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Advanced Soft Tissue SarcomaPhase 1ATLASNCT07156565

Actinium Therapy for Late-stage Aggressive Sarcomas

A Phase 1 study testing [Ac 225]RTX-2358 and [Cu64]LNTH-1363S in people with Advanced Soft Tissue Sarcoma, run by Ratio Therapeutics, Inc. at 6 sites in 2 countries.

Recruiting

Looking for participants now. Contact the study team or a site below to ask about taking part.

Sites and contacts
Official record
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Trial summary

The goal of this clinical trial is to learn about the safety of drug [Ac225]RTX-2358 and the diagnostic imaging agent [Cu64]LNTH-1363S. Additionally Ratio Therapeutics will learn if [Ac225]RTX-2358 drug is effective in treating advanced sarcoma. Read more about this study

Who is it for?
Ages 18 years and older
Conditions the study is for
Advanced Soft Tissue Sarcoma
What is being tested?
[Ac 225]RTX-2358 and [Cu64]LNTH-1363S
Type of treatment
Injection or infusion
Could I receive a placebo?
No, everyone receives an active treatment
How many people
48 participants
Where
6 sitesin 2 countries
Study dates
12 Nov 2025to Jan 2032
Phase
Phase 1
Are there results?
Not yet: the study is under way

Key requirements

Who can take part, as listed by the sponsor. Only the study team can confirm whether you are eligible, so take this list to your doctor.

  • Ages 18 years and older
  • All sexes
  • No healthy volunteers
  • Adult, Older adult

Who may be able to join

  • 18 years of age or older.
  • History of relapsed or refractory soft tissue sarcoma.
  • Histological confirmation of sarcoma at any point since diagnosis
  • At least 1 prior treatment regimen
  • Measurable disease as per RECIST (v1.1) for soft tissue sarcoma that is also positive on FAPi PET/CT or PET/MRI scan.
  • ECOG performance status of 0 or 1.
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  • Adequate Organ reserve and renal function as evidenced by:
  • Neutrophil count ≥ 1200 µL without granulocyte colony stimulating factor
  • Platelet count ≥ 100,000 µL
  • Hemoglobin ≥ 8 g/dL
  • Total bilirubin level ≤ 1.5 × upper limit of normal (ULN).
  • Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5 × ULN.
  • Calculated creatinine clearance ≥ 60 mL/min. 6. Coagulation parameters (prothrombin time, international normalized ratio and activated partial thromboplastin time) ≤ 1.5 × ULN 7. All patients (males and females of childbearing potential) must agree to practice 2 forms of highly effective contraceptive precautions to prevent pregnancy from the time consent is signed and throughout the trial.
  • 8. The patient or the patient's legal representative must be willing and able to provide written informed consent.

Who cannot join

  • History of whole pelvic irradiation or lung irradiation.
  • History of radioligand therapy.
  • Treatment within 14 days prior to first administration with:
  • Palliative surgery or external beam radiation.
  • Approved anticancer therapy including chemotherapy or immunotherapy.
  • Any investigational therapy.
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  • Any major surgery (e.g., requiring general anesthesia).
  • Patients who are scheduled for external beam radiation therapy or radioligand therapy during the study period.
  • Congestive heart failure > Class II New York Heart Association Functional Classification, current pericarditis, myocardial infarction within 6 months, or symptomatic coronary artery disease.
  • Patients with uncontrollable incontinence.
  • Unstable or clinically significant concurrent medical condition that would, in the opinion of the Investigator, jeopardize the safety of a patient and/or their compliance with the protocol.
  • Active infection requiring systemic antibacterial/antibiotic, antifungal, or antiviral therapy. Chronic antiviral therapy for stable viral disease can be included.
  • History of active fibrotic condition.
  • Any active malignancy other than sarcoma that requires treatment.
  • Stable brain metastasis is allowed but documented untreated metastases to brain or meninges are excluded.
  • Currently pregnant or lactating.
  • Known allergy to any of the study drugs or their excipients.
  • Inability or unwillingness to comply with any of the required imaging studies or study assessments.
  • History of pneumonitis, interstitial lung disease, chronic obstructive pulmonary disease (COPD) or other inflammatory/fibrotic pulmonary condition.

Trial locations and contacts

6 sites in 2 countries. Contact the study team or a site near you to ask about taking part.

