Clinical Pharmacogenetics of Olanzapine : with Focus on FMO Gene Polymorphisms

Pharmacogenetics is the study of variability in drug response attributed to genetic variation. Olanzapine (OLA) is a widely used antipsychotic drug for schizophrenia treatment. The pharmacokinetics of OLA display large inter-individual variation leading to multiple-fold differences in drug exposure between patients at a given dose. This variation in turn gives rise to the need of individualized dosing in order to avoid concentration-dependent adverse effects and therapeutic failure. The observed variability has been partially explained by environmental and physiological factors. Genetically determined differences in drug metabolism represent a less studied source of variability. Precluded contribution by cytochrome P450 (CYP) 2D6 calls for evaluation of the other major OLA metabolizing enzymes. The objective of this thesis was to study pharmacogenetic influence of flavin-containing monooxygenase (FMO) 1 and 3, CYP1A2 and uridine diphosphate-glucuronosyltransferase (UGT) 1A4 on therapeutic OLA exposure. We conducted genetic association studies applying gene re-sequencing and genotyping of candidate and tagging SNPs. Patients carrying the FMO1*6 allele displayed increased dose-adjusted concentrations (C/Ds) of OLA, in serum as well as cerebrospinal fluid. Patients who were homozygous for the FMO3 K158-G308 compound variant showed reduced C/Ds of OLA N-oxide metabolite, but no alteration in OLA exposure. This compound variant is expected to have clinical relevance primarily for non-African populations, since low frequencies were detected among native Africans. Deviation in OLA exposure was observed in carrier of a rare FMO3 mutation, predicted in silico to affect gene splicing. Reduced OLA exposure was observed in UGT1A4*3 carriers. The CYP1A2 -163(A) (CYP1A2*1F) variant was not associated with increase in CYP1A2-catalyzed OLA metabolism or reduction in OLA exposure. Correlations were detected for two cis-acting variants within the inter-genetic region of the CYP1A cluster and a trans-acting variant located upstream the locus encoding aryl hydrocarbon receptor. The inconsistent data reported for CYP1A2*1F could be explained by presence of ethnic specific haplotype structures incorporating the -163(A) variant. A continuously improved understanding of the wide range of factors that can influence pharmacokinetics and pharmacodynamics will increase the likelihood of achieving optimal treatment response for individual patients