Preclinical Characterization of ARX517, a Site-specific Stable PSMA-Targeted Antibody Drug Conjugate for Treatment of Metastatic Castration-Resistant Prostate Cancer

Metastatic castration-resistant prostate cancer (mCRPC) is an advanced disease in which patients ultimately fail standard-of-care androgen deprivation therapies and exhibit poor survival rates. The prostate-specific membrane antigen (PSMA) has been validated as an mCRPC tumor antigen with overexpression in tumors and low expression in healthy tissues. Using our proprietary technology for incorporating synthetic amino acids into proteins at selected sites, we have developed ARX517, an antibody-drug conjugate composed of a humanized anti-PSMA antibody site-specifically conjugated to a tubulin inhibitor at a drug-to-antibody ratio of 2. After binding PSMA, ARX517 is internalized and catabolized, leading to cytotoxic payload delivery and apoptosis. To minimize premature payload release and maximize delivery to tumor cells, ARX517 employs a noncleavable polyethylene glycol linker and stable oxime conjugation enabled via synthetic amino acid protein incorporation to ensure its overall stability. In vitro studies demonstrate that ARX517 selectively induces cytotoxicity of PSMA-expressing tumor cell lines. ARX517 exhibited a long terminal half-life and high serum exposure in mice and dose-dependent antitumor activity in both enzalutamide-sensitive and -resistant cell line-derived xenograft and patient-derived xenograft models of prostate cancer. Repeat-dose toxicokinetic studies in nonhuman primates demonstrated that ARX517 was tolerated at exposures well above therapeutic exposures in mouse pharmacology studies, indicating a wide therapeutic index. In summary, ARX517 inhibited tumor growth in diverse mCRPC models, demonstrated a tolerable safety profile in monkeys, and had a wide therapeutic index based on preclinical exposure data. Based on the encouraging preclinical data, ARX517 is currently being evaluated in a phase I clinical trial (NCT04662580).