This study will investigate the safety, symptoms and biomarker response of subjects with biopsy-proven well-differentiated, low-to-intermediate-grade, unresectable, or metastatic pancreatic neuroendocrine tumors (PNETs) or or Gastrointestinal Neuroendocrine tumors (GI-NETs) with elevated biochemical markers who have relapsed during or after receiving prior standard of care therapies, including octreotide, chemotherapy or targeted therapy.