Feasibility and limitations of oxcarbazepine monitoring using salivary monohydroxycarbamazepine (MHD)

The purpose of this study is to determine the feasibility of using 10-hydroxy-10,11-dihydrocarbazepine (MHD) concentration in saliva as an alternative to serum for the therapeutic monitoring of oxcarbazepine (OXC) treatment. Investigators identified subjects seen in neurology clinics at the University of Kentucky Chandler Medical Center. Patients were eligible if they agreed to participate in this study, were taking oxcarbazepine, and if a serum MHD concentration had been ordered by their physician. Unstimulated saliva specimens (0.25 mL minimum) were collected in the clinic and frozen until analysis. Blood samples were obtained by phlebotomy. Serum specimens were analyzed by a reference laboratory. Saliva MHD concentrations were determined by high-performance liquid chromatography in the Clinical Laboratory at the Cincinnati Children's Hospital Medical Center. Linear regression analysis was used to evaluate correlations. Saliva and blood specimens were collected from 28 epilepsy patients, but usable samples were obtained from only 23. The mean serum MHD concentration was 23.9 ± 10.0 μg/mL, and the mean saliva concentration was 23.1 ± 10.1 μg/mL. There was a significant positive correlation between the serum and saliva concentrations: saliva (y) = 0.95 serum (x) + 0.39; r = 0.941; n = 23; P < 0.001). The mean saliva:serum MHD concentration ratio was 0.96 ± 0.15. The results of the current study indicate that the relationship between freely flowing (unstimulated) saliva and serum concentrations of MHD is sufficient for therapeutic drug monitoring. A limitation of saliva MHD monitoring is that individuals who have difficulty producing small quantities of saliva or who have viscous saliva should generally be avoided for this type of monitoring. It is also recommended to avoid saliva collection within 8 hours after OXC dosing to allow complete absorption and transformation of the parent drug.