Association of genetic and nongenetic variabilities with phenytoin and carbamazepine response phenotypes.

Abstract

In spite of the availability of an increasing numbers of antiepileptic drugs (AEDs), drug treatment of epilepsy remains symptomatic with a manifestation of a large variability in drug response to AEDs among patients. At present, it is unclear how drug-resistant epilepsy as well as the variability in response among individuals to AEDs happens. Drug-resistance and variability to drug response are proposed to be a multifactorial and complex genetic trait. The variability is under the influence of several putative factors including patient’s genetic and nongenetic variability that affect drug disposition and action. Therefore, the objective of the two pharmacogenetic studies include in this dissertation was to investigate the combined associations of common genetic variations in genes encoding drug metabolizing enzymes, drug transporters and drug target along with nongenetic variations on the clinical phenotypes of drug response to phenytoin (PHT) and carbamazepine (CBZ) namely maintenance dose and drug exposure. These two studies were cross-sectional genetic association studies using a candidate gene approach. Retrospective data were used. The study populations were patients with epilepsy who were unrelated Caucasian Americans or African Americans that were enrolled in the P50 studies. For PHT pharmacogenetic study, a dataset of 54 adult Caucasian patients with epilepsy on PHT maintenance therapy was used. Demographic and clinical variables were retrieved. Genomic DNA samples were used to genotype for 5 candidate single nucleotide polymorphisms (SNPs): SCN1A c.IVSN5+5 G>A, ABCB1c.3435C>T, CYP2C9*2 (g.3608C>T), CYP2C9*3 (g.42614A>C) and CYP2C19*2 (g.19154G>A). Steady-state AUC0- 12 hr was determined from PHT plasma concentrations at 0, 0.08, 0.25, 0.5, 1, 2, 4, 6 and 12 hours after an oral dose. Bivariate analysis as well as stepwise multiple linear regression analysis were used to determine the association of genetic and nongenetic variants with PHT maintenance dose and AUC0-12 hr. This study identified two non-genetic factors (body weight and age) and three genetic variants (CYP2C9*2, CYP2C9*3 and CYP2C19*2) that were strongly associated with variability in PHT maintenance dose in adult Caucasian patients with epilepsy. These covariates explained about 40% of variability in PHT dose requirement in this sample. Moreover, PHT dose and a genetic factors including CYP2C9*3 and ABCB1c.3435C>T were found to be associated with increase in phenytoin AUC 0-12 hr in the same group of adult Caucasian patients. These covariates explained about 42% (R2= 0.422, P= 0.007) and 76% (R2= 0.760, p< 0.001) of variability in phenytoin AUC 0-12 hr in two different multiple linear regression models. For CBZ pharmacogenetic study, demographic and clinical variables were retrieved from datasets of 55 unrelated adult Caucasian American and 32 African American patients with epilepsy on CBZ maintenance therapy. Genomic DNA samples were used to genotype for 5 candidate SNPs including CYP3A5*3 (g.6986A>G), CYP3A5*6 (g.14690G>A), CYP3A5*7 (g.27131_27132insT), SCN1A c.IVSN5+5 G>A and ABCB1c.3435C>T. Steady-state AUC0-24 hr was determined from CBZ plasma concentrations at 0, 0.08, 0.25, 0.5, 1, 2, 4, 6, 12 and 24 hours after oral doses. Bivariate analysis as well as stepwise multiple linear regression analysis were used to determine the association of genetic and non-genetic variants with CBZ response phenotypes, namely CBZ maintenance dose and AUC0-24 hr -to-dose ratio (ADR) in Caucasians or African American patients. By using multiple linear regression analysis, this study found a significant association between CBZ maintenance dose and a non-genetic factor, age, and a genetic variant, CYP3A5*3. The two covariates explained about 9% (R2 = 0.089, P = 0.020) of inter-individual variability in CBZ maintenance dose. In line with that, carbamazepine AUC 0-24 hr -to- dose ratio was found to be associated with the presence of CYP3A5*3 alleles. The model explains about 32% of the variability in carbamazepine AUC 0-24 hr-to-dose ratio (R2= 0.324, P< 0.001). In addition, it was found that carbamazepine AUC 0-24 hr -to- dose ratio was significantly different between African and Caucasian American patients. In bivariate analysis, carbamazepine AUC 0-24 hr -to- dose ratio was found to be associated with race, CYP3A5*3, CYP3A5*7 or ABCB1c.3435C>T allele suggesting that there might be the influence of multiple polymorphisms on CBZ pharmacokinetics which may resulted in the different exposure to CBZ between African and Caucasian patients. These two pharmacogenetic studies clearly confirm that drug response is a complex multifactorial phenotype influenced by many genetic and nongenetic factors. However, the genes and variants identified so far explain only a small fraction of variability in response to AEDs, and still needed to be replicated by other independent study.