VEGFR‐2 Targeted Microbubble Contrast Agents for Ultrasound Molecular Imaging of Tumor Angiogenesis

Abstract

Ultrasound molecular imaging is an emerging technique for detecting intravascular targets. We have developed a novel microbubble (MB) ultrasound contrast agent covalently coupled to a recombinant single‐chain VEGF construct (scVEGF), which binds VEGFR‐2 (KDR) on the angiogenic endothelium. Ligand conjugation was first validated with a fluorophore (BODIPY‐cystine), and covalently bound dye was detected by fluorometry and flow cytometry. MB were subsequently conjugated to scVEGF, which bound at ~100,000 molecules/MB (ELISA). Targeted adhesion of scVEGF‐MB was demonstrated with in vitro flow assays. At 0.5 dynes/cm2, scVEGF‐MB exhibited 5‐fold higher adhesion to both recombinant murine VEGFR‐2 and porcine aortic endothelial cells transfected with VEGFR‐2 (PAE/KDR) compared to non‐targeted control MB. Additionally, scVEGF‐MB targeted to immobilized VEGFR‐2 in an ultrasound flow phantom showed an 8‐fold increase in mean acoustic signal relative to casein‐coated control channels. A murine model of colon adenocarcinoma was used to assess targeted adhesion in vivo, and scVEGF MB, but not control MB, showed significantly higher ultrasound contrast signal enhancement in tumors. These results demonstrate the functionality of a novel scVEGF‐bearing microbubble contrast agent, which could be a valuable research tool for molecular imaging of VEGFR‐2. Supported by NIH 1R43EB007857.