Rationale: Metastatic melanoma is the most aggressive and dangerous form of skin cancer. The introduction of immunotherapy with Immune checkpoint Inhibitors (ICI) and of targeted therapy with BRAF and MEK inhibitors for BRAF mutated melanoma, has greatly improved the clinical outcome of these patients. Nevertheless, response to therapy remains highly variable and the development of drug resistance continues to be a daunting challenge. Within this context there is a need to develop diagnostic tools capable of predicting response or resistance to therapy in order to select the best therapeutic approach. Over the years, accumulating evidence brought to light the role of microRNAs (miRNAs) as disease biomarkers. Methods: In particular, the detection of miRNAs in whole blood or specific blood components such as serum or plasma, allows these molecules to be good candidates for diagnosis, prognosis and for monitoring response to anticancer therapy. In this paper, we evaluated circulating basal levels of 6 previously identified miRNAs in serum samples of 70 BRAF-mutant melanoma patients before starting targeted therapy. Results: Results show that the circulating levels of the oncosuppressor miR-579-3p and of the oncomiR miR-4488 are able to predict progression free survival (PFS) but not overall survival (OS). Most importantly, we observed that the best predictor of disease outcome is represented by the ratio of circulating miR-4488 vs. miR-579-3p (miRatio). Finally, the combination of the Lactate dehydrogenase (LDH) blood levels with the two circulating miRNAs alone or together did not produce any improvement in predicting PFS indicating that miR-579-3p and miR-4488 are independent predictors of PFS as compared to LDH. Conclusions: All together these data underscored the relevance of circulating miRNAs as suitable tools to predict therapy response in melanoma and maybe further developed as companion diagnostics in the clinic.

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