GRPR-targeted SPECT imaging using a novel bombesin-based peptide for colorectal cancer detection.

Colorectal cancer (CRC) is the third most common cancer worldwide, and the prognosis of CRC is better with an earlier diagnosis. The presence of the gastrin-releasing peptide receptor (GRPR) has been documented in very high numbers on colorectal cancer cells, which makes it an ideal biomarker for the diagnosis of CRC. Bombesin (BBN) peptide analogs have been extensively investigated for the imaging of human cancers with GRPR overexpression. Recently, we have reported a novel GRPR-targeted peptide named the GB-6 peptide. The GB-6 peptide based on BBN was designed to improve in vivo metabolic stability and decrease intestinal uptake. Meanwhile, GB-6 greatly retained the original GRPR-binding affinity of BBN. In this study, the GB-6 peptide was labeled with radionuclide Tc or fluorescent dye for colorectal cancer imaging. In vitro receptor binding was studied in Caco-2 cells, and the GRPR targeting capacity and kinetics in vivo were evaluated using Caco-2 tumor xenografted mice models. In addition, cells and mice were also subjected to the corresponding BBN conjugations for comparison. The GB-6 peptide exhibited specific GRPR binding in vitro with a high affinity similar to that of BBN. Furthermore, we observed that GB-6 showed higher tumor uptake and displayed lower intestinal activity than corresponding unmodified probe BBN in Caco-2 tumor-bearing mice. Overall, our studies demonstrated that GB-6 has the potential for early detection of CRC patients, and it may also serve as a valuable tool for non-invasive monitoring of colorectal tumor growth.