e14050 Background: The extracellular matrix (ECM) is a major component of tumors. Several recent findings link ECM composition with outcome in cancer patients treated with immune checkpoint inhibitors (ICIs). The aim here was to explore circulating ECM- and tissue-derived metabolites to enable clustering of patients with metastatic melanoma (MM) into putative endotypes and evaluate patient characteristics and outcome when treated with Ipilimumab accordingly. Methods: Serum was collected from MM patients prior to treatment with Ipilimumab (n = 64). Wards hierarchical clustering of patients was based on 15 ECM- and tissue-derived metabolites measured in serum by ELISA. Identified clusters (endotypes) were compared to clinical characteristics and evaluated for associations with disease control rate (DCR), progression free survival (PFS) and overall survival (OS). Results: Three putative endotypes (cluster A, B, C) were identified including 14, 30 and 20 patients, respectively. Overall a stepwise increase in ECM metabolite median levels were detected from C-B-A with the largest absolute change seen from B-A. There was no difference between A, B, and C according to age, gender, lactate dehydrogenase levels, number of metastasized organs and whether patients were treated previously. At follow-up, the DCR was 0%, 60% and 45% in A, B, and C, respectively. Likewise, patients with endotype A had a median PFS/OS time of 69/85 days versus 174/520 and 165/589 days for endotype B and C. In support, endotype A predicted for poor survival outcomes (PFS_AvsB+C:HR = 3.9, 95%CI:2.0-7.6, p = 0.0001; OS_AvsB+C:HR:2.5, 95%CI:1.2-4.9, p = 0.0108). Conclusions: Hierarchical clustering of MM patients based on 15 ECM- and tissue-derived metabolites measured in a liquid biopsy identifies 3 putative endotypes. One endotype (A) seems to reflect patients with an overall high and differentiated ECM turnover profile. These patients experience poor outcome when treated with Ipilimumab. If validated, this supports a link between ECM remodeling and outcome in cancer patients treated with ICIs.