A novel PET tracer for targeted imaging of atherosclerosis

Non-oncological applications of RGD-based single-photon emission tomography and positron emission tomography agents

INTRODUCTION

Non-invasive imaging techniques (especially single-photon emission tomography and positron emission tomography) apply several RGD-based imaging ligands developed during a vast number of preclinical and clinical investigations. The RGD (Arg-Gly-Asp) sequence is a binding moiety for a large selection of adhesive extracellular matrix and cell surface proteins. Since the first identification of this sequence as the shortest sequence required for recognition in fibronectin during the 1980s, fundamental research regarding the molecular mechanisms of integrin action have paved the way for development of several pharmaceuticals and radiopharmaceuticals with clinical applications. Ligands recognizing RGD may be developed for use in the monitoring of these interactions (benign or pathological). Although RGD-based molecular imaging has been actively investigated for oncological purposes, their utilization towards non-oncology applications remains relatively under-exploited.

METHODS AND SCOPE

This review highlights the new non-oncologic applications of RGD-based tracers (with the focus on single-photon emission tomography and positron emission tomography). The focus is on the last 10 years of scientific literature (2009-2020). It is proposed that these imaging agents will be used for off-label indications that may provide options for disease monitoring where there are no approved tracers available, for instance Crohn's disease or osteoporosis. Fundamental science investigations have made progress in elucidating the involvement of integrin in various diseases not pertaining to oncology. Furthermore, RGD-based radiopharmaceuticals have been evaluated extensively for safety during clinical evaluations of various natures.

CONCLUSION

Clinical translation of non-oncological applications for RGD-based radiopharmaceuticals and other imaging tracers without going through time-consuming extensive development is therefore highly plausible. Graphical abstract.

Synthesis, Chemical Characterization and Multiscale Biological Evaluation of a Dimeric-cRGD Peptide for Targeted Imaging of α V β 3 Integrin Activity

Cyclic peptides containing the Arg-Gly-Asp (RGD) sequence have been shown to specifically bind the angiogenesis biomarker α_Vβ₃ integrin. We report the synthesis, chemical characterization, and biological evaluation of two novel dimeric cyclic RGD-based molecular probes for the targeted imaging of α_Vβ₃ activity (a radiolabeled version, ⁶⁴Cu-NOTA-PEG₄-cRGD₂, for PET imaging, and a fluorescent version, FITC-PEG₄-cRGD₂, for in vitro work). We investigated the performance of this probe at the receptor, cell, organ, and whole-body levels, including its use to detect diabetes associated impairment of ischemia-induced myocardial angiogenesis. Both versions of the probe were found to be stable, demonstrated fast receptor association constants, and showed high specificity for α_Vβ₃ in HUVECs (K_d ~ 35 nM). Dynamic PET-CT imaging indicated rapid blood clearance via kidney filtration, and accumulation within α_Vβ₃-positive infarcted myocardium. ⁶⁴Cu-NOTA-PEG₄-cRGD₂ demonstrated a favorable biodistribution, slow washout, and excellent performance with respect to the quality of the PET-CT images obtained. Importantly, the ratio of probe uptake in infarcted heart tissue compared to normal tissue was significantly higher in non-diabetic rats than in diabetic ones. Overall, our probes are promising agents for non-invasive quantitative imaging of α_Vβ₃ expression, both in vitro and in vivo.