title: Genetic predictors of the maximum doses patients receive during clinical use of the anti-epileptic drugs carbamazepine and phenytoin.
abstract: Phenytoin and carbamazepine are effective and inexpensive anti-epileptic drugs (AEDs). As with many AEDs, a broad range of doses is used, with the final "maintenance" dose normally determined by trial and error. Although many genes could influence response to these medicines, there are obvious candidates. Both drugs target the alpha-subunit of the sodium channel, encoded by the SCN family of genes. Phenytoin is principally metabolized by CYP2C9, and both are probable substrates of the drug transporter P-glycoprotein. We therefore assessed whether variation in these genes associates with the clinical use of carbamazepine and phenytoin in cohorts of 425 and 281 patients, respectively. We report that a known functional polymorphism in CYP2C9 is highly associated with the maximum dose of phenytoin (P = 0.0066). We also show that an intronic polymorphism in the SCN1A gene shows significant association with maximum doses in regular usage of both carbamazepine and phenytoin (P = 0.0051 and P = 0.014, respectively). This polymorphism disrupts the consensus sequence of the 5' splice donor site of a highly conserved alternative exon (5N), and it significantly affects the proportions of the alternative transcripts in individuals with a history of epilepsy. These results provide evidence of a drug target polymorphism associated with the clinical use of AEDs and set the stage for a prospective evaluation of how pharmacogenetic diagnostics can be used to improve dosing decisions in the use of phenytoin and carbamazepine. Although the case made here is compelling, our results cannot be considered definitive or ready for clinical application until they are confirmed by independent replication.
authors.affiliation: Department of Biology, University College London, Darwin Building, Gower Street, London WC1E 6BT, United Kingdom.
authors.forename: Sarah K
authors.initials: SK
authors.lastname: Tate
authors.forename: Chantal
authors.initials: C
authors.lastname: Depondt
authors.forename: Sanjay M
authors.initials: SM
authors.lastname: Sisodiya
authors.forename: Gianpiero L
authors.initials: GL
authors.lastname: Cavalleri
authors.forename: Stephanie
authors.initials: S
authors.lastname: Schorge
authors.forename: Nicole
authors.initials: N
authors.lastname: Soranzo
authors.forename: Maria
authors.initials: M
authors.lastname: Thom
authors.forename: Arjune
authors.initials: A
authors.lastname: Sen
authors.forename: Simon D
authors.initials: SD
authors.lastname: Shorvon
authors.forename: Josemir W
authors.initials: JW
authors.lastname: Sander
authors.forename: Nicholas W
authors.initials: NW
authors.lastname: Wood
authors.forename: David B
authors.initials: DB
authors.lastname: Goldstein
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000483:Alleles; D000595:Amino Acid Sequence; D000927:Anticonvulsants / Q000008:administration & dosage* / Q000627:therapeutic use; D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics; D001483:Base Sequence; D001921:Brain / Q000378:metabolism; D002220:Carbamazepine / Q000008:administration & dosage* / Q000627:therapeutic use*; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D065729:Cytochrome P-450 CYP2C9; D004827:Epilepsy / Q000188:drug therapy*; D005091:Exons / Q000235:genetics; D005260:Female; D005819:Genetic Markers / Q000235:genetics; D005838:Genotype; D006801:Humans; D007223:Infant; D007438:Introns / Q000235:genetics; D008297:Male; D008875:Middle Aged; D008969:Molecular Sequence Data; D062550:NAV1.1 Voltage-Gated Sodium Channel; D009419:Nerve Tissue Proteins / Q000235:genetics; D010672:Phenytoin / Q000008:administration & dosage* / Q000627:therapeutic use*; D011110:Polymorphism, Genetic / Q000235:genetics*; D015222:Sodium Channels / Q000235:genetics
chemical_list: D000927:Anticonvulsants; D005819:Genetic Markers; D062550:NAV1.1 Voltage-Gated Sodium Channel; D009419:Nerve Tissue Proteins; C568242:SCN1A protein, human; D015222:Sodium Channels; D002220:Carbamazepine; D010672:Phenytoin; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; D001189:Aryl Hydrocarbon Hydroxylases