Interrogating the Tumor Microenvironment (TME) in Patients (pts) with Head and Neck Mucosal Squamous Cell Carcinoma (hnmscc) Treated with Programmed Death-1 (PD-1) Inhibitors.

6048 Background: Survival in recurrent/metastatic HNmSCC remain poor. PD-1 inhibitors have become standard of care, demonstrating improved overall survival and toxicity when compared to chemotherapy and targeted therapy. Biomarkers such as PD-Ligand(L)1 combined proportion score (CPS) remain rudimentary, with CPS >20 showing a response rate of only 23% to pembrolizumab (KEYNOTE-048). We used high-dimensional imaging mass cytometry (IMC) to explore predictive biomarkers in HNmSCC pts receiving PD-1 inhibitor-based therapy. Methods: We retrospectively analysed 27 formalin-fixed paraffin embedded tissue samples from 24 pts prior to receiving PD-1 inhibitor-based therapy between May 2016 – April 2021. Clinicopathological characteristics including PD-L1, p16 status, prior treatment and survival data were collected. Pts were classified into responders (RES, Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 complete response (CR) or partial response (PR), stable disease (SD) >6 months (mths)) and non-responders (non-RES, RECIST SD <6 mths or disease progression (PD)). An antibody panel (n = 40) was created to interrogate specific components of interest within the TME and was analysed by IMC using the Hyperion TM Imaging System. Results: Of the 24 patients, 16/24 were male and median age 57.6 year. 8 pts were RES (RECIST CR, n = 1; PR, n = 3; SD > 6 mths, n = 4) and 14 pts were non-RES (RECIST SD < 6 mths, n = 1; PD, n = 11; clinical PD, n = 4). Four patients underwent rapid clinical disease progression prior to progress imaging and were categorised as non-RES. At time of data cut off on January 2024, and 23/24 pts had progressed on treatment. The cellular landscape within the TME was similar, irrespective of the location of the primary and p16 status. However, distinct immune profiles were observed between RES vs non-RES: RES showed higher infiltrates of CD4+ T cells, B cells, PD-1+ CD8+ T cells (P < 0.05) and both central memory and effector memory T cell subsets (p < 0.01). In contrast, non-RES showed high frequencies of CD44+ NK cells. Key cell interactions within the TME identified proliferating malignant squamous cells closely interacting with CD8+ T cells, CD4+ Tregs and endothelial cell in RES but not interacting in non-RES. Further spatial regional analysis identified a distinct tissue architecture with hallmarks of Tertiary Lymphoid-like Structures (TLS), present in higher proportions in RES. RES pts with TLS proportions >20% (n = 3) had a progression free survival of 80.3 mths, 26.8 mths and NE (unrelated death at 15.6 mths). Conclusions: The findings of this study identify mechanisms of PD-1 inhibitor response and resistance in HNmSCC pts, providing a unique opportunity to guide combination strategies and improve outcome.