PRE-CLINICAL STUDIES FOR ADVANCING CIRMTUZUMAB-BASED ANTI-ROR1 THERAPIES IN METASTATIC PROSTATE CANCER

INTRODUCTION AND OBJECTIVE: Increased levels of the Wnt ligand, WNT5A, are correlated with bone metastatic, castrate resistant PCa (CRPC). Wnt signaling is comprised of canonical (bcatenin dependent) and noncanonical pathways which are critical for normal development and physiology. Alterations are frequently associated with tumorigenesis and metastasis in many cancers including prostate cancer. Noncanonical Wnt5A signaling is mediated in part through ROR1, a fetal oncoprotein for which a therapeutic ROR1 inhibitory antibody, Cirmtuzumab, has been developed. Its safety has been clinically proven in trials for chronic lymphocytic leukemia (CLL) and metastatic breast cancer.Hypothesis: WNT5A may activate a stem-cell-like program via ROR1 that leads to castration resistance in bone metastatic prostate cancer. The anti-ROR1 biologic, Cirmtuzumab, may inhibit this mechanism of CRPC and re-sensitize bone metastatic prostate cancer to standard of care therapies. METHODS: We used patient-derived xenograft (PDX) bone metastatic prostate cancer models and PCa cell lines to test mechanism of action of a new therapeutic target, the WNT5A/ROR1 signaling pathway in prostate cancer for which a therapeutic ROR1 inhibitor antibody, Cirmtuzumab, has been developed and clinically tested in CLL and metastatic breast cancer patients. RESULTS: Preclinical studies using RNASeq, qRT-PCR, FACS and Westerns showed increased expression of ROR1 in the neuroendocrine prostate cancer cell lines, PC3 and DU145, and a patient derived xenograft (PDX) model of small cell bone metastatic prostate cancer, PCSD13. Pretreatment of the PCSD13 PDX with Cirmtuzumab increased the efficacy of the taxane docetaxel at inhibiting tumor growth. Intravenous infusion of anti-ROR1 CART cells into mice bearing subcutaneous, ROR1-expressing, PC3 xenografts, showed marked reduction in tumor growth. ROR1 Knock out using CRISPR-Cas9 in PC3 and DU145 cells showed specificity of Cirmtuzumab-based immunotherapies. We have developed immunohistochemistry (IHC) assays to detect ROR1 in patient biopsy samples which will allow us to define the prostate cancer patient population that expressed ROR1 and to be able to evaluate the treatment in patients for a future clinical trial. CONCLUSIONS: ROR1 was expressed at high levels on castration resistant small cell PCa and neuroendocrine PCa models in PCa cell lines and PDX models. Cirmtuzumab has demonstrated efficacy in our patient derived xenograft organoid cultures. These studies have formed the rationale for development of a Phase 1b clinical trial of Cirmtuzumab in combination with docetaxel in metastatic CRPC patients.