Valproic Acid Significantly Lowers Serum Concentrations of Olanzapine—An Interaction Effect Comparable With Smoking

Background: Preliminary reports have indicated that valproic acid (VPA) reduces serum concentrations of olanzapine (OLZ). The aim of this study was to investigate the impact of VPA and other antiepileptic drugs (AEDs) on serum concentrations of OLZ and 3 of its major metabolites in a large-scale material of therapeutic drug monitoring samples. Methods: OLZ-treated patients were stratified into subgroups according to coadministration of various AEDs, that is, lamotrigine (LTG; 110 patients/153 samples), VPA (92/166), LTG + VPA (7/12), carbamazepine (CBZ) (8/8), oxcarbazepine (2/3), gabapentin (3/4), levetiracetam (2/3), and topiramate (2/2). A control group treated with OLZ without AEDs was also included (205/247). Dose-adjusted serum concentrations (C:D ratios) of OLZ and its major metabolites (N-desmethyl, N-oxide, and 10-N-glucuronide) were compared between AED subgroups and controls, using linear mixed model analyses with age, gender, and cigarette smoking as covariates. Results: Significantly lower OLZ C:D ratios were found in patients comedicated with VPA (―32%, P < 0.001), VPA + LTG (―31%, P < 0.01), and CBZ (―50%, P < 0.001), compared with controls. The 10-N-glucuronide concentration was significantly lower in patients comedicated with VPA (―26%, P < 0.001), whereas CBZ significantly lowered N-desmethyl (―42%, P = 0.001) and N-oxide (―52%, P < 0.001) metabolite concentrations. C:D ratios of OLZ and metabolites were not significantly affected by comedication with LTG or any of the other AEDs. All covariates were significant determinants of OLZ C:D ratio, that is, age 60 years or above +35% (P < 0.001), female gender +11% (P < 0.01) and smoking ―32% (P < 0.001). Conclusions: Concurrent use of VPA significantly decreases serum concentrations of OLZ to an extent comparable with smoking. The mechanism behind the interaction could not be derived from the results of this study.