INVIVO APPLICATION OF [IN-111-DTPA-D-PHE1]-OCTREOTIDE FOR DETECTION OF SOMATOSTATIN RECEPTOR-POSITIVE TUMORS IN RATS

Radioiodinated somatostatin analogues are useful ligands for the in vitro and in vivo detection of somatostatin receptors. [In-111-DTPA-D-Phe1]-octreotide, a somatostatin analogue labeled with a different radionuclide, also binds specifically to somatostatin receptors in vitro. In this study we investigated its in vivo application in the visualization of somatostatin receptor-positive tumors in rats. The distribution of the radiopharmaceutical was investigated after intravenous injection in normal rats and in rats bearing the somatostatin receptor-positive rat pancreatic carcinoma CA 20948. After injection the radiopharmaceutical was rapidly cleared (50 % decrease in maximal blood radioactivity in 4 min), predominantly by the kidneys. Excreted radioactivity was mainly in the form of the intact radiopharmaceutical. Ex vivo autoradiographic studies showed that specific accumulation of radioactivity occurred in somatostatin receptor-containing tissue (anterior pituitary gland). However, in contrast to the adrenals and pituitary, the tracer accumulation in the kidneys was not mediated by somatostatin receptors. Increasing radioactivity over the somatostatin receptor-positive tumors was measured rapidly after injection and the tumors were clearly visualized by gamma camera scintigraphy. In rats pretreated with 1 mg octreotide accumulation of [In-111-DTPA-D-Phe1]-octreotide in the tumors was prevented. Because of its relatively long effective half-life, [In-111-DTPA-D-Phe1]-octreotide is a radionuclide-coupled somatostatin analogue which can be used to visualize somatostatin receptor-bearing tumors efficiently after 24 hr, when interfering background radioactivity is minimized by renal clearance. This is an advantage over the previously used [I-123-Tyr3]-octreotide which has a shorter effective half-life and shows high abdominal interference due to its hepato-biliary clearance. Therefore, [In-111-DTPA-D-Phe1]-octreotide seems a better alternative for scintigraphic imaging of somatostatin receptor-bearing tumors.