Abstract CT211: A phase 1 dose-escalation and dose-expansion study to evaluate the safety, pharmacokinetics, and anti- tumor activity of ARX517 in subjects with advanced solid tumors resistant or refractory to standard therapies (APEX-01 trial, NCT04662580)

Abstract Background: Novel anti-PSMA (prostate-specific membrane antigen) targeted therapies have exhibited encouraging antitumor activity in prostate cancer and most non-prostate solid tumors express PSMA in tumor neovasculature. ARX517 is an antibody drug conjugate composed of a fully humanized anti-PSMA monoclonal antibody linked to pAF-AS269, a potent microtubule inhibitor. Upon binding to PSMA on the surface of cancer cells, ARX517 is internalized and pAF-AS269 is released following lysosomal metabolism. Methods: This Phase 1, dose-escalation and dose-expansion study is evaluating the safety, pharmacokinetics, and preliminary anti-tumor activity of ARX517 in adults with prostate cancer or other solid tumors whose disease is resistant or refractory to standard therapies. Patients must have metastatic castration-resistant prostate cancer refractory to standard therapies, including abiraterone, darolutamide, apalutamide or enzalutamide and progression by Prostate Cancer Working Group criteria. Non-prostate advanced solid tumors must be pathologically confirmed, metastatic, or unresectable solid tumors that have disease progression despite at least 1 prior standard of care systemic treatment and have no satisfactory treatment alternatives. All patients must have adequate organ function and any brain metastasis must demonstrate radiographic stability. Exploratory pharmacodynamic assessment includes PSMA-PET/CT imaging, which is optional for low dose cohorts but required starting at 1.4 mg/kg dose. Dose-escalation will consist of ascending dose levels of ARX517 administered as a single agent and will use a i3+3 design. The number of subjects in the dose-expansion will be based on the number of doses to be expanded for further evaluation of safety, PK, and clinical activity. Through dose levels one and two, no dose limiting toxicities have been observed. Ascending dose levels of ARX517 as a single agent will be administered until the recommended Phase 2 dose or maximum tolerated dose is determined. Status: ARX517-2011 began recruiting patients in July 2021 and as of January 2022 has completed enrollment in the 3rd cohort of dose-escalation. Citation Format: Russell K. Pachynski, Luke Nordquist, Michael T. Schweizer, John Shen, Nabil Adra, Mehmet A. Bilen, Zachery R. Reichert, Rahul Aggarwal, Bilal A. Siddiqui, Matt Li, Dong Xu, Richard Huhn, Darryl Z. L'Heureux, Jinchun Yan, Ying Buechler, Sulan Yao, John Simitzi, Wenge Shi, Shawn Zhang, Scott T. Tagawa. A phase 1 dose-escalation and dose-expansion study to evaluate the safety, pharmacokinetics, and anti- tumor activity of ARX517 in subjects with advanced solid tumors resistant or refractory to standard therapies (APEX-01 trial, NCT04662580) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr CT211.