Oxcarbazepine

Indications

• FDA-approved as monotherapy for partial-onset seizures in adults and in children aged 4 years and older.
• FDA-approved as adjunctive therapy for partial-onset seizures in adults and in children as young as 2 years for the immediate-release form.
• The extended-release tablet is FDA-approved as adjunctive therapy for partial-onset seizures in adults and children aged 6 to 17 years.
• Off-label use as a mood stabilizer in bipolar disorder, where controlled trials have been largely negative and guidelines rank it low.
• Off-label first-line alternative to carbamazepine for trigeminal neuralgia, with comparable efficacy and easier monitoring.
• Off-label options include alcohol withdrawal and neuropathic pain, generally where carbamazepine interactions are unacceptable.

Mechanism of action

• Blocks voltage-sensitive sodium channels in a use-dependent manner, stabilizing hyperexcited membranes and limiting repetitive firing.
• Most activity comes from the 10-monohydroxy derivative, so the parent compound behaves largely as a prodrug after oral dosing.
• Modulates high-voltage-activated calcium currents and increases potassium conductance, further reducing synaptic transmission.
• Unlike carbamazepine it is not converted to a 10,11-epoxide, which accounts for less neurotoxicity and fewer hematologic effects.
• Potentiation of antidiuretic hormone action at the collecting duct produces hyponatremia more often than with carbamazepine.

Pharmacokinetics

• Absorption is essentially complete, and the parent drug is rapidly reduced by cytosolic enzymes to the active monohydroxy derivative.
• The parent half-life is 1 to 2.5 hours while the active metabolite has a half-life near 9 hours, supporting twice-daily dosing.
• There is no autoinduction, so a stable dose keeps stable levels rather than drifting downward over the first month like carbamazepine.
• It is a weak inducer of CYP3A4 and CYP3A5 and an inhibitor of CYP2C19, giving a narrower interaction profile than carbamazepine.
• The metabolite is cleared renally as a glucuronide, so dose reduction is required when creatinine clearance falls below 30 mL/min.

Dosing

• Start 300 mg twice daily for adults and increase by no more than 600 mg per day at weekly intervals as tolerated.
• Usual maintenance for adjunctive epilepsy therapy is 1200 mg/day, with the labeled maximum of 2400 mg/day in divided doses.
• When converting from carbamazepine use roughly 1.5 times the carbamazepine dose, since 200 mg carbamazepine approximates 300 mg oxcarbazepine.
• Off-label mood or neuralgia use typically targets 900-1800 mg/day, titrated by response rather than by a defined serum level.
• Halve the starting dose and titrate slowly if creatinine clearance is below 30 mL/min; no adjustment is needed for mild hepatic impairment.
• Taper over 1 to 2 weeks when discontinuing to avoid withdrawal seizures, even in patients treated for nonepileptic indications.

Adverse effects

• Dizziness, somnolence, headache, diplopia, and ataxia are the most common effects and are dose-related during rapid titration.
• Hyponatremia below 125 mmol/L occurs in roughly 2 to 3 percent of patients and mild sodium drops are far more frequent.
• Nausea, vomiting, and abdominal discomfort are common early and often lessen when the dose is taken with food.
• Rash develops in about 3 percent, and 25 to 30 percent of patients with carbamazepine hypersensitivity react to oxcarbazepine as well.
• Stevens-Johnson syndrome, toxic epidermal necrolysis, and multiorgan hypersensitivity are rare but reported, with HLA-B*1502 raising risk.
• Cognitive slowing, tremor, and gait unsteadiness limit dosing in older adults and in those taking other sodium-channel drugs.

Monitoring

• Check serum sodium at baseline, at 2 to 4 weeks, after dose increases, and then periodically, especially in older or diuretic-treated patients.
• Recheck sodium promptly for new headache, nausea, confusion, lethargy, or increased seizure frequency, which suggest hyponatremia.
• Consider HLA-B*1502 testing in patients of Asian ancestry, since the allele raises risk of severe cutaneous reactions with this drug as well.
• Routine serum levels are not required, but monitoring the monohydroxy derivative can help assess adherence or unexplained toxicity.
• Assess renal function at baseline and review gait, balance, and cognition at each visit during titration.

Interactions

• Doses above 1200 mg per day meaningfully induce CYP3A4 and can lower ethinyl estradiol exposure enough to cause contraceptive failure.
• Inhibition of CYP2C19 raises phenytoin concentrations, sometimes requiring a phenytoin dose reduction of 20 to 30 percent.
• Strong enzyme inducers such as carbamazepine, phenytoin, phenobarbital, and rifampin lower monohydroxy derivative levels substantially.
• Additive hyponatremia with thiazides, SSRIs, and desmopressin, and additive sedation and ataxia with alcohol and other CNS depressants.
• Avoid in patients with prior severe cutaneous reactions to carbamazepine given the substantial cross-reactivity between the two drugs.

Special populations

• Human pregnancy data are more limited than for older anticonvulsants, but registry signals suggest lower malformation risk than valproate.
• Clearance of the active metabolite increases substantially during pregnancy, so levels and seizure control should be followed each trimester.
• Breast milk transfer is modest and nursing is generally acceptable with monitoring for infant sedation and poor feeding.
• Pediatric use is established for adjunctive therapy from age 2 and monotherapy from age 4, with weight-based dosing and faster clearance.
• Older adults need lower doses and closer sodium monitoring because hyponatremia, dizziness, and falls are considerably more frequent.

Clinical pearls

• Check sodium at 2-4 weeks; hyponatremia is the effect most likely to force a stop.
• Not a carbamazepine substitute for bipolar disorder; controlled evidence is weak and mostly negative.
• Above 1200 mg/day it induces enough CYP3A4 to compromise hormonal contraception.