Radionuclides

The four radionuclide conjugates most commonly used in the treatment of neuroendocrine tumors are iodine I 131-metaiodobenzylguanidine (131I-MIBG), indium In 111 (111In), yttrium Y 90, and lutetium Lu 177 (177Lu), with the latter three bound to a variety of somatostatin analogues. However, the median tumor response rate for the patients treated with 131I-MIBG is less than 5%, although the modality appears somewhat more effective in achieving biochemical stability (~50%) or tumor stability (~70%).[1] Although 111In-labeled somatostatin analogues are the most commonly studied radiopeptides to date, largely reflecting their availability, and with therapeutic benefits similar to 131I-MIBG, the most promising advance in radiopeptide therapeutics has been the development of 177Lu-octreotate, which emits both beta and gamma radiation.[1] In the largest patient series treated to date with lutetium-labeled somatostatin analogues (n = 131; 65 with gastrointestinal neuroendocrine tumors), remission rates were correlated positively with high pretherapy octreotide scintigraphy uptake and limited hepatic tumor load.[27] In patients with extensive liver involvement, median time to progression was shorter (26 months) compared with patients who had either stable disease or tumor regression (>36 months).