Unraveling the Impact of Drug Metabolism on Tamoxifen Response in Breast Cancer.

The selective estrogen receptor modulator tamoxifen is a mainstay of endocrine breast cancer therapy. However, the clinical response rates of tamoxifen are inferior to those of aromatase inhibitors, which may be partially explained by variable drug exposure due to the pharmacogenetics of the drug-metabolizing enzyme cytochrome P450 (CYP) 2D6. Clinical trials investigating the association between CYP2D6 impairment and tamoxifen outcomes have yielded conflicting results. The results of a comprehensive meta-analysis of 33 single-center tamoxifen trials reported here address this inconsistency by adjusting for two biases that may affect the validity of previous association studies: allele coverage of CYP2D6 genotyping and loss of heterozygosity of the CYP2D6 locus in tumor-derived DNA. After adjustment for bias, meta-analyses show significantly reduced study heterogeneity and a higher risk of recurrence or death in patients with impaired CYP2D6 metabolism compared with those with normal activity. These data may support the use of pharmacogenetics-guided tamoxifen therapy to improve outcomes in patients with CYP2D6-compromised breast cancer. Prospective studies should be considered. See related article by MacLehose et al., p. 224.