Abstract PS4-07-15: A single preoperative pembrolizumab dose plus a single subablative radiotherapy fraction (7 Gy) elicits anti-tumor immune response and increases stromal tumor infiltrating lymphocytes in triple negative breast cancer: a phase 1b/2 study

Abstract Background Neoadjuvant chemotherapy combined with immune checkpoint blockade (ICB) improves outcomes in high-risk TNBC, but this regimen is not approved and may be overtreatment for cT1N0 TNBC. Preclinical studies show that low dose radiotherapy (RT) can enhance antitumor immunity. In this study, we evaluated whether a neoadjuvant chemotherapy-free regimen of a single pembrolizumab (pembro) dose + a single subablative RT fraction (7Gy) may elicit antitumor immune response and increase stromal tumor infiltrating lymphocytes (sTILs) in TNBC to high levels, defined here as 50%, which is a biomarker known to correlate with excellent outcomes without chemotherapy. Methods In this open-label phase 1b/2 trial (NCT04454528), participants with histologically confirmed early stage invasive breast cancer planned for standard of care (SOC) upfront surgery were enrolled. Each participant received a single preop dose of pembro either alone or in combination with RT given before or after pembro. The co-primary endpoints were demonstration of 1) feasibility of completing RT + pembro regimen within 21 days without delaying planned surgery 14 days and 2) immune activation, defined as a) increase in % sTILs, and, b) increase in % proliferating (Ki67+) circulating T cells. Secondary objectives included assessment of tumor response by change in size from baseline. Responders are defined as those with tumor size reduction 30% (TΔ30). Study sample size was powered for TΔ30. Assuming 40% of treated subjects achieve TΔ30 versus 0% in historical controls, 15 subjects yield 80% power at α=0.05. Exploratory analyses included immune profiling of peripheral blood mononuclear cells (PBMC) by flow cytometry for markers of T-cell activation, proliferation, differentiation and exhaustion, T-cell receptor repertoire (TCR) dynamics, and digital spatial profiling of the tumor immune microenvironment pre and post treatment. All participants received SOC adjuvant therapy after surgery. Findings Between January 6, 2021, and February 18, 2025, 30 participants (12 with HR+HER2-, 17 TNBC and one HER2+) enrolled and completed the preop regimen. All participants proceeded to surgery without delay. Median follow-up was 29 months (IQR 14 - 45). There were no adverse events (AEs) ≥ grade 2 which included one grade 2 immune-related AE (hypothyroidism). TΔ30 rate was significantly higher in RT + pembro arms (14 of 25, 56%, including 3 with pCR) compared to pembro only arm (0 of 5, 0%), p = 0.024. Post-treatment, sTILs shifted rightward from 20% (IQR 5 - 50 ) to 50% (IQR 10 - 60) overall and from 40% (IQR 20 - 55) to 60% (IQR 50 - 70) in TNBC. The proportion of TNBC with high sTILs (50%) increased from 3 of 11 (27%) to 7 of 13 (54%) post treatment. Immune profiling of PBMC from responders demonstrated increased proportions of early-memory CD4+ and CD8+ T cells at baseline, marked expansion of activated, Ki67+ CD4+ and CD8+ T cells post treatment, and a higher and more diverse TCR repertoire both pre- and post-treatment. Spatial transcriptomic analyses demonstrated that tumor regions directly interacting with T-cells in responders were enriched for gene signatures of cGAS-STING-pathway activation, antigen processing and presentation, and response to ICB. Interpretation In this small single institution study, we demonstrate that a single pembro dose + a single sub-ablative RT fraction can elicit anti-tumor immune response and increase sTILs from low to high levels in TNBC. As only ∼20% TNBC have high sTILs, this well tolerated neoadjuvant chemotherapy-free regimen may be evaluated in future studies as an immune induction strategy to expand the number of patients eligible to enroll in adjuvant chemotherapy omission trials such as the OPTImaL study. Citation Format: J. Tchou, A. Nayak, H. N. Xu, J. Kollmar, E. H. Smith, M. Z. Mazur, L. Keele, A. Clark, N. Taunk, J. Fraietta, on behalf of the BreastVax Study Team. A single preoperative pembrolizumab dose plus a single subablative radiotherapy fraction (7 Gy) elicits anti-tumor immune response and increases stromal tumor infiltrating lymphocytes in triple negative breast cancer: a phase 1b/2 study abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-07-15.