Synthesis, 18F-Labeling, and in Vitro and in Vivo Studies of Bombesin Peptides Modified with Silicon-Based Building Blocks

Abstract

The gastrin-releasing peptide receptor (GRPr) is overexpressed on various human tumors. The goal of our study was the synthesis of new ¹⁸F-labeled bombesin analogues for the PET imaging of GRPr expression in prostate tumor using a silicon-based one-step n. c. a. radiolabeling method. The silicon-containing building blocks were efficiently coupled to the N-terminus of the peptides via solid-phase synthesis. Radiolabeling of the obtained peptide precursors proceeded smoothly under acidic conditions (34−85% conversion). Using the di-tert-butyl silyl building block as labeling moiety, products containing a hydrolytically stable ¹⁸F-label were obtained. In in vitro receptor binding experiments 2-(4-(di-tert-butylfluorosilyl)phenyl)acetyl-Arg-Ava-Gln-Trp-Ala-Val-NMeGly-His-Sta-Leu-NH₂ (4b, IC₅₀ = 22.9 nM) displayed a 12-fold higher binding affinity than 2-(4-(di-tert-butylfluorosilyl)phenyl)acetyl-Arg-Ava-Gln-Trp-Ala-Val-Gly-His(3Me)-Sta-Leu-NH₂ (3b, IC₅₀ = 276.6 nM), and 4b was therefore chosen for further evaluation. In vitro and ex vivo metabolite studies of [¹⁸F]4b showed no significant degradation. In biodistribution experiments, tumor uptake of [¹⁸F]4b was low and unspecific, whereas the GRPr-rich pancreas revealed a high and specific accumulation of the radiotracer. This study demonstrates the applicability of our silicon-based one-step n. c. a. radiolabeling method for the synthesis of new ¹⁸F-labeled bombesin derivatives. This innovative approach represents a general, straightforward access to radiolabeled peptides as PET imaging probes.