Dose Optimization of Rosuvastatin in Early Stage and Metastatic Estrogen Receptor Positive Breast Cancer Patients on Endocrine Therapy

Inclusion Criteria: * Women with ER+/PR+ HER2-negative breast cancer initiating neoadjuvant endocrine therapy with curative intent OR initiating endocrine therapy for the treatment of metastatic breast cancer with a biopsy accessible primary breast tumor * The patients may start any FDA approved endocrine therapy (with which they have not been previously treated) at week 4 of the trial except for tamoxifen * Palbociclib can be started at week 4, if indicated * Patients with metastatic disease currently on endocrine therapy must be willing to stop endocrine therapy for 2 weeks prior to starting the study and to switch to a new endocrine therapy on the study (at week 4) * Intact breast tumor present and size measuring at least 1cm at enrollment * Post-menopausal at enrollment (age ≥ 60, age ≤ 60 and amenorrhea for ≥12 months in the absence of chemotherapy, tamoxifen, ovarian suppression and FSH/estradiol in the post-menopausal range) * ECOG ≤ 2 * Can be on other endocrine therapy if willing to change a different endocrine therapy agent for the trial * Must have at least one FDA approved endocrine therapy option with which the patient has not received prior treatment * Life expectancy \> 12 weeks * Laboratory criteria: normal renal function: creatinine \< 1.5 x upper limit of normal (ULN)), liver function: bilirubin \< 2 x ULN, transaminases \< 2 x ULN and blood counts: WBC ≥ 2.0, Neutrophils ≥1250, platelets ≥50,000, Hemoglobin ≥ 8. * Age \> 18 years * Patients must have the ability to give informed consent. * Patients must have a signed informed consent form prior to enrollment on study. - Exclusion Criteria: * Statin use in the last 6 months * Patient has been treated with all FDA approved endocrine therapies or has been treated with all FDA approved endocrine therapies except for tamoxifen (tamoxifen is excluded from the trial) * Active liver disease with elevated transaminases \> 2x ULN * Known hypersensitivity to rosuvastatin * Any history of any other cancer (except non-melanoma skin cancer or carcinoma in-situ of the cervix) unless the patient has been in remission and off all other cancer therapy for at least 3 years. * Patients should have no significant psychiatric illness or medical illness that would preclude the ability to comply with the protocol. * Patients currently taking medications with known rosuvastatin interactions including cyclosporine, gemfibrozil, lopinavir/ritonavir, atazanavir/ritonavir, coumarin anticoagulants, colchicine, fenofibrates, and niacin.