Pharmacogenetics concerns the influence of an individual's genetic background on the pharmacokinetics and pharmacodynamics of
xenobiotics. Much of the pharmacogenetic data in the field of
epilepsy deals with the pharmacokinetics of
antiepileptic drugs (AEDs). In particular, two polymorphisms of
cytochrome P450 2C9 are known to slow down the metabolism of
phenytoin to a degree that increases the risk of the neurotoxic adverse effects of this
drug among carriers of these polymorphisms. A significant number of patients with
epilepsy do not respond to AEDs and such pharmacoresistance is a major, largely unsolved, problem that is likely to be multifactorial in nature. In this regard, genetic factors may influence transmembrane
drug transporter
proteins, thereby modifying the intracerebral penetration of AEDs. Monogenic idiopathic
epilepsies are rare and frequently associated with
ion channel mutations; however, to date, a consistent relationship between changes in channel properties and clinical phenotype has not been established nor has any association between genotype and response to specific treatment options. Polymorphisms of
drug targets may represent another genetic facet in
epilepsy: a recent study demonstrated for the first time a polymorphism of a
drug target (the alpha-subunit of a
voltage-gated sodium channel) associated in clinical practice with differing response to two classic AEDs.
Adverse drug reactions and teratogenicity of AEDs remain a major concern. Whole-genome single nucleotide polymorphism profiling might in the future help to determine genetic predisposing factors for
adverse drug reactions. Recently, in Han Chinese treated with
carbamazepine and presenting with
Stevens-Johnson syndrome, a strong association was found with
HLA B*1502. If genetically targeted
drug development becomes more affordable/cost efficient in the near future, the development of new drugs for relatively
rare diseases could become economically viable for the pharmaceutical industry. The synergy of lower trial costs and efficacy-based prescribing may reduce the cost of medical treatment for a particular disease. This hypothetical advantage of the practical use of pharmacogenetics is, however, counterbalanced by several possible dangers, including illicit data mining and the development of a human 'genetic underclass' with the risk of exclusion from, for example employment or health insurance, because of an 'unfavourable' genetic profile.