Strategies for Optimized Radiolabeling of Nanoparticles for in vivo PET Imaging

Driven by the motivation for optimizing Cu radiolabeling efficiency of nanoparticles for in vivo positron emission tomography (PET) imaging, a new strategy has been developed. This strategy involved a complete redesign of the nanoparticle system, utilizing macromolecular precursors that were preloaded with labeling sites and programmed for supramolecular assembly into discrete, functional nanoscale objects. A series of shell-crosslinked nanoparticles (SCKs) have been constructed by grafting a copper chelating agent (DOTAlysine) onto amphiphilic block copolymers PAA-b-PS, self assembling the functionalized block copolymer precursors into micelles, and crosslinking the micellar corona to afford the expected nanoobjects. These pre-DOTAlysine-SCKs showed impressive results on Cu radiolabeling (∼ 400 copper atoms per spherical nanoparticle). Among the molecular imaging modalities, PET is widely used as a powerful diagnostic tool by clinicians and scientists. Compared with other imaging methods, it bears the advantages of high sensitivity (the level of detection approaches 10 M of tracer) and isotropism (i.e., ability to detect expression accurately, regardless of tissue depth), which provide reliability for quantitative imaging analyses of in vivo abnormalities. As the pharmaceutical industry began applying PET imaging for assisting drug discovery, small animal PET scanners with spatial resolution up to 1 mm were developed and have been considered to be one of the major achievements for PET technology during the past two decades. Cu is an attractive radionuclide for PET imaging because of its suitable half-life (t1/2 = 12.7 h) and positron emission energy (0.65 MeV), as well as the relatively convenient radiolabeling via coordination with specially designed ligands (chelators). The formation of thermodynamically-stable metal complexes reduces the copper binding with plasma proteins which minimizes its non-specific background activity and its accumulation and resultant toxicity in the liver and kidney. Under the present instrumental conditions, optimizations and improvements of the specific activity of radiopharmaceuticals are of special interest to Cu-based PET systems for achieving high quality images even at low doses, especially when the targets can be readily saturated in vivo. One practical resolution is to encapsulate or conjugate the chelating agents with nanocarriers, which have already been utilized by many research groups including ourselves 8] and have been found to exhibit exciting potential in both high loading capacities and redirection of the bio-distributions of small molecule ligands (e.g. for tissue targeting) or guests (e.g. for pharmaceutical effect). Our research has focused upon SCKs as the nanoscale framework for the attachment of macrocyclic chelators and labeling by Cu radionuclides. SCKs have been established from the self-assembly of amphiphilic block copolymers to afford micelles with core-shell morphology that are then covalently crosslinked throughout the shell domain. Recently, it was confirmed that by tuning the nanoparticle properties, especially the size and rigidity, increased in vivo circulation times and improved bio-distributions could be reached for Cu-TETA SCK conjugates. Although these preliminary results are promising, several challenges require further investigations. Among them, efficient radiolabeling takes the highest priority. Previously, the direct conjugation of macrocyclic chelators onto pre-established SCKs afforded limited coupling and radiolabeling yields, due to steric and electrostatic factors. 11] As part of our ongoing efforts, we now report an alternative strategy to construct chelator-SCK conjugates with high radiolabeling efficiencies, which is expected to lead to nanoscale objects that can be administered in small quantities for ultra-sensitive PET imaging. C O M M U N IC A IO N