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.
University of Minnesota Ph.D. dissertation. May 2012. Major: Social, Administrative, and Clinical Pharmacy. Advisor: Prof. Richard Brundage. 1 computer file (PDF); xiii, 102 pages.
Association of genetic and nongenetic variabilities with phenytoin and carbamazepine response phenotypes..
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