O-Desmethyltramadol Enhanced Anti-Cancer Efficacy over Tramadol through Non-μ-Opioid Receptor and Differential Cellular Contexts of Human Breast Cancer Cells

Tramadol, a widely used analgesic, has recently been explored for its potential anti-cancer effects. However, the antitumor dosage of tramadol is over its current clinical application. Its primary metabolite, O-desmethyltramadol, has a higher affinity for the μ-opioid receptor and exhibits stronger pharmacological activity. Hence, we sought to examine whether the cytotoxic effect of O-desmethyltramadol was better than tramadol on breast cancer cells. Our results showed that O-desmethyltramadol significantly reduced cell viability in MDA-MB-231 and MCF-7 breast cancer cells, at least, ten-fold efficacy than tramadol. The presence of a μ-opioid receptor antagonist alvimopan did not alter these cytotoxic effects of tramadol and O-desmethyltramadol, indicating a non-opioid receptor-mediated mechanism. Compared with antitumor activity of tramadol mediated through ER stress, we confirmed that O-desmethyltramadol induced endoplasmic reticulum stress proteins, p-eIF2α/ eIF2α ratio, ATF4, and CHOP in western blot analysis and ATF4, DDIT3, and CHAC1 mRNAs in real-time polymerase chain reaction. Beyond the opioid receptor-mediated pathway, we further analyzed the differential functions of O-desmethyltramadol than tramadol using the RNA-seq analysis. The pathway enrichment analyses revealed that O-desmethyltramadol influenced immune and inflammatory pathways, such as TNF and IL-6/JAK/STAT3 signaling in MDA-MB-231 cells, while in MCF-7 cells, it affected metabolic and transcriptional pathways, including mTOR and MAPK signaling. Gene Set Enrichment Analysis further highlighted O-desmethyltramadol’s role in interferon response and tumor microenvironment modulation. Four upregulated genes and five downregulated genes were modulated by O-desmethyltramadol in MDA-MB-231 and MCF-7 cells. Overall, our findings indicated that O-desmethyltramadol exerted potent anti-cancer effects through multiple non-opioid mechanisms, with distinct response from tramadol depending on breast cancer subtype.