6 sites in 2 countries

United States5 sites, 5 recruiting

  • Case Western

    Cleveland, Ohio, 44106

    Ankit Mangla, MD, 216-844-3951, ankit.mangla@uhhospitals.org

    Ankit Mangla, MD

    Recruiting

  • MD Anderson Cancer Center

    Houston, Texas, 77230

    Fiona Ouyang, 713-470-8008, fouyang@mdanerson.org

    Michael Nakazawa, MD

    Recruiting

  • UCLA

    Los Angeles, California, 90095

    Jacqueline Banuelos, 310-869-7014, jbanuelosmurillo@mednet.ucla.edu

    Arun Singh, MD

    Recruiting

  • Memorial Sloane Kettering Cancer Center

    New York, New York, 10065

    Sandra D'Angelo, MD, 646-392-1037, dangelos@mskcc.org

    Sandra D'Angelo, MD

    Recruiting

  • Mayo Clinic

    Rochester, Minnesota, 55905

    Clinical Trials Referral Office, 855-776-0015, CANCERCTR@mayo.edu

    Brittany Siontis, MD

    Recruiting

Other trials in United States

Canada1 site, 1 recruiting

  • Princess Margaret Cancer Centre

    Toronto, Ontario, M5G 2M9

    David Shultz, MD, 416-946-4501 ext. 6899, david.shultz@uhn.ca

    Recruiting

Other trials in Canada

About this study

Official title: A Seamless Phase 1/2 Open-label Study to Evaluate the Safety, Determine the Maximum Tolerated Dose of Administered Activity, and Evaluate the Efficacy of the Therapeutic Radiopharmaceutical [Ac 225]-RTX-2358 in the Treatment of Relapsed or Refractory Sarcoma

The goal of this clinical trial is to learn about the safety of drug [Ac225]RTX-2358 and the diagnostic imaging agent [Cu64]LNTH-1363S. Additionally Ratio Therapeutics will learn if [Ac225]RTX-2358 drug is effective in treating advanced sarcoma.

The main questions the study aims to answer in Phase/Part 1 of the trial are:

Read more
  • Is [Ac225]RTX-2358 tolerable or does it cause toxicities (medical problems) in patients.
  • What is the most tolerable dose of [Ac225]RTX-2358
  • Does the treatment show effectiveness on advanced sarcoma

Participants will:

  • Take drug [Ac225]RTX-2358 once every 8 weeks (4 cycle target; 6 cycle maximum) over a period of 12 months
  • Visit the clinic three times for the first week of cycle one, then once a week for the remaining 7 weeks of cycle 1 for check-ups and tests.
  • For cycles 2-6: patient will visit the clinic once every 2 weeks for checkups and tests
  • Remain in long term follow-up for a period of four additional years
Read the detailed description

RTX-2358-101 is an open-label, seamless, Phase 1/2 clinical study to evaluate safety, tolerability, dosimetry, biodistribution, pharmacokinetic (PK), and anti-tumor activity of [Ac225]RTX-2358. [Ac 225]-RTX-2358 is for patients with relapsed or treatment-resistant soft tissue sarcoma.

The study consists of two phases, an ascending administered activity phase (Phase1) and an expansion phase (Phase 2). In the Phase 1 portion of the trial patients will be divided into three groups, each group receiving a higher dose than the last. A modified "3 + 3" dose escalation with Phase 1 queue (IQ) design approach will be used, with potential backfill in administered activity cohorts.

Patients must be 18 or older, have soft tissue sarcoma that can be measured by imaging, and meet specific health requirements, including good organ function and a positive PET scan. Each treatment cycle in the phase 1 portion of the study involves a single injection on Day 1, followed by eight weeks of monitoring. Patients have a target of four cycles of treatment, with the possibility of two more (six maximum), if their side effects are mild and their disease hasn't worsened.

A Safety Review Committee (SRC) will review the data from each group in the Phase 1 portion to decide when it's safe to move to the next dose level. The SRC will also provide recommendations on timing and administered activity for backfill cohorts to gather additional safety and preliminary data on effectiveness. Patients that receive at least one treatment in the Phase 1 portion of the study, will be followed in Active (AFUP) or Long-term follow-up (LTFU). The AFUP will follow patients until confirmation of disease progression or start of subsequent anti-cancer therapy. After AFUP, the patient will enter LTFUP. Patients who discontinue with confirmed progressive disease will be monitored for LTFU via remote or onsite visits every 6 months until 5 years after the start of treatment to track long-term safety, disease progression, and overall survival.

The Phase 2 portion of the study will evaluate the efficacy and safety of [Ac225]RTX-2358 of up to 50 patients.

