The pharmacokinetics of lamotrigine, a new anticonvulsant, were studied in three studies in normal volunteers. In the first study, five subjects received oral doses of lamotrigine up to 240 mg. A linear relationship was observed between dose administration and both peak drug concentration and AUC. In a second study 10 subjects received 120 mg lamotrigine and the mean (+/- SD) of the elimination half-life (t1/2) was 24.1 +/- 5.7 hours and of volume of distribution/bioavailability 1.2 +/- 0.12 L/kg. Saliva concentrations were 46% of the plasma concentration. Total urinary recovery of drug over 144 hours was 70.5% of the oral dose. A glucuronide conjugate accounted for 89.4% of the urinary recovery. In a third study the kinetics of repeated administration were studied. Fifteen subjects were randomized to lamotrigine (n = 10) or placebo (n = 5) and received multiple doses over 7 days. The overall plasma elimination t1/2 calculated from data during the 7 days was 25.5 +/- 10.2 hours. Observed pharmacokinetics on multiple administration obeyed closely those predicted from the single-dose experiment, suggesting the absence of autoinduction of metabolism. No clinically important side effects or changes in central nervous system or cardiovascular system variables, hematology, biochemistry, or urinalysis occurred during the 7 days.
Concomitant administration of sodium valproate (VPA) reduced lamotrigine (LTG) total clearance by approximately 21% and increased elimination half-life and AUC. Reduced elimination occurred acutely within the first hour. Renal elimination of LTG was not impaired. The most probable explanation for this effect is hepatic competition between VPA and LTG for glucuronidation. Volume of distribution and parameters related to absorption, Cmax and tmax were unchanged.
1 Twelve healthy male volunteers received phenytoin 0.5 and 1 g, lamotrigine (a new anticonvulsant) 120 and 240 mg, diazepam 10 mg and placebo orally in a double-blind, cross-over, randomized trial.2 Maximum drug concentrations at 4 h, measured in plasma were 11.5 + 2.2 ,ug ml-' for phenytoin and 2.7 + 0.4 ,ug ml-' for lamotrigine. These levels were in the therapeutic range for phenytoin and the putative therapeutic range for lamotrigine.3 Side effects after diazepam (mainly sedation) and phenytoin (mainly unsteadiness) differed markedly from lamotrigine which produced no important side effects. Subjective effects as measured by visual analogue scales were caused by phenytoin and diazepam but not by lamotrigine. 4 Diazepam impaired eye movements, adaptive tracking and body sway. Phenytoin impaired adaptive tracking, increased body sway and impaired smooth pursuit eye movement. Lamotrigine produced only a possible slight increase in body sway. 5 There were significant correlations between performance and saliva levels of phenytoin and diazepam. 6 It was concluded that the tests used were suitable for monitoring CNS effects of anticonvulsants and that lamotrigine possibly could have a more favourable CNS side effect profile than phenytoin.
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