[6-K] Radiopharm Theranostics Ltd Current Report (Foreign Issuer)

Rhea-AI Filing Summary

Radiopharm Theranostics Limited reported initial first-in-human findings from its Phase 0/1 HEAT clinical trial of 177Lu-RAD202, a HER2-targeted radiopharmaceutical for advanced solid tumors. In the initial lowest dose cohort of three heavily pre-treated HER2-positive patients given 30 mCi, 177Lu-RAD202 showed encouraging tumor uptake and a favorable safety profile. Based on these data, the independent Data Safety and Monitoring Committee recently recommended advancing to the next dose level, allowing escalation to the third cohort at 130 mCi. The HEAT trial is an open-label, multicenter study in HER2-positive locally-advanced or metastatic solid tumors, building on earlier diagnostic proof-of-concept for RAD202 in HER2-positive breast cancer.

Insights

Biotech clinical development analyst

Early 177Lu-RAD202 human data show acceptable safety and permit dose escalation.

Radiopharm Theranostics is advancing 177Lu-RAD202, a HER2-targeted radiopharmaceutical, through the Phase 0/1 HEAT trial in heavily pre-treated HER2-positive solid tumors. The initial lowest dose cohort at 30 mCi demonstrated encouraging tumor uptake and a favorable safety profile.

An independent Data Safety and Monitoring Committee reviewed the early data and approved escalation to the third cohort at 130 mCi. Since higher doses are typically needed for therapeutic effect, this step is important for exploring potential antitumor activity while maintaining safety oversight.

The program builds on prior diagnostic results where RAD202 showed positive safety and biodistribution in ten HER2-positive breast cancer patients. Subsequent disclosures from this integrated Phase 0/1 study will be needed to understand efficacy signals and how 177Lu-RAD202 might fit alongside existing HER2-targeted therapies.

Key Figures

Initial cohort size: 3 patients Initial dose level: 30 mCi Next planned dose: 130 mCi +3 more

6 metrics

Initial cohort size 3 patients First, lowest dose cohort in HEAT trial

Initial dose level 30 mCi Dose used in lowest cohort of HEAT trial

Next planned dose 130 mCi Dose for third cohort approved by DSMC

Trial phase Phase 0/1 HEAT first-in-human study design

Prior diagnostic study size 10 patients HER2-positive breast cancer diagnostic proof-of-concept

Active trials in pipeline 1 Phase 2 and 5 Phase 1 Radiopharm’s solid tumor clinical program

Key Terms

Data Safety and Monitoring Committee, Phase 0/1, HER2-positive, radiopharmaceutical, +2 more

6 terms

Data Safety and Monitoring Committee clinical

"The DSMC is a multidisciplinary committee that conducts detailed reviews of study data"

An independent panel of experts that regularly reviews patient safety data and trial results during a clinical study to protect participants and ensure the study is producing reliable information. Like impartial referees who can pause, modify, or stop a trial if safety concerns or clear benefits appear, their findings can alter a drug or device’s development timeline, regulatory prospects, and investor risk, so their assessments matter for company value and timelines.

Phase 0/1 clinical

"ongoing Phase 0/1 HEAT trial (NCT06824155), evaluating 177Lu-RAD202"

Phase 0/1 describes the earliest stages of testing a new drug or treatment in people: Phase 0 involves very small, exploratory tests to see how the human body handles the drug, and Phase 1 expands to determine a safe dose and look for initial signs of side effects. For investors, these steps are a first checkpoint on scientific and regulatory risk—successful results reduce uncertainty and keep a program on schedule, while failures can sharply change a company’s value, much like an early test drive revealing major mechanical problems.

HER2-positive medical

"patients with HER2-positive locally-advanced or metastatic solid tumors"

HER2-positive describes cancer cells that have too many copies of the HER2 gene or make too much of the HER2 protein, which acts like an overactive growth switch that drives tumor growth. For investors, HER2 status matters because it determines whether patients can receive specific, often expensive targeted therapies and diagnostic tests, so trial results, approvals, or competing drugs tied to HER2 can strongly affect drug sales and company value.

radiopharmaceutical medical

"a first-in-class HER2-targeted radiopharmaceutical therapy"

A radiopharmaceutical is a special type of medicine that contains a small amount of radioactive material, used primarily for medical imaging or treatment. It can be thought of as a tiny, targeted signal that helps doctors see inside the body or deliver therapy directly to affected areas. For investors, understanding radiopharmaceuticals is important because they represent a growing field within healthcare, driven by advancements in diagnostics and personalized treatments.

single-domain antibody (sdAb) medical

"177Lu-RAD202 is a Lutetium-177–labeled single-domain antibody (sdAb) designed to target HER2-expressing tumors"

dosimetry clinical

"The AACR poster highlights first-in-human safety, biodistribution, dosimetry and tumor uptake clinical findings"

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.

