Phase 1 First-in-Human, Dose Escalation and Dose Expansion Study of RYZ401, a Novel Radiopharmaceutical Therapy Labeled With Actinium-225, in Subjects With Neuroendocrine Tumors and Other Solid Tumors Expressing Somatostatin Receptors.

Inclusion criteria: * At least 18 years old at the time of signing the main study informed consent form (ICF). * Histologically confirmed: Dose Escalation (all cohorts): Grades 1-3 WD, metastatic or locally advanced and unresectable NET at any primary site, including, but not limited to, GI, pancreatic, lung, thyroid, breast, urogenital, and adrenal tumors. * Dose Expansion Cohort A: Grades 1-3 WD, metastatic or locally advanced and unresectable GEP-NETs * Dose Expansion Cohort B: metastatic or locally advanced and unresectable tumors known to frequently express SSTR, limited to the following indications: * WD NETs (Grades 1-3) from primary sites other than gastrointestinal and pancreatic (including, but not limited to, lung, thyroid, breast, urogenital, and adrenal tumors) Meningioma (Grades 1-3) * SSTR-positive disease, as assessed by SSTR-PET imaging * Adequate renal, hematologic and hepatic function Exclusion criteria: * Prior RPT, including Lu-177. * Prior solid organ or bone marrow transplantation. * Use of chronic systemic steroid therapy. * Significant cardiovascular disease * Resistant hypertension * Uncontrolled diabetes * Prior history of liver cirrhosis * HIV, hepatitis B infection or known active hepatitis C virus infection. Note: Additional criteria may apply and will be assessed by the study site