Somatostatin-Receptors (SSTR)-Agonist [212Pb]VMT-alpha-NET in Metastatic or Inoperable SSTR+ Gastrointestinal Neuroendocrine Tumor and Pheochromocytoma/Paraganglioma Previously Treated With Systemic Targeted Radioligand Therapy

Background: Gastrointestinal neuroendocrine tumors (GI NET) are a type of cancer that affects the stomach and intestines; pheochromocytoma/paragangliomas (PPGL) are tumors that grow in or near the adrenal glands. Both of these types of tumor have high levels of a protein called somatostatin receptors (SSTR) on their surfaces. Researchers want to test a treatment that targets SSTR. Objective: To test a drug (\[212Pb\]VMT-alpha-NET) in people with GI NET or PPGL. The drug has 2 components: a protein to bind to SSTR and a radioactive agent to kill the cancer cells. Eligibility: Adults aged 18 years or older with GI NET or PPGL tumors that have spread and cannot be removed with surgery. Design: Participants will be screened. They will have a physical exam, with imaging scans, blood tests, and tests of their heart function. \[212Pb\]VMT-alpha-NET is given through a tube attached to a needle inserted into a vein (infusion). Treatment will be given in four 8 week cycles. Participants will receive the drug on the first day of each cycle. They will remain in the clinic at least 4 hours after each infusion and may need to stay in the hospital for up to 48 hour for monitoring and testing. They will have blood tests every week of each cycle. Some participants will also get a related study drug (\[203Pb\]VMT-alpha-NET). They will receive this drug a few days before the first 2 cycles. At 4, 24, and 48 hours after each infusion, they will have whole body scans. These scans will show where the study drug went in their body. Follow-up visits will continue for 10 years....

* INCLUSION CRITERIA: * Participants must have histopathologically confirmed gastrointestinal neuroendocrine tumors (GI NET) or pheochromocytoma/paraganglioma (PPGL) cancers that are metastatic or inoperable per Standard of Care. * Have received at least 1 prior systemic radioligand therapy for definitive therapeutic purposes. Note: Participants with prior external beam radiation treatment (EBRT) will also be eligible as long as they have had at least 1 prior administration of a systemic radioligand therapy. * Must have at least 1 measurable lesion by RECIST 1.1 (phase II only). * History of progression by imaging per RECIST 1.1 or clinically (defined as increase in severity or frequency of symptoms related to disease) within the past 36 months prior to the first dose of \[203Pb\]VMT-alpha-NET. * Evidence of somatostatin receptors (SSTR) expression on at least 50 percent of the radiographically identifiable (i.e., visible on an anatomic scan such as CT or magnetic resonance imaging \[MRI\]) tumor, as indicated by a positive (uptake qualitatively identifiable as above the local background) on SSTR PET scan. * Age \>= 18 years. * ECOG performance status \= 9.0 g/dL * Total bilirubin: within normal institutional limits. Note: \