RecruitingDrug trialACTinium in Castrate-RESistant Prostate Cancer After LUTEtiumAdvanced metastatic castration-resistant prostate cancer is a medical condition for which additional effective and tolerable treatments are urgently needed in order to improve patient outcomes and quality of life. The goal of this clinical trial is to learn more about Actinium (225Ac) radiohybrid prostate-specific membrane antigen-10.1 (rhPSMA-10.1) injection in men with prostate cancer that has spread and progressed after previous treatments, particularly after Lutetium-PSMA. Actinium (225Ac) rhPSMA-10.1 is an injectable radioactive medication that aims to attach to prostate cancer cells in the body and destroy them using ionising radiation. It is a new medication that has not yet been studied in humans. Participants will receive a dose of Actinium (225Ac) rhPSMA-10.1 every 6 weeks, to a maximum of 6 doses. They will be reviewed regularly by the trial researchers to monitor side effects and safety signals. A range of medication doses will be administered so that researchers can find out what doses of the medication are safe for men with prostate cancer. The trial will also aim to determine how effective this medication is for treating advanced prostate cancer.