Improved prognosis and evidence of enhanced immunogenicity in tumor and circulation of high-risk melanoma patients with unknown primary

Background Melanoma of unknown primary (MUP) represents a poorly understood group of patients both clinically and immunologically. We investigated differences in prognosis and candidate immune biomarkers in patients with unknown compared with those with known primary melanoma enrolled in the E1609 adjuvant trial that tested ipilimumab at 3 and 10 mg/kg vs high-dose interferon-alfa (HDI). Patients and methods MUP status was defined as initial presentation with cutaneous, nodal or distant metastasis without a known primary. Relapse-free survival (RFS) and overall survival (OS) rates were estimated by the Kaplan-Meier method. Stratified (by stage) log-rank test was used to compare RFS and OS by primary tumor status. Gene expression profiling (GEP) was performed on the tumor biopsies of a subset of patients. Similarly, peripheral blood samples were tested for candidate soluble and cellular immune biomarkers. Results MUP cases represented 12.8% of the total population (N=1699) including 11.7% on the ipilimumab arms and 14.7% on the HDI arm. Stratifying by stage, RFS (p=0.001) and overall survival (OS) (p=0.009) showed outcomes significantly better for patients with unknown primary. The primary tumor status remained prognostically significant after adjusting for treatment and stage in multivariate Cox proportional hazards models. Including only ipilimumab-treated patients, RFS (p=0.005) and OS (p=0.023) were significantly better in favor of those with unknown primary. Among patients with GEP data (n=718; 102 MUP, 616 known), GEP identified pathways and genes related to autoimmunity, inflammation, immune cell infiltration and immune activation that were significantly enriched in the MUP tumors compared with known primaries. Further investigation into infiltrating immune cell types estimated significant enrichment with CD8 +and CD4+T cells, B cells and NK cells as well as significantly higher major histocompatibility complex (MHC)-I and MHC-II scores in MUP compared with known primary. Among patients tested for circulating biomarkers (n=321; 66 unknown and 255 known), patients with MUP had significantly higher circulating levels of IL-2R (p=0.04). Conclusion Patients with MUP and high-risk melanoma had significantly better prognosis and evidence of significantly enhanced immune activation within the TME and the circulation, supporting the designation of MUP as a distinct prognostic marker in patients with high-risk melanoma.