Mechanisms of Resistance in Gastroenteropancreatic Neuroendocrine Tumors

SIMPLE SUMMARY: Advanced neuroendocrine tumors originating in the intestinal tract and pancreas are treated with biologics that activate somatostatin receptors (lanreotide and octreotide), small molecule drugs that target the mTOR (everolimus) and VEGF and other signaling pathways (sunitinib), chemotherapies (temozolomide, capecitabine, fluorouracil, platinums), and receptor-targeted radionuclides (Lu177-DOTATATE). These treatments eventually fail to control tumor progression, but the mechanisms for therapeutic resistance are poorly understood. We will review preclinical and early clinical studies that provide insight into potential etiologies and new therapies and combinations that may address this resistance. ABSTRACT: Gastroenteropancreatic neuroendocrine tumors (GEP-NETs), although curable when localized, frequently metastasize and require management with systemic therapies, including somatostatin analogues, peptide receptor radiotherapy, small-molecule targeted therapies, and chemotherapy. Although effective for disease control, these therapies eventually fail as a result of primary or secondary resistance. For small-molecule targeted therapies, the feedback activation of the targeted signaling pathways and activation of alternative pathways are prominent mechanisms, whereas the acquisition of additional genetic alterations only rarely occurs. For somatostatin receptor (SSTR)-targeted therapy, the heterogeneity of tumor SSTR expression and dedifferentiation with a downregulated expression of SSTR likely predominate. Hypoxia in the tumor microenvironment and stromal constituents contribute to resistance to all modalities. Current studies on mechanisms underlying therapeutic resistance and options for management in human GEP-NETs are scant; however, preclinical and early-phase human studies have suggested that combination therapy targeting multiple pathways or novel tyrosine kinase inhibitors with broader kinase inhibition may be promising.