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Background Several CAR-T therapies have been approved for hematologic malignancies. However despite starting hundreds of clinical trials in solid tumors, no CAR-T therapy has been successful against solid tumors. 1 Efficacy in solid tumors is hampered by various factors such as antigen variation, lack of infiltration of T-cells in tumors from blood vessels, and an immune suppressive tumor micro-environment (TME).2 These factors lack in current in vitro and preclinical animal models. Therefore CAR-T cells are not evaluated on their efficacy with these critical parameters in place, which makes it impossible to select promising candidates for the clinic.Methods This study aims to reduce the in vitro-in vivo gap by introducing these parameters in our vascularized tumor models. In earlier work we evaluated T-cell infiltration from a vessel towards chemotactic gradients and tumor cells3. In our organ-chip platform we now have created a model which contains a blood vessel, through which CAR-T cells can be perfused, and an adjacent co-culture of cancer tissue. The adjacent co-culture can consist of cancer cell lines, cancer cells and stroma, or primary tissue containing cancer cells and stroma to recapitulate the TME.Results With an EPCAM targeting second generation CAR-T, extravasation and migration into the tumor compartment could be quantified. Furthermore, after 72h the CAR-T cells reduced tumor volume in the EPCAM-positive HT29 colorectal cancer cell line, while volume of the EPCAM-negative A375 melanoma cell line grew. There was a clear dose response. The vasculature was shown to be intact at most concentrations of CAR-T cells, while being destroyed at highest concentrations. Ultimately there was a ‘therapeutic window’ in which the CAR-T cells reduced tumor mass but left a viable vessel.With the model the effects of modifications to the CAR-T cells or stimulatory cytokines could be evaluated. Changing the co-stimulatory domain from CD28 to 4-1BB reduced the potency of CAR-T, whereas increasing IL-2 concentrations in the expansion or treatment phase enhanced the potency of CAR-T.Conclusions Hence we created a co-culture of a vessel and cancer tissue for evaluating CAR-T functionality. In the future the system will be used for testing novel cell therapy variants in panels of patient cancers to support drug discovery and translational medicine by evaluating the efficacy and therapeutic index of different CAR-T variants and identifying responders.References Saez, et al. The changing landscape of cancer cell therapies: clinical trials and real-world data. Nature Reviews Drug Discovery (2024).Hou, et al. Navigating CAR-T cells through the solid tumor micro-environment. Nature Reviews Drug Discovery (2021).