Lamotrigine 100 mg (ER) · Tablets

The precise mechanism(s) by which lamotrigine exerts its anticonvulsant action are unknown. In animal models designed to detect anticonvulsant activity, lamotrigine was effective in preventing seizure spread in the maximum electroshock (MES) and pentylenetetrazol (scMet) tests, and prevented seizures in the visually and electrically evoked after-discharge (EEAD) tests for antiepileptic activity. Lamotrigine also displayed inhibitory properties in the kindling model in rats both during kindling development and in the fully kindled state. The relevance of these models to human epilepsy, however, is not known.

One proposed mechanism of action of lamotrigine, the relevance of which remains to be established in humans, involves an effect on sodium channels. In vitro pharmacological studies suggest that lamotrigine inhibits voltage-sensitive sodium channels, thereby stabilizing neuronal membranes and consequently modulating presynaptic transmitter release of excitatory amino acids (e.g., glutamate and aspartate).

Effect of Lamotrigine on N-Methyl d-Aspartate-Receptor-Mediated Activity — Lamotrigine did not inhibit N-methyl d-aspartate (NMDA)-induced depolarizations in rat cortical slices or NMDA-induced cyclic GMP formation in immature rat cerebellum, nor did lamotrigine displace compounds that are either competitive or noncompetitive ligands at this glutamate receptor complex (CNQX, CGS, TCHP). The IC50 for lamotrigine effects on NMDA-induced currents (in the presence of 3 μM of glycine) in cultured hippocampal neurons exceeded 100 μM.

In comparison with immediate-release lamotrigine, the plasma lamotrigine levels following administration of lamotrigine extended-release are not associated with any significant changes in trough plasma concentrations, and are characterized by lower peaks, longer time to peaks and lower peak-to-trough fluctuation, as described in detail below.

Absorption — Lamotrigine is absorbed after oral administration with negligible first-pass metabolism. The bioavailability of lamotrigine is not affected by food.

In an open-label, crossover study of 44 subjects with epilepsy receiving concomitant AEDs, the steady-state pharmacokinetics of lamotrigine were compared following administration of equivalent total doses of lamotrigine extended-release given once daily with those of lamotrigine immediate-release given twice daily. In this study, the median time to peak concentration (Tmax) following administration of lamotrigine extended-release was 4 to 6 hours in subjects taking carbamazepine, phenytoin, phenobarbital, or primidone; 9 to 11 hours in subjects taking valproate; and 6 to 10 hours in subjects taking AEDs other than carbamazepine, phenytoin, phenobarbital, primidone, or valproate. In comparison, the median Tmax following administration of immediate-release lamotrigine was between 1 and 1.5 hours.

The steady-state trough concentrations for extended-release lamotrigine were similar to or higher than those of immediate-release lamotrigine depending on concomitant AED (see Table 6). A mean reduction in the lamotrigine Cmax by 11% to 29% was observed for lamotrigine extended-release compared with immediate-release lamotrigine, resulting in a decrease in the peak-to-trough fluctuation in serum lamotrigine concentrations. However, in some subjects receiving enzyme-inducing AEDs, a reduction in Cmax of 44% to 77% was observed. The degree of fluctuation was reduced by 17% in subjects taking enzyme-inducing AEDs; 34% in subjects taking valproate; and 37% in subjects taking AEDs other than carbamazepine, phenytoin, phenobarbital, primidone, or valproate. Lamotrigine extended-release and immediate-release lamotrigine regimens were similar with respect to area under the curve (AUC, a measure of the extent of bioavailability) for subjects receiving AEDs other than those known to induce the metabolism of lamotrigine. The relative bioavailability of extended-release lamotrigine was approximately 21% lower than immediate-release lamotrigine in subjects receiving enzyme-inducing AEDs. However, a reduction in exposure of up to 70% was observed in some subjects in this group when they switched to lamotrigine extended-release. Therefore, doses may need to be adjusted in some patients based on therapeutic response.

Dose Proportionality — In healthy volunteers not receiving any other medications and given lamotrigine extended-release once daily, the systemic exposure to lamotrigine increased in direct proportion to the dose administered over the range of 50 mg to 200 mg. At doses between 25 mg and 50 mg, the increase was less than dose proportional, with a 2-fold increase in dose resulting in an approximately 1.6-fold increase in systemic exposure.

Distribution — Estimates of the mean apparent volume of distribution (Vd/F) of lamotrigine following oral administration ranged from 0.9 to 1.3 L/kg. Vd/F is independent of dose and is similar following single and multiple doses in both patients with epilepsy and in healthy volunteers.

Protein Binding — Data from in vitro studies indicate that lamotrigine is approximately 55% bound to human plasma proteins at plasma lamotrigine concentrations from 1 to 10 mcg/mL (10 mcg/mL is 4 to 6 times the trough plasma concentration observed in the controlled efficacy trials). Because lamotrigine is not highly bound to plasma proteins, clinically significant interactions with other drugs through competition for protein binding sites are unlikely. The binding of lamotrigine to plasma proteins did not change in the presence of therapeutic concentrations of phenytoin, phenobarbital, or valproate. Lamotrigine did not displace other AEDs (carbamazepine, phenytoin, phenobarbital) from protein-binding sites.