Abstract C092: ATNM-400, a first-in-class Actinium-225 antibody radioconjugate, demonstrates potent anti-tumor activity and overcomes osimertinib resistance in lung cancer models

Abstract Background: Non-small cell lung cancer (NSCLC) is the most prevalent type of lung cancer (LC), comprising of about 85% of all lung cancer cases. EGFR (Epidermal Growth Factor Receptor) mutant NSCLC is detected in about 10-15% of NSCLC cases in Western countries and up to 50% in some Asian populations. EGFR tyrosine kinase inhibitors (TKIs) SOC (standard-of-care), such as Osimertinib (Osi), have revolutionized treatment for advanced EGFR-mutated NSCLC but acquired resistance to Osi inevitably develops, requiring innovative strategies to overcome resistance. Here, we report on the development of ATNM-400, a novel antibody radioconjugate using Actinium-225 (Ac-225). ATNM-400 targets a protein that is associated with poor prognosis in NSCLC, is overexpressed in LC and is linked to increased metastasis and resistance to treatments like EGFR TKIs. In this study, we assessed the anti-tumor efficacy of ATNM-400 in preclinical LC models to evaluate its potential to overcome current therapeutic limitations and improve outcomes beyond what is achievable with Osi. Methods: ATNM-400 was synthesized by conjugating ATNM400 antibody with p-SCN-Bn-DOTA, followed by radiolabeling with Ac-225 and product characterization included radio-iTLC, HPLC and mass spectrometry. Target expression, binding affinity, and internalization were assessed using flow cytometry and radioactivity quantification in NCI-H1975 (L858R and T790M EGFR mutations, and TP53 mutation) and Calu-3 (amplified HER2, wild-type EGFR, mutant K-Ras (G13D), and TP53 and CDKN2A mutations). In vivo biodistribution of ATNM-400 and efficacy of ATNM-400 alone and in combination with SOC Osi was assessed in NCI-H1975 LC in vivo resistance models. ATNM400 efficacy was also tested in Calu-3 LC model. Results: ATNM-400 exhibited strong specific binding and significant internalization in target-positive LC cell lines. In vitro, target expression was significantly elevated in NCI-H1975 cells after Osi treatment, supporting its mechanistic link to resistance. ATNM-400 induced potent, dose-dependent cytotoxicity as monotherapy alone, in combination with Osi and post-Osi resistance. The increased target expression on Osi treated NCI-H1975 cells resulted in significantly enhanced ATNM-400 cytotoxicity, even at low non-cytotoxic Osi doses. In vivo, ATNM-400 achieved durable tumor growth regressions in Calu-3 xenografts at both 20 and 40 μCi/kg, with both doses being well tolerated. In NCI-H1975 xenografts, tumor growth inhibition (%TGI) reached 65% with 40 μCi/kg ATNM-400, 35% with Osi, and 99% with 40 μCi/kg ATNM-400 and Osi combo, all of which were well tolerated. This demonstrates the potential of ATNM-400 to overcome Osi resistance. Conclusions: ATNM-400 shows strong anti-tumor efficacy and favorable tolerability across multiple LC subtypes, including Osi resistant models. These findings support further development of ATNM-400 as a novel therapeutic approach for LC patients with limited options following TKI therapy failure- both as monotherapy and in rational combination regimens or post-resistance to TKI. Citation Format: Sumit Mukherjee, Amanda S. Chin, Karina Peregrina, Jason Li, Debbie Lewis, Heer Sethi, Le-Cun Xu, Dhiren Patel, Monideepa Roy, Adeela Kamal. ATNM-400, a first-in-class Actinium-225 antibody radioconjugate, demonstrates potent anti-tumor activity and overcomes osimertinib resistance in lung cancer models [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 C092.