On November 2, 2026 Actinium Pharmaceuticals, Inc. (NYSE American: ATNM) (Actinium or the Company), a pioneer in differentiated targeted radiotherapies with the potential to transform cancer treatment, reported that an abstract highlighting preclinical efficacy of ATNM-400 across therapy-resistant breast cancer subtypes has been accepted for poster presentation at the 2026 San Antonio Breast Cancer Symposium (SABCS), taking place December 8–11, 2026, in San Antonio, Texas.
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The presentation will feature ATNM-400, the Company’s investigational, first-in-class Actinium-225 (²²⁵Ac) antibody radioconjugate targeting HER3, an antigen implicated in tumor survival and treatment resistance. ATNM-400 combines the targeting specificity of a monoclonal antibody with potent alpha-particle radiation to deliver a tumor-directed cytotoxic payload.
The breast cancer findings build on previously reported preclinical results demonstrating antitumor activity in non-small cell lung cancer (NSCLC) models across EGFR, KRAS and ALK alterations, enhanced responses with standard-of-care therapies in colorectal cancer (CRC) models, and durable activity in ovarian cancer models, including treatment-resistant disease. Together, these findings reinforce ATNM-400’s pan-tumor potential and support further evaluation as a standalone therapy and in combination with established treatments across multiple solid tumor indications, complementing its ongoing clinical development in prostate cancer.
"We are excited to showcase ATNM-400’s activity in therapy-resistant breast cancer models at SABCS," said Sandesh Seth, Actinium’s Chairman and CEO. "Building on our previously reported NSCLC, colorectal and ovarian cancer results, these findings strengthen our vision for developing ATNM-400 as a pan-cancer radiotherapy. HER3 is implicated in resistance to standard-of-care therapies across several solid tumors and treatment settings, which implies ATNM-400’s potential extends well beyond a single oncology setting. Together with our ongoing first-in-human prostate cancer program, this growing body of evidence supports our strategy to pursue ATNM-400’s broad potential and address treatment-resistant disease across large patient populations."
ATNM-400 SABCS 2026 Presentation Details
Abstract Title: ATNM-400 a first-in-class targeted alpha therapy targeting a driver of tumor biology and treatment resistance demonstrates efficacy across therapy-resistant breast cancer subtypes
Abstract Number: 1626
Presentation Number: P2-12-17
Presentation Type: Poster presentation
Presentation Date & Time: Wednesday, December 9, 2026, 5:00–6:30 p.m. CST
Location: Exhibit Hall
About ATNM-400 and HER3
ATNM-400 is an investigational antibody radioconjugate that pairs the HER3-targeting monoclonal antibody barecetamab with the alpha-emitting isotope Actinium-225. Its antibody-based design combines antigen-specific targeting with the potential for sustained tumor localization, supporting prolonged delivery of high-energy alpha radiation. Tumor uptake and retention observed through Day 7 with the ⁸⁹Zr-labeled imaging counterpart support this development rationale alongside the approximately 10-day half-life of ²²⁵Ac. The theranostic approach enables noninvasive visualization of antibody distribution to inform therapeutic development.
ATNM-400 is differentiated from PSMA-targeted radiotherapies by its focus on HER3, an antigen implicated in tumor survival and treatment resistance. PSMA-directed approaches include Novartis’ Pluvicto and a broad pipeline of investigational therapies, including Telix Pharmaceuticals’ TLX591-Tx, Clarity Pharmaceuticals’ ⁶⁷Cu-SAR-bisPSMA, Convergent Therapeutics’ CONV01-α, AstraZeneca’s AZD2265 (formerly FPI-2265), Curium’s lutetium-177 zadavotide guraxetan (¹⁷⁷Lu-PSMA-I&T), ARTBIO’s AB001 and AdvanCell’s ADVC001. These programs span small-molecule and antibody-based therapies using beta- or alpha-emitting isotopes. ATNM-400 combines HER3 targeting with an Actinium-225 antibody radioconjugate, offering a distinct biological approach with the potential to expand radiotherapy across the broad mCRPC population, including patients with PSMA-low or PSMA-negative disease and those who do not respond to or progress following PSMA-directed treatment. Its HER3-directed mechanism also supports investigation beyond prostate cancer in other HER3-expressing solid tumors.
HER3, also known as ERBB3, is a member of the epidermal growth factor receptor family involved in signaling pathways that support tumor growth, survival and resistance to treatment. HER3-directed antibody-drug conjugates in clinical development include BioNTech and MediLink’s BNT326/YL202, Bristol Myers Squibb and SystImmune’s EGFR × HER3 bispecific ADC izalontamab brengitecan (iza-bren), and Daiichi Sankyo and Merck’s patritumab deruxtecan (HER3-DXd). These programs underscore the clinical development interest in HER3 as an oncology target. ATNM-400 applies a distinct payload strategy, using alpha radiation in place of a conventional cytotoxic drug. Its preclinical activity in prostate, non-small cell lung and breast cancer models supports investigation as a potential pan-cancer radiotherapy, as a monotherapy and in rational combinations.
(Press release, Actinium Pharmaceuticals, NOV 2, 2026, View Source [SID1234674052])