BackgroundMammary cancer is a common disease affecting female dogs, where approximately 50% of the cases are malignant. There is a subpopulation of cancer cells with stem cell-like features within the tumour microenvironment, which can form in vitro spheres, cell structures that grow in anchor-free conditions. This cell population shows resistance to conventional antitumor treatments explaining in part the recurrence of some type of cancers. It has been previously reported that spheres derived from CF41.Mg canine mammary carcinoma cells exhibit several stemness features. Melatonin has shown antitumor effects on cancer mammary cells; nevertheless, its effects have been poorly evaluated on canine mammary cancer stem-like cells. In this regard, it has described that melatonin decreases the expression of OCT-4 in CMT-U2229 mammary cancer cells, a transcription factor that participates in the modulation of self-renewal and drug resistance in cancer stem-like cells. The aim of this study was to compare the effects of melatonin on viability and migration of canine mammary carcinoma CF41.Mg-spheres, and CF41.Mg-parental cells. CF41.Mg cells were grown in DMEM high-glucose medium containing 10% bovine foetal serum. CF41.Mg-spheres were cultured in ultra-low attachment plates with serum-free DMEM/F12 containing several growth factors. Cell viability (MTS reduction) and migration (transwell) assays were conducted in presence of melatonin (0.01, 0.1 or 1 mM).ResultsMelatonin decreased cell viability at 1 mM (P < 0.05), with a significant reduction in spheres compared to parental cells at 24 and 48 h (P < 0.05). Cell migration was inhibited in response to non-cytotoxic concentration of melatonin (0.1 mM) (P < 0.05) in spheres and monolayer of cells, no significant differences were detected between both cell subtypes.ConclusionsThese results indicate that melatonin reduces viability and migration of CF41.Mg cells, where spheres exhibit greater sensitivity to the hormone. Thus, melatonin represents a valuable potential agent against mammary cancer cells, especially cancer stem-like cells.