Actinium Pharmaceuticals Unveils ATNM‑400 Data at EANM 2026 with a First‑In‑Human HER3‑Targeted Radiotherapy Approach Beyond PSMA in Prostate Cancer

Repeated dosing in the initial three-patient cohort produced no observed dose-limiting toxicities during the reported observation period.

Rhea-AI Summary

Actinium Pharmaceuticals (NYSE American: ATNM) reported initial Phase 1 results for ATNM‑400 in advanced prostate cancer and advanced dose escalation. ATNM‑400 targets HER3 and carries Actinium‑225 radiation; the study treats metastatic castration‑resistant prostate cancer, which progresses despite testosterone suppression. The first three patients received 1.0 µCi/kg, with no dose-limiting toxicities, dry mouth or Grade 3/4 blood-related adverse events observed during the toxicity window or reported follow-up. All received at least two cycles; two received three.

HER3-targeted PET imaging showed tumor uptake retained through Day 7. These imaging findings concern the labeled imaging agent, not a reported clinical tumor response. Dose escalation has reached 1.5 µCi/kg. The study aims to establish a recommended Phase 2 dose. Preclinical activity extended across prostate cancer models with high, low or absent PSMA expression.

Positive

• Minor pointInitial three-patient, 1.0 µCi/kg cohort had no observed dose-limiting toxicities, dry mouth or Grade 3/4 blood-related adverse events.
• Minor pointOther safety observations included no ALT/AST or creatinine elevations, injection-site reactions or infusion-related reactions.
• Minor pointRepeat dosing reached at least two cycles in all three patients, including three cycles in two patients.
• Minor pointHER3-targeted imaging showed tumor uptake retained through Day 7.
• Minor pointDose escalation advanced to 1.5 µCi/kg after clearance of the first cohort.

3 minor points

• Minor pointPreclinical antitumor activity covered PSMA-high, PSMA-low and PSMA-negative prostate cancer models, including one with low HER3 expression.
• Minor pointPreclinical findings included sustained tumor control after enzalutamide failure and durable activity combined with enzalutamide.
• Minor point. Forward-looking: it has not happened yet and may not happen.Actinium's development strategy spans monotherapy and ARPI combinations, with investigation beyond prostate cancer.

Negative

• None.

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Key Figures

Initial cohort: 3 patients Initial dose level: 1.0 µCi/kg Dose escalation: 1.5 µCi/kg +3 more

Initial cohort

3 patients

First dose level in the first-in-human Phase 1 mCRPC study

Initial dose level

1.0 µCi/kg

Dose received by the initial cohort

Dose escalation

1.5 µCi/kg

Second dose level; escalation is ongoing

Observed safety events

None observed

No dose-limiting toxicities, xerostomia, or Grade 3/4 hematologic adverse events reported

Imaging retention

Through Day 7

HER3-directed imaging showed lesion uptake retained through Day 7

Actinium-225 half-life

10 days

The reported imaging retention timeframe was consistent with this half-life

Key Terms

mcrpc, dose-limiting toxicities, pharmacokinetics, dosimetry, +2 more

6 terms

mcrpc medical

"in metastatic castration-resistant prostate cancer (mCRPC)"

mCRPC stands for metastatic castration‑resistant prostate cancer, a form of prostate cancer that has spread beyond the prostate and keeps progressing despite treatments that lower male hormones. It matters to investors because this stage is harder to treat, drives demand for new therapies, and often involves large, expensive clinical trials and regulatory decisions that can strongly influence a drug maker’s future revenue and stock value—think of it as a stubborn problem that creates both medical need and commercial opportunity.

dose-limiting toxicities medical

"No dose-limiting toxicities, xerostomia (dry mouth)"

Dose-limiting toxicities are the harmful side effects seen in early clinical trials that are severe enough to stop researchers from raising a drug’s dose. Like a car’s speed limiter marking the safe top speed, DLTs define the maximum tolerable dose, and they matter to investors because they determine whether a medicine can reach effective levels, influence development timelines, costs, and regulatory chances, and thus affect a drug’s commercial prospects.

pharmacokinetics medical

"The study evaluates safety, dosimetry and pharmacokinetics"

