ZONISADE
Clinical safety rating: caution
Comprehensive clinical and safety monograph for ZONISADE (ZONISADE).
Zonisamide is a sulfonamide anticonvulsant. Its precise mechanism of action is unknown, but it is believed to inhibit voltage-sensitive sodium channels and reduce T-type calcium currents, thereby stabilizing neuronal membranes and suppressing neuronal hypersynchronization. It may also modulate GABA and glutamate neurotransmission.
| Metabolism | Primarily metabolized by CYP3A4 via reductive cleavage, with secondary pathways involving acetylation and glucuronidation. Less than 30% is excreted unchanged in urine. |
| Excretion | Renal: approximately 62% (35% unchanged, 27% as glucuronide conjugate); fecal: 3%; biliary: negligible |
| Half-life | Terminal elimination half-life: 63-69 hours in adults; allows once-daily dosing; steady-state achieved in 14-21 days |
| Protein binding | 40-50% bound to plasma proteins (primarily albumin) |
| Volume of Distribution | Vd: 0.8 L/kg; indicates extensive tissue distribution including brain |
| Bioavailability | Oral: approximately 97% (almost complete absorption) |
| Onset of Action | Oral: onset of seizure protection occurs within 1-2 weeks; peak plasma concentration reached in 2-5 hours |
| Duration of Action | Duration of anticonvulsant effect: approximately 24 hours with once-daily dosing due to long half-life |
100-200 mg orally every 8 hours; maximum 600 mg/day.
| Dosage form | SUSPENSION |
| Renal impairment | CrCl 30-50 mL/min: 100 mg every 12 hours; CrCl 15-29 mL/min: 100 mg every 24 hours; CrCl <15 mL/min or hemodialysis: not recommended. |
| Liver impairment | Child-Pugh Class A: no adjustment; Class B: reduce dose by 50%; Class C: not recommended. |
| Pediatric use | Body weight <45 kg: 1-2 mg/kg/dose every 8 hours; maximum 4 mg/kg/day. |
| Geriatric use | Start at lower end of dosing range (100 mg every 12 hours) due to age-related renal function decline; monitor renal function. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for ZONISADE (ZONISADE).
| Breastfeeding | Zonisamide is excreted into human breast milk. The milk-to-plasma (M/P) ratio is approximately 0.93, indicating extensive transfer. Infant serum concentrations may reach up to 25% of maternal levels. Cases of drowsiness, poor feeding, and hypernatremia have been reported in breastfed infants. Because of the potential for serious adverse reactions, breastfeeding is generally not recommended during zonisamide therapy. |
| Teratogenic Risk | Zonisamide (ZONISADE) is classified as FDA Pregnancy Category C. In animal studies, it caused developmental toxicity (increased malformations, embryolethality, reduced fetal weight) at clinically relevant doses. Human data are limited; however, sulfonamide derivatives have been associated with an increased risk of congenital malformations (e.g., neural tube defects, cardiovascular anomalies) when used in the first trimester. During the second and third trimesters, exposure may lead to neonatal hemorrhage, metabolic acidosis, or withdrawal symptoms. The drug crosses the placenta. Use only if potential benefit justifies risk. |
■ FDA Black Box Warning
Zonisamide can cause serious or fatal reactions in patients with a history of sulfonamide allergy. Cross-sensitivity may occur. Discontinue if signs of hypersensitivity develop.
| Serious Effects |
Hypersensitivity to zonisamide or sulfonamides, history of metabolic acidosis, concomitant use with carbonic anhydrase inhibitors (e.g., topiramate, acetazolamide) due to increased risk of kidney stones and metabolic acidosis.
| Precautions | Sulfonamide hypersensitivity reactions (including Stevens-Johnson syndrome), oligohidrosis and hyperthermia (especially in children), metabolic acidosis, somnolence and fatigue, serious skin reactions, blood dyscrasias, increased seizure frequency with abrupt withdrawal, kidney stones, teratogenicity (pregnancy category D). |
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| Fetal Monitoring | Monitor serum zonisamide levels to maintain therapeutic range (typically 10-40 mcg/mL) due to pregnancy-induced pharmacokinetic changes. Perform periodic complete blood count (CBC), liver function tests, renal function tests, and serum bicarbonate (risk of metabolic acidosis). Fetal monitoring includes ultrasound for structural anomalies (e.g., neural tube defects, cardiac malformations) in the first trimester and growth scans in the second and third trimesters. Neonates should be observed for signs of withdrawal, metabolic acidosis, and hemorrhage. |
| Fertility Effects | Zonisamide may impair male and female fertility. In animal studies, it caused decreased spermatogenesis, reduced fertility indices, and increased preimplantation loss. In humans, menstrual irregularities and anovulatory cycles have been reported. The drug may also interfere with hormonal contraception via enzyme induction, reducing contraceptive efficacy. |