HPLC analysis of risperidone and fexofenadine for TDM purposes: preliminary results within a multidisciplinary study on p-glycoprotein (p-GP) activity

The Therapeutic Drug Monitoring (TDM) is one of the most important tools for the optimization and personalization of pharmacological psychiatric therapies. It can be useful not only for routine treatments, but also for research purposes within multidisciplinary studies. One of these projects is currently in progress: it aims to discover correlations between risperidone (an atypical antipsychotic drug) and 9-hydroxyrisperidone (its main active metabolite) plasma levels, central receptor occupancy and P-glycoprotein (P-gp) activity. P-gp is a membrane transporter that mediates the removal of drugs from the brain back to the peripheral blood compartment and hence, serves as a functional complement to the physico-chemical blood-brain-barrier. Since risperidone is a known P-gp substrate, it is probable that the individual variability in risperidone dopamine D2 receptor occupancy in the brain can be explained by variations in P-gp activity in vivo. If these hypotheses are correct, baseline P-gp activity as measured by single dose fexofenadine (a P-gp phenotyping probe) plasma clearance will significantly correlate with striatal dopamine D2 receptor occupancy following risperidone administration. Moreover, co-administration of the potent P-gp inhibitor ketoconazole with risperidone will significantly increase D2 receptor occupancy in the brain. Patients with schizophrenia or schizoaffective disorder were treated with oral or long acting risperidone and received a single oral dose of fexofenadine. It is evident the importance of reliably analyzing the plasma levels of all the involved compounds: risperidone, 9-hydroxyrisperidone and fexofenadine. The method used for the determination of the analytes is based on HPLC with UV detection, using a C8 reversed phase column as the stationary phase and an acetonitrile / acidic phosphate buffer mixture as the mobile phase. All the analytes and the internal standard (diphenhydramine) were detected at 238 nm. A fast and original sample pre-treatment procedure was developed, based on solid phase extraction (SPE). This procedure needed small amounts of plasma, granting high extraction yields (>90%) and good selectivity. The method was successfully applied to patient plasma samples, obtaining high accuracy (>85%) and allowing the quantitative determination of risperidone, 9-hydroxyrisperidone and fexofenadine in all of them.