Predicting Response to Capecitabine in Women With Metastatic Breast Cancer

OBJECTIVES: Primary * To determine the sensitivity, specificity, and positive and negative predictive values of dihydrouracil/uracil (UH\_2/U) ratio measured before starting treatment on grade 3-4 capecitabine-related toxicity in women with metastatic breast cancer. Secondary * To prospectively test the value of the germinal genotype of thymidylate synthase (TS) and methylenetetrahydrofolate reductase (MTHFR) as predictors of resistance to capecitabine. * To evaluate the practical feasibility of such pre-therapeutic screening. * To determine the sensitivity, specificity, and positive and negative predictive values of dihydropyrimidine dehydrogenase genotyping on grade 3-4 capecitabine-related toxicity in the first and second courses. * To evaluate the predictive gain provided by genotyping relative to phenotyping alone. * To evaluate the influence of TS and MTHFR gene polymorphisms on clinical response and duration of response. * To evaluate the pharmacokinetics of capecitabine and its metabolites and their relationship with UH\_2/U and genotype. * To evaluate the total cost of pre-therapeutic phenotyping alone and the combination of phenotyping and genotyping. * To exhaustively analyze the 23 exons of the dihydropyrimidine dehydrogenase (DPYD) gene in patients who developed toxicity. OUTLINE: This is a multicenter study. Patients receive oral capecitabine twice daily on days 1-14. Treatment repeats every 21 days in the absence of disease progression or unacceptable toxicity. Blood samples are collected 8-15 days before the start of treatment and periodically on the first day of treatment for dihydropyrimidine dehydrogenase phenotyping (dihydrouracil/uracil ratio and high performance liquid chromatography analysis), genotyping (4 most relevant single nucleotide polymorphisms), and pharmacokinetic analysis.