ABCB1 polymorphisms influence steady-state plasma levels of 9-hydroxyrisperidone and risperidone active moiety

Genetically determined differences in drug metabolism and disposition and drug targets play a pivotal role in the interindividual variability in the clinical outcome of antipsychotic treatment. The aim of this thesis was to study the impact of polymorphisms in genes involved in the pharmacokinetics and pharmacodynamics of antipsychotics, with special focus on their extrapyramidal and metabolic adverse effects. Polymorphisms in serotonin 2A and 2C receptor coding genes (HTR2A and HTR2C) were found to be associated with the risk to develop extrapyramidal side effects (EPS) in patients on short term perphenazine treatment. A further study in a larger group of patients on long term treatment with various classical antipsychotics confirmed the association between occurrence of EPS and HTR2C polymorphisms. In another study, dose corrected steady state serum clozapine and N-desmethylclozapine concentrations (C/D) and insulin elevation during clozapine therapy were found to correlate with CYP1A2 but not with CYP2D6 polymorphisms. Furthermore, HTR2C and HTR2A polymorphisms were found to have significant influences on BMI and C-peptide levels in patients treated with olanzapine and clozapine. Evaluation of the impact of polymorphisms in genes encoding CYP3A4, CYP3A5 and P-glycoprotein (ABCB1) in addition to CYP2D6 on the steady state plasma levels of risperidone, 9-hydroxyrisperidone and their active moiety revealed a significant influence of ABCB1 genotype on 9-hydroxyrisperidone and active moiety C/Ds, while CYP2D6 genotype associated with risperidone C/Ds but not with 9-hydroxyrisperidone or active moiety C/D. We have shown that polymorphisms in genes involved in the pharmacokinetics and the pharmacodynamics of antipsychotic drugs play a role in the occurrence of adverse effects, both EPS and metabolic disturbances, induced by antipsychotic treatment. Genotyping for HTR2A, HTR2C, CYP1A2, CYP2D6 and ABCB1 polymorphisms may therefore potentially provide useful information to identify patients at higher risk to develop EPS or metabolic adverse during schizophrenia treatment with antipsychotic drugs.