In vivo cellular distribution and endocytosis of the somatostatin receptor-ligand complex.

Radioactive tumor targeting agents are highly interesting and for treatment of neuroendocrine tumors expressing somatostatin receptors, radiolabeled somatostatin analogues (including [(111)In-DTPA-D-Phe1]-octreotide) has been tried in a small number of patients with encouraging results. To increase our knowledge about the in vivo processing of administered [(111)In-DTPA-D-Phe1]-octreotide we have examined tumor and normal tissue material from a patient with a midgut carcinoid tumor. By ultrastructural autoradiography, silver grains indicating the presence of [(111)In-DTPA-D-Phe1]-octreotide could be identified within tumor cells, both in the primary tumor and in the mesenteric metastases. Silver grains were also found in leukocytes and in blood vessels. However, normal enterocytes did not show any specific radioligand uptake. This study indicates that the binding and endocytosis of [(111)In-DTPA-D-Phe1]-octreotide is a specific process that takes place in cells expressing somatostatin receptors. However, the importance of the number of somatostatin receptors and subtypes expressed will have to be further studied.