Lamotrigine

Mechanism

Chemical and Commercial Profile

Common trade names: Lamictal, Crisomet, Labileno.
Group: Antiepileptic and ion channel stabilizer, derived from phenyltriazines.

Mechanism of Action

It shows a double molecular mechanism of synaptic stabilization:

1. Inhibition of voltage-gated Sodium channels (Na_v 1.2 and 1.6): Selectively blocks channels in the inactive state in hyperactive presynaptic neurons with rapid and repetitive firing.
2. Inhibition of presynaptic glutamate release: By stabilizing the presynaptic membrane and modulating N- and P-type voltage-dependent calcium channels, it preferentially decreases the release of excitatory amino acid neurotransmitters (glutamate and aspartate) in the prefrontal cortex and hippocampus, preventing excitatory neurotoxicity and stabilizing the affective state in the depressive pole.

Pharmacokinetics

Pharmacokinetics

• Absorption: Fast and complete, bioavailability of 98%. C_max in 1.5 to 3 hours.
• Metabolism: Hepatic through conjugation via the UDP-glucuronosyltransferase pathway (mainly UGT1A4), without significant intervention of the CYP450 system.
• Excretion: Renal (90%) in the form of inactive glucuronide metabolites.
• Half-life (t_1/2): 24 to 35 hours in healthy adults.

Indicators and dose

Indications

• Prevention and long-term maintenance of depressive episodes in Bipolar Disorder type I and II. (Note: Not effective for controlling acute manic episodes).
• Partial and generalized epilepsy, Lennox-Gastaut syndrome (adjuvant or monotherapy).

Dosage and Settings

• Mandatory standard titration schedule (in monotherapy):
  • Weeks 1 and 2: 25 mg once a day.
  • Weeks 3 and 4: 50 mg once a day.
  • Week 5: 100 mg once a day.
  • Week 6+: Maintenance range of 100 mg to 200 mg/day total.
• Valproic Acid Guideline: Start with 12.5 mg every other day during the first two weeks; maintenance reduced to 100 mg/day.
• Schedule with Inducers (Carbamazepine): Start with 50 mg/day during the first two weeks; upper maintenance range of 200 mg to 400 mg/day.
• Kidney / Liver Failure:
  • Moderate to severe (Cl_Cr < 30 ml/min): Reduce the therapeutic dose by 50%.
  • Liver cirrhosis with ascites: Reduce initial and maintenance doses by at least 50-75%, closely monitoring the appearance of rash.

Security

Contraindications

• Known hypersensitivity to the compound or components of the formulation.

Adverse Effects (ADR)

Critical alert · Stevens-Johnson syndrome (SJS)

Lamotrigine is associated with a ~0.1% risk of inducing serious and life-threatening dermatological reactions, such as Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN). This risk multiplies exponentially if high initial doses are associated or if it is co-administered with valproic acid. Any skin rash of recent onset accompanied by fever, oral or ophthalmic mucosal involvement or lymphadenopathy requires immediate and irreversible suspension of treatment.

• Dermatological: Benign uncomplicated rash in the first 8 weeks (occurs in 10% of patients; monitor closely).
• CNS: Diffuse headache, rotational dizziness, transient reversible diplopia, mild ataxia, secondary sleep insomnia.
• Immune: Drug hypersensitivity syndrome with eosinophilia and systemic symptoms (DRESS).

Clinical Interactions

• Valproic Acid (Critical Interaction): Valproate is a potent glucuronidation (UGT) inhibitor. Doubles the half-life of lamotrigine to 70 hours. It requires reducing the initial dose of lamotrigine by half (12.5 mg every other day) and performing an extremely slow titration.
• Glucuronidation inducers: Carbamazepine, phenytoin and oral contraceptives containing estrogens halve the half-life of lamotrigine, requiring doubling the usual dose to maintain stable levels.

Pregnancy and Breastfeeding

FDA Category: C. Extensive human data (pregnancy records of more than 10,000 exposed) demonstrate that it is one of the safest antiepileptics in pregnancy, with a slightly increased but very low absolute risk of cleft lip. Plasma levels of lamotrigine fall by up to 60% during pregnancy due to hormonal induction of glucuronidation, requiring therapeutic monitoring. Breastfeeding: Compatible with extreme caution. It is excreted in milk reaching considerable concentrations in the infant; monitor rash or apnea.