NCT07156565

Actinium Therapy for Late-stage Aggressive Sarcomas

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

• Status: Recruiting
• Phase: Phase 1
• Study type: Interventional
• Enrollment: 30 (estimated)
• Sponsor: Ratio Therapeutics, Inc. · Industry
• Sex: All
• Age: 18 Years
• Healthy volunteers: Not accepted

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

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 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.

Conditions

• Advanced Soft Tissue Sarcoma

Interventions

Type	Name	Description
DRUG	[Ac 225]RTX-2358	\[Ac225\]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.
DIAGNOSTIC_TEST	[Cu64]LNTH-1363S	\[Cu64\]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 13-35 days before the first administration of study drug \[AC225\]RTX-2358 to confirm that the sarcoma has FAP expression.

Timeline

• Start date: 2025-11-12
• Primary completion: 2028-01-01
• Completion: 2032-01-01
• First posted: 2025-09-05
• Last updated: 2026-03-17

Locations

6 sites across 2 countries: United States, Canada

Regulatory

• FDA-regulated drug study