The in vitro adsorption of some antiepileptics on antacids.

The adsorption of the antiepileptics sulthiame, phenytoin, mephenytoin, mesuximide, phensuximide, ethosuximide, and primidone on various antacids or adsorbents was studied at 37 degrees C. The antacids or adsorbents used were magnesium trisilicate, aluminium hydroxide, bismuth oxidcarbonate, magnesium oxide, talc, kaolin and calcium carbonate. Magnesium trisilicate was found to be the strongest adsorbent for most of the antiepileptics tested. The other antacids or adsorbents were without an appreciable effect. Sulthiame exhibited the highest degree of interaction with magnesium trisilicate. Mesuximide, phensuximide and primidone showed intermediate adsorption properties. Mephenytoin and phenytoin had lower adsorption characters, while ethosuximide was the least adsorbed antiepileptic tested. The extent of elution was found to be inversely proportional to adsorption. Alkaline solution gave relatively higher eluting power than acid solution. The mechanism of adsorption of the various antiepileptics on antacids was discussed. The effect of magnesium trisilicate on the bioavailability of coadministered antiepileptics has still to be confirmed by in vivo testing.