Pharmacokinetics is the study of how a substance, such as a drug or chemical, moves through and is processed by the body over time. It tracks how it is absorbed, distributed, broken down, and eventually eliminated. For investors, understanding pharmacokinetics helps gauge the effectiveness, safety, and potential risks of new medications or treatments, which can influence a company’s success and valuation in the healthcare industry.

dosimetry medical

"The study evaluates safety, dosimetry and pharmacokinetics"

Dosimetry is the measurement and calculation of how much ionizing radiation is absorbed by people, tissues, or devices, similar to a thermostat tracking temperature in different rooms to know where and how much heat is present. Investors should care because accurate dosimetry underpins safety, treatment effectiveness, regulatory approval, and liability for products and services that use radiation—affecting market access, costs, and commercial risk.

objective response rate medical

"including PSA responses, objective response rate, radiographic progression-free survival"

The objective response rate (ORR) is the percentage of patients in a clinical trial whose tumors measurably shrink or disappear according to preset rules. Investors use it as a quick, objective signal of a drug’s ability to produce a clear treatment effect—like counting how many plants visibly respond after applying a new fertilizer—and higher ORR can improve odds of regulatory approval, commercial success, and company valuation.

radiographic progression-free survival medical

"objective response rate, radiographic progression-free survival and overall survival"

The length of time during which a patient’s cancer does not get worse on medical imaging scans; it is measured from the start of treatment until scans show tumor growth or new lesions. Investors care because it’s a common measure used in clinical trials to signal whether a therapy is working — like a stopwatch showing how long a treatment keeps disease stable — and strong results can influence regulatory approval, sales potential and stock value.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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• HER3‑targeted PET imaging demonstrated tumor uptake and retention, supporting ATNM‑400's potential to deliver sustained alpha radiation to tumors over a timeframe consistent with Actinium‑225's 10-day half‑life
• Initial three‑patient cohort completed the first dose level of 1.0 µCi/kg with no dose‑limiting toxicities, xerostomia or Grade 3/4 hematologic adverse events observed
• ATNM‑400 advanced to the second dose level of 1.5 µCi/kg, marking continued clinical progress for a first‑in‑class Actinium‑225 antibody radiotherapy with potential across the broad mCRPC population

NEW YORK, Oct. 20, 2026 /PRNewswire/ -- Actinium Pharmaceuticals, Inc. (NYSE American: ATNM) (Actinium or the Company), a pioneer in differentiated targeted radiotherapies with the potential to transform cancer treatment, today announced encouraging initial clinical results from the first‑in‑human Phase 1 study of ATNM‑400 in metastatic castration‑resistant prostate cancer (mCRPC). The findings, presented in an oral presentation on October 19 at the 39th Annual Congress of the European Association of Nuclear Medicine (EANM) in Vienna, Austria, showed a favorable initial safety profile at the first dose level and tumor uptake and retention with the HER3‑targeted imaging agent. Dose escalation has advanced to 1.5 µCi/kg and is ongoing.

ATNM‑400 is a first‑in‑class, HER3‑targeted alpha radiotherapy that conjugates the monoclonal antibody barecetamab with Actinium‑225 (²²⁵Ac). HER3 is implicated in tumor progression and resistance to androgen receptor pathway inhibitors (ARPIs), providing a biology‑driven rationale for treatment beyond PSMA. ATNM‑400 combines the antigen specificity of a monoclonal antibody with the potent, high‑energy alpha‑particle radiation of ²²⁵Ac. The study employs a theranostic approach, using Zirconium‑89 (⁸⁹Zr)‑labeled barecetamab PET imaging to visualize tumor uptake and retention and characterize biodistribution and dosimetry to inform ATNM‑400 development.

Sandesh Seth, Actinium's Chairman and CEO, said, "These first‑in‑human results mark an important milestone for a differentiated radiotherapy with the potential to expand treatment across the broad mCRPC population. HER3's role in tumor survival and treatment resistance provides a compelling opportunity to address disease that remains underserved by existing approaches. Despite the progress of PSMA‑directed therapies, patients with PSMA‑low or PSMA‑negative disease and those who do not respond to or progress following these treatments need additional options. ATNM‑400 is designed to meet that need through a distinct target and potent alpha‑radiation payload. The initial tumor uptake and tolerability findings support continued clinical development in prostate cancer and our broader vision for ATNM‑400 as a pan‑cancer radiotherapy."

