Abstract B091: A Phase 1b study of neoadjuvant-adjuvant pembrolizumab in resectable advanced basal cell carcinoma reveals robust T cell expansion, driving a memory-like subtype with promising therapeutic potential

Abstract Background: Despite advances in checkpoint inhibitors (CPI) and other systemic treatments for unresectable BCC, options for high-risk resectable disease remain limited, with no approved neoadjuvant or adjuvant therapies. Our group conducted a phase IB trial of neoadjuvant-adjuvant pembrolizumab (PEMBRO) in patients with resectable locally advanced BCC of the head and neck. Clinical outcomes were previously presented. We report here our work to assess immune cell function and spatial modulation of the tumor microenvironment among tumors of enrolled patients. Method: 13 patients enrolled from July 2020 to May 2023 and received at least one dose of PEMBRO. Our analysis included 11 patients who proceeded to surgery. We performed 33-plex immunofluorescence on tumor biopsies using the Akoya Phenocycler, followed by spatial analysis with QuPath. Immune modulation was assessed via 32-color spectral flow cytometry on peripheral blood pre- and post-CPI, with T cell analysis using FlowJo. Functional cytokine profiling of plasma was conducted with IsoPlexis. Data were analyzed in GraphPad Prism. Results: Three patients (27%) had a path complete response (CR) at time of surgery and were designated as responders. With 30 months of median follow-up there have not been a relapse. Spatial analysis of tumor biopsies identified 204,601 cells across 45 sections, averaging 4,547 cells per section, with a range from 236 to 14,246 cells. Compared to non-responder, responder have significant higher number of T cell infiltration, with increased cell counts in both CD4 and CD8 T cells. Notably, responders exhibit elevated cell count of CD45RO-expressing CD4 and CD8 T cells compared non-responders. Patients who underwent therapy showed significant increase in CD8 T cell percentage compared to pre-treatment. Flow cytometry analysis using population percentages and mean fluorescence intensity values showed variable CD4 levels across groups, with some patients exhibiting up to a 2-fold increase post-CPI, along with higher CD8 levels, consistent with spatial analysis. Cluster analyses showed increased CD4 central memory (TCM) and CD8 effector memory (TEM) T cells post-CPI, with naïve and exhausted T cells more prominent pre-CPI. The observation of those clusters in responders highlights their critical role in fostering a robust, long-lasting anti-tumor response and improving therapeutic outcomes. Additionally, post-CPI IFNg, IL-6, IL-17 and soluble CD137 levels were significantly elevated in responders, reflecting heightened immune activation and ultimately contributing to an effective therapeutic response. Conclusion: Our findings show that neoadjuvant-adjuvant PEMBRO boosts T cell expansion and preferentially promotes memory T cells in patients with locally advanced BCC. The increase in memory T cells post-CPI suggests a more durable immune response, correlating with improved recurrence-free survival in our trial cohort. These T cell changes provide insights into CPI response also in other immune-responsive tumor types. Citation Format: Yee Peng Phoon, Beatriz Coutinho de Oliveria, Grace Jones, James Isaacs, Jennifer Ko, Lucy Boyce Kennedy, Pauline Funchain, Thach-Giao Truong, Brian Gastman, Jan J Melenhorst. A Phase 1b study of neoadjuvant-adjuvant pembrolizumab in resectable advanced basal cell carcinoma reveals robust T cell expansion, driving a memory-like subtype with promising therapeutic potential [abstract]. In: Proceedings of the AACR IO Conference: Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2025 Feb 23-26; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2025;13(2 Suppl):Abstract nr B091.