PB:PRM ratio in patients with epilepsy treated with primidone

Phenobarbitone (PB) and primidone (PRM) plasma concentrations were measured in 88 patients of both sexes with different types of epilepsy and treated with PRM, alone or in association with carbamazepine (CBZ), phenytoin (PHT), ethosuximide (ESM) or valproic acid (VPA). A correlation was observed between the dose and the levels of both PRM and PB. Plasma PB:PRM ratio was high variable, especially interindividually. These changes seemed to be linked to age and particularly to pharmacological associations. In fact, PB:PRM ratio was increased with CBZ, ESM, VPA and PHT respectively. Moreover, a correlation was observed between the PB:PRM ratio and PHT plasma levels. The possibility of monitoring PB and PRM plasma levels during long-term treatment with PRM is discussed.