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Saul J. Priceman, Yukiko Yamaguchi, Elizabeth Epps, John Burnett and Stephen J. Forman

Repairing defects in antitumor immunity has been a longstanding challenge in prostate cancer, and in recent years cellular immunotherapy has emerged as a promising approach for controlling advanced disease. To date, therapies including tumor vaccine and adoptive T-cell immunotherapy have made remarkable headway in solid cancers. Several validated prostate-specific tumor antigens are available as targets for T-cell therapy, including prostate stem cell antigen (PSCA), which is overexpressed in metastatic disease. We are in late-stage preclinical development of PSCA-specific chimeric antigen receptor (CAR)-engineered T cells with plans to initiate a clinical trial early 2019 for the treatment of metastatic castration-resistant prostate cancer. Immune checkpoint pathways, including the programmed cell death protein-1 (PD-1) and the cytotoxic T lymphocyte-associated protein-4 (CTLA4), have emerged as critical drivers of immunosuppression in solid cancers, by limiting both adaptive antitumor immunity as well as adoptive T-cell therapies. Unfortunately in prostate cancer, checkpoint inhibition has led to underwhelming responses, likely due to the low mutational load and immunologically “cold” tumor microenvironment. We hypothesize that antitumor activity of PSCA-CAR T cells will elicit checkpoint pathways that dampen antitumor immune responses and reduce overall clinical outcomes. Herein, we utilize an shRNA approach to knockdown checkpoint receptors as a rational combinatorial strategy that targets checkpoint pathways to improve overall therapy with PSCA-CAR T cells for metastatic prostate cancer. We have successfully developed a multiple shRNA knockdown approach to simultaneously disrupt 3 pathways that may hamper CAR T-cell activity in the tumor. These CAR T cells with shRNA knockdown of checkpoint receptors will be directly compared with checkpoint pathway inhibitor antibody therapies in xenograft models of prostate cancer, with the hope that next generation CARs will resist this break on the immune system in solid tumors.