Abstract 5646: A novel <i>in vivo</i> self-assembled peptide-drug conjugate targeting HER2 with potent anti-tumor efficacy overcoming resistance of HER2 ADCs

Background: Several HER2-targeting antibody-drug conjugates (ADCs) have demonstrated significant clinical effectiveness on HER2-expressing cancer. However, these ADCs still face challenge of disease progression in most patients due to diverse resistance mechanisms. Herein, we report preclinical studies of iSAP-0913, a novel in vivo self-assembled peptide-drug conjugate (PDC) targeting HER2. iSAP-0913 demonstrates significant antitumor activity in both in vitro and in vivo models, overcoming the resistance of HER2 ADCs with Topo I inhibitor payload. Method: The strategy of in vivo self-assembly was leveraged to construct iSAP-0913 with a HER2 targeting peptide possessing binding epitopes located in domain II of HER2, a tubulin inhibitor MMAE, and a self-assembling peptide linker. A series of in vitro assays were conducted to evaluate biological functions and anti-tumor activities. The in vivo anti-tumor efficacy was assessed in a DS8201-induced NCI-N87 resistance model. Preliminary safety of iSAP-0913 was evaluated by repeated dose toxicity (DRF) study in cynomolgus monkeys. Results: iSAP-0913 binds HER2 receptor to form nano-assemblies on tumor cells, contributed by the design of self-assembling peptide sequence in linker. As a result, in vivo self-assembled PDC monomers accumulate in tumor lesions through assembly induced retention (AIR) effect. In NCI-N87 cells, the payload delivery efficiency of iSAP-0913 was 2-3 times higher than that of HER2 ADCs. Desorption Electrospray Ionization imaging results demonstrated that iSAP-0913 exhibits significantly enhanced tumor tissue penetration compared to HER2 ADCs (DS8201, RC-48) in NCI-N87 xenograft tumor models. iSAP-0913 induced tumor-growth suppression, even regression in in vivo models with 1-3 mg/kg dosage. iSAP-0913 also showed more effective tumor suppression with TGI% of 73.4% in xenograft models established with DS8201 induced resistant cells, whereas DS8201 exerted negligible anti-tumor activity (TGI%&amp;lt;19.6%). Tissue distribution studies in NCI-N87 tumor-bearing mice revealed that both iSAP-0913 and MMAE are rapidly cleared from plasma. However, MMAE exhibited an elimination half-life of up to 57 hours in the tumor lesion, which may be attributed to the AIR effect. Preliminary repeat-dose toxicological assessment of iSAP-0913 in pharmacologically relevant species cynomolgus monkey demonstrated that iSAP-0913 is well tolerated with no signs of lung toxicity. TK data showed that free MMAE in plasma is 1/100th of iSAP-0913, without MMAE accumulation during dosage. Conclusion: Comprehensive preclinical data have supported iSAP-0913 as a promising PDC candidate for HER2-low expression and/or patients insensitive to other HER2-targeting ADCs, particularly as a sequential treatment following resistance to HER2 ADCs. iSAP-0913 is now undergoing preclinical development. Citation Format: Gantian Lyu, Min Han, Yuzhen Qian, Yingbin Zhang, Lin Fu, Hao Wang. A novel in vivo self-assembled peptide-drug conjugate targeting HER2 with potent anti-tumor efficacy overcoming resistance of HER2 ADCs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5646.