Enchancement of Toremifene Anti-Tumor Action by Metformin and Unusual Side Effect of Toremifene in Male Transgenic Mice with HER2-Positive Breast Tumor

HER2-positive breast tumors are found in 25–30% of patients with breast cancer and are characterized by aggressive course and reduced sensitivity to both chemotherapy and hormone therapy. The aim of the work was to study the possibilities of enhancing the therapeutic effect of anti-estrogen drug toremifene by combining it with biguanide, metformin, on the HER2-positive breast cancer model in FVB/N HER-2/neu transgenic mouse. Male FBV/N mice with intramuscularly transplanted HER2-positive mammary gland tumor from a female mouse of the same strain have been given toremifene (30 mg/kg, orally daily) or metformin (100 mg/kg, orally daily) that had a moderate antitumor effect (decrease the area under the kinetic curve of tumor growth by 1.6 and 1.5 times, respectively, when compared with intact control). Co-administration of these drugs in the same doses had a more pronounced effect (the area under the kinetic curve of tumor growth decreased by 3.1 times compared to intact control; p