PROJECT 2 – PROJECT SUMMARY Immune-checkpoint blockade has resulted in unprecedented treatment advances in multiple tumor types, despite yielding modest results in prostate cancer (PCa). While CTLA-4 and PD-L1 are infrequently expressed in PCa, B7-H3 (another B7 superfamily member) is highly expressed in many PCas, modulates anti-tumor immune responses, and is associated with worse prognosis. Targeting B7-H3 is now clinically feasible with the development of Enoblituzumab, an investigational humanized Fc-optimized (for antibody-dependent cellular cytotoxicity [ADCC]) monoclonal antibody that binds B7-H3 with high affinity, and with MGC018, a duocarmycincontaining humanized antibody-drug conjugate (ADC) that also binds non-competitively to B7-H3 with high affinity. Notably, enoblituzumab and MGC018 bind to different epitopes on B7-H3, permitting rational combination of these two agents. Here, we propose to test the hypothesis that B7-H3–targeted treatment (using the ADC with or without the monoclonal antibody) in patients with metastatic castrate resistant prostate cancer will be safe, tolerable and lead to PSA and radiographic responses, by modulating T cell immunity in the tumor microenvironment (TME) as well as direct tumor killing by duocarmycin ± ADCC-directed activity. We will build on our prior experience from our recently completed investigator initiated Enoblituzumab monotherapy trial (n=32, NCT02923180) to test the hypothesis that therapeutic efficacy can be enhanced through a combination of enoblituzumab’s myeloid-enhancing and T cell-enhancing activity plus direct B7-H3–targeted cytotoxicity with MGC018’s duocarmycin payload. Our specific aims are: (1) To investigate the safety and clinical efficacy of B7H3–targeted treatment (MGC018 alone or combined with enoblituzumab) in the mCRPC setting, (2) To assess the pathologic and adaptive immunologic effects of single (MGC018) or dual (MGC018 plus enoblituzumab) B7H3 targeting in PCa, and (3) To interrogate functional antigen-specific T cell repertoires induced by single or dual targeting of B7-H3 in PCa. These studies will provide the first in-human assessment of immunotherapy- mediated gene expression changes, both peripheral and intratumoral, in relation to targeted treatment against B7-H3 mediated by cytotoxic (MGC018) +/- ADCC activity (enoblituzumab) to determine if synergistic anti-tumor activity is observed. These studies will be the first to integrate novel spatial proteomic/transcriptomic, T cell repertoires, and functional analyses in PCa patients thereby potentially identifying critical immune molecules and pathways that can be targeted for additional combinatorial immunotherapy PCa trials in the future.