Abstract
BACKGROUND: Naloxone is viewed as a specific competitive opioid antagonist acting at the level of opioid receptors (μ, δ, and κ) with blended agonist-adversary or agonist action. The role of naloxone in tumor cell growth has been poorly studied in human cancer cell lines. MATERIALS AND METHODS: In the present study, we report findings from in vitro and in vivo experiments performed to evaluate the effects of naloxone on human breast cancer cell growth and progression. In vitro assays were conducted on estrogen receptor-negative human breast carcinoma cells, MDA.MB231, treated with naloxone at different concentrations (10-100 μM). In vivo experiments were performed on a mouse model of human triple-negative breast cancer generated by using MDA.MB231 injected subcutaneously in mice. Naloxone was daily intraperitoneally injected in mice at 0.357 mg/kg for 2 weeks and at 0.714 mg/kg for the next 2 weeks. Microvessels formation was detected by fluorescein isothiocyanate-dextran (100 μL) injected into the tail vein of mice and confirmed by immunohistochemistry with CD31 on mice tumor sections. RESULTS: In vitro tests showed that the cell proliferation of MDA.MB231 was inhibited by naloxone in a dose-dependent manner, whereas the cell death was increased. In vivo studies demonstrated that tumors of mice treated with naloxone were significantly smaller than those observed in the control groups, as long as naloxone was administered. Finally, naloxone was not able to impair the microvessel formation in tumors of treated mice. CONCLUSION: Our data showed, for the first time, that naloxone reduced breast cancer progression without affecting angiogenesis.