Which antiepileptic drug is a prodrug that requires hepatic metabolism to its active metabolite, phenobarbital?

Explanation

## Prodrug Metabolism of Primidone **Key Point:** Primidone is a prodrug that undergoes hepatic metabolism to form two active metabolites: phenobarbital (major) and phenylethylmalonamide (PEMA). ### Mechanism of Action Primidone itself has weak anticonvulsant activity. The clinical efficacy is primarily due to: 1. Conversion to phenobarbital (40–50% of the dose) 2. Conversion to PEMA (minor metabolite) 3. Direct action of primidone (minimal) ### Clinical Implications - **Onset of action:** Delayed (days to weeks) because phenobarbital accumulates gradually - **Steady-state:** Achieved over 2–3 weeks due to long half-life of phenobarbital - **Drug interactions:** Induces CYP450 enzymes (like phenobarbital) - **Adverse effects:** Similar to phenobarbital (sedation, ataxia, megaloblastic anemia) **High-Yield:** Primidone is the only antiepileptic that is a true prodrug requiring conversion to an active metabolite for therapeutic effect. This distinguishes it from phenytoin (active as-is) and valproic acid (active as-is). **Clinical Pearl:** Because of slow accumulation of phenobarbital, primidone should be started at low doses and titrated gradually to avoid toxicity. [cite:KD Tripathi 8e Ch 12]