Clinical pharmacology of lamotrigine

The pharmacokinetics and pharmacodynamics of lamotrigine (LTG), a new antiepileptic drug (AED), were studied in healthy volunteers. In an open dose-escalating study, LTG 240 mg produced peak plasma concentrations of around 3 micrograms/mg with no significant adverse events. Subsequent pharmacokinetic studies revealed complete oral absorption, first-order kinetics with a mean half-life of approximately 1 day, and elimination mainly as a glucuronide in the urine. Early studies in patients with epilepsy revealed more rapid metabolism when given with enzyme-inducing AEDs and delayed metabolism by valproate. A placebo-controlled, double-blind study compared LTG 120 and 240 mg with phenytoin (PHT) 500 and 1,000 mg, and diazepam (DZP) 10 mg. Visual analogue scales showed sedation after PHT 1,000 mg and DZP 10 mg, but not after LTG. Smooth pursuit eye movements and adaptive tracking were impaired by DZP and PHT 1,000 mg. LTG did not affect these variables. A comparison of LTG 150 and 300 mg and carbamazepine (CBZ) 200, 400, and 600 mg demonstrated impairment of smooth pursuit and saccadic eye movements by CBZ 600 and 400 mg, but not by LTG. Additionally, CBZ 600 mg impaired adaptive tracking and increased body sway and heart rate. These studies have shown LTG to have desirable and predictable pharmacokinetic properties for an AED. Pharmacodynamic effects were absent, suggesting a high therapeutic index.