Background Obesity and overweight are well known risk factors for breast cancer and also are associated with higher recurrence and mortality rates.

Main goal of this study is to evaluate the efficacy of metformin plus chemotherapy in terms of pathologic complete response in comparison with placebo plus the same chemotherapy regimen Design: Randomized, double blind, clinical trial. This study will be performed at National Cancer Institute of Mexico City, at breast cancer unit.

Patients with ER+ or PR+, HER2 negative, breast cancer are candidates to participate.

After completion of chemotherapy all patients will have a breast surgery to assess pathologic response.

Complete pathologic response is defined as the abscence of malignant cells in breast tissue and lymph nodes. The presence of DCIS is considered as pCR

Definitions Luminal A: tumors ER + PR + HER2 Negative and Ki67 <14% Luminal B: tumors ER +/-, PR +/-, HER2 negative and Ki67 >15% Complete pathologic response is defined as the abscence of malignant cells in breast tissue and lymph nodes. The presence of DCIS is considered as pCR

Inclusion Criteria:

• Women older than 18 years old and younger than 70 years old

  • Invasive breast cancer confirmed by core biopsy, any histology
  • Tumor ≥2 cm and/or lymph node positive (proven by FNA)(T2-4b, N0-3, M0)
  • Must have ER/PR positive and HER2 negative
  • Must have full staging and extent disease and clinically and radiographically tumor measure
  • Without previous treatment for breast cancer (including surgery, hormonotherapy or chemotherapy)

• Normal liver, kidney and blood tests

• Performance Status ECOG 0-2 o Karnofsky ≥70%

• Fasten glucose levels <125 mg/dl

• Signed consent

Exclusion Criteria:

• Previous use of metformin for any indication
• Presence of Diabetes Mellitus