Radioiodinated somatostatin analogue RC-160: preparation,biological activity,in vivo application in rats and comparison with [123I-Tyr3]octreotide

We have evaluated the potential usefulness of the radioiodinated octapeptide RC-160, a somatostatin analogue, which might serve as a radiopharmaceutical for the in vivo detection of somatostatin receptor-positive tumours. For this purpose, iodine-123 and iodine-125 labelled RC-160 was tested for biological activity and applied in vivo in rats bearing the transplantable rat pancreatic tumour CA20948, which expresses somatostatin receptors. Our group has recently described the in vivo visualization of such tumours in rats and in humans with the radioiodinated somatostatin analogue [Tyr³]octreotide. Like [¹²³I-Tyr³]octreotide, ¹²³I-RC-160 showed uptake in and specific binding in vivo to somatostatin receptor-positive organs and tumours. However, blood radioactivity (background) was higher, resulting in a lower tumour to blood (background) ratio. We therefore conclude that in this animal model ¹²³I-RC-160 has no advantage over [¹²³I-Tyr³]octreotide as a radiopharmaceutical for the in vivo use as a somatostatin receptor imager, although, like [¹²³I-Tyr³]octreotide, ¹²³I-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 those of the breast, ovary, exocrine pancreas, prostate and colon. Therefore ¹²³I-RC-160 might be of interest for future use in humans as a radiopharmaceutical for imaging octreotide receptor-negative tumours.Correspondence to: W.A.P. Breeman, Department of Nuclear Medicine, University Hospital Dijkzigt, Dr. Molewaterplein 40, NL-3015 GD Rotterdam, The NetherlandsThe authors wish to thank Dr. Wil Kort, Ineke Hekking-Weyma, Reno Mekes, Marcello Harms and Ina Loeve for their expert assistance during the experiments.