Design, Synthesis, and in Vitro and in Vivo Evaluation of High Affinity and Specificity Near-Infrared Fluorescent Bombesin Antagonists for Tumor Imaging

The
bombesin (BBN) antagonist binds with high affinity to the gastrin
releasing peptide receptor (GRPr), a receptor overexpressed on many
human cancers. We present an investigation employing BBN antagonist
for highly specific near-infrared fluorescence (NIRF) imaging of GRPr-positive
tumors. Nine NIRF-dye labeled BBN antagonists with differing linkers
and dyes were synthesized and characterized to screen for the optimal
agent. Three novel agents, AF750-G-pip-Sta-BBN (1), AF750-GSG-Sta-BBN
(2), and AF750-6Ahx-Sta-BBN (3), exhibited
an excellent binding-specificity and affinity to human PC-3 prostate
cancer cells in vitro, and a remarkable in vivo tumor-selectivity
and NIRF imaging sensitivity in PC-3 tumor-bearing mice. Compound 1 showed the fastest, and 3, the slowest, pharmacokinetics
on the tumor sites. Despite of high tumor uptake, 2 had
a low pancreas uptake distinct from 1 and 3 at 0.44 nmol dose. This difference was attributed to the inherent
linker properties such as the hydrophilicity, polarity, and charge.