RADIOIODINATED SOMATOSTATIN ANALOG RC-160 - PREPARATION, BIOLOGICAL-ACTIVITY, IN-VIVO APPLICATION IN RATS AND COMPARISON WITH [I-123-TYR(3)]OCTREOTIDE

We have evaluated the potential usefulness of the radioiodinated octap eptide RC-160, a somatostatin analogue, which might serve as a radioph armaceutical for the in vivo detection of somatostatin receptor-positi ve tumours. For this purpose, iodine-123 and iodine-125 labelled RC-16 0 was tested for biological activity and applied in vivo in rats beari ng the transplantable rat pancreatic tumour CA20948, which expresses s omatostatin receptors. Our group has recently described the in vivo vi sualization of such tumours in rats and in humans with the radioiodina ted somatostatin analogue [Tyr3]octreotide. Like [I-123-Tyr3]octreotid e, I-123-RC-160 showed uptake in and specific binding in vivo to somat ostatin receptor-positive organs and tumours. However, blood radioacti vity (background) was higher, resulting in a lower tumour to blood (ba ckground) ratio. We therefore conclude that in this animal model I-123 -RC-160 has no advantage over [I-123-Tyr3] octreotide as a radiopharma ceutical for the in vivo use as a somatostatin receptor imager, althou gh, like [I-123-Tyr]octreotide, I-123-RC-160 shows specific binding to different somatostatin receptor-positive organs. Recently differences were reported in affinity between somatostatin and its analogues for somatostatin receptors expressed in different human cancers, like thos e of the breast, ovary, exocrine pancreas, prostate and colon. Therefo re I-123-RC-160 might be of interest for future use in humans as a rad iopharmaceutical for imaging octreotide receptor-negative tumours.