INTRODUCTION: The anti-diabetic drug metformin exhibits potential anti-cancer properties, which are believed to involve both direct (insulin-independent) and indirect (insulin-dependent) actions. Direct effects are linked to activation of adenosine monophosphate (AMP)-activated protein kinase (AMPK) and an inhibition of mammalian target of rapamycin (mTOR) signaling, while indirect effects are mediated by reductions in circulating insulin, leading to reduced insulin receptor (IR)-mediated signaling. However, the in vivo impact of metformin on cancer cell signaling, and the factors governing sensitivity in patients remain unknown. METHODS: Therefore, we conducted a neoadjuvant, single arm, "window of opportunity" trial examining the clinical and biological effects of metformin on breast cancer patients. Non-diabetic women with untreated breast cancer were given 500 mg of metformin three times a day for >=2 weeks post diagnostic biopsy until surgery. Fasting blood and tumour samples were collected at diagnosis and surgery. Blood glucose and insulin were assayed to assess the physiologic effects of metformin, while immunohistochemical analysis of tumours was used to characterize cellular markers before and after treatment. RESULTS: Levels of IR expression decreased significantly in tumours (P = 0.04) as did the phosphorylation status of protein kinase B (PKB)/Akt (S473), extracellular signal-regulated kinase (ERK1/2, T202/Y204), AMPK (T172), and acetyl coA carboxylase (ACC, S79) (P = 0.0001, P <0.0001, P <0.005, and P = 0.02, respectively). All tumours expressed organic cation transporter 1 (OCT1), with 90% (35/39) exhibiting an Allred score of 5 or higher. CONCLUSIONS: Reduced PKB/Akt and ERK1/2 phosphorylation, coupled with decreased insulin and IR levels, suggest insulin-dependent effects are important in the clinical setting. These results are consistent with beneficial anti-cancer effects of metformin and highlight key factors involved in sensitivity, which could be used to identify breast cancer patients who may be responsive to metformin based therapies. TRIAL REGISTRATION: ClinicalTrials.gov NCT00897884 , May 8(th), 2009.
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