Abstract LB365: Mechanistic and early clinical characterization of VRN101099, a selective HER2 inhibitor with receptor degradation activity

Abstract ERBB2 (HER2) is a key oncogenic driver in multiple solid tumors, where amplification or activating mutations promote aberrant kinase signaling. Beyond catalytic activity, HER2 signaling is regulated by receptor dimerization, trafficking, and degradation, indicating that therapeutic strategies combining kinase inhibition with receptor downregulation may achieve more complete pathway suppression. Several HER2-directed tyrosine kinase inhibitors (TKIs), including tucatinib, zongertinib, and sevabertinib, have demonstrated clinical activity in HER2-driven cancers. However, these agents primarily function as ATP-competitive inhibitors and do not directly promote HER2 internalization or degradation. Prior observations with multi-target covalent TKIs suggest that specific covalent binding modes can render HER2 susceptible to proteolytic turnover. VRN101099 is a novel covalent HER2 kinase inhibitor with high selectivity over off-target kinases, including EGFR. Mechanistic studies demonstrate that VRN101099 induces a unique HER2 conformational state associated with lysosome-dependent degradation. In HER2-amplified (BT474, SKBR3, N87, and HCC1569) and in some HER2-mutant (5637 cells with HER2-S310F) cancer cells, VRN101099 markedly reduced both total and phosphorylated HER2 levels compared with other HER2 TKIs. In In-Cell Western assays, VRN101099 achieved maximal inhibition (100%) of phosphorylated HER2, exceeding that observed with tucatinib (56%), zongertinib (62%), and sevabertinib (72%) under comparable conditions, indicating that receptor degradation enables suppression of HER2 signaling. This effect was abrogated by bafilomycin A1, confirming lysosome-dependent degradation. Consistently, flow cytometry demonstrated reduced cell-surface HER2 following VRN101099 treatment, whereas tucatinib and zongertinib increased surface HER2 levels. Given that HER2-directed antibody-drug conjugates (ADCs), including trastuzumab deruxtecan (T-DXd) and trastuzumab emtansine (T-DM1), require receptor internalization and lysosomal processing for activity, VRN101099 showed synergistic antitumor activity with these ADCs in HER2-driven xenograft models. A first-in-human, phase 1 dose-escalation study of VRN101099 monotherapy is ongoing. As of January 2026, partial responses (30% tumor reduction) have been observed at 160-mg and 240-mg dose levels in patients with HER2-activating mutations (including S310Y, V777L) or HER2 amplification, including individuals previously treated with multiple HER2-targeted therapies such as T-DXd. Collectively, these data support VRN101099 as a mechanistically differentiated, HER2-selective covalent inhibitor that promotes receptor internalization and degradation, providing a rationale for its continued clinical development both as monotherapy and in combination with HER2-directed ADCs. Citation Format: Jun-Young Park, Jihye Yoo, Rasha Cosman, Jee Hung Kim, Christopher Steer, Jieun Lee, Yeon Hee Park, Seock-ah Im, Sun Young Rha, Jee Hyun Kim, Kyung Hae Jung. Mechanistic and early clinical characterization of VRN101099, a selective HER2 inhibitor with receptor degradation activity abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86 (8Suppl): Abstract nr LB365.