Abstract 1812: Development of novel topoisomerase 1 inhibitor PBX-7 payload-based ADC including tandem cleavable linker system

Abstract Antibody-Drug Conjugates (ADC) employing camptothecin-based payloads are gaining high interests as a new modality to treat various cancers, especially solid tumors. Among them, novel ADCs containing synthetic camptothecin-based payloads specially optimized for ADC applications (e.g., DXd) are of special interest due to their successful clinical results. Despite the remarkable success of ADCs exploiting these payloads, such as Enhertu (DS-8201a), there are still clear unmet needs, including the improvement of safety profile (minimization of ILD and neutropenia) and the development of novel ADCs with multiple MoA payloads to combat cancer heterogeneity. To address these unmet needs, we have synthesized and evaluated a series of PBX-7 payloads, derived from novel camptothecin FL118, a potent, dual inhibitor of Top 1/anti-apoptotic pathway known for its favorable safety profile. These compounds showed strong in vitro cytotoxicity across various cancer cell lines and inhibition of Top 1 comparable to DXd or other related camptothecin compounds. They had excellent safety profile in the preliminary toxicity study in mice that can be attributed to their physicochemical properties. PBX-7 series payloads-based ADC exhibited superior cellular cytotoxicity and in vivo tumor regression compared to Enhertu in Her2-positive cell line and CDX mouse model. Notably, PBX-7 payload-based ADC demonstrated remarkable efficacy in reducing tumor volume more than Enhertu in the T-DM1-resistant JIMT-1 xenograft mouse model. Furthermore, PBX-7 payload-based ADC have bystander effects similar to Enhertu, leading to reduced viability of antigen-negative cells when co-cultured with antigen-positive cell lines. In NHP toxicity study in Cynomolgus monkeys, no sign of toxicity was observed when dosed with anti-Her2 PBX-7 payload-based ADC up to 30mg/kg. As another way of addressing unmet needs, we synthesized novel linker including tandem cleavage site to reduce payload mediated toxicity. Two distinct enzyme, beta-glucuronidase and cathepsin B cleave linker sequentially. This linker system has a higher stability during circulation in the body and higher specific payload release in the tumor tissue than single cleavable linker. In conclusion, our research shows the promising potential of novel camptothecin PBX-7 payload-based ADCs and tandem cleavable linker system as potent and safe ADC candidates for cancer therapy. These advancements hold great promise in addressing the current challenges in ADC development with their enhanced safety profiles and targeted efficacy. Citation Format: Younghwa Chun, Hyun Yong Cho, Byeong Sung Lee, Areum Go, Seunggun Shin, JaeHyeon Lee, Sohui Kweon, Hyemin Han, Suhyun Park, Doo Young Jung. Development of novel topoisomerase 1 inhibitor PBX-7 payload-based ADC including tandem cleavable linker system [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1812.