Background
Excessive extracellular matrix (ECM) remodeling and a reactive stroma can affect T-cell infiltration and T-cell activity in the tumor and hereby influence response to immune checkpoint inhibitors (ICI). In the pursuit of finding biomarkers that predict treatment response, we evaluated the association between serum biomarkers of collagen and vimentin turnover and outcomes in metastatic melanoma patients treated with the anti-CTLA-4 antibody ipilimumab (IPI).

Methods
Type III collagen formation (PRO-C3), MMP-degraded type I, type III and type IV collagens (C1M, C3M and C4M), and citrullinated and MMP-degraded vimentin (VICM) were measured with ELISAs in serum from metastatic melanoma patients before (n = 66) and 3 weeks after (n = 52) initiation of IPI treatment. Biomarker levels were associated with Disease Control Rate (DCR) and survival outcomes.

Results
We found that baseline levels of PRO-C3 (p = 0.011), C1M (p = 0.003), C3M (p = 0.013) and C4M (p = 0.027) were significantly elevated in patients with progressive disease (PD). Univariate Cox regression analysis identified high PRO-C3 (p = 0.021) and C4M (p = 0.008) as predictors of poor overall survival (OS) and the biomarkers remained significant when evaluated with other covariates (PRO-C3 (p = 0.049) and C4M (p = 0.046)). Multivariate analysis identified VICM as a predictor of longer OS (p = 0.026). Similarly, a high C3M/PRO-C3 ratio predicted for increased OS (p = 0.034). Only C3M (p = 0.003) and VICM (p < 0.0001) increased 3 weeks after treatment.

Conclusions
ECM and tissue remodeling quantified in pre-treatment serum were associated with response and survival outcomes in metastatic melanoma patients treated with IPI. This highlights the importance of addressing the ECM and stromal component non-invasively in future ICI studies.