Pharmacokinetics of mood stabilizers and new anticonvulsants

Mechanisms of action, efficacy spectra, pharmacokinetics, and adverse effects differentiate the mood stabilizers lithium, carbamazepine (CBZ), and valproate (VPA). Lithium, which has a low therapeutic index, is excreted through the kidneys, resulting in renally mediated, but not hepatically mediated, drug-drug interactions. CBZ also has a low therapeutic index and is metabolized primarily by a single isoform (CYP3A3/4). It has an active epoxide metabolite, is susceptible to CYP3A3/4 or epoxide hydrolase inhibitors, and is able to induce drug metabolism (both via cytochrome P450 oxidation and conjugation). CBZ thus has multiple problematic drug-drug interactions. In contrast, VPA has less prominent neurotoxicity and three principal metabolic pathways, and it is less susceptible to pharmacokinetic drug interactions. Still, VPA can increase plasma concentrations of some drugs by inhibiting metabolism and can increase the free fractions of certain medications by displacing them from plasma proteins. The newer anticonvulsants lamotrigine, topiramate, and tiagabine have different, generally less problematic, hepatically mediated drug-drug interactions. Gabapentin, which is renally excreted, lacks hepatic drug-drug interactions, though bioavailability may be reduced at higher doses. Recently approved anticonvulsants, including oxcarbazepine, zonisamide, and levetiracetam, may have improved pharmacokinetic profiles compared to older agents. Novel psychotropic effects of these drugs may also be demonstrated, based on their mechanisms of action and preliminary clinical data.