Simple SummaryCannabinoids are mainly used for recreational purposes but find their way into oncology due to ongoing legalization efforts and anti-cancerous hints in the scientific literature. The goal of this study was to elucidate the mode of action of a clinically used cannabis medication in metastatic melanoma as well as its clinical value in combination with targeted therapy. By cell viability and apoptosis assays, we could demonstrate that cannabinoids mediate their apoptotic effect in a caspase-mediated fashion by disturbing mitochondrial integrity. With in vivo experiments, we could demonstrate that clinically used cannabinoid medication does not interfere with the commonly used anti-cancerous drug trametinib. Our results suggest that cannabinoids are effective in metastatic melanoma and pave the way for further clinical trials.AbstractBackground:Cannabinoids are mainly used for recreational purposes, but also made their way into oncology, since these substances can be taken to increase appetite in tumour cachexia. Since there are some hints in the literature that cannabinoids might have some anti-cancerous effects, the aim of this study was to study if and how cannabinoids mediate pro-apoptotic effects in metastatic melanoma in vivo and in vitro and its value besides conventional targeted therapy in vivo.Methods:Several melanoma cell lines were treated with different concentrations of cannabinoids, and anti-cancerous efficacy was assessed by proliferation and apoptosis assays. Subsequent pathway analysis was performed using apoptosis, proliferation, flow cytometry and confocal microscopy data. The efficacy of cannabinoids in combination with trametinib was studied in NSG mice in vivo.Results:Cannabinoids reduced cell viability in multiple melanoma cell lines in a dose-dependent way. The effect was mediated by CB1, TRPV1 and PPARα receptors, whereby pharmacological blockade of all three receptors protected from cannabinoid-induced apoptosis. Cannabinoids initiated apoptosis by mitochondrial cytochrome c release with consecutive activation of different caspases. Essentially, cannabinoids significantly decreased tumour growth in vivo and were as potent as the MEK inhibitor trametinib.Conclusions:We could demonstrate that cannabinoids reduce cell viability in several melanoma cell lines, initiate apoptosis via the intrinsic apoptotic pathway by cytochrome c release and caspase activation and do not interfere with commonly used targeted therapy.