Characterisation of zuclopenthixol metabolism by in vitro and therapeutic drug monitoring studies

Davies SJC, Westin AA, Castberg I, Lewis G, Lennard MS, Taylor S, Spigset O. Characterisation of zuclopenthixol metabolism by in vitro and therapeutic drug monitoring studies. Objective: Zuclopenthixol pharmacokinetics is incompletely characterised. We investigated potential interactions mediated through cytochrome P450 enzymes. Method: In vitro, we examined the impact of CYP2D6 and CYP3A4 inhibitors on zuclopenthixol metabolism in microsomes from six human livers. Subsequently, we compared dose‐corrected serum zuclopenthixol concentrations in 923 samples from a therapeutic drug monitoring database from patients prescribed oral (n = 490) or injected (n = 423) zuclopenthixol alone or with fluoxetine, paroxetine, levomepromazine or carbamazepine. Results: In vitro fluoxetine, paroxetine, ketoconazole and quinidine all significantly inhibited zuclopenthixol metabolism. Ketoconazole and quinidine together abolished zuclopenthixol disappearance. Clinically, dose‐corrected oral zuclopenthixol serum concentrations increased significantly, after adjustment, by 93%, 78% and 46% during co‐treatment with fluoxetine, paroxetine and levomepromazine and decreased 67% with carbamazepine. Carbamazepine caused dose‐dependent reductions in the oral zuclopenthixol concentration–dose ratio (P < 0.001), fluoxetine (P < 0.001) and paroxetine (P = 0.011) dose‐dependent increases and levomepromazine an increase related to its serum concentration (P < 0.001). Results for injected zuclopenthixol were similar but not all reached statistical significance. Conclusion: The In vitro study suggests zuclopenthixol is metabolised primarily by CYP2D6 and CYP3A4. The clinical study supports this, demonstrating the impact of co‐prescribed inhibitors or inducers. Guidelines should incorporate these interactions noting the potential for zuclopenthixol‐related toxicity or treatment failure.