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Background: Surgery plays a significant role in the comprehensive treatment of breast cancer, and opioids are often the first-choice analgesics in the perioperative period. However, recent studies showed that opioids may enhance the angiogenesis of breast cancer and the recurrence and metastasis of tumor cells. Objectives: We aim to investigate the influence of opioids on recurrence and metastasis of breast cancer in nude mice. Methods: Forty female nude mice with breast tumor were randomly divided into 4 groups (n = 10). They were treated with (i) normal saline (10 mL/kg), (ii) morphine (10 mg/kg), (iii) morphine plus naloxone (10 + 4 mg/kg), and (iv) naloxone (4 mg/kg) for 2 weeks. Four groups of MDA-MB-231 cells were administered (i) Dulbecco’s Modified Eagle’s Medium, (ii) morphine (10 μmol/mL), (iii) morphine plus naloxone (10 + 10 μmol/mL), and (iv) naloxone (10 μmol/mL). The influence of morphine in each treated group was evaluated by immunocytochemistry and Western blotting. Results: Mice in the morphine group had higher rates of Ki67-positive cells, lower rates of apoptotic index, and a significant increase in the microvessels density of the tumor as evidenced by CD31 staining (p < 0.05). Furthermore, the MDA-MB-231 cells in the morphine group showed an increase in p-Akt, c-Myc, and thrombosponin-1 expression. Conclusion: In the current study, we found that morphine promotes the angiogenesis of the recurrent postoperative tumors of nude mice with breast cancer and the proliferation of tumor cells and such promotion may be related to the PI3K-c-Myc signaling pathway.

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