Background: Human epidermal growth factor receptor 3 (HER3), a member of the ErbB receptor tyrosine kinase family, is overexpressed in a variety of human cancers and plays an important role in cell proliferation and survival. U3-1402 is a novel HER3-targeting antibody-drug conjugate (ADC) consisting of a fully human anti-HER3 antibody (patritumab), a tetrapeptide-based linker, and a topoisomerase I inhibitor payload. It is currently being investigated in clinical trials for HER3-positive breast cancer (phase 1/2) and NSCLC (phase 1). In principle, effective action of ADCs depends on multiple dynamic factors including antigen-specific binding, internalization, trafficking to lysosomes, and payload release, as well as other inherent characteristics such as payload potency and linker stability.

Methods: To assess the targeted delivery potential of U3-1402, the HER3-mediated molecular dynamics of U3-1402 (cell surface binding, internalization, trafficking, and payload release) were investigated in 8 cancer cell lines with various HER3 expression levels, along with cell growth inhibition activity. In vitro molecular dynamics were compared between U3-1402 and trastuzumab (anti-HER2 antibody) using the MDA-MB-453 cell line, which expresses both HER2 and HER3. The activity of patritumab (naked antibody of U3-1402) was tested. In vivo efficacy of U3-1402 was also evaluated and immunostaining of HER3 and γH2AX (marker of DNA double-stranded breaks) as well as drug concentration assessment were performed.

Results: U3-1402 bound to the cell surface of HER3-positive cell lines in HER3 expression level-dependent manner, and was then internalized into cells. Interestingly, the internalization rate of U3-1402 in MDA-MB-453 cells at 1 h was 64%, which was higher than that observed with trastuzumab (2%). Subsequently, U3-1402 was translocated to the lysosome, followed by payload release, leading to an improved cell growth inhibition compared to that noted with patritumab. In vivo antitumor efficacy was also demonstrated in a HER3-dependent manner in MDA-MB-453 xenograft model. In the study, internalization of HER3 from cell membrane into the intracellular compartment by U3-1402 treatment was observed, followed by increased free payload and γH2AX levels in tumor tissue.

Conclusion: U3-1402 has a high internalization property for effective payload delivery to HER3-expressing cancer cells, resulting in a favorable ADC-driven efficacy.

Citation Format: Kumiko Koyama, Yuuri Hashimoto, Yasuki Kamai, Yoshinobu Shiose, Manabu Abe, Yuki Kaneda, Naoyuki Maeda, Kenji Hirotani, Yusuke Ogitani, Tsuyoshi Karibe, Takashi Kagari, Kenichi Wakita, Suguru Ueno, Toshinori Agatsuma, Masato Murakami. U3-1402, a novel HER3-targeting antibody-drug conjugate, exhibits its antitumor activity through increased payload intracellular delivery via highly efficient drug internalization [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-275.