Synthesis and biological evaluation of potent α_vβ₃-integrin receptor antagonists

Abstract 

Introduction

α_vβ₃ Integrin is expressed in sprouting endothelial cells in growing tumors, whereas it is absent in quiescent blood vessels. In addition, various tumor cell types express α_vβ₃ integrin. α_vβ₃ Integrin, a transmembrane heterodimeric protein, binds to the arginine–glycine–aspartic acid (RGD) amino acid sequence of extracellular matrix proteins such as vitronectin and plays a pivotal role in invasion, proliferation and metastasis. Due to the selective expression of α_vβ₃ integrin in tumors, radiolabeled RGD peptides and peptidomimetics are attractive candidates for tumor targeting.

Methods

A cyclic RGD peptide, a peptoid–peptide hybrid, an all-peptoid and a peptidomimetic compound were synthesized, conjugated with 1,4,7,10-tetraazadodecane-N,N′,N″,N‴-tetraacetic acid (DOTA) and radiolabeled with ¹¹¹In. Their in vitro and in vivo α_vβ₃-binding characteristics were determined.

Results

IC₅₀ values were 236 nM for DOTA-E-c(RGDfK), 219 nM for DOTA-peptidomimetic, >10 mM for DOTA-all-peptoid and 9.25 mM for the peptoid–peptide hybrid DOTA-E-c(nRGDfK). ¹¹¹In-labeled compounds, except for [¹¹¹In]DOTA-all-peptoid, showed specific uptake in human α_vβ₃-expressing tumors xenografted in athymic mice. Tumor uptake for [¹¹¹In]DOTA-E-c(RGDfK) was 1.73±0.4% ID/g (2 h postinjection) and that of [¹¹¹In]DOTA-peptidomimetic was 2.04±0.3% ID/g. Tumor uptake for the peptoid–peptide hybrid [¹¹¹In]DOTA-E-c(nRGDfK) was markedly lower (0.45±0.07% ID/g). The all-peptoid [¹¹¹In]DOTA-E-c(nRGnDnFnK) did not show specific uptake in tumors (0.11±0.04% ID/g).

Conclusions

The peptidomimetic compound and the cyclic RGD peptide have a high affinity for α_vβ₃ integrin, and these compounds have better tumor-targeting characteristics than the peptoid–peptide hybrid and the all-peptoid.

Keywords: α_vβ₃ Integrin, RGD peptide, Peptidomimetic, Peptoid, ¹¹¹In