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Head and neck cancer (HNC) is the sixth leading cause of cancer in the world, and the third most common neoplasia in developing countries [1]. Patients with HNC may suffer significant pain, for which opioids are commonly given. However, recent reports suggest that opioids may promote cancer progression by action on the mu-opioid receptor (MOR) [2, 3]. The goal of this research was to test the hypothesis that the MOR antagonist methylnaltrexone (MTNX) can decrease tumorigenesis in HN squamous cell carcinoma (SCC). First, we confirmed via RNAseq and western blot analyses that FaDu cells (a HNSCC line) express MOR. Then, we performed cyclic adenosine monophosphate (cAMP) activation enzyme-linked immunosorbent assay to evaluate the activity of MOR after activation with [D-Ala2N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO), a selective MOR agonist. Short term (1hr) treatment with DAMGO decreased cAMP levels, suggesting full activity of the receptor. We conducted similar cAMP activity assays using MNTX. Our dose-dependent (10nM-1µM) experiments revealed that 1hr exposure of cells to MTNX inhibited cAMP activity, but not after 48 hrs of treatment, consistent with an inverse or partial agonist effect at low doses. To assess the impact of our findings on FaDu cell behaviors, we conducted in vitro proliferation, colony formation, and migration assays. MTNX at different concentrations (0.1nM, 1nM, 10nM, 100nM and 1µM) inhibited FaDu cell proliferative and metastatic cell behaviors (p 1. Siegel, R.L. et al, Cancer statistics, 2018. CA Cancer J Clin, 2018. 68(1): p. 7-30. 2. Lennon, F.E., et al, The mu-opioid receptor in cancer progression: is there a direct effect? Anesthesiology, 2012. 116(4): p. 940-5. 3. Patino, M.A., et al., The impact of intraoperative opioid use on survival after oral cancer surgery. Oral Oncol, 2017. 74: p. 1-7. Citation Format: Aysegul Gorur, Miguel Patino, Ted Shi, Hideaki Takahashi, Jeffrey N. Myers, Juan P. Cata. The mu-opioid receptor antagonist methylnaltrexone can decrease tumorigenesis in head and neck squamous cell carcinoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5027.