Actinium Pharmaceuticals Advances Trop-2 Targeted Alpha Radiotherapy Into Phase 1/2 Trials in the U.S. and Australia

Actinium Pharmaceuticals (NYSE American: ATNM) is advancing its Trop-2-targeted ATNM-400 construct into Phase 1/2 trials across the United States and Australia, marking an important step as the company broadens its presence in solid-tumor radiopharmaceuticals. With targeted alpha radiotherapy gaining momentum worldwide, the program reinforces Actinium's position as one of the more established players developing actinium-225-based therapeutics for treatment-resistant and late-stage cancers.

Why Trop-2 Remains One of Oncology's Most Valuable Targets

Trop-2 has become a central antigen in oncology drug development due to its strong expression across several major tumor types, including breast cancer, NSCLC, ovarian cancer, pancreatic cancer, and other epithelial malignancies. The success of prior Trop-2-directed therapies helped validate its commercial relevance, but many tumors eventually develop resistance to standard modalities. Actinium's approach differs by delivering actinium-225-based alpha radiation directly to Trop-2-positive cells, a payload capable of producing deeper DNA damage and potentially offering activity in tumors that have failed multiple prior lines of therapy.

Actinium-225 as a Driver of Next-Generation Radiotherapeutics

Actinium-225 is recognized for its ability to emit high-energy alpha particles that travel only a few cell diameters, minimizing systemic toxicity while inducing irreparable DNA double-strand breaks. Its potency, short path length, and ability to function across diverse microenvironments make it an attractive isotope for difficult-to-treat solid tumors.

A major barrier for many companies pursuing alpha therapies is reliable isotope supply. Actinium Pharmaceuticals remains one of the few developers with established actinium-225 production and radiochemistry capabilities. This internal infrastructure supports multiple clinical-stage and preclinical programs simultaneously, including Trop-2, B7-H3, Nectin-4, and STEAP-1, and reduces the supply constraints that have limited trial expansion for competing platforms.

Global Trial Strategy Across the U.S. and Australia

The Trop-2 program is one of the few actinium-225 constructs entering clinical evaluation simultaneously in two major regulatory regions. Multi-country trial execution provides broader enrollment access, accelerates data generation, and enables visual confirmation of tumor uptake through imaging, which can validate targeting early in clinical development.

The Phase 1 stage will define safety, dose levels, and biodistribution, while Phase 2 expansion cohorts are expected to explore activity across multiple Trop-2-positive cancers. Operating in established radiopharmaceutical ecosystems in both the U.S. and Australia also enhances collaboration opportunities and global visibility.

Reinforcement From New Breast Cancer Data Presented at SABCS 2025

New preclinical data for ATNM-400 presented at the San Antonio Breast Cancer Symposium further support its clinical advancement. The construct showed robust anti-tumor activity in hormone-receptor positive, HER2-positive, and triple-negative breast cancer, including models resistant to tamoxifen and trastuzumab. These findings underscore ATNM-400's ability to overcome resistance mechanisms that limit the effectiveness of endocrine therapy, HER2-targeted agents, and certain antibody-drug conjugates.

Importantly, the targeted antigen associated with ATNM-400 is significantly upregulated in resistant disease settings, suggesting an opportunity for patient-selection strategies that could sharpen clinical benefit. The therapy also demonstrated favorable tolerability and combination potential with standards of care.

Pan-Tumor Potential Across Prostate Cancer and NSCLC

Beyond breast cancer, ATNM-400 has produced strong preclinical results in metastatic castration-resistant prostate cancer and non-small cell lung cancer. In prostate cancer, the agent targets a non-PSMA antigen associated with progression and therapy resistance, offering a differentiated path for patients who fail ARPI therapy or PSMA-directed radioligands. In NSCLC, ATNM-400 outperformed EGFR-targeting approaches and showed synergy with osimertinib, including activity in post-EGFR-resistant settings.

These data highlight the multi-indication versatility of the ATNM-400 platform and support development strategies that include monotherapy, combinations, and sequencing with standard treatments.

Conclusion

The advancement of Trop-2-targeted ATNM-400 into Phase 1/2 trials across the U.S. and Australia represents a meaningful evolution for Actinium Pharmaceuticals as it transitions into a multi-program, multi-region radiopharmaceutical company. New breast cancer data add further credibility to the platform's potential, while strong preclinical performance in prostate cancer and NSCLC demonstrates the breadth of its applicability.

Actinium's established actinium-225 supply chain, validated targeting strategies, and growing global clinical footprint position the company to compete effectively in one of oncology's fastest-advancing therapeutic categories. As alpha radiotherapy gains wider recognition for its ability to target resistant and heterogeneous tumors, the ATNM-400 platform stands out as a clinically ambitious and strategically differentiated approach with the potential for long-term impact across several major solid-tumor markets.

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