Abstract 5647: GS24-B057, a potential best-in-class Nectin-4- and Trop-2-directed bispecific ADC, for the treatment of urothelial carcinoma and other solid tumors.

Abstract Background: Nectin-4 and Trop-2 are transmembrane proteins commonly co-overexpressed in various solid tumors and associated with tumor progression. ADCs targeting Nectin-4 or Trop-2 have shown promising therapeutic potential in urothelial carcinoma (UC), non-small cell lung cancer (NSCLC), and triple-negative breast cancer (TNBC). Notably, the combination of Padcev™ (a Nectin-4-directed ADC; also referred to as EV) and Trodelvy™ (a Trop-2-directed ADC; also referred to as SG) as second-line therapy for metastatic UC has demonstrated significantly improved efficacy, with the overall response rate (ORR) reaching 70%. However, the safety profile of this combination remains to be a concern, with ≥Grade 3 treatment-emergent adverse events (TEAEs) occurring in 78% of patients. To maintain clinical efficacy while minimizing toxicity, we developed GS24-B057, a dual Nectin-4- and Trop-2-targeting ADC with a drug-to-antibody ratio (DAR) of 6, integrating an optimally engineered bispecific antibody, a tailored topoisomerase I inhibitor payload, and site-specific conjugation. Materials and Methods: The in vitro tumor cell binding activity of GS24-B057 was assessed by flow cytometry. For internalization analysis, Nectin-4- and Trop-2-expressing cells were incubated with pHrodo-conjugated ADC, followed by fluorescence intensity quantification. Cytotoxicity and bystander killing effect were assessed in a Cell Titer-Glo assay and in a luciferase assay, respectively. The in vivo anti-tumor efficacy of GS24-B057 was systematically examined in mouse models bearing human cancer cell line-derived xenografts (CDX) and patient-derived xenografts (PDX). Pharmacokinetics was studied in cynomolgus monkeys. Results: In preclinical studies, GS24-B057 exhibited significantly stronger in vitro cell binding and internalization than the EV and SG combination, as well as robust direct cytotoxicity and bystander killing effect in tumor cell lines. GS24-B057 demonstrated potent anti-tumor activity across multiple CDX and PDX models of human cancer, including UC, TNBC, and NSCLC, with superior efficacy versus EV, SG, and SKB264 as single agent, and comparable efficacy to the EV and SG combination but at a lower dose level and administration frequency. Furthermore, GS24-B057 featured a favorable pharmacokinetic profile in cynomolgus monkeys. Conclusion: These data suggest the potential of GS24-B057 as an effective therapeutic option for patients with UC and other solid tumors and support its further evaluation in IND-enabling studies. Citation Format: Meihan Li, Fu Li, Qiuru Che, Xiaojing Huang, Dongxia Chu, Qiyang Zhang, Nan Li, Qingjuan Ma, Weiming He, Wenqiang Zhai, Yihui Lin, Xiaozhen Wang, Fanglong Yang, Siqing Wang, Lei Jin, John L. Xu. GS24-B057, a potential best-in-class Nectin-4- and Trop-2-directed bispecific ADC, for the treatment of urothelial carcinoma and other solid tumors 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 5647.