmacology is correct?
- A It is excreted unchanged by the kidney and requires no hepatic metabolism
- B It blocks both sodium channels and T-type calcium channels and is extensively hepatically metabolised ✓
- C It selectively inhibits GABA transaminase and causes permanent visual field defects
- D It acts solely as an AMPA receptor antagonist
Correct answer: B. It blocks both sodium channels and T-type calcium channels and is extensively hepatically metabolised
Explanation
Zonisamide combines blockade of voltage-gated sodium channels with inhibition of T-type calcium currents, giving it broad-spectrum activity. It undergoes extensive hepatic metabolism including acetylation, relevant because as a sulfonamide it carries risks of rash and hypersensitivity, so caution applies in sulfonamide-allergic patients. Visual field loss is the hallmark of vigabatrin, not zonisamide, and AMPA antagonism alone defines perampanel.
Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.
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