On November 11, 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 the RADIANZ ("Radiolabeled Antibody Distribution In Advanced Neoplasms with Zirconium-89") clinical trial is enrolling patients. RADIANZ is an imaging study of ⁸⁹Zr-Barecetamab-DFO using PET to guide the pan-tumor development of ²²⁵Ac-Barecetamab-DOTA (ATNM-400) in solid tumors outside of prostate cancer.
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RADIANZ builds on ATNM-400’s first-in-human Phase 1 study in metastatic castration-resistant prostate cancer (mCRPC). ATNM-400 is an investigational, first-in-class Actinium-225 antibody radioconjugate targeting HER3, an antigen implicated in tumor survival and treatment resistance across multiple solid tumors. Initial results from that study were presented in October 2026 as an oral presentation at the 39th Annual Congress of the European Association of Nuclear Medicine (EANM). The first three patients received ATNM-400 at 1.0 µCi/kg with no dose-limiting toxicities, HER3-targeted PET imaging showed tumor uptake retained through Day 7, and dose escalation advanced to 1.5 µCi/kg. The mCRPC study was also featured in at the European Society for Medical Oncology (ESMO) (Free ESMO Whitepaper) Congress 2026. At EANM 2026, Actinium presented preclinical data in NSCLC, colorectal and ovarian cancer models demonstrating the potential of ATNM-400 either as a single agent of in combination with approved drugs. The Company will also present additional breast cancer data for ATNM-400 at the San Antonio Breast Cancer Symposium (SABCS) taking place December 8-11, 2026.
"Dosing the first patient in RADIANZ is a major step in our pan-cancer strategy for ATNM-400," said Sandesh Seth, Actinium’s Chairman and CEO. "The initial clinical results we presented at EANM and the study we highlighted at ESMO (Free ESMO Whitepaper) established ATNM-400 in the clinic in prostate cancer. Our preclinical work across lung, breast, colorectal and ovarian cancer models demonstrates HER3 targeting potential beyond prostate cancer. With RADIANZ, we are now testing that potential in patients and building a clinical path for ATNM-400 across several large, treatment-resistant patient populations."
RADIANZ Study Design
The RADIANZ imaging study will include patients with various tumor types, including NSCLC, breast, colorectal, pancreatic and ovarian cancers who have progressed on standard-of-care therapy. Patients receive Zirconium-89-barecetamab PET imaging with time points up to two weeks to demonstrate tumor retention of the antibody radioconjugate (ARC) in different cancer types over this time span. The study evaluates biodistribution, tumor uptake, pharmacokinetics (PK), and dosimetry across different tumor types.
"Many patients with advanced solid tumors have few options once standard therapies stop working," said Steffen Heeger, MD, Actinium’s Chief Medical Officer. "HER3 has been implicated in resistance to targeted therapies, chemotherapy and hormonal therapies across several solid tumors. The RADIANZ Imaging study will give us further insights on how ATNM-400 can be further developed in different solid tumors beyond prostate cancer. We are excited at the potential ATNM-400 offers as the first of its kind HER3 targeting radiotherapy and the opportunity to develop a truly differentiated pan cancer product."
About ATNM-400
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.
About HER3
HER3, also known as ERBB3, belongs to the epidermal growth factor receptor family and participates in signaling pathways that promote tumor growth, survival and treatment resistance. HER3-directed antibody-drug conjugates (ADCs) 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 highlight clinical development interest in HER3 as an oncology target. ATNM-400 takes a different payload approach, delivering alpha radiation rather than a conventional cytotoxic drug. Preclinical activity in prostate, non-small cell lung, breast, colorectal and ovarian cancer models supports investigating ATNM-400 as a potential pan-cancer radiotherapy, both as monotherapy and in rational combinations.
(Press release, Actinium Pharmaceuticals, NOV 11, 2026, View Source [SID1234674609])