A NEW RADIOLABELED SOMATOSTATIN ANALOG [IN-111-DTPA-D-PHE(1)]RC-160 -PREPARATION, BIOLOGICAL-ACTIVITY, RECEPTOR SCINTIGRAPHY IN RATS AND COMPARISON WITH [IN-111-DTPA-D-PHE(1)]OCTREOTIDE

We have evaluated the potential usefulness of indium-111 labelled [DTP A-D-Phe(1)]RC-160, derived from the octapeptide somatostatin analogue RC-160, as a radiopharmaceutical for the in vivo detection of somatost atin receptor-positive tumours. For this purpose In-111- and In-115-la belled [DTPA-D-Phe(1)]RC-160 was tested for its biological activity, a nd applied for somatostatin receptor scintigraphy in vivo to rats bear ing the transplantable rat pancreatic tumour CA20948, which expresses somatostatin receptors. We previously described the development of the In-111-labelled somatostatin analogue [DTPA-D-Phe(1)]octreotide and i ts use in the in vivo visualization of somatostatin receptor-positive tumours in rats and in humans. Like [In-111-DTPA-D-Phe(1)]octreotide, [In-111-DTPA-D-Phe(1)]RC-160 showed uptake in and specific binding in vivo to somatostatin receptor-positive organs and tumours, and the tum ours were clearly visualized by gamma camera scintigraphy. However, as compared to [In-111-DTPA-D-Phe(1)]octreotide, blood radioactivity (ba ckground) was higher, resulting in a lower tumour to blood (background ) ratio. Using this animal model we therefore conclude that [In-111-DT PA-D-Phe(1)]RC-160 has no advantage over [In-111-DTPA-D-Phe(1)]octreot ide as a radiopharmaceutical in the visualization of somatostatin rece ptors which bind both analogues. However, recent reports suggest the e xistence of different somatostatin receptor subtypes on some human can cers, which differentially bind RC-160 and not octreotide. These rumou rs include cancers of the breast, ovary, exocrine pancreas, prostate a nd colon. [In-111-DTPA-D-Phe(1)]RC-160 might be of interest for future use in such cancer patients as a radiopharmaceutical for imaging soma tostatin receptor-positive tumours, which do not bind octreotide.