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 s omatostatin analogues (including [In-111-DTPA-D-Phe(1)]-octreotide) has bee n tried in a small number of patients with encouraging results. To increase our knowledge about the in vivo processing of administered [In-111-DTPA D- Phe(1)]-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 [In-111-DTPA-D-Phe(1)]-octreotide could be identified within tumor cells, both in the primary tumor and in t he mesenteric metastases. Silver grains were also found in leukocytes and i n blood vessels. However, normal enterocytes did not show any specific radi oligand uptake. This study indicates that the binding and endocytosis of [I n-111-DTPA-D-Phe(1)]-octreolide is a specific process that takes place in c ells expressing somatostatin receptors. However, the importance of the numb er of somatostatin receptors and subtypes expressed will have to be further studied.