Therapy of neuroendocrine tumors with radiolabeled MIBG and somatostatin analogues.

The increased understanding of the neuroendocrine tumors at a cellular and molecular level has led to the development of new radiopharmaceuticals for imaging. Two of the imaging agents include 131I metaiodobenzylguanidine (131I-MIBG) and 111In-DTPA-D-Phe1-octreotide (111In-pentetreotide) each having specific localization in certain neuroendocrine tumors. The selective uptake of these radiopharmaceuticals by the tumor cells has generated interest in potential use for targeted radiotherapy for neuroendocrine tumors. 131I-MIBG has been used to treat patients with pheochromocytoma, neuroblastoma, carcinoid tumors, medullary thyroid carcinoma, and paragangliomas. The tumor responses have been variable with the most encouraging results being in patients with pheochromocytoma. The dose-limiting toxicity has been thrombocytopenia or granulocytopenia. 111In-pentetreotide has been used as therapy in only a few patients and has resulted in objective evidence of tumor responses. A therapeutic agent using a somatostatin analogue will most likely require radiolabeling with a beta- or possibly an alpha-emitting radionuclide to achieve significant and durable tumor responses.