Intraindividual and Interindividual Variability of Olanzapine Trough Concentrations in Patients Treated With the Long-Acting Injectable Formulation

BACKGROUND: A long-acting injectable (LAI) formulation of olanzapine has been developed as an alternative to oral regimens. A therapeutic range of 20 to 80 ng/mL for oral olanzapine trough concentrations has been proposed. Here, we sought to investigate the intraindividual and interindividual variability of olanzapine concentrations with time in patients on maintenance therapy with the LAI formulation carried out in the routine clinical practice. METHODS: To address this issue, we carried out a retrospective analysis of therapeutic drug monitoring of olanzapine concentrations in 21 schizophrenic patients on maintenance LAI olanzapine. Drug concentrations were correlated with LAI olanzapine doses, duration of treatment, and main clinical characteristics. RESULTS: Fifty percent of the patients had olanzapine trough concentrations lower than 20 ng/mL. Only drug doses significantly correlated with olanzapine exposure. Mean interindividual and intraindividual coefficients of variations of olanzapine concentrations were 56% (range, 21%-97%) and 34% (range, 15%-69%), respectively. CONCLUSIONS: We have documented that, in a real-life setting, a large proportion of patients treated with olanzapine LAI had drug trough concentrations of less than 20 ng/mL; wide intraindividual and interindividual variability of olanzapine concentrations has been also observed. Our results could provide the rationale for the design of larger prospective, concentration-controlled clinical trials specifically designed with the goal to identify ad hoc therapeutic ranges of drug concentrations for olanzapine LAI.