Actinium To Present New Breast Cancer Data On Its ATNM-400 Radiotherapy At San Antonio Symposium

Author: Benzinga Newsdesk | November 04, 2025 07:34am

Actinium Pharmaceuticals, Inc. (NYSE American: ATNM) ("Actinium" or the "Company"), a leader in the development of differentiated targeted radiotherapies, announced today that new preclinical data for its lead antibody radioconjugate program, ATNM‑400, will be presented at the 2025 San Antonio Breast Cancer Symposium (SABCS) taking place December 10‑14, 2025 in San Antonio, Texas.

 

Title: Anti‑tumor activity of ATNM‑400, a first‑in‑class Actinium‑225 antibody radioconjugate, in tamoxifen and trastuzumab resistant breast cancer models 

Abstract Number: 2069

Presentation Number: PS4‑04‑26

Date/Time: Thursday, December 11, 2025, 5:00 PM–6:30 PM CT

Session: Poster Session 4

The ATNM-400 data presentation at SABCS follows prostate cancer data presentations at the American Association for Cancer Research (AACR) annual meeting and the Society of Nuclear Medicine and Molecular Imaging (SNMMI) annual meeting, NSCLC data was presented at the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics, from which key findings are summarized below.

ATNM‑400: Program and Multi-Indication Opportunity Overview

ATNM-400 is a first-in-class antibody-radioconjugate powered by Actinium-225 (Ac-225), designed to deliver potent alpha-particle radiation directly to tumor cells. The therapy's high linear energy transfer (LET) enables precise tumor cell killing with minimal off-target exposure, while its target - a disease-driving protein linked to resistance and poor prognosis - is overexpressed across multiple solid tumors, including prostate, lung, and breast cancers.

Preclinical studies show ATNM-400 delivers best-in-class efficacy, overcomes resistance to, and synergizes with, standard-of-care therapies, supporting broad development as a monotherapy, combination, or treatment alternative across multiple solid tumors.

Prostate Cancer Data Highlights

1. Superior efficacy and durability vs 177Lu-PSMA-617 (active agent in Pluvicto®), 225Ac-PSMA-617, and ARPI enzalutamide (active agent in Xtandi®), with durable tumor control beyond 100 days.
2. Improved overall survival compared to 177Lu-PSMA-617 and enzalutamide.
3. Overcomes resistance to these standard-of-care agents, showing sustained tumor control and survival benefit in 177Lu-PSMA-617 and enzalutamide resistant prostate cancer models.
4. Synergy with ARPI therapy as the ATNM-400 target is upregulated after enzalutamide; combination achieved complete tumor regression in 40% of animals.
5. PSMA-independent activity enables treatment of patient populations not eligible for or progressing on 177Lu-PSMA-617.

NSCLC Data Highlights

1. 3–5x greater tumor growth inhibition vs. front-line osimertinib (Tagrisso®, AstraZeneca) and EGFR tyrosine kinase inhibitor (TKI), second-line Dato-DXd (Datroway®, AstraZeneca/Daiichi Sankyo) a Trop-2 ADC, and third-line amivantamab (Rybrevant®, Johnson & Johnson) an EGFR-cMET bispecific. Combined 2024 sales of these agents exceeded $7B.
2. Target upregulation following EGFR inhibition; ATNM-400 + osimertinib achieved complete tumor regression in 100% of tumor-bearing animals demonstrating synergy of the combination.
3. Clinical rationale for combination supported by a study that showed EBRT or external-beam radiotherapy + osimertinib improved PFS to 32.2 months vs. 20 months with osimertinib alone (Sampath et al.1, Lancet eClinicalMedicine, 2025). ATNM-400 has the potential to deliver precision targeted, powerful alpha radiation via Ac-225 which on a per-cell basis is ~4–8x more biologically lethal than diffuse, low-energy EBRT beams. Clinically, this may translate to higher response rates, lower toxicity, and entry into previously untreatable market segments when osimertinib is combined with EBRT.

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