OctreoScan 111 for imaging of a somatostatin receptor-positive islet cell tumor in rat

Somatostatin (SRIF) receptors are present in a variety of human tumors such as pituitary and endocrine pancreatic tumors, brain tumors, small cell lung cancers and malignant breast tumors. The 111In-labeled SRIF analog SDZ 215-811 (OctreoScan 111) binds with a high affinity to somatostatin receptors and exhibits SRIF-like biological properties, as demonstrated by the inhibition of growth hormone release from pituitary cells. We report here the in vitro characterization of SDZ 215-811 and the in vivo imaging of an islet cell tumor grown in rats using [111In]SDZ 215-811. In vitro autoradiographies revealed a high density of SRIF receptors on the pancreatic tumor tissue. As early as 5 min after intravenous injection of [111In]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. The radioligand was mainly cleared from the circulation via the kidneys, with a rapid alpha-phase (t1/2 = 5.6 min) and a slow elimination phase (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 [111In]SDZ 215-811 was observed for the tumor tissue (0.92 +/- 0.07% ID/g; 1 h post injection). Interestingly, 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 accumulation of radioactivity in the islet cell tumor.(ABSTRACT TRUNCATED AT 250 WORDS)