Abstract 5201: U3-1402, a novel HER3-targeting antibody-drug conjugate, exhibits in vitro antitumor activity against breast cancer cells expressing HER3 mutations without dependence on HER2 overexpression
Authors: Koyama Kumiko , Ishikawa Hirokazu , Abe Manabu , Shiose Yoshinobu , Ueno Suguru , Nakamaru Kenji , Murakami Masato 
Published: September 2020

Abstract
Background: U3-1402, a novel HER3-targeting antibody-drug conjugate (ADC) composed of a fully human anti-HER3 antibody (patritumab), tetrapeptide-based linker, and topoisomerase I inhibitor payload, is currently being studied in clinical trials for patients with HER3-positive breast cancer (phase 1/2, NCT02980341) and NSCLC (phase 1, NCT03260491). HER3 is overexpressed in multiple human cancers and plays an important role in cell proliferation and survival. Furthermore, HER3 somatic mutations have been reported in several cancers, including breast cancer, and certain HER3 mutants demonstrates oncogenic activity in the presence or absence of HER2 overexpression. Here, we evaluate if U3-1402 shows an antitumor activity against clinically reported HER3 mutations as observed with HER3 wild-type (HER3WT).
Methods: MDA-MB-231, a triple negative breast cancer cell line (HER3-negative), was transduced with lentiviral vectors encoding flag-tagged HER3WT or 11 HER3 mutations (V104L, V104M, A232V, P262H, G284R, D297Y, G325R, T355I, Q809R, S846I, and E928G), along with HER3 empty vector (HER3EV), in the presence and absence of HER2 overexpression. To assess the targeted delivery of U3-1402 to the HER3 transfectants, we studied cell surface binding of U3-1402 (0.1–100 nM) using flow cytometry, and assessed lysosomal trafficking of U3-1402 (0.1–10 nM) using high-content imaging system with utilizing pH-sensitive pHrodo iFL conjugated antibody. Cell growth inhibition activity of U3-1402 was also evaluated.
Results: U3-1402 bound to HER3WT cell surface in a concentration-dependent manner, but not to HER3EV cell surface. Each of the above HER3 mutant cells demonstrated the same cell surface binding property of U3-1402 as HER3WT cells. Subsequently, U3-1402 was translocated to the lysosome, where pHrodo-labeled U3-1402-derived signals were observed in time- and concentration-dependent manners in any of the HER3 mutant cells, at comparable levels to those in the HER3WT cells. Cell growth inhibition activity of U3-1402 was observed in both HER3WT cells and all HER3 mutant cells tested. HER2 overexpression had no significant impact on cell surface binding, trafficking and cell growth inhibition activity of U3-1402.
Conclusion: U3-1402 has a potential to show antitumor activity against HER3 mutant cells with the same potency as HER3WT cells, regardless of HER2 overexpression. This finding provides insights into HER3-targeted therapy, in which an efficient payload delivery via ADC-mediated internalization could be achieved for HER3 mutations.
Citation Format: Kumiko Koyama, Hirokazu Ishikawa, Manabu Abe, Yoshinobu Shiose, Suguru Ueno, Kenji Nakamaru, Masato Murakami. U3-1402, a novel HER3-targeting antibody-drug conjugate, exhibits in vitro antitumor activity against breast cancer cells expressing HER3 mutations without dependence on HER2 overexpression [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5201.