Abstract 1685: The HER2-targeting dual-payload antibody-drug conjugate combining a topoisomerase I inhibitor and a microtubule inhibitor demonstrates superior efficacy and overcomes resistance to single-payload ADCs in xenograft models

Abstract The genetic and phenotypic heterogeneity of human cancers is a primary driver of drug resistance, posing a major challenge to achieving durable therapeutic responses. To overcome this, Sutro has developed a dual-payload antibody-drug conjugate (dpADC) platform that enables precise co-delivery of two cytotoxic payloads via a single, homogeneous molecule. Sutro’s HER2-targeting dpADC demonstrates that simultaneous delivery of two cytotoxic payloads provides greater anti-tumor activity than single-payload ADCs and overcomes preclinical, in-vivo-derived treatment resistance. Combining Sutro Biopharma’s XpressCF+® cell-free expression system with site-specific conjugation technology, we engineered a HER2-targeting dpADC combining exatecan (topoisomerase I inhibitor; TOPO1i) and monomethyl auristatin E (MMAE) (microtubule inhibitor; MTi) payloads at an 8:4 ratio. The resulting dpADC exhibited favorable pharmacokinetics in vivo and minimal linker-payload loss over a 21-day study. In in vitro cell killing assays, the dpADC performed better than Enhertu and DAR8 exatecan ADC across multiple tumor cell lines. In in vivo efficacy studies, the dpADC exhibited greater anti-tumor activity than both single-payload ADC comparators across multiple xenograft models. To further evaluate the dpADC concept’s potential, we examined its utility in addressing resistance induced by ADC treatment. To mimic resistances to single payload ADCs observed in the clinic, we continuously dosed xenograft tumors with Enhertu on a weekly dosing schedule until the tumors progressed. These Enhertu-resistant tumors were then continuously dosed with a MTi ADC until de novo resistance developed. Dual-resistant tumors were then treated with a dpADC resulting in deep and durable anti-tumor responses. Even in tumors that were rendered resistant to both payloads, the dpADC treatment was able to achieve substantial tumor regression, demonstrating the potential for benefit in mono payload ADC refractory settings. Overall, these results indicate that Sutro’s HER2 dpADC can enhance anti-tumor activity beyond that of single-payload ADCs and can overcome prior treatment-induced resistance in a preclinical setting. Citation Format: Angela Matcham, Robert Yuan, Brian Vuillemenot, Rhoneil Pena, Young Park, Abigail Yu, Jeffrey Hanson, Cuong Tran, Xiaofan Li, Miao Wen, Daniel Calarese, Werner Rubas, Krishna Bajjuri, Guifen Xu, Alice Yam, Hanspeter Gerber, . The HER2-targeting dual-payload antibody-drug conjugate combining a topoisomerase I inhibitor and a microtubule inhibitor demonstrates superior efficacy and overcomes resistance to single-payload ADCs in xenograft models [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 1685.