The cytochrome P450-catalyzed metabolism of levomepromazine: a phenothiazine neuroleptic with a wide spectrum of clinical application.

The aim of the present study was to identify cytochrome P450 isoenzymes (CYPs) involved in the 5-sulfoxidation and N-demethylation of the aliphatic-type phenothiazine neuroleptic levomepromazine in human liver. Experiments were performed in vitro using cDNA-expressed human CYP isoforms (Supersomes 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4), liver microsomes from different donors and CYP-selective inhibitors. The obtained results indicate that CYP3A4 is the main isoform responsible for levomepromazine 5-sulfoxidation (72%) and N-demethylation (78%) at a therapeutic concentration of the drug (10μM). CYP1A2 contributes to a lesser degree to levomepromazine 5-sulfoxidation (20%). The role of CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6 and CYP2E1 in catalyzing the above-mentioned reactions is negligible (0.1-8%). Moreover, at a higher, toxicological concentration of the neuroleptic (100μM), the relative contribution of CYP1A2 to levomepromazine metabolism visibly increases (from 20% to 28% for 5-sufoxidation, and from 8% to 32% for N-demethylation), while the role of CYP3A4 significantly decreases (from 72% to 59% for 5-sulfoxidation, and from 78% to 47% for N-demethylation). The obtained results indicate that the catalysis of levomepromazine 5-sulfoxidation and N-demethylation in humans shows a strict CYP3A4 preference, especially at a therapeutic drug concentration. Hence pharmacokinetic interactions involving levomepromazine and CYP3A4 substrates (e.g. tricyclic antidepressants, calcium channel antagonists, macrolide antibiotics, testosterone), inhibitors (e.g. ketoconazole, erythromycin, SSRIs) or inducers (e.g. rifampicin, carbamazepine) are likely to occur.