ErbB3, a member of the ErbB receptor family, is a potent mediator in the development and progression of cancer, and its activation plays pivotal roles in acquired resistance against anti-EGFR therapies and other standard-of-care therapies. Upon ligand (NRG1) binding, ErbB3 forms heterodimers with other ErbB proteins (i.e., EGFR and ErbB2), which allows activation of downstream PI3K/Akt signaling. In this study, we developed a fully human anti-ErbB3 antibody, named ISU104, as an anticancer agent. ISU104 binds potently and specifically to the domain 3 of ErbB3. The complex structure of ErbB3-domain 3::ISU104-Fab revealed that ISU104 binds to the NRG1 binding region of domain 3. The elucidated structure suggested that the binding of ISU104 to ErbB3 would hinder not only ligand binding but also the structural changes required for heterodimerization. Biochemical studies confirmed these predictions. ISU104 inhibited ligand binding, ligand-dependent heterodimerization and phosphorylation, and induced the internalization of ErbB3. As a result, downstream Akt phosphorylation and cell proliferation were inhibited. The anticancer efficacy of ISU104 was demonstrated in xenograft models of various cancers. In summary, a highly potent ErbB3 targeting antibody, ISU104, is suitable for clinical development.

ErbB3 is noted as one of major causes of acquired cetuximab resistance in colorectal and head and neck cancers. ErbB3 causes activation of alternative signaling pathways that bypass the original target and sustained PI3K/AKT activation, and these are associated with cetuximab resistance. We confirmed the induced ErbB3 activation by the cetuximab treatment in FaDu head and neck squamous-cell carcinoma (HNSCC) xenograft model. Immunoblot analysis was shown that twice weekly 10 mg/kg of cetuximab treatment upregulated ErbB3 expression and phosphorylation even though tumor growth was well controlled. To investigate whether ErbB3 activation by ligand, heregulin (HRG,) might induce cetuximab resistance, two cetuximab-sensitive colorectal cancer cell lines, DiFi and LIM1215, were treated 0-25 ng/mL of HRG. HRG induced dose-dependent cetuximab resistance in cell proliferation assay, and this was reversed via ISU104 treatment. To evaluate if ISU104 could overcome resistance to cetuximab in vivo, acquired cetuximab-resistant FaDu xenograft model was established. Tumors that had acquired resistance to 5 mg/kg of cetuximab treatment were significantly regressed by replaced treatment of ISU104 alone (10 mg/kg) or combination treatment of ISU104 and cetuximab, while mice continued on cetuximab only showed uncontrolled tumor growth like vehicle-treated group. Our results suggest that ISU104 effectively overcomes cetuximab resistance and may provide clinical benefit to cetuximab-resistant patients. Citation Format: Miyoung Kim, Youngmi Hur, Mirim Hong, Youngsoo Sohn, Kum-Joo Shin, Kyungyong Kim, Seung-Beom Hong, Kum-Joo Shin, Donggoo Bae. ISU104, a fully human anti-ErbB3 antibody, overcomes acquired cetuximab resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 836.

ErbB3, a heterodimeric partner of EGFR or ErbB2, plays important roles in the survival and growth of cancer cells through activation of PI3K/AKT pathway. ErbB3 can be activated by NRG1 expressed either by cancer cells or adjacent mesenchymal cells, or NRG1-fusion proteins produced by genetic alterations in cancer cells. Oncogenic driver mutations in ErbB3 also induce its activation. Previously, we have demonstrated preclinical anti-cancer efficacy of a monoclonal anti-ErbB3 antibody, ISU104 as a monotherapy or in combination with anti-EGFR antibody in various preclinical models. Biomarker analysis of the models demonstrated that there is a significant positive correlation between tumor growth inhibition by ISU104 and NRG1 mRNA/pERBB3 protein expression levels. Based on this analysis, anti-cancer efficacy of ISU104 was further explored in cancer cells and cell line- or patient-derived xenograft cancer models with high NRG1 expression, NRG1-fusion or oncogenic ErbB3 mutations. ISU104 potently inhibited phosphorylation of ERBB3 and AKT, cell proliferation and tumor growth of such models. Potential impacts of other genetic alterations on the anti-cancer efficacy of ISU104 were also investigated. Overall, the presented data suggest that ISU104, an anti-ErbB3 agent in the early stage of clinical development, can be applied for the treatment of the solid tumors expressing high level of NRG1 or harboring genetic alterations such as NRG1-fusion or oncogenic ErbB3 mutations. Citation Format: Seung-Beom Hong, Mirim Hong, Tae-Eun Kim, Ju Yeon Kim, Jung Won Kim, Jeongin Cho, Bongseok Shin, Donggoo Bae. Anti-cancer efficacy of an anti-ErbB3 antibody, ISU104, against the cancers with NRG1-overexpression, NRG1-fusion, or oncogenic ErbB3 mutations [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 3042.

<p>Figure S1-S6, Table S1-S7</p>

<p>Figure S1-S6, Table S1-S7</p>