title: The use of the 13C-dextromethorphan breath test for phenotyping CYP2D6 in breast cancer patients using tamoxifen: association with CYP2D6 genotype and serum endoxifen levels.
abstract: OBJECTIVE
  Adjuvant therapy with tamoxifen significantly reduces breast cancer recurrence and mortality in estrogen receptor positive disease. CYP2D6 is the main enzyme involved in the activation of the prodrug tamoxifen into the anti-estrogen endoxifen. Endoxifen is thought to be a main determinant for clinical efficacy in breast cancer patients using tamoxifen. As the large interindividual variation in endoxifen levels is only partly explained by CYP2D6 genotype, we explored the use of the (13)C-dextromethorphan breath test (DM-BT) for phenotyping CYP2D6 and to predict serum steady-state endoxifen levels as a marker for clinical outcome in breast cancer patients using tamoxifen.
  
  
  METHODS
  In 65 patients with early breast cancer using tamoxifen, CYP2D6 phenotype was assessed by DM-BT. CYP2D6 genotype using Amplichip and serum steady-state levels of endoxifen were determined. Genotype was translated into the gene activity score and into ultrarapid, extensive, heterozygous extensive, intermediate or poor metabolizer CYP2D6 predicted phenotype.
  
  
  RESULTS
  CYP2D6 phenotype determined by the DM-BT explained variation in serum steady-state endoxifen levels for 47.5% (R(2) = 0.475, p < 0.001). Positive and negative predictive values for a recently suggested threshold serum level of endoxifen (5.97 ng/mL) for breast cancer recurrence rate were 100 and 90%, respectively, for both CYP2D6 phenotype by DM-BT (delta-over-baseline at t = 50 min (DOB(50)) values of 0.7-0.9) and genotype (CYP2D6 gene activity score of 1.0).
  
  
  CONCLUSIONS
  DM-BT might be, along with CYP2D6 genotyping, of value in selection of individualized endocrine therapy in patients with early breast cancer, especially when concomitant use of CYP2D6 inhibiting medication alters the phenotype.
authors.affiliation: Department of Clinical Oncology, K-1, Leiden University Medical Center, P.O. box 9600, 2300 RC Leiden, The Netherlands. f.l.opdam@lumc.nl
authors.forename: F L
authors.initials: FL
authors.lastname: Opdam
authors.forename: V O
authors.initials: VO
authors.lastname: Dezentje
authors.forename: J
authors.initials: J
authors.lastname: den Hartigh
authors.forename: A S
authors.initials: AS
authors.lastname: Modak
authors.forename: R
authors.initials: R
authors.lastname: Vree
authors.forename: E
authors.initials: E
authors.lastname: Batman
authors.forename: C H
authors.initials: CH
authors.lastname: Smorenburg
authors.forename: J W R
authors.initials: JW
authors.lastname: Nortier
authors.forename: H
authors.initials: H
authors.lastname: Gelderblom
authors.forename: H-J
authors.initials: HJ
authors.lastname: Guchelaar
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000996:Antitussive Agents; D001943:Breast Neoplasms / Q000201:enzymology*; D001944:Breath Tests / Q000379:methods*; D019389:Cytochrome P-450 CYP2D6 / Q000235:genetics*; D003915:Dextromethorphan; D005260:Female; D005838:Genotype; D006801:Humans; D008875:Middle Aged; D010641:Phenotype; D011446:Prospective Studies; D013629:Tamoxifen / Q000031:analogs & derivatives* / Q000097:blood; D055815:Young Adult
chemical_list: D000996:Antitussive Agents; D013629:Tamoxifen; C055492:4-hydroxy-N-desmethyltamoxifen; D003915:Dextromethorphan; D019389:Cytochrome P-450 CYP2D6