Form 6-K: How Foreign Companies Report to the SEC →

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Form 6-K

REPORT OF FOREIGN PRIVATE ISSUER

PURSUANT TO RULE 13a-16 OR 15d-16 UNDER

THE SECURITIES EXCHANGE ACT OF 1934

For the Month of April 2026

Commission File Number: 001-41621

RADIOPHARM THERANOSTICS LIMITED

(Name of Registrant)

Level 3, 62 Lygon Street, Carlton South, Victoria, 3053, Australia

(Address of principal executive office)

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F.

Form 20-F ☒       Form 40-F ☐

Indicate by check mark whether by furnishing the information contained in this Form, the registrant is also thereby furnishing the information to the Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934.

Yes ☐       No ☒

If “Yes” is marked, indicate below the file number assigned to the registrant in connection with Rule 12g3-2(b): 82-

RADIOPHARM THERANOSTICS LIMITED

EXPLANATORY NOTE

Radiopharm Theranostics Limited (the “Company”) published one announcement (the “Public Notice”) to the Australian Securities Exchange on April 20, 2026 titled:

“RAD Initial Findings from Phase 1 177Lu-RAD202 Trial” 

A copy of the Public Notice is attached as an exhibit to this report on Form 6-K.

This report on Form 6-K (including the exhibit hereto) shall not be deemed to be “filed” for purposes of the Securities Exchange Act of 1934, as amended (the “Exchange Act”) and shall not be incorporated by reference into any filing under the Securities Act of 1933, as amended, except as shall be expressly set forth by specific reference in such filing.

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EXHIBITS

Exhibit Number		Description
99.1		RAD Initial Findings from Phase 1 177Lu-RAD202 Trial

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SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

	RADIOPHARM THERANOSTICS LIMITED
Date: April 20, 2026	By:	/s/ Phillip Hains
		Phillip Hains
		Company Secretary

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Exhibit 99.1

ASX ANNOUNCEMENT
April 20, 2026

Radiopharm Theranostics Presents Initial Findings from Phase 1 First-in-Human HEAT Clinical Trial for 177Lu-RAD202 in HER2+ Solid Tumors at American Association for Cancer Research 2026

177Lu-RAD202 demonstrated encouraging tumor uptake and a favorable safety profile in the lowest dose cohort

Data Safety and Monitoring Committee Recently Approved Advancing 177Lu-RAD202 to Next Highest Dose at 130 mCi

Sydney, Australia – 20 April 2026 – Radiopharm Theranostics (ASX: RAD, Nasdaq: RADX, “Radiopharm” or the “Company”), a clinical-stage biopharmaceutical company focused on developing innovative oncology radiopharmaceuticals for areas of high unmet medical need, today announced that new data from the ongoing Phase 0/1 HEAT trial (NCT06824155), evaluating 177Lu-RAD202, a first-in-class HER2-targeted radiopharmaceutical therapy, will be presented as a poster at the American Association for Cancer Research (AACR) Annual Meeting 2026, being held April 17–22, 2026 in San Diego, California.

“These first-in-human results represent an important early milestone for our HER2-targeted radiopharmaceutical program,” said Dr. Dimitris Voliotis, Chief Medical Officer of Radiopharm Theranostics. “In a heavily pre-treated patient population with significant unmet need, 177Lu-RAD202 demonstrated encouraging tumor uptake and a favorable safety profile at the lowest dose level. Importantly, the observed dosimetry supports continued dose escalation, which was recently approved by the Data Safety and Monitoring Committee (DSMC) to advance to the third cohort at 130 mCi dosing. We look forward to further evaluating the therapeutic potential of this novel approach and expect to see signs of antitumor activity at higher, more therapeutic dose levels.”

The AACR poster highlights first-in-human safety, biodistribution, dosimetry and tumor uptake clinical findings from the initial lowest dose cohort of three patients with advanced HER2-positive breast and urothelial cancers who had received multiple prior metastatic therapies and were dosed at 30 mCi.

