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_vβ ₃ 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.