Abstract 3210: Neoadjuvant-adjuvant pembrolizumab (PEMBRO) in advanced basal cell carcinoma (BCC) induces memory-like T cell expansion with therapeutic promise

Background: BCC is a prevalent form of skin cancer, and immunotherapy has been explored as a treatment option. Advancements in checkpoint inhibitors (CPI) and other systemic therapies have improved outcomes for unresectable BCC. However, alternatives for high-risk resectable disease remain scarce, with no approved neoadjuvant or adjuvant therapies. Our team conducted a phase IB trial evaluating PEMBRO in patients with resectable locally advanced BCC of the head and neck, with clinical outcomes previously reported. Here, we present our findings on immune cell function and the spatial tumor microenvironment dynamics in the enrolled patients. Method: 13 patients were enrolled from July 2020 to May 2023, with analysis focusing on 11 who received surgery after at least one dose of PEMBRO. Immune modulation was assessed using 32-color flow cytometry on peripheral blood pre- and post-CPI, T cell analysis in FlowJo, and 33-plex spatial multiplexed immunofluorescence (mIF) analysis in QuPath for tumor biopsies. Cytokine profiling was conducted with IsoPlexis, and data were analyzed in GraphPad Prism. Results: In this trial, 3 patients (27%) achieved a pathologic complete response at surgery and were classified as responders. With a median follow-up of 30 months, none of the responders experienced a relapse. Flow cytometry analysis showed variation in CD4 levels across the groups. In some patients, CD4 levels increased by up to 2-fold post-CPI, accompanied by higher CD8 levels. Cluster analysis revealed an increase in CD4 central memory (TCM) and CD8 effector memory (TEM) T cells post-CPI, while naïve and exhausted T cells were more prominent pre-CPI. These clusters, particularly in responders, underscore the importance of a sustained anti-tumor immune response for better therapeutic outcomes. Spatial mIF of tumor biopsies identified 204,601 cells across 45 sections, averaging 4,547 cells per section. Responders showed a significantly higher level of T cell infiltration than non-responders, with increased CD4 and CD8 T cell counts. Notably, responders had elevated CD45RO-expressing CD4 and CD8 T cell counts compared to non-responders. Patients showed a marked increase in CD8 T cell percentage post-CPI. In responders, post-CPI plasma levels of IL-6, IL-17, and IFNγ were significantly elevated, indicating increased immune activation that likely enhanced treatment effectiveness. Conclusion: Our study demonstrates that neoadjuvant-adjuvant PEMBRO stimulates T cell expansion and selectively enhances memory T cells in locally advanced BCC. The rise in memory T cells after CPI therapy indicates a more sustained immune response, which is associated with better recurrence-free survival in our trial cohort. These T cell modifications offer valuable insights into the potential effectiveness of CPI in other immune-sensitive tumor types. Citation Format: Yee Peng Phoon, Beatriz Coutinho de Oliveira, Grace Jones, James M. Isaacs, Jennifer Ko, Lucy B. Kennedy, Pauline Funchain, Thach-Giao Truong, Brian Gastman, Jan J. Melenhorst. Neoadjuvant-adjuvant pembrolizumab (PEMBRO) in advanced basal cell carcinoma (BCC) induces memory-like T cell expansion with therapeutic promise [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3210.