Near-infrared fluorescence imaging of tumor integrin αvβ3 expression with Cy7-labeled RGD multimers

Purpose: Cell adhesion molecule integrin alpha(v)beta(3) is an excellent target for tumor interventions because of its unique expression on the surface of several types of solid tumor cells and on almost all sprouting tumor vasculatures. Here, we describe the development of near-infrared (NIR) fluorochrome Cy7-labeled RGD peptides for tumor integrin targeting. Procedures: Mono-, di-, and tetrameric RGD peptides were synthesized and conjugated with Cy7. The integrin specificity of these fluorescent probes was tested in vitro for receptor binding assay and fluorescence microscopy and in vivo for subcutaneous U87MG tumor targeting. Results: The tetrameric RGD peptide probe with the highest integrin affinity showed the highest tumor activity accumulation and strongest tumor-to-normal tissue contrast. This uptake is integrin-specific as the signal accumulated in the tumor can be effectively blocked by unconjugated RGD peptide antagonist of integrin alpha(v)beta(3). Conclusions: Noninvasive NIR fluorescence imaging is able to detect and semiquantify tumor integrin expression based upon the highly potent tetrameric RGD peptide probe.