Pharmacogenetics in Breast Cancer Therapy

Interindividual and interethnic variability of drug pharmacokinetics and pharmacodynamics may be contributed by commonly occurring genetic polymorphisms of drug-metabolizing enzymes and transporters. Polymorphisms of CYP2D6 in particular have been associated with effects on tamoxifen disposition and clinical efficacy, with interethnic differences in distribution of functional alleles that affect metabolizer phenotype. Other tamoxifen-related genetic variants of CYP3A4, CYP3A5, and sulfotransferase1A1 (SULT1A1) are also briefly reviewed here. Polymorphisms of CYP19A1 (aromatase gene) have been reported to correlate with clinical outcomes from aromatase inhibitors in small studies but require further confirmation. Many studies on chemotherapy are based on hypothesis-generating association studies and need to be validated through larger-scale cooperative group studies. For anthracyclines, polymorphisms in genes such as carbonyl reductase 3 (CBR3), ATP-binding cassette subfamily B, member 1 (ABCB1), glutathione-related transporter genes, and oxidative stress - related genes have been reported to correlate with clinical outcomes. The pharmacogenetics of taxanes has been extensively investigated, but associations of genetic polymorphisms in drug - metabolizing enzymes and transporters reported in earlier small studies have not been validated in a recent large clinical trial. Allelic variants associated with gemcitabine, capecitabine/5-fluorouracil, vinorelbine, and platinum disposition are reviewed. No pharmacogenetic studies have been published for targeted agents thus far, although several potential candidate genes warrant investigation, Future pharmacogenetic studies will need to focus on integration of multiple drug pathways to allow a more comprehensive analysis of genetic factors influencing drug efficacy and toxicity.