Oxcarbazepine

Mechanism

Chemical and Commercial Profile

Common trade names: Trileptal, Oxcarbazepine Normon, Oxcarbazepine Kern Pharma.
Group: Second-generation antiepileptic, keto-homologous derivative of carbamazepine.

Mechanism of Action

Oxcarbazepine acts as a **prodrug**. It is rapidly reduced in the liver to its active metabolite, the 10-monohydroxy derivative (MHD). MHD selectively blocks voltage-gated sodium channels in their inactive state, limiting high-frequency repetitive neuronal firing and stabilizing hypercytable membranes. It also modulates N- and P-type calcium channels.

Pharmacokinetics

Pharmacokinetics

• Absorption: Rapid and complete after oral intake; 95% bioavailability. It is not altered by food. The plasma peak of MHD is reached after 4-6 hours.
• Distribution: Moderate binding of MHD to plasma proteins (40%), which reduces the risk of protein displacement interactions. Volume of distribution of MHD: 0.8 L/kg.
• Metabolism: Rapid presystemic reduction in the liver by cytosolic enzymes (carbonyl-reductases) to MHD. This metabolite is not oxidized through the cytochrome CYP450 system, avoiding the toxic epoxide pathway common to carbamazepine. MHD is eliminated by conjugation with glucuronic acid via UGT.
• Excretion: Renal (>95%), mainly in the form of MHD glucuronide (80%) and unchanged MHD.
• Half-life (t_1/2): Oxcarbazepine parent: 1-2 hours (transient phase). Active MHD metabolite: 8-12 hours in patients with preserved renal function.

Indicators and dose

Indications

• Monotherapy and adjuvant therapy in focal seizures in adults and children over 6 years of age (with or without secondary generalization).
• Symptomatic treatment of refractory neuropathic pain (second line).

Dosage and Settings

• Starting Dose: 300 mg twice a day (600 mg/day total).
• Titration: Increase 300 mg/day every week until therapeutic response is obtained.
• Usual therapeutic range: 900-1800 mg/day divided into two equivalent doses.
• Renal Failure: In patients with creatinine clearance Cl_Cr < 30 ml/min, the starting dose should be reduced by half (300 mg/day total) and titration should be spaced at two-week intervals.
• Liver Failure: Does not require adjustments in mild to moderate hepatic dysfunction through the non-cytochrome-mediated metabolism pathway. Avoid in Child-Pugh C cirrhosis due to lack of data.

Security

Contraindications

• Documented cross-hypersensitivity to carbamazepine (occurs in 25-30% of patients).
• High-grade atrioventricular block.

Adverse Effects (ADR)

Safety Alert · Acute Hyponatremia due to MHD

Oxcarbazepine has a rate of induction of severe dilutional hyponatremia (serum sodium <125 mEq/L) higher than that observed with carbamazepine, affecting 15% of exposed elderly patients. It generally appears in the first 6 weeks of treatment, asymptomatic or appearing with nausea, headache, lethargy or confusion. Requires basal and periodic serum sodium control if thiazide diuretics are associated.

• Neurological: Daytime sleepiness, postural dizziness, reversible diplopia, mild intention tremor, horizontal nystagmus, generalized fatigue.
• Dermatological: Benign maculopapular rash (3-5%). Extremely rare cases of Stevens-Johnson Syndrome associated with the HLA-B*1502 allele.
• Gastrointestinal: Temporary nausea, vomiting, moderate watery diarrhea.

Clinical Interactions

• Weak enzyme inducer and inhibitor: Unlike carbamazepine, oxcarbazepine does not produce metabolic autoinduction. However, it moderately inhibits the CYP2C19 isoenzyme (which can increase the levels of phenobarbital or phenytoin) and moderately induces the CYP3A4 isoenzyme and glucuronidation (which reduces the effectiveness of oral contraceptives by 30%).

Pregnancy and Breastfeeding

FDA Category: C. It has a lower rate of major congenital malformations compared to carbamazepine, although cases of cardiac septal defects and facial clefts have been reported in experimental studies. It is recommended to supplement with folic acid at a dose of 5 mg/day. Breastfeeding: Compatible with monitoring. The MHD metabolite is excreted in milk with a milk/plasma ratio of 0.5. Monitor for lethargy or weak sucking in the infant.