Biodegradable dendritic positron-emitting nanoprobes for the noninvasive imaging of angiogenesis

A biodegradable positron-emitting dendritic nanoprobe targeted at α_vβ₃ integrin, a biological marker known to modulate angiogenesis, was developed for the noninvasive imaging of angiogenesis. The nanoprobe has a modular multivalent core-shell architecture consisting of a biodegradable heterobifunctional dendritic core chemoselectively functionalized with heterobifunctional polyethylene oxide (PEO) chains that form a protective shell, which imparts biological stealth and dictates the pharmacokinetics. Each of the 8 branches of the dendritic core was functionalized for labeling with radiohalogens. Placement of radioactive moieties at the core was designed to prevent in vivo dehalogenation, a potential problem for radiohalogens in imaging and therapy. Targeting peptides of cyclic arginine–glycine–aspartic acid (RGD) motifs were installed at the terminal ends of the PEO chains to enhance their accessibility to α_vβ₃ integrin receptors. This nanoscale design enabled a 50-fold enhancement of the binding affinity to α_vβ₃ integrin receptors with respect to the monovalent RGD peptide alone, from 10.40 nM to 0.18 nM IC₅₀. Cell-based assays of the ¹²⁵I-labeled dendritic nanoprobes using α_vβ₃-positive cells showed a 6-fold increase in α_vβ₃ receptor-mediated endocytosis of the targeted nanoprobe compared with the nontargeted nanoprobe, whereas α_vβ₃-negative cells showed no enhancement of cell uptake over time. In vivo biodistribution studies of ⁷⁶Br-labeled dendritic nanoprobes showed excellent bioavailability for the targeted and nontargeted nanoprobes. In vivo studies in a murine hindlimb ischemia model for angiogenesis revealed high specific accumulation of ⁷⁶Br-labeled dendritic nanoprobes targeted at α_vβ₃ integrins in angiogenic muscles, allowing highly selective imaging of this critically important process.