Abstract 3121: Therapeutic potential of a HER3 antibody-drug conjugate for the treatment of HER3-expressing cancers

Abstract To address the need for targeted therapy in the treatment of HER3-positive solid tumors, the human HER3 mAb, seribantumab, was evaluated in the context of an antibody-drug conjugate (ADC). As a proof-of-concept, seribantumab was conjugated with a cleavable valine-citrulline linker and monomethyl auristatin E (MMAE) payload via the stochastic cysteine conjugation method to yield HER3-ADC1. A drug-antibody ratio of 4 was selected for preclinical assessment in HER3-expressing models. Methods: HER3-ADC1 was evaluated in vitro and in vivo, with patritumab deruxtecan (patri-DXd) as a comparator. Binding to BT474 breast carcinoma cells (HER3 high; immunohistochemical (IHC) staining intensity 3+) was measured by flow cytometry. Internalization properties were established using seribantumab and patritumab coupled to Fab-AF488 following 4 hours incubation in SK-BR-3 breast adenocarcinoma (HER3 high; IHC 3+) and HCC1569 breast carcinoma (HER3 low; IHC 0-1+) cell lines. In vitro cytotoxicity was evaluated for HER3-ADC1, isotype-MMAE and free MMAE payload as well as patri-DXd, isotype-DXd and free deruxtecan payload in BT474 cells, SK-BR-3 cells and NCI-H446 lung carcinoma (HER3 low; IHC 0-1+) cells. In vivo anti-tumor activity was assessed for HER3-ADC1, isotype-MMAE, patri-DXd and isotype-DXd in patient derived xenograft (PDX) models of pancreatic (HER3 high; IHC 3+) and breast cancer (HER3 low; IHC 0-1+). Results: HER3-ADC1 binding to cancer cells, endocytosis, MMAE release, and inhibition of proliferation were dependent on HER3 expression. In assays investigating antibody internalization, seribantumab displayed similar internalization capacity as patritumab. For both HER3 mAbs, internalization was greater in a HER3 high than a HER3 low cell line. In cytotoxicity assays, HER3-ADC1 and patri-DXd displayed HER3 expression level-dependent cell killing. HER3-ADC1 outperformed patri-DXd in HER3 high cell lines based on in vitro cytotoxicity. HER3-ADC1 and patri-DXd were ineffective against NCI-H466 cells with low HER3 expression. In a PDX model of pancreatic cancer (HER3 high), HER3-ADC1 induced tumor regression, while isotype-MMAE had only a moderate anti-tumor effect. Both patri-DXd and isotype-DXd had moderate anti-tumor effects, indicating a lack of HER3 target dependence. In a PDX model of breast cancer (HER3 low), HER3-ADC1 and isotype-MMAE lacked anti-tumor activity; isotype-DXd had a moderate anti-tumor effect and patri-DXd had a substantial anti-tumor effect. Conclusions: HER3-ADC1 demonstrated target-dependent in vitro cytotoxicity and in vivo anti-tumor activity in a HER3-expressing pancreatic cancer PDX model. Results from in vitro and in vivo studies highlight the promising therapeutic potential of a seribantumab-based ADC for patients with HER3-expressing cancers. Additional results on the optimization and characterization of HER3-ADC1 will be presented. Citation Format: Thomas O'Hare, Jaclyn Cleveland, Valerie M. Jansen, David Dornan. Therapeutic potential of a HER3 antibody-drug conjugate for the treatment of HER3-expressing cancers abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 3121.