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

The development of novel 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 saturation binding assay format. Since GPCRs are transmembrane proteins, receptor samples for binding assays are prepared from tissue sections, cell membranes, cell homogenates, or intact cells. As part of our investigations on modulating the pharmacokinetics of radiolabeled peptides for improved theranostic targeting of neuroendocrine tumors with a high abundance of the somatostatin receptor sub-type 2 (SST), we characterized a series of Cu-labeled [Tyr]octreotate (TATE) derivatives in vitro in saturation binding assays. Herein, we report on the SST binding parameters measured toward intact mouse pheochromocytoma cells and corresponding cell homogenates and discuss the observed differences taking the physiology of SST and GPCRs in general into account. Furthermore, we point out method-specific advantages and limitations.