Gd3+:DOTA-Modified 2-Hydroxypropyl-β-Cyclodextrin/4-Sulfobutyl Ether-β-Cyclodextrin-Based Polyrotaxanes as Long Circulating High Relaxivity MRI Contrast Agents

Multifunctional polysaccharide nanoprobes for biological imaging

Imaging and tracking biological targets or processes play an important role in revealing molecular mechanisms and disease states. Bioimaging via optical, nuclear, or magnetic resonance techniques enables high resolution, high sensitivity, and high depth imaging from the whole animal down to single cells via advanced functional nanoprobes. To overcome the limitations of single-modality imaging, multimodality nanoprobes have been engineered with a variety of imaging modalities and functionalities. Polysaccharides are sugar-containing bioactive polymers with superior biocompatibility, biodegradability, and solubility. The combination of polysaccharides with single or multiple contrast agents facilitates the development of novel nanoprobes with enhanced functions for biological imaging. Nanoprobes constructed with clinically applicable polysaccharides and contrast agents hold great potential for clinical translations. This review briefly introduces the basics of different imaging modalities and polysaccharides, then summarizes the recent progress of polysaccharide-based nanoprobes for biological imaging in various diseases, emphasizing bioimaging with optical, nuclear, and magnetic resonance techniques. The current issues and future directions regarding the development and applications of polysaccharide nanoprobes are further discussed.

Structure-function relationships of cholesterol mobilization from the endo-lysosome compartment of NPC1-deficient human cells by β-CD polyrotaxanes

Niemann-Pick Type C is a rare metabolic disorder characterized by the cellular accumulation of cholesterol within endosomal and lysosomal compartments. 2-Hydroxypropyl-β-cyclodextrin (HP-β-CD) containing polyrotaxanes represent an attractive approach for treating this disease due to their ability to circulate in the blood stream for longer periods of time as a prodrug form of HP-β-CD. Once inside the cell, the macromolecular structure is thought to break down into the Pluronic precursor and the active cyclodextrin agent that promotes cholesterol mobilization from the aberrant accumulations within NPC-deficient cells. We now report that both cholesterol and decaarginine (R10) endcapped polyrotaxanes are able to remove cholesterol from NPC1 patient fibroblasts. R10 endcapped materials enter these cells and are localized within endosomes after 16 h. The cholesterol mobilization from endo-lysosomal compartments of NPC1 cells by the polyrotaxanes was directly related to their extent of endcapping and their threading efficiency. Incorporation of 4-sulfobutylether-β-cyclodextrin (SBE-β-CD) significantly improved cholesterol mobilization due to the improved solubility of the compounds. Additionally, in our efforts to scale-up the synthesis for preclinical studies, we prepared a library of polyrotaxanes using a solid phase synthesis method. These compounds also led to significant cholesterol mobilization from the cells, however, cytotoxicity studies showed that they were substantially more toxic than those prepared by the solvent-assisted method, thus limiting the therapeutic utility of agents prepared by this expedited method. Our findings demonstrate that complete endcapping of the polyrotaxanes and improved solubility are important design features for delivering high copy numbers of therapeutic β-CD to promote enhanced sterol clearance in human NPC1-deficient cells.

Cyclodextrins: promising scaffolds for MRI contrast agents

Magnetic resonance imaging (MRI) is a powerful tool for non-invasive, high-resolution three-dimensional medical imaging of anatomical structures such as organs and tissues. The use of contrast agents based on gadolinium chelates started in 1988 to improve the quality of the image, since researchers and industry focused their attention on the development of more efficient and stable structures. This review is about the state of the art of MRI contrast agents based on cyclodextrin scaffolds. Chemical engineering strategies are herein reported including host-guest inclusion complexation and covalent linkages. It also offers descriptions of the MRI properties and in vitro and in vivo biomedical applications of these emerging macrostructures. It highlights that these supramolecular associations can improve the image contrast, the sensitivity, and the efficiency of MRI diagnosis by targeting cancer tumors and other diseases with success proving the great potential of this natural macrocycle.

Quantum Dots and Gd3+ Chelates: Advances and Challenges Towards Bimodal Nanoprobes for Magnetic Resonance and Optical Imaging

The development of multimodal nanoprobes has been growing in recent years. Among these novel nanostructures are bimodal systems based on quantum dots (QDs) and low molecular weight Gd³⁺ chelates, prepared for magnetic resonance imaging (MRI) and optical analyses. MRI is a technique used worldwide that provides anatomic resolution and allows distinguishing of physiological differences at tissue and organ level. On the other hand, optical techniques are very sensitive and allow events to be followed at the cellular or molecular level. Thus, the association of these two techniques has the potential to achieve a more complete comprehension of biological processes. In this review, we present state-of-the-art research concerning the development of potential multimodal optical/paramagnetic nanoprobes based on Gd³⁺ chelates and QDs, highlighting their preparation strategies and overall properties.

