Studies toward the Development of New Silicon-Containing Building Blocks for the Direct 18F-Labeling of Peptides

Silicon-containing prosthetic groups have been conjugated to peptides to allow for a single-step labeling with F-18 radioisotope. The fairly lipophilic di-tert-butylphenylsilane building block contributes unfavorably to the pharmacokinetic profile of bombesin conjugates. In this article, theoretical and experimental studies toward the development of more hydrophilic silicon-based building blocks are presented. Density functional theory calculations were used to predict the hydrolytic stability of di-tert-butylfluorosilanes 2-23 with the aim to improve the in vivo properties of F-18-labeled silicon-containing biomolecules. As a further step toward improving the pharmacokinetic profile, hydrophilic linkers were introduced between the lipophilic di-tert-butylphenylsilane building block and the bombesin congeners. Increased tumor uptake was shown with two of these peptides in xenograft-bearing mice using positron emission tomography and biodistribution studies. The introduction of a hydrophilic linker is thus a viable approach to improve the tumor uptake of F-18-labeled silicon bombesin conjugates.