The influence of amitriptyline and carbamazepine on levomepromazine metabolism in human liver: an in vitro study.

Background: Joint administration of phenothiazine neuroleptics and an antidepressant or carbamazepine is applied in the therapy of many complex psychiatric disorders. The aim of the present study was to investigate possible effects of the tricyclic antidepressant drug amitriptyline and the anticonvulsant drug carbamazepine on the metabolism of the aliphatic-type phenothiazine neuroleptic levomepromazine in human liver.

Methods: The experiment was performed in vitro using human liver microsomes. The rates of levomepromazine 5-sulfoxidation and N-demethylation (levomepromazine concentrations: 5, 10, 25 and 50μM) were assessed in the absence and presence of amitriptyline or carbamazepine added in vitro (drug concentrations: 1, 2.5, 5, 10, 25μM).

Results: A kinetic analysis of levomepromazine metabolism carried out in the absence or presence of carbamazepine showed that the anticonvulsant drug potently inhibited levomepromazine 5-sulfoxidation (Ki=7.6μM, non-competitive inhibition), and moderately decreased the rate of levomepromazine N-demethylation (Ki=15.4μM, mixed inhibition) at therapeutic drug concentrations. On the other hand, amitriptyline weakly diminished the rate of levomepromazine 5-sulfoxidation (Ki=63μM, mixed inhibition) and N-demethylation (Ki=47.7μM, mixed inhibition).

Conclusion: Regarding the central and peripheral effects of levomepromazine and some of its metabolites, the observed metabolic interaction between this neuroleptic and carbamazepine may be of pharmacological and clinical importance.