Background We recently reported data supporting the unknown primary status as a potentially distinct prognostic group among high-risk melanoma patients treated with ipilimumab and high dose interferon-alfa (HDI) in the ECOG-ACRIN E1609 trial (N=1670) with improved RFS and OS outcomes compared to known primary. Therefore, we investigated differences in candidate immune biomarkers in the circulation and tumor microenvironment (TME) of patients with unknown compared to those with known primary melanoma enrolled in this trial that tested adjuvant ipilimumab at 3 and 10 mg/kg versus HDI.

Methods Gene expression profiling (GEP) was performed on the tumor biopsies of 718 (102 unknown, 616 known primary) melanoma patients. The primary endpoint was mRNA expression profiling using U133A 2.0 Affymetrix gene chips. Raw microarray data sets were normalized by using the Robust Multi-array Average (RMA) method using Affymetrix Power Tools (APT) as previously published. Multiple probe sets representing the same genes were collapsed by using the probe with maximum gene expression. Gene set enrichment analysis (GSEA) was performed by comparing the unknown and known primary tumor samples, and gene sets with FDR q-value <0.1 were deemed as significant. Similarly, peripheral blood (serum and PBMC) samples were tested for soluble (Luminex) and cellular (multicolor flow cytometry) immune biomarkers in a subset of patients (N=321; 66 unknown and 255 known primary). All patients provided an IRB-approved written informed consent.

Results Unknown primary melanoma cases represented 12.8% of the total E1609 study population (N=1670) including 11.7% on the ipilimumab arms and 14.7% on the HDI arm. Stratifying by stage, relapse free survival (RFS) (P=0.001) and overall survival (OS) (P=0.009) were significantly better for patients with unknown primary tumor compared to known primary. Including only ipilimumab-treated patients, RFS (P=0.005) and OS (P=0.023) were significantly better in favor of the unknown primary status. Among the cohort of patients with tumor GEP data (N=718), GEP identified pathways and genes related to autoimmunity, inflammation, immune cell infiltration and immune activation that were significantly enriched in the unknown primary tumors compared to known primaries (table 1). Among the subset of patients tested for circulating biomarkers, patients with unknown primary melanoma had significantly higher circulating levels of IL-2R than those with known primary (P=0.04).

Conclusions Unknown primary high-risk melanoma patients had significantly better prognosis and evidence of significantly enhanced immune activation within the TME and the circulation, supporting the designation of unknown primary melanoma as a distinct prognostic marker in patients with high-risk melanoma.

Acknowledgements We are grateful to the patients and family members who participated in the E1609 trial and the E1609 trial investigators. This study was conducted by the ECOG-ACRIN Cancer Research Group (Peter J. O’Dwyer, MD and Mitchell D. Schnall, MD, PhD, Group Co-Chairs) and supported by the National Cancer Institute of the National Institutes of Health under the following award numbers: U10CA180794, U10CA180820, U10CA180888, UG1CA233180, UG1CA233184. Biomarkers studies were supported under the following award number: P50CA12197310 (Tarhini and Kirkwood). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

Trial Registration NCT01274338

Ethics Approval The E1609 study protocol was approved by the institutional review board of each participating institution and conducted in accordance with Good Clinical Practice guidelines as defined by the International Conference on Harmonization. All patients provided an IRB-approved written informed consent.

Consent Not applicable.