ARTICLE pubs.acs.org/Biomac
Dendritic MRI Contrast Agents: An Efficient Prelabeling Approach Based on CuAAC Francisco Fernandez-Trillo,† Jesus Pacheco-Torres,‡ Juan Correa,† Paloma Ballesteros,§ Pilar Lopez-Larrubia,‡ Sebastian Cerdan,‡ Ricardo Riguera,*,† and Eduardo Fernandez-Megia*,† †
Department of Organic Chemistry and Center for Research in Biological Chemistry and Molecular Materials (CIQUS), University of Santiago de Compostela, Jenaro de la Fuente s/n, 15782 Santiago de Compostela, Spain ‡ Instituto de Investigaciones Biomedicas “Alberto Sols” CSIC-UAM, Madrid, Spain § Instituto Universitario de Investigacion - UNED, Madrid, Spain
bS Supporting Information ABSTRACT: The Cu(I)-catalyzed azidealkyne cycloaddition (CuAAC) allows the efficient and complete functionalization of dendrimers with preformed Gd chelates (prelabeling) to give monodisperse macromolecular contrast agents (CAs) for magnetic resonance imaging (MRI). This monodispersity contrasts with the typical distribution of materials obtained by classical routes and facilitates the characterization and quality control demanded for clinical applications. The potential of a new family of PEG-dendritic CA based on a gallic acidtriethylene glycol (GATG) core functionalized with up to 27 Gd complexes has been explored in vitro and in vivo, showing contrast enhancements similar to those of Gadomer-17, which reveals them to be a promising platform for the development of CA for MRI.
’ INTRODUCTION Contrast agents (CAs ) based on Gd(III) chelates are increasingly popular in magnetic resonance imaging (MRI) for diagnosis.1,2 With the aim of reducing the 1H relaxation times of H2O and producing a contrast enhancement in MR images, CAs currently used in clinical radiology have low molecular weight, which determines their passive distribution into the interstitial space. The use of macromolecular CA is envisioned to provide longer circulation times in the bloodstream (adquisition windows) and selective diffusion through angiogenic tissue, along with an increased relaxivity per Gd.36 Among the macromolecular CA available, dendrimers are especially appealing.712 Their branched structure imparts rigidity and a high density of functional groups for the multivalent display of Gd and other synergistically integrated agents for therapy and diagnosis.36,13,14 In addition, the intrinsically monodisperse nature of dendrimers and absolute control of their size allows their selection for specific applications as a function of generation. Indeed, the pharmacokinetics and pharmacodynamics of dendrimers, their permeability, excretion routes, and recognition by the reticuloendothelial system can be controlled by generation.15 The synthesis of dendritic and other macromolecular CA has been conventionally achieved in a stepwise fashion through postlabeling approaches, which involve the incorporation of a suitable metal ligand onto the macromolecular scaffold (mainly by active esters and isothiocyanates), followed by complexation to the desired metal ion (Scheme 1).36 Commercially available poly(amido amine) (PAMAM) and poly(propylene imine) (PPI) dendrimers have been postlabeled in this way by incorporating r 2011 American Chemical Society
chelates of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and diethylenetriaminepentaacetic acid (DTPA). Unfortunately, these postlabeling approaches suffer from incomplete ligand functionalization and in some instances from slow complexation rates to Gd, which results in mixtures of polydisperse compounds with varying degrees of functionalization (ca. 80% of the terminal groups functionalized with the chelating agent and 80% of these ligands complexed to Gd).16,17 In this way, not only is the advantage of starting from monodisperse macromolecular materials is lost, but on top of that the final functionalized dendrimers become strongly batch-dependent, which complicates characterization and ultimately limits their clinical application. In addition, incomplete complexation to Gd leaves free carboxylate groups able to coordinate neighboring metal ions and so reduce their water exchange rate and the relaxivity of the CA.1820 Free carboxylates have also been claimed to impact relaxivity negatively by altering the overall hydrophilicity and charge.21,22 With the aim of addressing these shortcomings, Brechbiel and coworkers have recently reported on the direct incorporation of Gd chelates (isothiocyanate-activated 1B4M-DTPA-Gd and CDOTA-Gd) onto PAMAM dendrimers, as opposed to uncomplexed ligands (Scheme 1).23,24 Although the resulting prelabeled CA revealed increased relaxivities compared with postlabeled Received: March 31, 2011 Revised: July 4, 2011 Published: July 05, 2011 2902
dx.doi.org/10.1021/bm2004466 | Biomacromolecules 2011, 12, 2902–2907
Biomacromolecules
ARTICLE
Scheme 1. Pre- and Post-Labeling Approaches to Dendritic CA
analogs,16 the low coupling efficiencies (