Gd3+ Complexes Conjugated to Cyclodextrins: Hydroxyl Functions Influence the Relaxation Properties

In the search for improvement in the properties of gadolinium-based contrast agents, cyclodextrins (CDs) are interesting hydrophilic scaffolds with high molecular weight. The impact of the hydrophilicity of these systems on the MRI efficacy has been studied using five β-CDs substituted with DOTA or TTHA ligands which, respectively, allow for one (q = 1) or no water molecule (q = 0) in the inner coordination sphere of the Gd3+ ion. Original synthetic pathways were developed to immobilize the ligands at C-6 position of various hydroxylated and permethylated β-CDs via an amide bond. To describe the influence of alcohol and ether oxide functions of the CD macrocycle on the relaxation properties of the Gd3+ complexes, 1H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles, and 17O transverse relaxation rates have been measured at various temperatures. The differences observed between the hydroxylated and permethylated β-CDs bearing non-hydrated GdTTHA complexes can be rationalized by a second sphere contribution to the relaxivity in the case of the hydroxylated derivatives, induced by hydrogen-bound water molecules around the hydroxyl groups. In contrast, for the DOTA analogs the exchange rate of the water molecule directly coordinated to the Gd3+ is clearly influenced by the number of hydroxyl groups present on the CD, which in turn influences the relaxivity and gives rise to a very complex behavior of these hydrophilic systems.

Lanthanide-Containing Rotaxanes, Catenanes, and Knots

The valuable luminescence, magnetic, and catalytic properties of lanthanide cations are beginning to be exploited in conjunction with structurally exotic mechanically interlocked molecules (MIMs) such as rotaxanes, catenanes and knots. This Minireview provides an account of this rapidly developing research area commencing with the use of lanthanides in extended MIM-containing frameworks. Then, attention turns to discrete lanthanide-containing pseudorotaxanes, followed by fully interlocked rotaxanes, catenanes and knots - where lanthanides have not only been incorporated into MIM architectures but have also been used to template formation of the interlocked structure. Particular focus is paid to examples where the lanthanide MIMs have been put to useful applications, in what is still a relatively youthful avenue of research in both lanthanide coordination chemistry and the chemistry of mechanically interlocked molecules.

Specific Modification with TPGS and Drug Loading of Cyclodextrin Polyrotaxanes and the Enhanced Antitumor Activity Study in Vitro and in Vivo

A kind of specific cyclodextrin polyrotaxanes (PRs) drug delivery system was developed for an effective drug delivery and enhancing antitumor effect. In this work, we prepared the PR by using α-CD derivatives and dicarboxyl-PEG (M_n = 4200) self-assembling and end-capping with β-CD derivatives. Then, we chose d-a-Tocopheryl polyethylene glycol 1000 succinate (TPGS) with an antitumor effect to modify the PR. The modified PRs have a certain anticancer effect and can assist the anticancer drug to treat cancer. The 10-hydroxycamptothecin (HCPT) was combined to the specific PRs by covalent bonds to prepare drug-loaded specificity PRs (PR-TPGS-HCPT). The enhanced antitumor activities of PR-TPGS-HCPT were studied by in vitro and in vivo experiments, and the experiment results proved that the TPGS could effectively assist the drug to treat cancer and prolong the lifetime of the tumor-bearing mice. Therefore, this research provides a promising drug-loaded material for the cancer treatment and the specific water-soluble PRs will have potential applications in the biomedical field.

A Supramolecular Nanocarrier for Delivery of Amiodarone Anti-Arrhythmic Therapy to the Heart

Amiodarone is an effective antiarrhythmic drug used to treat and prevent different types of cardiac arrhythmias. However, amiodarone can have considerable side effects resulting from accumulation in off-target tissues. Cardiac macrophages are highly prevalent tissue-resident immune cells with importance in homeostatic functions, including immune response and modulation of cardiac conduction. We hypothesized that amiodarone could be more efficiently delivered to the heart via cardiac macrophages, an important step toward reducing overall dose and off-target tissue accumulation. Toward this goal, we synthesized a nanoparticle drug carrier composed of l-lysine cross-linked succinyl-β-cyclodextrin that demonstrates amiodarone binding through supramolecular host-guest interaction as well as a high macrophage affinity. Biodistribution analyses at the organ and single-cell level demonstrate accumulation of nanoparticles in the heart resulting from rapid uptake by cardiac macrophages. Nanoparticle assisted delivery of amiodarone resulted in a 250% enhancement in the selective delivery of the drug to cardiac tissue in part due to a concomitant decrease of pulmonary accumulation, the main source of off-target toxicity.