Highlights from the EANM 2026 Oral Presentation

• Preclinical findings support broad potential beyond PSMA. ATNM‑400 demonstrated antitumor activity across PSMA‑high, PSMA‑low and PSMA‑negative prostate cancer models, including a model with low HER3 expression, supporting its potential to address patients underserved by PSMA‑directed therapies.
  
  
• HER3‑directed imaging demonstrated tumor uptake and retention. Serial imaging demonstrated lesion uptake retained through Day 7, supporting the use of ATNM‑400 to deliver high‑energy alpha‑particle radiation to tumors, with sustained retention aligned with the 10-day half‑life of ²²⁵Ac.
  
  
• First dose level cleared in three patients with previously treated mCRPC. Patients received ATNM‑400 at 1.0 µCi/kg. All had received prior ARPI therapy, two had received chemotherapy, and one had also received ¹⁷⁷Lu‑PSMA therapy. At the time of the presentation, all three had received at least two cycles of ATNM‑400, including two patients who had received three cycles.
  
  
• Encouraging initial safety findings supported continued dose escalation. No dose‑limiting toxicities, xerostomia (dry mouth), or Grade 3/4 hematologic adverse events were observed during the dose‑limiting toxicity window or subsequent follow‑up reported in the presentation. No elevations in liver enzymes (ALT/AST) or creatinine, injection‑site reactions, or infusion‑related reactions were observed.
  
  
• Study advanced to the second dose level. Following clearance of the 1.0 µCi/kg cohort, dose escalation proceeded to 1.5 µCi/kg. The study is designed to assess safety and tolerability and establish a recommended Phase 2 dose.

Professor Joe Cardaci of GenesisCare Murdoch, a study investigator and presenting author, commented, "Patients whose prostate cancer progresses after established therapies need additional treatment options. In this initial cohort, we observed HER3‑positive tumor uptake with the imaging agent and encouraging tolerability with repeated ATNM‑400 dosing, including in a patient previously treated with ¹⁷⁷Lu‑PSMA. The absence of dose‑limiting toxicities, xerostomia and Grade 3/4 hematologic adverse events is encouraging at this early stage. These findings support continued dose escalation and evaluation in additional patients."

Steffen Heeger, MD, MSc, Actinium's Chief Medical Officer, said, "HER3 is involved in tumor survival signaling and treatment resistance across multiple solid tumors, making it a compelling target for antibody‑directed alpha radiotherapy. In prostate cancer, its association with ARPI resistance provides a strong rationale for ATNM‑400, both as a monotherapy and in combination with established treatments. Preclinical activity across PSMA‑high, PSMA‑low and PSMA‑negative prostate cancer models, including a model with low HER3 expression, underscores the opportunity to reach patients underserved by PSMA‑directed therapies. Together with sustained tumor control after enzalutamide failure and durable activity in combination with enzalutamide, these findings support a broad mCRPC development strategy spanning monotherapy and ARPI combinations. We believe the combination of HER3 biology, sustained antibody localization and potent alpha radiation also supports investigation of ATNM‑400's pan‑cancer potential, building on our preclinical findings in lung and breast cancer."

About the ATNM‑400 Phase 1 Study
The ongoing first‑in‑human study uses a 3+3 dose‑escalation design followed by dose expansion in patients with progressive mCRPC who have received at least one ARPI and at least one taxane regimen, unless chemotherapy is unsuitable or declined. Prior ¹⁷⁷Lu‑PSMA treatment is permitted. Patients undergo HER3‑targeted PET imaging with [⁸⁹Zr]Zr‑barecetamab‑DFO before treatment with ATNM‑400, administered intravenously every six weeks for up to four cycles. Planned dose levels are 1.0, 1.5, 2.25 and 3.0 µCi/kg. The study evaluates safety, dosimetry and pharmacokinetics, with dose expansion intended to establish the recommended Phase 2 dose and further assess tolerability and antitumor activity, including PSA responses, objective response rate, radiographic progression‑free survival and overall survival.

