The effect of melatonin treatment regimen on mammary adenocarcinoma development in HER‐2/neu transgenic mice

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Abstract
The effect of various regimens of treatment with melatonin on the development of mammary tumors in HER2/neu transgenic mice was investigated. Female HER‐2/neu mice starting from the age of 2 months were kept under standard light/dark regimen and as given melatonin with tap water (20 mg/l) during the night time 5 times monthly (interrupted treatments) or constantly to natural death. Intact mice served as controls. Treatment with melatonin slowed down age‐related disturbances in estrous function most in the group exposed to interrupted treatment with the hormone. Constant treatment with melatonin decreased incidence and size of mammary adenocarcinomas, and incidence of lung metastases, compared to controls. The number of mice bearing 4 and more tumors was reduced in the group with constant melatonin treatment. Interrupted treatment with melatonin promote mammary carcinogenesis in HER‐2/neu transgenic mice. The data demonstrate the regimen‐dependent inhibitory effect of melatonin on the development of spontaneous mammary tumors in HER‐2/neu mice but not on overall survival with implication about the likely cause of the effect. Polycystic kidney disease is common in this transgenic line. Adverse effect of melatonin on the life span in our study may be unique to the transgenic model used and may not be relevant to the suppressive effect of melatonin in delay of mammary cancer.