RATIONALE: Estrogen can promote growth of endocrine sensitive breast cancer cells. Endocrine therapy with tamoxifen citrate may fight breast cancer by blocking the use of estrogen by the tumor cells. Pharmacokinetics and -genomics can have an impact on the efficacy of the treatment.

PURPOSE: This phase III trial is studying blood samples to see if the level of active metabolites of tamoxifen can be improved in patients with breast cancer.

OBJECTIVES:

Primary

• To determine how the increase of tamoxifen citrate dose influences the level of its major metabolites in patients with hormone-sensitive breast cancer.

Secondary

• To characterize the population pharmacokinetic profile
• To investigate the role of the other CYPs
• To assess the relation between clinical symptoms and CYP2D6 genotypes and/or active metabolites levels
• To explore the correlation between genotypes/metabolites levels and clinical outcomes in terms of tumor relapse.
• To assess the feasibility, efficacy, and safety of concentration-guided adjustment of tamoxifen citrate dosage.
• To conduct other exploratory analysis based on the eventual new data coming up in the future.

OUTLINE: Patients receive oral tamoxifen citrate (at a dose of 40 mg/day) daily for 4 months in the absence of disease progression or unacceptable toxicity.

Blood samples are collected for PK, genotyping, phenotyping, and further analysis.

DISEASE CHARACTERISTICS:

• Diagnosis of breast cancer

  • Hormone-sensitive breast cancer defined as > 10% estrogen receptor and/or > 10% progesterone receptor positivity by immunohistochemistry

• Receiving treatment with tamoxifen citrate and must be eligible for exposure to higher doses

PATIENT CHARACTERISTICS:

• No history of deep venous thrombosis or pulmonary embolism
• No history of endometrial carcinoma
• No known history of vaginal bleeding, endometriosis, endometrial hyperplasia, endometrial hypertrophy, and/or polyps
• Not pregnant or nursing
• No contraindication to tamoxifen citrate treatment
• No known allergy to midazolam or dextromethorphan

PRIOR CONCURRENT THERAPY:

• See Disease Characteristics