Brain binding of anticonvulsants: carbamazepine and valproic acid.

We investigated the binding of radioactive carbamazepine and valproic acid to brain homogenates, lipid-free extracts, brain lipid, and phospholipid fractions. Carbamazepine was bound to each of these components in a manner that is qualitatively indistinguishable from the binding of phenytoin, in spite of major physical-chemical differences in these molecules but consistent with the similarities in pharmacologic action. These data support the concept that protein and phospholipid binding may be required for the activity of membrane-stabilizing anticonvulsants. Neither valproic acid nor its metabolites exhibited binding to brain or to any of the individual components tested. Therefore, binding cannot explain the reported long duration of action of this drug.