e21543 Background: Immune checkpoint inhibitors are extensively employed in the management of various solid cancers. Our study aims to assess the potential prognostic and predictive value of peripheral blood biomarkers, as well as their correlation with adverse events. Methods: In this study, we conducted a retrospective analysis of data retrieved from a cohort of 23 patients diagnosed with metastatic melanoma at a single institution between 2015 and 2019. Specifically, we focused on analyzing the neutrophil-to-lymphocyte ratio (NLR), absolute neutrophil+monocyte-to-absolute lymphocyte+eosinophil ratio (ANMC/ALEC) from complete blood count (CBC), and lactate dehydrogenase (LDH) level from peripheral blood to assess their correlation with progression-free survival (PFS), overall survival (OS), and immune-related adverse events (irAEs). Results: Our findings revealed that NLR values less than 0.34 and ANMC/ALEC ratio less than 3.45 were associated with improved PFS and OS, as well as a lower incidence of immune-related adverse events, which supports our initial hypothesis. However, these results were not statistically significant due to the low power of our study. On the other hand, high NLR values were significantly associated with higher pre-treatment LDH levels (P-value = 0.037), which is a well-established adverse prognostic factor in melanoma. Additionally, high ANMC/ALEC ratios were also found to be associated with high LDH levels, although this relationship was not statistically significant (P-value = 0.133). Conclusions: The results of our study suggest that patients with lower pre-treatment NLR and ANMC/ALEC ratios have better PFS and OS, as well as lower incidence of irAEs. Conversely, those with high NLR and high ANMC/ALEC ratios exhibited lower PFS and OS, as well as higher incidence of irAEs. High NLR and ANMC/ALEC ratios were also found to be associated with higher pre-treatment LDH levels, which have previously been demonstrated to be an adverse prognostic factor in melanoma. [Table: see text]