Being tested
[Ac 225]RTX-2358, [Cu64]LNTH-1363S
Keywords
Radioligand Therapy, Sarcoma, Relapsed or refractory sarcoma, Soft tissue sarcoma, Radiopharmaceuticals, Theranostics, advanced soft tissue sarcoma
Medical subject headings
Sarcoma, Recurrence

What participants receive

What participants receive. In a randomised study each person is assigned to one group by chance.

  • [Ac 225]RTX-2358Drug

    [Ac225]RTX-2358 will be given as an intravenous injection once every 8 weeks for up to 6 doses. Each group will receive a set administered activity level (dose) of [Ac 225]RTX 2358 given once every 8 weeks. The first group will received the lower dose, the second group will received the mid level dose, and the third group the highest dose. A Safety Review Committee will determine when/if it is appropriate to allow the next group to proceed.

  • [Cu64]LNTH-1363SDiagnostic test

    [Cu64]LNTH-1363S is a investigational radioactive diagnostic imaging agent to demonstrate that the tumor expresses Fibroblast Activation Protein. An imaging test is performed after a single injection with [Cu64]LNTH-1363S 14-30 days before the first administration of study drug [AC225]RTX-2358 to confirm that the sarcoma has FAP expression.

Study groups in NCT07156565
Study groupRoleWhat they receive
Treatment with [Ac 225]RTX-2358ExperimentalThe study design is a modified 3+3 with Phase 1 queue (IQ) escalating group design.
Receives: Drug: [Ac 225]RTX-2358; Diagnostic Test: [Cu64]LNTH-1363S

What is being measured

The primary outcome is the main question the study is designed to answer.

  1. Evaluate the safety (Frequency and severity of treatment emergent adverse events (TEAEs), Dose Limiting Toxicities (DLTs), serious adverse events (SAEs) and adverse events of special interest) of [Ac225-]RTX-2358

    Frequency and severity of treatment emergent adverse events (TEAEs), Dose Limiting Toxicities (DLTs), serious adverse events (SAEs) and adverse events of special interest (AESIs).

    Measured: 48 weeks

  2. Evaluate the preliminary anti-tumor activity of [Ac225]RTX-2358 in patients with relapsed or refractory soft tissue sarcoma

    Objective response rate (ORR), defined as the proportion of patient with a complete response (CR) or partial response (PR), per RECISTv1.1. Duration of response (DOR) defined as the duration of time from first date of observed response (CR or PR), to first date of disease progression or death from any cause.

    Measured: 48 weeks

  3. Determine the MTD of the administered activity of [Ac-225]RTX-2358 and select the administered activity and schedule for further clinical evaluation

    Evaluate dose levels in Phase 1 escalating group design to identify the MTD and the administered activity to be used in the expansion cohort

    Measured: 8 weeks

Secondary outcomes (9)

  1. Further evaluate the preliminary efficacy of [Ac225]RTX-2358 in patients with relapsed or refractory soft tissue sarcoma

    Duration of progression-free survival (PFS): the duration of time from first treatment administration to first date of disease from any cause.

    Measured: Up to 5 years

  2. Further evaluate the preliminary efficacy of [Ac225]RTX-2358 in patients with relapsed or refractory soft tissue sarcoma

    Overall Survival (OS): the duration of time from first treatment administration to date of death from any cause.

    Measured: Up to 5 years

  3. Determine the normal tissue radiation dosimetry of [Ac225]RTX-2358

    Radiation absorbed dose estimates for selected normal tissues. Analysis of radioactive blood samples for select timepoints

    Measured: Days 1, 3, 8, and Week 4

  4. Determine the maximum concentration (Cmax) of radiation (Pharmacokinetics) in the blood after a single dose of [Ac225]RTX-2358

    Blood samples will be collected Pre-dose, immediately post-dose, 15 minutes, 30 minutes, 60 minutes, 4 hours, Day 3, Day 8, and 4 weeks post dose. Radiation (PK) to be measured by gamma counter (KBq/mL).

    Measured: Up to 4 weeks post injection

  5. Determine the area under the curve (AUC) in blood after a single dose of [Ac225]RTX-2358

    Blood samples will be collected Pre-dose, immediately post-dose, 15 minutes, 30 minutes, 60 minutes, 4 hours, Day 3, Day 8, and 4 weeks post dose. AUC to be calculated by kilobecquerel x hours/milliliter (KBq x h/mL)

    Measured: Up to 4 weeks post injection

  6. Determine the elimination half-life in the blood after a single dose of [Ac225]RTX-2358

    Blood samples will be collected Pre-dose, immediately post-dose, 15 minutes, 30 minutes, 60 minutes, 4 hours, Day 3, Day 8, and 4 weeks post dose. Elimination to be measured by time to half of the product being excreted from the blood (as measured by gamma counter) in hours (h)

    Measured: Up to 4 weeks post injection

  7. Determine the clearance in the blood after a single dose of [Ac225]RTX-2358

    Blood samples will be collected Pre-dose, immediately post-dose, 15 minutes, 30 minutes, 60 minutes, 4 hours, Day 3, Day 8, and 4 weeks post dose. Clearance to be measured by milliliter/minute.

