Background: Most patients with R/M SCCHN do not derive durable benefit from standard-of-care anti-PD-1-based therapy. The currently utilized biomarker, PD-ligand-1 combined proportion score is limited by the availability and heterogeneity of tumor samples. There is an unmet need to develop easily accessible peripheral blood-based biomarkers for response to immunotherapy. Methods: We are conducting a prospective multi-center study to identify peripheral blood derived DNA-based methylation biomarkers measured in patients with R/M SCCHN on treatment with FDA-approved anti-PD-1 based therapy. DNA isolated from these samples undergo bisulfite conversion, methylation profiling using Illumina EPIC microarray, and cellular deconvolution to generate peripheral blood immune profiles (PMID 35140201), including granulocytic (g) and monocytic (m) myeloid derived suppressor cell (MDSC) scores. Associations between baseline peripheral blood immune profiles before the start of treatment with progression-free survival (PFS) and overall survival (OS) were evaluated using Kaplan Meier curves and Cox proportional hazards models adjusted for age and sex. Additionally, logistic regression models adjusted for age and sex were utilized to investigate the baseline immune profiles between subgroups of patients who received anti-PD-1 monotherapy with durable clinical benefit for at least 1 year (Group A) versus progression or death within 100 days (Group B) from start of therapy. Results: 61 patients with R/M SCCHN who received anti-PD-1 based therapy as of Oct 31, 2022, were eligible for this preliminary analysis. Mean (SD) age of the entire population was 66.4 (11.8) years, 82% were male. gMDSC score was the only immune parameter with a statistically significant association with PFS (HR 2.6 (1.40 – 4.87) independent of age and sex, with higher scores stratified by median value associated with shorter PFS (Median PFS: 75 days vs. 145 days). Memory CD4 T cells, total CD4 T cells, percentage of T regulatory /CD4 cells, and absolute memory B cells were significantly associated with OS in this population, independent of age and sex (Table). Among the subgroup treated with anti-PD-1 monotherapy, Group A with durable benefit (n = 7, 86% male, mean age: 64.7 years) had significantly higher proportion of CD4 naïve/total CD4 cells (p = 0.03), and significantly lower gMDSC (p = 0.015) & mMDSC (p = 0.000049) scores, compared with Group B (n = 27, 78% male, mean age: 66.4 years). Conclusions: Peripheral blood DNA-based epigenetic immune profiling can recognize clinically relevant methylation biomarkers of benefit from anti-PD-1 therapy before the start of treatment.