Actinium Pharmaceuticals, Inc. filed an 8-K on November 2, 2026, announcing that a preclinical abstract for its investigational compound ATNM-400 has been accepted for poster presentation at the 2026 San Antonio Breast Cancer Symposium.

Key Highlights

  • Accepted abstract: Abstract Number 1626, Presentation Number P2-12-17, will appear as a poster at the 2026 San Antonio Breast Cancer Symposium, taking place December 8-11, 2026, in San Antonio, Texas.
  • Presentation slot: The poster is scheduled for Wednesday, December 9, 2026, from 5:00-6:30 p.m. CST in the Exhibit Hall.
  • Compound highlighted: ATNM-400 is described in the filing as a first-in-class Actinium-225 antibody radioconjugate targeting HER3, pairing the monoclonal antibody barecetamab with the alpha-emitting isotope Actinium-225.
  • Breast cancer scope: According to the filing, the data cover efficacy across "therapy-resistant breast cancer subtypes," building on prior preclinical results in non-small cell lung, colorectal, and ovarian cancers.
  • Clinical program noted: The company disclosed that ATNM-400 is currently in a first-in-human Phase 1 study in metastatic castration-resistant prostate cancer (mCRPC).
  • Patent portfolio: Actinium reported holding approximately 250 patents and patent applications, including intellectual property related to cyclotron-based production of Ac-225.

ATNM-400 Breast Cancer Abstract Details and Pan-Tumor Development Context

According to the filing, Actinium Pharmaceuticals (NYSE American:ATNM) announced the acceptance of a preclinical abstract for ATNM-400 at the 2026 San Antonio Breast Cancer Symposium. The abstract, titled "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," carries Abstract Number 1626 and will be presented as Poster P2-12-17 on December 9, 2026.

The filing describes ATNM-400 as combining barecetamab, a HER3-targeting monoclonal antibody, with Actinium-225, an alpha-emitting isotope with an approximately 10-day half-life. The company disclosed that tumor uptake and retention were observed through Day 7 in studies using a zirconium-89-labeled imaging counterpart. The filing characterizes HER3, also known as ERBB3, as "a member of the epidermal growth factor receptor family involved in signaling pathways that support tumor growth, survival and resistance to treatment."

The filing states that the breast cancer findings build on previously reported preclinical activity in non-small cell lung cancer models across EGFR, KRAS, and ALK alterations; in colorectal cancer models with standard-of-care therapies; and in ovarian cancer models including treatment-resistant disease. The company also noted that ATNM-400 is differentiated from PSMA-targeted radiotherapies by its HER3 focus, and cited a range of competing PSMA-directed programs including Novartis' Pluvicto. The filing contains forward-looking statements, which the company states are subject to risks and uncertainties and which it undertakes no obligation to update.

What Was Disclosed

Actinium Pharmaceuticals disclosed the acceptance of a preclinical ATNM-400 abstract for poster presentation at SABCS 2026, providing session details, compound background, and context on the company's broader HER3-directed and radiotherapy pipeline.