Abstract A118: Discovery of HMPL-A251, a first-in-class HER2-directed antibody-targeted therapy conjugate (ATTC) with a novel PI3K/PIKK inhibitor payload

Abstract Background: Although toxin-based ADCs have revolutionized cancer therapy, their long-term use remains hindered by severe toxicities that mirror those of their cytotoxic payloads. These safety challenges further limit their combination potential with chemotherapy, highlighting the need for innovative payload design. Aberrant activation of PI3K-AKT-mTOR pathway (PAM) is associated with poor prognosis and resistance to anti-HER2 therapies. Despite the synergistic effects of dual HER2 and PAM inhibition, systemic toxicity associated with PAM inhibitors limits their clinical application, providing the rationale for developing HMPL-A251, a novel HER2-targeted ATTC conjugated with a potent PI3K/PIKK inhibitor. Methods: HER2 binding and internalization was evaluated by FACS. Confocal microscopic imaging was used to monitor endocytosis. Cell viability was measured by CCK-8 or luminescence 3D cell viability assays. Multiple human xenograft tumors were applied in immune-deficient mice to investigate the anti-tumor activity of HMPL-A251. Results: The payload of HMPL-A251 potently inhibited class I PI3K and PIKK family kinases, with IC50 ranging around 1 to 10 nM. Eurofins profiling across 418 kinases revealed the payload has excellent selectivity. By inhibition of PI3K and PIKK signaling pathways, the payload demonstrated broad anti-tumor activity across a panel of 130 tumor cell lines, with a median IC50 <1 nM. By conjugating this potent payload with an anti-HER2 antibody via a hydrophilic linker, the ATTC compound HMPL-A251 was discovered and demonstrated robust anti-tumor effect. Upon binding to HER2-positive breast cancer cell line with PIK3CAH1047R, HMPL-A251 underwent rapid internalization, lysosomal trafficking, payload release, and PAM and PIKK signaling inhibition to induce tumor cell apoptosis. In a 26-human solid tumor cell line panel, HMPL-A251 showed HER2-dependent cytotoxicity, potently inhibiting HER2-positive tumor cell growth regardless of PAM pathway alterations, with moderately reduced activity in HER2-low/PAM-altered cell lines. HMPL-A251 showed a strong bystander effect when HER2-null cells co-cultured with HER2-positive cells. In vivo, HMPL-A251 demonstrated superior anti-tumor efficacy and tolerability to the naked antibody and payload combination. An intravenous single dose of HMPL-A251 at ≥ 3 mg/kg induced tumor regression across multiple models including HER2-positive and -low models with or without PAM alteration. Its efficacy was well-correlated with payload concentration and target inhibition in tumor tissue. Notably, HMPL-A251 achieved superior or comparable efficacy to T-DXd at the equivalent dose in most tumor models tested. Conclusion: HMPL-A251 demonstrates strong anti-tumor activity in preclinical models, supporting further clinical evaluation. Initiation of phase I human study is expected in December 2025. Citation Format: Jia Hu, Junqing Liang, Yan Xu, Haibin Yang, Min Cheng, Nelson Ng, Peihua Liu, Jiahuan Zhu, Fangfang Mao, Xuelei Ge, Wei Zhang, Qihang Zhang, Shaohui Shen, Pan Wang, Leilei Wu, Xiaoyan Xu, Na Yang, Yu Cai, Jian Wang, Weihan Zhang, Yongxin Ren, Guangxiu Dai, Michael Shi, Weiguo Su. Discovery of HMPL-A251, a first-in-class HER2-directed antibody-targeted therapy conjugate (ATTC) with a novel PI3K/PIKK inhibitor payload [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr A118.