Background: Multiple systemic therapies have resulted in improved overall survival (OS) for mHSPC, including several AR-targeted agents (ARTA) and docetaxel chemotherapy. However, patients (pts) with high volume de novo metastatic prostate cancer still only have an OS of approximately 50 months. Thus, more effective combination strategies for initial treatment of mHSPC are urgently needed. CABO, a multi-tyrosine kinase inhibitor (including MET, VEGFR-1, -2,-3, AXL) has shown synergy with ABI as well as with CPI in preclinical studies. CABO and ABI have demonstrated an acceptable safety profile in metastatic castration resistant prostate cancer (mCRPC) (Choudhury et al Prostate 2018), and CABO and atezolizumab (ATEZO) in combination have showed a promising efficacy signal and manageable safety profile in mCRPC (COSMIC-021, NCT03170960; Agarwal ESMO 2021). The phase 3 CONTACT-02 trial (NCT04446117) is randomizing mCRPC pts to either CABO and ATEZO or a second generation ARTA. Given the robust preclinical and emerging clinical data for the use of both CABO and CPI therapy in advanced prostate cancer and the need to optimize therapy earlier in the course of disease, in the present trial we intend to evaluate the safety and tolerability of ABI, CABO, and CPI in mHSPC. Methods: CABIOS (NCT04477512) is a phase Ib, single center, open label trial of ABI, CABO and the CPI nivolumab (NIVO), an anti-PD-1 monoclonal antibody, in pts with mHSPC. Key inclusion criteria include histologically or cytologically confirmed metastatic prostate adenocarcinoma without neuroendocrine/small cell differentiation and radiographic evidence of metastatic disease. Ongoing androgen deprivation (ADT) within 12 weeks of study start is allowed. Key exclusion criteria include evidence of castration resistance, prior treatment with second-generation androgen receptor inhibitors, CYP17 inhibitors, CABO, or checkpoint inhibitor immunotherapy (anti-PD-1/PD-L1, CTLA-4), active autoimmune disease, and ongoing treatment with systemic corticosteroids daily. The primary objective of the trial is evaluation of the safety and tolerability of this combination therapy in the trial population; the primary endpoint is the frequency of dose-limiting toxicities (DLTs) as measured by CTCAE v5.0. The study will employ a 3 + 3 design evaluating ABI 1,000 mg once daily with prednisone 5 mg daily, NIVO 480 mg IV every 4 weeks, and a CABO starting dose of 20 mg once daily at dose level 1 (DL1). If 0 of 3 pts at DL1 experience a DLT, 3 pts will be enrolled at DL2 with CABO 40 mg once daily. If 1 of 3 pts at DL1 experience a DLT, 3 additional pts will be enrolled at DL1. If 0 of 3 pts experience a DLT at DL2, the study will move to a recommended phase 2 dose (RP2D) expansion cohort. Total sample size of approximately 20 pts is expected depending on DLT frequencies. Enrollment is ongoing. Clinical trial information: NCT04477512.