Abstract Background: Hormone receptor positive (HR+) is the most common subtype of breast cancer (BC) and comprises 70% of BC diagnosis. Tamoxifen is the most widely prescribed endocrine therapy (ET) for modulating estrogen receptors, however, ∼20-30% of BC patients have disease recurrence despite ET. Similarly, resistance to HER2-targeted therapy trastuzumab (Herceptin) can develop in HER2+ BC. While antibody drug conjugates (ADCs) like trastuzumab deruxtecan (Enhertu) show strong efficacy in HER2+ and HER2-low BC, their use is limited by interstitial lung disease toxicities. In contrast, Actinium-225-based radioconjugates may provide potent tumor killing with less off-target lung toxicity. Here, we report the development of ATNM-400, a novel antibody radioconjugate using Actinium-225 to target a protein that is overexpressed in BC. The target is linked to disease progression and metastasis, with overexpression correlating with poor clinical outcomes in BC patients. Furthermore, target expression is elevated in patients who develop ET and HER2 therapy resistance. Here, we evaluated the anti-tumor efficacy of ATNM-400 in preclinical BC models and evaluated ATNM-400’s potential to overcome current therapeutic limitations and improve outcomes beyond what is achievable with tamoxifen or trastuzumab. Methods: ATNM-400 was synthesized by conjugating ATNM400 antibody with p-SCN-Bn-DOTA, followed by radiolabeling with Ac-225 and characterized by radio-iTLC, HPLC and mass spectrometry. Target expression, binding affinity, and internalization were assessed using flow cytometry and radioactivity quantification in parental and drug-resistant BC cell lines, including MCF-7 (HR+), MCF-7 Tam1 (tamoxifen-resistant), BT-474 (HER2+), BT-474 Clone 5 (trastuzumab-resistant), and MDA-MB-468 (triple-negative). In vivo biodistribution and efficacy were evaluated in murine xenograft models. Efficacy of ATNM-400 alone and in combination with standard-of-care (SOC) therapies was assessed in both in vitro and in vivo resistance models. Results: ATNM-400 demonstrated over 98% radiochemical purity. ATNM-400 exhibited strong and specific binding and internalized in target-positive BC cell lines. Target expression was significantly elevated in tamoxifen- and trastuzumab-resistant lines, supporting its mechanistic link to resistance. In vitro, ATNM-400 induced potent, dose-dependent cytotoxicity as monotherapy, which was further enhanced when combined with SOC agents. In vivo, ATNM-400 achieved tumor growth inhibition (TGI) of 76.4% and 111.5% in MCF-7 xenografts at 20 and 40 μCi/kg, respectively, with the lower dose being well tolerated. In MDA-MB-468 xenografts, TGI reached 66.3% and 103.4% at corresponding doses, both of which were well tolerated. Notably, ATNM-400 retained efficacy in both tamoxifen- and trastuzumab-resistant models, demonstrating its potential to overcome resistance mechanisms. Conclusions: ATNM-400 shows strong anti-tumor efficacy and favorable tolerability across multiple BC subtypes, including models resistant to tamoxifen and trastuzumab. These findings support further development of ATNM-400 as a novel therapeutic approach for patients with limited options following endocrine or HER2-targeted therapy failure-both as monotherapy and rational combination regimens. Citation Format: A. Kamal, A. S. Chin, S. Mukherjee, J. Li, K. Peregrina, D. Lewis, H. Sethi, L. Xu, D. Patel, M. Roy, A. Bardia. Anti-tumor activity of ATNM-400, a first-in-class Actinium-225 antibody radioconjugate, in tamoxifen and trastuzumab resistant breast cancer models abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-04-26.
Kamal et al. (Tue,) studied this question.