Metformin extends life span of HER-2/neu transgenic mice and in combination with melatonin inhibits growth of transplantable tumors in vivo.

Population studies have shown that treatment with the antidiabetic biguanide metformin significantly reduced cancer risk. In our animal studies, metformin delayed the onset of mammary adenocarcinoma (MAC) in transgenic HER-2/neu mice but not the onset of spontaneous mammary tumors in female SHR mice. Pineal hormone also inhibits mammary carcinoma development in HER2/neu transgenic mice as well as in female SHR mice. Here we demonstrated that a combination of metformin and melatonin significantly inhibits growth of transplanted tumors in mice. Metformin (0.5 mg/ml in drinking water) increased mean life span by 8% and MAC latency by 13.2% (p < 0.05) in HER2/neu mice. The treatment with melatonin alone (2 mg/L in drinking water during the night time) or combined treatment with metformin (0.5 mg/ ml in drinking water during the day time) + melatonin (2 mg/L in drinking water during the night time) did not influence mammary carcinogenesis in the mice. The treatment metformin alone inhibited the growth of transplantable HER2 mammary carcinoma in FVB/N male mice by 46% at the 45(th) day after transplantation (p < 0.001). The combined treatment with metformin + melatonin significantly suppressed Ehrlich tumor growth (by 40%, p < 0.001). These results suggest that metformin may be useful in prevention and treatment of breast cancer.