Neuroendocrine tumor cells frequently overexpress somatostatin receptors at their cell surfaces. To evaluate the possibility of using the somatostatin analog 111In-DTPA-D-Phe1-octreotide for radiation therapy, we studied the binding and subsequent internalization of 111In into three types of cultured human neuroendocrine tumor cells.
Methods: Primary cultures of gastric carcinoid, midgut carcinoid and glucagonoma cells were incubated with 111In-DTPA-D-Phe1-octreotide and cell-surface bound, internalized and released 111In activity was measured. Electron microscopic autoradiography was also performed.
Results: All three cell types specifically (80%-95%) bound 111In-DTPA-D-Phe1-octreotide and internalized 111In. After 1 hr pulse incubation with 111In-DTPA-D-Phe1-octreotide, there was an initial decrease in intracellular 111In to about 50% during the subsequent 6-hr incubation. Almost no further release was observed during the remaining 18-42 hr studied. Autoradiography showed that the internalized 111In was found in the cytoplasm and nucleus in the midgut carcinoid cells.
Conclusion: Indium-111 DTPA-D-Phe1-octreotide might be useful for radiation therapy of patients with surgically incurable tumors having high somatostatin receptor densities.