S100A9 and melanoma: Furthermore, phenformin, an antidiabetic biguanide class drug, inhibited MDSC of significantly decreased proportion of PMN-MDSCs and reduction of ARG1, S100A8 and S100A9 enhancing the efficacy of PD-1 blockade, which is reflected in more infiltration of CD8+ T cell in the BRAFV600E/null melanoma mouse model (140).