Abstract 144: A bispecific integrin α5β1/αv antibody downregulates c-myc/E2F and TGF-β signaling and induces immunological control of basal-type prostate cancer cells

Abstract Background: Prior studies demonstrated that integrin α5β1 and αv crosstalk in chemotaxis and clonogenic survival of prostate cancer cells is abrogated by a bispecific α5β1/αv antibody (BsAbα5β1/αv), which uniquely induces internalization and lysosomal degradation of target integrins (Joshi, MCR 2020). We hypothesized that the BsAbα5β1/αv would inactivate pathological signaling pathways which would be validated in the control of tumor xenografts in vivo and in correlative studies from patient samples. Methods and Results: In basal-type prostate cancer (BTPC) cells, the BsAbα5β1/αv uniquely abrogated nuclear localization of YAP, active β-catenin, FAK and EGFR, compared to monospecific integrin α5β1 and αv antibodies (MsAbs), individually or combined. Dual integrin αv and α5 knockdown alone phenocopied the BsAbα5β1/αv effect. In vivo, clonogenic survival of BTPC cells (PC-3, DU-145) was eliminated by the BsAbα5β1/αv in nude mice, with complete regressions in up to 50% of established xenografts, associated with mononuclear infiltration. Tumor response and survival with BsAbα5β1/αv-treatment was superior to MsAbs, individually or combined. In vitro, RNA-seq of BsAb-treated BTPC cells revealed control of c-myc/E2F and EMT signaling with upregulation of Type I/II interferon pathways. Concordantly, multiplex ELISA of BsAb-treated BTPC supernatants showed induction of CXCL10 and CCL5 cytokines, with influx of natural killer (NK) cells, monocytes and macrophages into BsAb-treated xenografts confirmed by flow cytometry. Tumor control by the BsAbα5β1/αv was abrogated by NK cell depletion, highlighting their critical role. In a rapid autopsy TMA of metastatic castration-resistant prostate cancer (CRPC), integrin αv was highly expressed in osseous and non-osseous metastases and co-expressed with integrin α5 in bone metastases, osteoblasts and endothelial cells. Relevant to the osteoblastic phenotype of metastatic prostate cancer, the BsAbα5β1/αv strongly downregulated osteogenic differentiation of human bone marrow-derived mesenchymal stromal cells, superior to individual MsAbs. RNA-seq from rapid autopsy samples correlated integrin αv expression with c-myc/E2F signaling, consistent with data from SU2C CRPC and DKFZ early-onset prostate cancer cohorts. Conclusions: The BsAbα5β1/αv represents a novel therapeutic strategy to reprogram multiple hallmark pathological pathways including c-myc/E2F, TGF-β and EMT. It induces immunologic control of basal-type prostate cancers which are a paradigm of stem-like treatment resistance and immune evasion. Further translational development of the BsAbα5β1/αv therapeutic strategy in prostate cancer is warranted. Citation Format: Raghav Joshi, Ming Zhou, Jeffrey Lin, Colm Morrissey, Kun Hu, Alexander Poltorak, Paul Mathew. A bispecific integrin α5β1/αv antibody downregulates c-myc/E2F and TGF-β signaling and induces immunological control of basal-type prostate cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 144.