    Measured: Up to 4 weeks post injection

  8. Assess impact of treatment to quality of life (QoL)- European Organization for Research and Treatment of Cancer Quality of Life Questionnaire (EORTC QLC-C30 Version 3)

    Change from baseline in the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire (EORTC QLC-C30 Version 3)

    Measured: 48 weeks

  9. Evaluate the safety and tolerability of [Cu-64]LNTH-1363S

    Incidence, duration, and severity of Treatment Emergent Adverse Events (TEAEs) and Serious Adverse Events (SAEs)

    Measured: Up to 2 weeks after the [Cu-64]LNTH-1363S injections

Timeline

From the start of the study to its completion; the mark is today. Estimated dates are the sponsor’s plan.

  1. First posted5 Sep 2025
  2. Study start12 Nov 2025
  3. Primary completionJan 2028 (estimated)
  4. Study completionJan 2032 (estimated)

Resources

More details

For doctors and researchers: how the study is designed, who runs it, and its registry IDs.

Study design
Study type
Interventional
Phase
Phase 1
Primary purpose
Treatment
Allocation
Not applicable
Intervention model
Sequential
Model description
The Phase 1 portion of the study is a modified "3+3" dose escalation with Phase 1 queue (IQ) design planned for 3 cohorts/groups/dose levels, with potential backfill in administered activity cohorts.
Masking
None (open label)
Enrolment
48 participants (estimated)
IDs, dates and publications
ClinicalTrials.gov ID
NCT07156565
Sponsor's protocol ID
RTX-2358-101
Acronym
ATLAS
Status last verified
Aug 2026
First submitted
17 Jan 2025
Last updated
28 Aug 2026
Results posted
No
Data monitoring committee
Yes
FDA-regulated drug
Yes
FDA-regulated device
No
Individual data sharing
No

Questions about this trial

What is this clinical trial studying?

A Phase 1 study testing [Ac 225]RTX-2358 and [Cu64]LNTH-1363S in people with Advanced Soft Tissue Sarcoma, run by Ratio Therapeutics, Inc. at 6 sites in 2 countries.

The full registered description is in the About this study section and the official record.

Who may be eligible to take part?

The sponsor lists 14 inclusion and 19 exclusion criteria. Eligibility is decided by the study team after screening, so read the full criteria and contact a listed site.

Is this clinical trial currently recruiting?

The record says: looking for participants now, as of the sponsor's last update on 28 Aug 2026. Status can change between updates, so confirm with the study contact before making plans.

Where is the study taking place?

The record lists 6 sites in United States, Canada. Whether a site is taking part right now should be confirmed with that site. See every site.

What are the possible benefits and risks?

They depend on the study and the treatment, and no study can promise a benefit. The study team explains the known risks during informed consent, before anyone joins; the record itself does not list them.

What does this phase mean?

Phase 1 studies test a treatment in a small group of people for the first time or in a new way, mainly to check safety and find the right dose.

Not every study follows the same path, and some studies (observational studies, device studies) have no phase.

Are results available for this study?

This study is still under way. Results are not available in the source record yet.

How can a doctor refer a patient or contact the research team?

Each site lists its own contact: choose the nearest one under Trial locations. AzTrials does not enrol participants or take referrals.

What does the primary outcome measure?

Evaluate the safety (Frequency and severity of treatment emergent adverse events (TEAEs), Dose Limiting Toxicities (DLTs), serious adverse events (SAEs) and adverse events of special interest) of [Ac225-]RTX-2358 (time frame: 48 weeks). The study lists 3 primary outcomes in all. It is what the study was designed to measure, not a finding.

Where can I read the original study information?

On the ClinicalTrials.gov record for NCT07156565.

Does a listing mean the treatment is approved or proven to work?

No. A listing means the study is registered. It does not show that the treatment is safe, works, or is approved by a regulator.

How often is this information updated?

AzTrials copies changed records from ClinicalTrials.gov every night and reloads the whole registry every week. The sponsor last updated this record on 28 Aug 2026; AzTrials last refreshed its copy on 30 Oct 2026.

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