Epistemis

Vigabatrin

  • Irreversible GABA Transaminase Inhibitor / GABA Modulator

Agent with a structure analogous to GABA that acts as a suicidal inhibitor of its enzymatic degradation, with an ophthalmic safety profile that restricts its clinical use.

Mechanism

Chemical and Commercial Profile

Common trade names: Sabrilex.
Group: Second generation antiepileptic, vinyl derivative of GABA.

Mechanism of Action

Vigabatrin acts by specifically and irreversible blocking the enzyme GABA transaminase (GABA-T), the mitochondrial enzyme responsible for the catabolic degradation of the neurotransmitter GABA in the intersynaptic space. By covalently binding to the active center of the enzyme (suicidal inhibition), it causes a persistent elevation of the cellular concentration of GABA in the presynaptic vesicles and in the synaptic space, enhancing the inhibition mediated by GABAA and GABAB receptors.

Pharmacokinetics

Pharmacokinetics

  • Absorption: Rapid and independent of food intake; oral bioavailability is 60-80%. The plasma peak is reached at the time of intake.
  • Distribution: Virtually no binding to plasma proteins (0%). Distribution volume: 0.8 L/kg. It easily crosses the blood-brain barrier.
  • Metabolism: None in the liver. It does not present interactions with cytochrome enzymes.
  • Excretion: Renal in unchanged form (>80%). Clearance is linked to basal glomerular filtration.
  • Half-life (t1/2): Short plasma 5-8 hours; however, **biological half-life is weeks** due to irreversible enzymatic inhibition. The anticonvulsant effect of a dose lasts until the GABA-T enzyme is synthesized *de novo* by the neural tissue (which requires 5 to 10 days).

Indicators and dose

Indications

  • First choice treatment for West Syndrome (pediatric infantile spasms), especially associated with **Tuberous Sclerosis**.
  • Adjuvant treatment in focal seizures resistant to multiple drugs in adults, when the rest of the therapeutic combinations have failed.

Dosage and Settings

  • Infantile Spasms (West Syndrome): Start with 50 mg/kg/day divided into two doses; progressive titration of 25-50 mg/kg/day every 3 days to a common range of 100-150 mg/kg/day (maximum of 150 mg/kg/day).
  • Focal Seizures in Adults: Start with 500 mg once or twice a day; therapeutic dose of 2 to 3 g/day total in two doses.
  • Kidney Failure:
    • ClCr [30-50 ml/min]: Reduce the starting and maintenance dose by 25-50%.
    • ClCr < 30 ml/min (Severe): Reduce the dose by at least 50-75% of the usual dose.

Security

Contraindications

  • Presence of pre-existing defects in the visual field (advanced glaucoma, optic neuritis).

Adverse Effects (ADR)

Critical Alert · Visual Field Loss (VIRT)

Vigabatrin can cause an irreversible **concentric reduction of the bilateral visual field (Vigabatrin Retinal Toxicity - VIRT)** in 30-50% of long-term treated patients. It is dose-dependent and causes diffuse atrophy of the retinal nerve fiber layer due to the accumulation of GABA metabolites. It is mandatory to perform **serial visual campimetry** (every 6 months) and electroretinogram in treated pediatric patients.

  • CNS: Progressive sedation, somnolence, chronic fatigue, weight gain (moderate), headache, acute psychosis, or severe behavioral changes (depression, agitation).
  • Pediatric: Changes in the echogenicity of cerebral white matter in infants (generally asymptomatic and reversible upon discontinuation of treatment).

Clinical Interactions

  • Phenytoin: Decreases the serum concentration of phenytoin by 20-30% due to absorption competition and indirect clearance, requiring analytical monitoring of phenytoin.

Pregnancy and Breastfeeding

FDA Category: D. Risk of bone malformations and fetal microphthalmia documented in animal models. It is absolutely not recommended during pregnancy due to systemic ophthalmic restriction. Breastfeeding: Relatively contraindicated. Excretion in milk in considerable concentrations; risk of lethargy or transient visual delay in children.

Epistemis is educational review material. It is not a medical device, does not diagnose or prescribe treatment, and does not replace formal medical training, current clinical guidelines, or professional clinical judgment.

PreviousPregabalin
System
Antiepileptics and Antiparkinsonians
Cluster
Irreversible GABA Transaminase Inhibitor / GABA Modulator
Download Epistemis