Key Findings from the AACR Abstract and Poster

● Meaningful tumor uptake of 177Lu-RAD202 was observed at the initial and lowest dose level of 30 mCi, particularly in breast cancer lesions

● 177Lu-RAD202 was generally well tolerated in the first three treated patients, with predominantly Grade 1–2 treatment-emergent adverse events

● No dose-limiting toxicities or treatment discontinuations due to adverse events were observed

● Organ-level absorbed radiation doses were within expected and clinically acceptable ranges, supporting continued dose escalation

Radiopharm Theranostics Limited
Suite 1, Level 3, 62 Lygon Street, Carlton South VIC 3053 Australia
ABN: 57 647 877 889

ASX ANNOUNCEMENT
April 20, 2026

Poster Presentation Details

● Title: A First-in-Class HER2-Targeted Radiopharmaceutical Therapy: Initial Findings from the Phase 0/1 HEAT Trial of 177Lu-RAD202 in HER2+ Advanced Solid Tumors

● Abstract Number: CT046

● Presenter: Dimitris Voliotis, M.D., Chief Medical Officer of Radiopharm Theranostics

● Session: Poster Session

● Dates: April 20, 2026 at 9:00 AM PT

The complete poster can be found on the Company’s website here (https://radiopharmtheranostics.com/clinical-trials/#key-publications).

On April 8, 2026, Radiopharm Theranostics announced the positive recommendation from the Data Safety and Monitoring Committee (DSMC) to advance 177Lu-RAD202 to the third cohort at a dose level of 130mCi in the Phase 1 ‘HEAT’ clinical trial in patients with HER2-positive advanced solid tumors¹. The DSMC is a multidisciplinary committee that conducts detailed reviews of study data, discusses potential safety events and provides recommendations regarding trial continuation.

About the HEAT Trial

177Lu-RAD202 is a Lutetium-177–labeled single-domain antibody (sdAb) designed to target HER2-expressing tumors. The sdAb format enables deep tumor penetration and rapid systemic clearance, while the beta-emitting isotope 177Lu delivers cytotoxic radiation with potential bystander effects independent of HER2 receptor density.

The HEAT trial (HER2-Antibody Therapy with Lutetium-177; (NCT06824155) is a first-in-human, open-label, multicenter integrated Phase 0/1 study evaluating 177Lu-RAD202 in patients with HER2-positive locally-advanced or metastatic solid tumors.

● Phase 0 evaluates biodistribution, pharmacokinetics, and radiation dosimetry using an imaging dose

● Phase 1 consists of multiple-dose escalation to assess safety, tolerability, tumor targeting, and to determine the recommended Phase 2 dose

About RAD202:

RAD202 is a proprietary single-domain monoclonal antibody (sdAb) that targets the Human Epidermal Growth Factor Receptor 2 (HER2)-positive expression in advanced solid tumors. HER2 is overexpressed in breast cancer and several other solid tumors and represents a validated target in oncology. In a previous diagnostic study of ten HER2-positive breast cancer patients, RAD202 demonstrated clinical proof-of-concept and had positive safety and biodistribution^2,3.

1	clinicaltrials.gov/study/NCT06824155
2	Zhao et al, Br Canc Res (2024)

3 Zhao et al, Mol Pharmaceut (2021)

Radiopharm Theranostics Limited
Suite 1, Level 3, 62 Lygon Street, Carlton South VIC 3053 Australia
ABN: 57 647 877 889

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ASX ANNOUNCEMENT
April 20, 2026

About Radiopharm Theranostics

Radiopharm Theranostics is a clinical stage radiotherapeutics company developing a world-class platform of innovative radiopharmaceutical products for diagnostic and therapeutic applications in areas of high unmet medical need. Radiopharm is listed on ASX (RAD) and on NASDAQ (RADX). The company has a pipeline of distinct and highly differentiated platform technologies spanning peptides, small molecules and monoclonal antibodies for use in cancer. The clinical program includes one Phase 2 and five Phase 1 trials in a variety of solid tumor cancers including lung, breast, prostate and brain. Learn more at radiopharmtheranostics.com.

Authorized on behalf of the Radiopharm Theranostics Board of Directors by Executive Chairman Paul Hopper.

For more information:

Investors:

Riccardo Canevari
CEO & Managing Director
P: +1 862 309 0293
E: rc@radiopharmtheranostics.com

Anne Marie Fields

Precision AQ (formerly Stern IR)

E: annemarie.fields@precisionaq.com

Media:

Matt Wright
NWR Communications
P: +61 451 896 420
E: matt@nwrcommunications.com.au

Follow Radiopharm Theranostics:

Website – https://radiopharmtheranostics.com/
X – https://x.com/TeamRadiopharm
LinkedIn – https://www.linkedin.com/company/radiopharm-theranostics/
InvestorHub – https://investorhub.radiopharmtheranostics.com/

Radiopharm Theranostics Limited
Suite 1, Level 3, 62 Lygon Street, Carlton South VIC 3053 Australia
ABN: 57 647 877 889

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Filing Exhibits & Attachments

1 document

Press Releases

• EX-99.1 RAD INITIAL FINDINGS FROM PHASE 1 177LU-RAD202 TRIAL 23.4 KB