Comparative Saturation Binding Analysis of 64Cu-Labeled Somatostatin Analogues Using Cell Homogenates and Intact Cells

The development ofnovel ligands for G-protein-coupled receptors(GPCRs) typically entails the characterization of their binding affinity,which is often performed with radioligands in a competition or saturationbinding assay format. Since GPCRs are transmembrane proteins, receptorsamples for binding assays are prepared from tissue sections, cellmembranes, cell homogenates, or intact cells. As part of our investigationson modulating the pharmacokinetics of radiolabeled peptides for improvedtheranostic targeting of neuroendocrine tumors with a high abundanceof the somatostatin receptor sub-type 2 (SST2), we characterizeda series of Cu-64-labeled [Tyr(3)]octreotate (TATE)derivatives in vitro in saturation binding assays. Herein, we reporton the SST2 binding parameters measured toward intact mousepheochromocytoma cells and corresponding cell homogenates and discussthe observed differences taking the physiology of SST2 andGPCRs in general into account. Furthermore, we point out method-specificadvantages and limitations.

Automated summary

The development of novel ligands for G-protein-coupled receptors (GPCRs) involves the characterization of their binding affinity, typically using radioligands in competition or saturation binding assays. GPCRs, being transmembrane proteins, require preparation of receptor samples from tissue sections, cell membranes, cell homogenates, or intact cells for binding assays. In this study, we characterized a series of Cu-64-labeled [Tyr(3)]octreotate (TATE) derivatives in vitro using saturation binding assays to investigate the binding parameters of the somatostatin receptor sub-type 2 (SST2) towards intact mouse pheochromocytoma cells and corresponding cell homogenates. We discuss the observed differences, considering the physiology of SST2 and GPCRs in general, and highlight the advantages and limitations of the methods used.