Secretin receptor as a target in gastrointestinal cancer: expression analysis and ligand development

Abstract

Secretin was originally discovered as a gastrointestinal peptide that stimulates fluid secretion from pancreas and liver and delays gastric emptying. In disease, secretin receptor (SCTR) was found to occur as a splice variant in gastrinoma and pancreatic adenocarcinoma. Overexpression of SCTR has been described for gastrinomas, carcinoid tumors of the lung and cholangiocarcinoma. SCTR therefore remains a candidate target for molecular tumor imaging as well as for peptide receptor radioligand therapy (PRRT) in a number of oncological indications. The aim of this study was to characterize SCTR expression in esophageal and pancreatic cancer. 65 of 70 PDAC tissues stained strongly positive for SCTR in immunohistochemistry, as were most of 151 esophageal cancer samples, with minor influence of grading in both entities. In addition, the aim of this study was to further delineate residues in human secretin that are critical for binding to and activation of the human SCTR. For a potential development of short and metabolically stable analogs for clinical use, it was intended to probe the peptide for its capacity to incorporate deletions and substitutions without losing its affinity to SCTR. A library of 146 secretin variants containing single amino acid substitutions as well as truncations on either end was tested using {beta}-arrestin2-GFP translocation and fluorescent ligand internalization high-content analysis and cAMP assays run in agonist and antagonist mode as well as a radioligand binding. The main structural determinants of SCTR binding and activation were localized to the N-terminus, with His1, Asp3 being among the most sensitive positions, followed by Phe6, Thr7 and Leu10. Aminoterminal truncation caused a rapid decline in receptor activity and most of these variants proved to be partial agonists showing antagonistic properties. In this study, the most potent antagonist showed an IC50 of 309 {+/-} 74 nM in the {beta}-arrestin2-GFP translocation assay on human SCTR while remaining a weak partial agonist. Future studies will have to demonstrate the utility of further enhanced secretin analogues as tracers for in vivo imaging and therapy.