Background: Treatment with immune checkpoint blockade (ICB) alone results in suboptimal response rates in prostate cancer. Prostvac-VF Tricom is a therapeutic vaccine that incorporates DNA for the shared self-antigen PSA. Personalized neoantigen vaccines based on specific mutated epitopes may have the ability to overcome immunoresistance seen with self-antigens. Even in low mutational burden tumors like prostate cancer, T cell responses against neoantigens have been correlated with favorable clinical outcomes. Thus, we hypothesized that the combination of shared antigen and neoantigen vaccines with dual ICB will induce robust immune responses and improve clinical outcomes. Methods: This Phase I clinical trial (NCT03532217) enrolled patients from 2018-2021. Eligible patients had histologically confirmed high risk mHSPC, must have completed a course of docetaxel and received continuous androgen deprivation therapy. Patients were treated with Prostvac-VF in combination with ipilimumab/nivolumab within 60 days of the last docetaxel dose. Then, patients were continued on monthly nivolumab with their personalized neoantigen vaccine administered via intramuscular electroporation. The primary objectives of this study were to assess the feasibility, safety/tolerability, and immune responses of this combination strategy. Key secondary objectives include failure free survival, milestone overall survival (OS), PSA responses, and radiographic progression free survival. Results: Nineteen patients were enrolled and treated on trial, and feasibility was shown with 15 (79%) receiving neoantigen vaccines. Four patients did not receive neoantigen vaccines (2 for progressive disease, 2 for ICB toxicity). Treatment was well-tolerated with only 2 (2.4%) grade 3 treatment related adverse events (TRAEs) of colitis, and no grade 4+ TRAEs. The common grade 1-2 TRAEs were diarrhea (10%), injection site reactions (10%), rash (7.4%), and fatigue (6%). Median follow-up to date is 22.6 (11.3-39.6) months, with median OS not yet reached and 2 year milestone OS of 75%. Six (31.5%) patients had PSA progression per PCWG2 criteria while on treatment, with the median time to PSA progression not yet reached for the total population. Increases in activation/co-stimulatory/co-inhibitory seen after treatment with Prostvac/ICB, suggest immune priming. Sample collection is complete and immune correlative analyses are ongoing. Final safety/tolerability and preliminary correlative and clinical outcomes will be reported. Conclusions: This is the first clinical trial evaluating the use of personalized neoantigen vaccines in a combination immunotherapeutic approach in mHSPC patients. Clinical trial information: NCT03532217.