Background: Adoptive immunotherapy with Chimeric Antigen Receptor (CAR)-T cells has transformative potential for the treatment of cancer. However, a primary challenge to the success of these therapies in prostate cancer is the immunosuppressive microenvironment, including high levels of Transforming Growth Factor-beta (TGFβ), encountered by re-directed T cells upon tumor infiltration. Importantly, these immunosuppressive functions of TGFβ can be abrogated in T cells using a dominant negative TGFβ receptor (TGFβRdn), thereby enhancing antitumor immunity. In in vivo disseminated prostate cancer models, co-expression of TGFβRdn on PSMA-redirected CAR-T cells led to increased T cell proliferation, enhanced cytokine secretion, resistance to exhaustion, long-term persistence, and greater tumor eradication. Methods: We initiated a first-in-human phase 1 clinical trial to evaluate the safety and preliminary efficacy of lentivirally-transduced PSMA-directed/TGFβ-insensitive CAR-T cells (CART-PSMA-TGFβRdn) in men with metastatic CRPC. In preliminary dose-escalation cohorts, patients received a single dose of 1-3 x 10⁷/m² (Cohort 1) or 1-3 x 10⁸/m² (Cohort 2) CART-PSMA-TGFβRdn cells without lymphodepleting chemotherapy in a 3+3 design. In Cohort 3, patients will receive the MTD of CART-PSMA-TGFβRdn following a lymphodepleting regimen of cyclophosphamide and fludarabine. All patients provide newly obtained metastatic tumor biopsies at baseline, as well as on day +10 following the CAR-T cell infusion and at disease progression. CAR-T expansion and persistence in peripheral blood and trafficking to target tissues is evaluated via quantitative PCR of CART-PSMA-TGFβRdn DNA. Bioactivity of CART-PSMA-TGFβRdn cells is evaluated via multiplex immunoassays. Additional correlative studies include enumeration and phenotyping of circulating tumor cells and DNA. Cohorts 1 and 2 have been completed without observed DLT. Interestingly, a reversible cytokine release syndrome has been observed that is responsive to tocilizumab. Enrollment in Cohort 3 began in September 2018. Cohort expansions will examine serial CART-PSMA-TGFβRdn re-treatment strategies. Clinical trial information: NCT03089203