Abstract 2209: Rapidly switchable universal CAR-T cells with improved safety profile allow for active targeting of PD-L1 expressing solid tumors

Despite remarkable therapeutic success using chimeric antigen receptor modified T-cells (CAR-T) in hematologic malignancies, targeting solid tumors still remains challenging, given their heterogeneity and immunosuppressive milieu, including the expression of inhibitory immune checkpoints such as PD-1/PD-L1. Preclinical models demonstrate that combining CAR-T therapy with checkpoint inhibitors targeting the PD-1/PD-L1 axis could be a promising strategy to enhance anti-cancer activity of CAR-T in solid tumors and on-going phase I trials are clinically evaluating this strategy. However, primary and acquired resistance to checkpoint blockade is frequently observed limiting its broad applicability and efficacy. In addition, lack of specificity results in frequent occurrence of immune-related adverse events affecting several organs. Alternatively, expression of PD-L1 by solid tumor cells could be an attractive target for CAR-T therapy, especially in order to prevent or reverse tumor re-growth and relapse after successful initial tumor cell lysis. The development of a rapidly switchable universal CAR-T platform (UniCAR) with enhanced safety features allowed us to explore the possibility to directly attack PD-L1 expressing solid tumor cells. An inherent key feature of the UniCAR-T platform is a rapid and reversible turn off mechanism (less than 4 hours) determined by the short pharmacokinetic half-life of soluble targeting modules (TMs) and fast internalization of cell-bound TMs. TMs provide the antigen-specificity for UniCAR gene-modified T-cells (UniCAR-T) and consist of an antigen-binding moiety, e.g. an scFv, linked to a small peptide motif recognized by the UniCAR. For redirecting UniCAR-T against the PD-1/PD-L1 axis, a PD-L1 specific TM (TM-PD-L1) was developed and its functionality confirmed in binding assays to human and murine PD-L1 using surface plasmon resonance as well as in cell-based cytotoxicity assays. Half maximal killing efficacy (EC50) mediated by TM-PD-L1 redirected UniCAR-T was in the low picomolar range. As PD-L1 is also expressed on healthy tissue, toxicity studies were performed in a humanized mouse model with human UniCAR-T. Humanized mice treated with TM-PD-L1 twice a day for ten consecutive days did not reveal any signs of toxicity. In a prostate cancer (PCa) xenograft model efficient suppression of tumor growth could be demonstrated by administration of TM-PD-L1. Our data are the first to demonstrate safety and feasibility of active tar