Abstract Background: Gamma9 Delta2 (γ9δ2) T cells function at the crossroad of innate and adaptive immunity with important roles in immune responses against pathogens and carcinogenesis, making them an attractive target for cancer immunotherapy. Butyrophilin (BTN) 3A (CD277) is an immune checkpoint molecule expressed on tumors and cells of the immune system that is required for γ9δ2 T cell activation. These observations led to the design and development of ICT01, a humanized, monoclonal antibody that binds the extracellular domain of all 3 isoforms of BTN3A1/A2/A3 and induces pAg-independent γ9δ2 T cell activation and killing of multiple tumors in nonclinical settings. Methods: EVICTION (www.clinicaltrials.gov NCT04243499; EudraCT Number: 2019-003847-31) is a first-in-human, open-label clinical study to assess the safety, tolerability, and activity of a range of IV doses of ICT01 as monotherapy and in combination with pembrolizumab, in patients with advanced-stage, relapsed/refractory cancer. Following Regulatory and Ethics Committee approvals, the study is being conducted at cancer centers in France, Belgium, Spain, Germany, the UK and USA. Following signed informed consent, patients received ICT01 every 3 weeks with blood samples collected at multiple timepoints for immunophenotyping by flow cytometry and cytokine analysis (IFNγ, TNFα, IL-1β, IL-2, IL-4, IL-6, IL-8, IL-13). Tumor biopsies are collected at baseline and on Day 28 for immunohistochemistry (e.g., BTN3A, γ9δ2 T cells) and expression profiling of cancer- and immunity-related genes. Results: Solid tumor (ST) dose cohorts 1-4 (total n=20) were enrolled and treated with ICT01 doses ranging from 20 μg to 20 mg, and dose cohort 1 of hematologic cancer patients (n=3, 200 μg) has been enrolled. ICT01 was well-tolerated, with no dose-limiting toxicities or related SAEs reported. Target occupancy on T cells at 30 min or 4 hours post first dose ranged from 34% (700 μg, n=2), 79% (2 mg, n=5), 93% (7 mg, n=4) and 100% (20 mg, n=1), with activated γ9δ2 T cells (CD69+) migrating from the blood within 30 min of dosing that led to a > 95% decrease from baseline in the number of circulating γ9δ2 T cells for all 4 doses by 24hrs. The 2nd dose of ICT01 induced a similar activation & migration of γ9δ2 T cells from the circulation. There were no effects on CD4/CD8 T cells, NK, or B cells. Available cytokine data from ST cohorts 1 & 2 showed no CRS although transient 2-3x increases in IFNγ, TNFα, & IL-2 were observed 4 hours post dose in patients with higher baseline γ9δ2 T cell counts. A 5-10x increase of IFNγ was observed on Days 7 & 28 in ST cohort 2, consistent with γ9δ2 T cell activation. Digital pathology analysis of tumor biopsies of a cohort 1 melanoma pt demonstrated a 25x and 8x increase of CD3-TCRgamma delta++ and CD3+ cell densities, respectively, as compared to baseline. Analyses of additional cohorts are ongoing and will be presented. Conclusion: The preliminary results demonstrate that ICT01 safely and potently activates the anti-tumor immune response of γ9δ2 T cells mediated by BTN3A. Citation Format: Aurelien Marabelle, Christiane Jungels, Johann De Bono, Norbert Vey, Martin Wermke, Elena Garralda, Steven Le Gouill, Patricia LoRusso, Aude De Gassart, Emmanuel Valentin, Patrick Brune, Marina Iche, Daniel Olive, Paul A. Frohna. Activation of the antitumor immune response of gamma9delta2 T cells in patients with solid or hematologic malignancies with ICT01, a first-in-class, monoclonal antibody targeting butyrophilin 3A: The EVICTION study [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr CT034.