PROJECT 2: SUMMARY/ABSTRACT Limited options are available for the treatment of mPC, a lethal prostate cancer. Immunotherapy has significantly improved survival in patients with a variety of solid tumors; however, it has had limited efficacy in patients with metastatic prostate cancer (mPC). There is a lack of understanding of underlying mechanisms for this de novo resistance. In our pre-clinical studies, we have identified a novel mechanism that prostate tumors use to subvert and evade the immune system. We have found that metastatic prostate tumor cells convert the stress-induced immune stimulatory cell surface molecule, the MHC I Chain related molecule (MIC), to a highly immune suppressive soluble MIC (sMIC), through proteolytic-mediated shedding. Importantly, patients with mPC have significantly elevated immune suppressive sMIC in the circulation and severely suppressed immune cell function. To overcome the immune suppression of sMIC, we have developed a first-in-class sMIC-targeting monoclonal antibody (mAb) B10G5 that has demonstrated remarkable efficacy in eliminating prostate metastasis as a single agent in preclinical models. When used in combination, B10G5 synergizes with immune checkpoint blockade and reduces immune checkpoint therapy-induced colon toxicity. The mAb B10G5 has been optimized for human use (termed as huB10G5), proven to be safe in non-human primates (NHP) in pilot toxicity assessments, and is currently under IND-enabling studies. The goal of this SPORE project is to fulfill critical pre-clinical studies and to translate the B10G5 therapy into a potential therapy for treating mPC. Very recently, we found that androgen receptor (AR) activity could protect PC cells from immune cytotoxicity by upregulating the granzyme inhibitor serpinB9 and that B10G5 therapy could activate FAS-mediated killing. Thus, we hypothesize that the sMICtargeting antibody huB10G5 can be an effective immunotherapeutic agent for treating mPC as a single agent or in combination with immune checkpoint blockade therapy and/or standard AR-targeted therapy. We propose three Specific Aims: 1) to define the landscape of serum MIC levels in mPC patients with clinical characteristics and association with tumor immune modulation; 2) to determine therapeutic efficacy of targeting sMIC alone or in combination with immune checkpoint blockade (ICB) and/or AR-targeting for treatment of concurrent visceral and bone mPC; 3) To conduct a first-in-human Phase I clinical study of huB10G5 in mCRPC patients. These studies will provide us critical information for future Phase I expansion and Phase II study.