Abstract 4630: CD46-targeted dual-modality alpha-particle and cytotoxic payload-loaded antibody for enhanced treatment of metastatic castration-resistant prostate cancer

Abstract Background: CD46 is a validated therapeutic target broadly expressed in prostate cancer, including metastatic castration-resistant and PSMA-low or neuroendocrine subtypes. The CD46-targeting YS5 antibody supports two complementary therapeutic platforms: a CD46-directed antibody-drug conjugate (YS5-MMAE) and a radioimmunoconjugate delivering the alpha-emitter actinium-225 (225Ac-YS5). Each monotherapy induces tumor regression, yet dose escalation increases systemic toxicity, underscoring the need for a combined approach. Methods: Two combination strategies were evaluated: (1) co-administration of YS5-MMAE with 225Ac-Macropa-PEG4-YS5, and (2) a dual-labeled construct, 225Ac-Macropa-PEG4-YS5-MMAE, integrating both payloads on a single scaffold. The dual construct was synthesized by conjugating MMAE to YS5, attaching the Macropa-PEG4 chelator, and radiolabeling with 225Ac. Radiochemical purity, antigen binding, and in vitro stability were confirmed. In vitro cytotoxicity was assessed in 22Rv1 prostate cancer cells, and in vivo efficacy was tested in 22Rv1 tumor-bearing mice (n = 9 per group) receiving saline, YS5-MMAE (1.8 mg/kg), 225Ac-YS5 (0.125 µCi), their combination, or the dual-labeled construct. Results: In vitro, combination therapy induced greater cytotoxicity than either monotherapy, indicating synergistic interaction. In vivo, median survival was 24 days for saline, 30 for YS5-MMAE, and 66 for 225Ac-YS5. Co-administration of YS5-MMAE and 225Ac-Macropa-PEG4-YS5 extended survival beyond 80 days, with delayed tumor progression. In contrast, the dual-labeled construct achieved survival exceeding 90 days, accompanied by durable tumor regression and minimal renal or hematologic toxicity. Both combination and dual-modality therapies exhibited enhanced therapeutic efficacy compared to single-agent treatments. Conclusion: Simultaneous alpha-particle and cytotoxic targeting of CD46 produces synergistic effects, improving tumor control while reducing dose-limiting toxicity. The dual-labeled construct represents a next-generation radiotheranostic platform combining targeted alpha therapy and cytotoxic payload delivery, offering enhanced potency, safety, and translational potential for advanced prostate cancer therapy. Citation Format: Anil Bidkar, Scott Bidlingmaier, Anju Wadhwa, Kondapa Naidu Bobba, Shubhankar Naik, Cheng Xue, Athira Raveendran, Megha Basak, Jonathan Chou, Rahul Aggarwal, Henry VanBrocklin, Youngho Seo, Bin Liu, Robert Flavell. CD46-targeted dual-modality alpha-particle and cytotoxic payload-loaded antibody for enhanced treatment of metastatic castration-resistant prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4630.