Developing CAR T Cell Therapies to Target Tumor Heterogeneity in Advanced Prostate Cancer Therapies that inhibit androgen biosynthesis and target the androgen receptor are the mainstay of treatment for patients with advanced prostate cancer. However, most men will eventually develop resistant disease that is called castration-resistant prostate cancer (CRPC). Metastatic CRPC is not curable and treatments at this stage are aimed at extending and improving quality of life. CRPC is a heterogeneous disease composed of at least two subtypes including prostate adenocarcinoma and neuroendocrine prostate cancer (NEPC). Both CRPC subtypes are found together in many lethal, treatment-resistant prostate cancers. New and potent therapies that account for and eliminate the heterogeneity of CRPC are urgently needed. Chimeric antigen receptor (CAR) T cell therapy is a revolutionary advance in oncology that combines precision targeting with powerful killing of tumor cells. In this approach, a patient’s own immune T cells are collected from the blood, genetically engineered to recognize and kill his/her specific cancer and reintroduced into the patient. This technology has the potential to transform the treatment of cancer including those that have been considered incurable. Our group has pushed forward the first CAR T cell therapy for metastatic CRPC to a clinical trial by building on a series of scientific accomplishments. We discovered that prostate stem cell antigen (PSCA) is a protein expressed on the surface of the majority of prostate adenocarcinomas, developed antibodies that bind specifically to PSCA, and extensively engineered and tested PSCA CAR T cell therapy in laboratory models of prostate cancer. Recently, we have also found that another protein, carcinoembryonic antigen related cell adhesion molecule 5 (CEACAM5), is expressed on the surface of most NEPCs. In the proposed research, we will initiate a phase I clinical trial to evaluate our PSCA CAR T cells in patients with metastatic CRPC, and interrogate patient specimens to elucidate mechanisms of treatment resistance with particular attention given to the emergence of NEPC. We will also engineer and evaluate CARs aimed at safely and specifically targeting CEACAM5 in laboratory models of NEPC. Lastly, we will determine whether a strategy combining PSCA CAR T and CEACAM5 CAR T cells can safely address tumor heterogeneity in CRPC by eradicating both prostate adenocarcinoma and NEPC.