Identification and stabilization of a highly selective gastrin-releasing peptide receptor agonist.

The gastrin-releasing peptide receptor (GRPR) is part of the bombesin receptor family and a well-known target in cancer diagnosis and therapy. In the last decade, promising results have been achieved by using peptide-drug conjugates, which allow selective targeting of GRPR expressing tumor cells. Most ligands, however, have been antagonists even though agonists can lead to higher tumor uptake owing to their internalization. So far, only a few studies focused on the identification of small GRPR-selective agonists that are metabolically stable. Here, we developed novel bombesin analogs with high selectivity for the GRPR and improved blood plasma stability. The most promising analog [d-Phe6 , β-Ala11 , NMe-Ala13 , Nle14 ]Bn(6-14) displays an activity of 0.3nM at the GRPR, a more than 4000-fold selectivity over the other two bombesin receptors and more than 75% stability in human blood plasma after 24 hours. This analog is proposed as a promising drug shuttle for the intracellular delivery of different payloads in targeted tumor therapy approaches.