http://repub.eur.nl/res/aut/53736/ List of Publications en http://repub.eur.nl/static-eur/img/logo.png http://repub.eur.nl http://repub.eur.nl/res/pub/34368/ 2011-08-01T00:00:00Z Aims: Tamoxifen is metabolized by cytochrome P450s, with an important role for CYP2D6. Recently, we demonstrated in 80 patients that CYP2C19*2 is associated with increased survival in breast cancer patients using tamoxifen. Here, we aimed to confirm this in a large group of 499 patients. Materials & methods: A total of 499 estrogen receptor-positive primary breast tumor specimens of advanced disease patients treated with first-line tamoxifen were genotyped for CYP2C19*2 and 17 variant alleles, with primary end point time-to-treatment failure (TTF). Effects of CYP2C19, independent of treatment, were analyzed in 243 primary systematic untreated patients. Results: CYP2C19*2 hetero-and homozygote patients combined showed significantly longer TTFs (hazard ratio [HR]: 0.72; 95% CI: 0.57-0.90; p = 0.004). In multivariate analysis, including CYP2D6*4 status, CYP2C19*2 remained independently associated with TTF (HR: 0.73; 95% CI: 0.58-0.91; p = 0.007). In untreated patients, the CYP2C19*17 allele was significantly associated with a longer disease-free interval (HR: 0.66; 95%CI: 0.46-0.95; p = 0.025). Conclusion: CYP2C19 genotyping is potentially important for tamoxifen therapy for advanced disease and for breast cancer prognosis.