[Ac 225]-RTX-2358

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