title: Cytochrome P450 2C9 phenotyping using low-dose tolbutamide.
abstract: OBJECTIVE
  The hypoglycaemic drug tolbutamide is used for assessment of CYP2C9 activity in vivo. However, therapeutically active doses of 500 mg bear the risk of hypoglycaemia, and a tolbutamide-derived parameter based on a single plasma or urine concentration reflecting CYP2C9 activity accurately is lacking.
  
  
  METHODS
  We examined tolbutamide and its metabolites 4'-hydroxy-tolbutamide and carboxytolbutamide in plasma and urine of 26 healthy, male volunteers up to 24 h after intake of 125 mg tolbutamide using liquid chromatography-tandem mass spectrometry. CYP2C9 genotypes were determined by sequencing of exons 3 and 7. Raw plasma and urine data were compared with pharmacokinetic parameters, CYP2C9 genotypes, and data from a study in 23 volunteers with all six CYP2C9*1-*3 combinations who received 500 mg tolbutamide.
  
  
  RESULTS
  Plasma clearance and tolbutamide plasma concentrations 24 h after drug intake reflected the genotypes: 0.85 l/h and 1.70 microg/ml (95% confidence interval, CI, 0.80-0.89 l/h and 1.50-1.90 microg/ml) for CYP2C9*1 homozygotes (n=15), 0.77 l/h and 2.14 microg/ml (95%CI, 0.67-0.88 l/h and 1.64-2.63 microg/ml) for *1/*2 genotypes (n=7), 0.60 l/h and 3.13 microg/ml (95%CI, 0.58-0.62 l/h and 2.68-3.58 microg/ml) for *1/*3 genotypes (n=3), and 0.57 l/h and 3.27 microg/ml in the single *2/*2 carrier. Natural logarithms of tolbutamide plasma concentrations 24 h after intake correlated to plasma clearance (r(2)=0.84, P<0.0000001). This correlation was confirmed in the comparison data set (r(2)=0.97, P<0.0000001).
  
  
  CONCLUSIONS
  A low dose of 125 mg tolbutamide can safely and accurately be used for CYP2C9 phenotyping. As a simple metric for CYP2C9 activity, we propose to determine tolbutamide in plasma 24 h after drug intake.
authors.affiliation: Department of Pharmacology, Clinical Pharmacology, University of Cologne, Gleueler Strasse 24, 50931 Cologne, Germany. alexander.jetter@medizin.uni-koeln.de
authors.forename: Alexander
authors.initials: A
authors.lastname: Jetter
authors.forename: Martina
authors.initials: M
authors.lastname: Kinzig-Schippers
authors.forename: Andreas
authors.initials: A
authors.lastname: Skott
authors.forename: Andreas
authors.initials: A
authors.lastname: Lazar
authors.forename: Dorota
authors.initials: D
authors.lastname: Tomalik-Scharte
authors.forename: Julia
authors.initials: J
authors.lastname: Kirchheiner
authors.forename: Monika
authors.initials: M
authors.lastname: Walchner-Bonjean
authors.forename: Ursula
authors.initials: U
authors.lastname: Hering
authors.forename: Verena
authors.initials: V
authors.lastname: Jakob
authors.forename: Michael
authors.initials: M
authors.lastname: Rodamer
authors.forename: Wafaâ
authors.initials: W
authors.lastname: Jabrane
authors.forename: Dirk
authors.initials: D
authors.lastname: Kasel
authors.forename: Jürgen
authors.initials: J
authors.lastname: Brockmöller
authors.forename: Uwe
authors.initials: U
authors.lastname: Fuhr
authors.forename: Fritz
authors.initials: F
authors.lastname: Sörgel
mesh_terms: D000284:Administration, Oral; D000328:Adult; D001189:Aryl Hydrocarbon Hydroxylases / Q000187:drug effects / Q000235:genetics*; D002853:Chromatography, Liquid / Q000379:methods; D018592:Cross-Over Studies; D065729:Cytochrome P-450 CYP2C9; D004305:Dose-Response Relationship, Drug; D005091:Exons / Q000187:drug effects / Q000235:genetics; D005838:Genotype; D006801:Humans; D008297:Male; D013058:Mass Spectrometry / Q000379:methods; D008657:Metabolic Clearance Rate / Q000187:drug effects / Q000502:physiology; D008875:Middle Aged; D010597:Pharmacogenetics / Q000379:methods; D010641:Phenotype; D010865:Pilot Projects; D017422:Sequence Analysis, DNA / Q000379:methods; D013997:Time Factors; D014044:Tolbutamide / Q000031:analogs & derivatives* / Q000097:blood / Q000378:metabolism / Q000494:pharmacology* / Q000652:urine; D000096926:Benzenesulfonamides
chemical_list: D001189:Aryl Hydrocarbon Hydroxylases; D065729:Cytochrome P-450 CYP2C9; D014044:Tolbutamide; C006913:carboxytolbutamide; C031014:hydroxymethyltolbutamide; D000096926:Benzenesulfonamides; C450260:CYP2C9 protein, human