OCTREOSCAN(R)111 FOR IMAGING OF A SOMATOSTATIN RECEPTOR-POSITIVEISLETCELL TUMOR IN RAT

Somatostatin (SRIF) receptors are present in a variety of human tumors such as pituitary and endocrine pancreatic tumors, brain tumors, smal l cell lung cancers and malignant breast tumors. The In-111 labeled SR IF analog SDZ 215-811 (OctreoScan(R) 111) binds with a high affinity t o somatostatin receptors and exhibits SRIF-like biological properties, as demonstrated by the inhibition of growth hormone release from pitu itary cells. We report here the in vitro characterization of SDZ 215-8 11 and the in vivo imaging of an islet cell tumor grown in rats using [In-111]SDZ 215-811. In vitro autoradiographies revealed a high densit y of SRIF receptors on the pancreatic tumor tissue. As early as 5 min after intravenous injection of [In-111]SDZ 215-811 into tumor-bearing rats, the tumors were clearly localized by gamma-camera scintigraphy. Even 24 h post injection, the islet cell tumor was still detectable. T he radio-ligand was mainly cleared from the circulation via the kidney s, with a rapid alpha-phase (t1/2 = 5.6 min) and a slow elimination ph ase (t1/2 = 7.3 h). Biodistribution studies revealed a relatively high accumulation of radioactivity in the kidneys, but low uptake into the liver and the intestine. High uptake of [In-111]SDZ 215-811 was obser ved for the tumor tissue (0.92+/-0.07% ID/g; 1 h post injection). Inte restingly, a tumor load of 0.14+/-0.01% ID/g was still measured after 24 h. The tumor/blood ratio was 4.93 after 24 h, indicating specific a ccumulation of radioactivity in the islet cell tumor. [In-111]SDZ 215- 811 appears to be a sensitive and specific ligand for SRIF receptor-po sitive tumors and offers an easy procedure for scintigraphic imaging o f such tumors in man.