EANM 2026 Presentation Details
Abstract Title: First‑In‑Human Phase 1 Study of ATNM‑400, Actinium‑225 Antibody Radioconjugate, in Metastatic Castration‑Resistant Prostate Cancer (mCRPC)
Abstract Number: OP‑525
Presentation Type: Oral presentation
Session: 1111 — Theranostics Track: Prostate Cancer Therapy II
Session Date and Time: Monday, October 19, 2026, 4:45–6:15 p.m. CEST
Location: Hall G1, Vienna, Austria

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.

About Actinium Pharmaceuticals, Inc.
Actinium is a pioneer in targeted radiotherapies designed to improve outcomes for patients with cancer. The company employs a biology‑driven approach to develop differentiated radiopharmaceuticals for solid tumors and hematologic malignancies. Its mission is to transform cancer treatment through innovative radioconjugates that maximize therapeutic efficacy while minimizing toxicity to healthy tissue by combining expertise in tumor biology, translational medicine, and radiochemistry. Since inception, Actinium has focused on developing innovative radiotherapies. Its pipeline reflects this strategy across three areas: (1) solid tumor therapeutics including ATNM‑400 and Actimab‑A with pan‑tumor potential; (2) Actimab‑A as a therapeutic backbone for acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) in collaboration with the National Cancer Institute (NCI); and (3) targeted conditioning agents including Iomab‑B for bone marrow transplant and Iomab‑ACT for cell and gene therapy conditioning. ATNM‑400 targets HER3, an antigen distinct from PSMA, and is in a first‑in‑human Phase 1 study in mCRPC, with preclinical activity across metastatic castration‑resistant prostate cancer (mCRPC), non‑small cell lung cancer (NSCLC), and breast cancer. Actimab‑A has shown improved survival in relapsed/refractory AML with CLAG‑M and is advancing toward a Phase 2/3 trial, with additional development ongoing through a CRADA with the NCI. Actinium is also advancing preclinical solid tumor programs and holds ~250 patents and patent applications, including intellectual property related to cyclotron‑based production of Ac‑225. For more information, please visit www.actiniumpharma.com.

Forward‑Looking Statements
This press release may contain projections or other "forward‑looking statements" within the meaning of the "safe‑harbor" provisions of the private securities litigation reform act of 1995 regarding future events or the future financial performance of the Company which the Company undertakes no obligation to update. These statements are based on management's current expectations and are subject to risks and uncertainties that may cause actual results to differ materially from the anticipated or estimated future results, including the risks and uncertainties associated with preliminary study results varying from final results, estimates of potential markets for drugs under development, clinical trials, actions by the FDA and other governmental agencies, regulatory clearances, responses to regulatory matters, the market demand for and acceptance of Actinium's products and services, performance of clinical research organizations and other risks detailed from time to time in Actinium's filings with the Securities and Exchange Commission (the "SEC"), including without limitation its most recent annual report on form 10‑K, subsequent quarterly reports on Forms 10‑Q and Forms 8‑K, each as amended and supplemented from time to time.

Investors: investorrelations@actiniumpharma.com

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SOURCE Actinium Pharmaceuticals, Inc.

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

What did Actinium's initial ATNM‑400 Phase 1 results show?

The initial three-patient cohort at 1.0 µCi/kg had no observed dose-limiting toxicities, dry mouth or Grade 3/4 blood-related adverse events during the toxicity window or reported follow-up. HER3-targeted imaging showed tumor uptake retained through Day 7, and dose escalation advanced to 1.5 µCi/kg.

How is ATNM‑400 administered in Actinium's Phase 1 study?

ATNM‑400 is administered intravenously every six weeks for up to four cycles, following HER3-targeted PET imaging. The study uses a 3+3 dose-escalation design followed by dose expansion, with planned dose levels of 1.0, 1.5, 2.25 and 3.0 µCi/kg.

Who is eligible for Actinium's ATNM‑400 Phase 1 study?

The study enrolls patients with progressive metastatic castration-resistant prostate cancer who have received at least one androgen receptor pathway inhibitor and at least one taxane regimen, unless chemotherapy is unsuitable or declined. Prior lutetium‑177 PSMA treatment is permitted.

What outcomes will Actinium evaluate in the ATNM‑400 dose-expansion phase?

Dose expansion is intended to establish the recommended Phase 2 dose and further assess tolerability and antitumor activity. Planned activity measures include prostate-specific antigen responses, objective response rate, radiographic progression-free survival and overall survival.