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Cancer is a significant public health problem worldwide. While there has been a steady decrease in the cancer death rate over the last two decades, the number of survivors has increased and, thus, cancer‐related sequela. Pain affects the life of patients with cancer and survivors. Prescription opioids continue as the analgesic of choice to treat moderate‐to‐severe cancer‐related pain. There has been controversy on whether opioids impact cancer progression by acting on cancer cells or the tumor microenvironment. The μ‐opioid receptor is the site of action of prescription opioids. This receptor can participate in an important mechanism of cancer spread, such as perineural invasion. In this review, current evidence on the role of the μ‐opioid receptor in cancer growth is summarized and preliminary evidence about its effect on the cross‐talk between sensory neurons and malignant cells is provided. [ABSTRACT FROM AUTHOR]

The Mu‐opioid receptor (MOR) has been implicated in tumorigenesis and metastasis. Methylnaltrexone (MNTX), an antagonist of MOR, has shown to inhibit tumor growth and metastasis in lung cancer cell lines. The effect of MNTX on other cell lines such as head and neck squamous cell carcinoma (HNSCC) has not been investigated. We measured the expression and activity of the receptor in different HNSCC cell lines. Then, we evaluated the impact of modulating the expression MOR and the effect of MNTX on the proliferation, clonogenic activity, invasion, and migration of two HNSCC (FaDu and MDA686Tu) cell lines expressing MOR and one cell line (UMSCC47) not expressing the receptor. We also evaluated the impact of MNTX on tumor growth and metastasis formation in vivo. Activation of the receptor with [d‐Ala2,N‐Me‐Phe4, Gly5‐ol] (DAMGO) caused a significant reduction in cyclic adenosine monophosphate levels in FaDu cells. Knockdown of MOR inhibited in vitro aggressive cell behaviors on FaDu and MDA686Tu cells and correlated with a reduction in markers of epithelial–mesenchymal transition. In vitro studies showed that MNTX strongly inhibited the proliferation, clonogenic activity, invasion, and migration of FaDu and MDA686Tu cells but has no effect on UMSCC47 cells. In vivo experiments demonstrated that MNTX suppresses tumor growth in HNSCC cell tumor‐bearing mice. Our studies indicate that MOR could be considered as a therapeutic target to treat HNSCC. [ABSTRACT FROM AUTHOR]