Abstract CT507: ATP128 vaccine with ezabenlimab promotes antigen-specific immune responses in stage IV colorectal cancer in the KISIMA-01 phase 1b trial

Abstract Background: KISIMA࣪ is a vaccine platform based on a single chimeric fusion protein, containing a proprietary cell-penetrating peptide (CPP) for antigen delivery, a proprietary Toll-like receptor (TLR)-peptide agonist with self-adjuvant properties and a modulable multi-antigenic domain (Mad). ATP128 vaccine targets 3 antigens: carcinoembryonic antigen (CEA), Survivin and Achaete-scute complex homolog 2 (ASCL2); it is used in combination with a PD-1 inhibitor in the treatment of MSS/MMR proficient stage IV colorectal cancer (CRC) patients, after first line of standard of care therapy or as perioperative administration in patients with resectable liver metastases. Method: KISIMA-01 (NCT04046445) is an open-label, multi-center Phase 1b trial to investigate the safety, tolerability and immunogenicity of AT128 alone or in combination with the anti-PD-1 antibody ezabenlimab in patients with stage IV CRC. ATP128 is given SC q2w for the first 3 immunizations (prime) and q4w for the last 3 immunizations (boost). Ezabenlimab is administered q3w starting with the first ATP128 administration. Blood and tissue samples are collected before, during and after ATP128 treatment to monitor the induction of a tumor associated antigen-specific immune response (ELISpot) and immune-related changes in the peripheral blood and in the tumor microenvironment by immunohistochemistry (IHC) and flow cytometry. Results: In more than 50% of evaluated patients treated with ATP128 alone or with ezabenlimab, a cellular immune response against at least one out of three antigens was observed as determined by IFN-γ ELISpot analyses of patient PBMCs after the 3rd vaccination. Analysis of liver metastases by IHC indicated that evaluated patients were positive for all 3 antigens in ATP128. Furthermore, for the patients with paired biopsies, a significant increase in CD8 T cells infiltration into the tumor parenchyma was observed after 3 vaccine administrations, along with a significantly higher proportion of CD45RO expressing memory cells within the CD4 population as compared to baseline. Tumor infiltrating lymphocytes (TILs) flow cytometry analysis comparing untreated (historical controls) and ATP128/ezabenlimab-treated patients showed a similar quantity of the different infiltrated subsets but an improved quality of infiltrated T cells, indicated by an increase (more than 2-fold) in proportion of central memory T cells and an impressive decrease of the proportion of cells positive for exhaustion markers expression in KISIMA-01 patients. Conclusions: Analyses indicate induction of ATP128-specific immune response in the peripheral circulation and increased infiltration of TILs into liver metastases with an improved quality of T cells. Citation Format: Heinz-Josef Lenz, Hanz Prenen, Eric Van Cutsem, Thibaud Kössler, Jean-François Mayol, Francesca Trapani, Matthieu Tihy, Laura Rubbia-Brandt, Christian Toso, Thomas Bogenrieder, Elodie Belnoue, Madiha Derouazi, Scott Kopetz. ATP128 vaccine with ezabenlimab promotes antigen-specific immune responses in stage IV colorectal cancer in the KISIMA-01 phase 1b trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr CT507.