The purpose of this dose finding study is to evaluate the safety and efficacy of 2 different dose levels of CLR 131 in children, adolescents and young adults with relapsed or refractory high-grade glioma (HGG).

This study is designed to further evaluate the safety and tolerability of CLR 131 at the selected doses in children, adolescents and young adults with relapsed or refractory malignant high-grade glioma. It will also determine the therapeutic activity defined as progression free survival and overall survival, antitumor activity (treatment response) defined as the reduction in tumor volume and identify the recommended Phase 2/3 dose of CLR 131 in children, adolescents and young adults with relapsed or refractory HGG.

Inclusion Criteria:

• Previously confirmed (histologically or cytologically) high grade glioma that is clinically or radiographically suspected to be relapsed, refractory, or recurrent
• ≥ 10 years of age and ≤ 25 years of age at time of consent/assent
• If ≥ age 16 years, Karnofsky performance status of ≥ 60. If < age 16 years, Lansky performance status ≥ 60
• Platelets ≥ 75,000/μL (last transfusion, if any, must be at least 1 week prior to study registration, and, unless deemed medically necessary, no transfusions are allowed between registration and dosing)
• Absolute neutrophil count ≥ 750/μL
• Hemoglobin ≥ 8 g/dL (last transfusion must be at least 1 week prior to study registration, and, unless deemed medically necessary, no transfusions are allowed between registration and dosing)
• Using the bedside Schwartz formula, estimated GFR (creatinine clearance) > 60 ml/min/1.73m2
• Alanine aminotransferase < 3 × ULN
• Bilirubin < 2 × ULN
• At least 1 measurable intracranial lesion with longest diameter of at least 10 mm on any imaging sequence.
• Patients with previously known neurological deficits must be clinically stable at time of enrollment and able to complete all study related procedures. Patients with documented or newly diagnosed neurological deficits will be enrolled at the investigator's discretion.
• If patient receives steroids for neurological symptom control, the dose must be stable (unchanged for three weeks prior to registration) or on a steroid tapering regimen. Initiation of steroids per routine care immediately prior to CLR 131 dosing is acceptable
• Patient or his or her legal representative is judged by the Investigator to have the initiative and means to be compliant with the protocol.
• Patient or his or her legal representative has the ability to read, understand, and provide written informed consent for the initiation of any study-related procedures.
• Female patients of childbearing potential must have a negative pregnancy test at screening and within 24 hours of dosing. It is recommended that female caregivers of childbearing potential have a negative pregnancy test within one week of dosing.
• Patients of childbearing potential must practice an effective method of birth control while participating on this study to avoid possible harm to the fetus.

Exclusion Criteria:

• Antitumor therapy or investigational therapy, within 3-half-lives of the agent preceding the present study. For certain types of radiation (craniospinal, total abdominal, whole lung [spot irradiation to skull-based metastases is not considered craniospinal radiation for the purposes of this study]), at least 3 months must have elapsed. Palliative focal radiation to non-target lesions should be completed at least 2 weeks prior to dosing. Patients participating in non-interventional clinical trials (i.e., non-drug) are allowed to participate in this trial
• History of hypersensitivity to thyroid protection medication (e.g., potassium iodide, Lugol's solution, etc.)
• Any other concomitant serious illness or organ system dysfunction (including cardiac and pulmonary dysfunction) that in the opinion of the Investigator would either compromise patient safety or interfere with the evaluation of the safety of the test drug.
• Major surgery within 6 weeks of enrollment unless delay in therapy poses unacceptable risk to the patient due to clinical progression (enrollment o such patients should be discussed with Medical Monitor)
• Known history of human immunodeficiency virus or uncontrolled, serious, active infection
• Pregnancy or breast-feeding

The IPD will be made available upon request of NCI, the funding agency partner. This has not yet been decided.