Amphiphilic Ditopic Bis-Aqua Gd-AAZTA-like Complexes Enhance Relaxivity of Lipidic MRI Nanoprobes

Engineered Nonviral Protein Cages Modified for MR Imaging

Diagnostic medical imaging utilizes magnetic resonance (MR) to provide anatomical, functional, and molecular information in a single scan. Nanoparticles are often labeled with Gd(III) complexes to amplify the MR signal of contrast agents (CAs) with large payloads and high proton relaxation efficiencies (relaxivity, r1). This study examined the MR performance of two structurally unique cages, AaLS-13 and OP, labeled with Gd(III). The cages have characteristics relevant for the development of theranostic platforms, including (i) well-defined structure, symmetry, and size; (ii) the amenability to extensive engineering; (iii) the adjustable loading of therapeutically relevant cargo molecules; (iv) high physical stability; and (v) facile manufacturing by microbial fermentation. The resulting conjugates showed significantly enhanced proton relaxivity (r1 = 11-18 mM-1 s-1 at 1.4 T) compared to the Gd(III) complex alone (r1 = 4 mM-1 s-1). Serum phantom images revealed 107% and 57% contrast enhancements for Gd(III)-labeled AaLS-13 and OP cages, respectively. Moreover, proton nuclear magnetic relaxation dispersion (1H NMRD) profiles showed maximum relaxivity values of 50 mM-1 s-1. Best-fit analyses of the 1H NMRD profiles attributed the high relaxivity of the Gd(III)-labeled cages to the slow molecular tumbling of the conjugates and restricted local motion of the conjugated Gd(III) complex.

Derivatives of GdAAZTA Conjugated to Amino Acids: A Multinuclear and Multifrequency NMR Study

The GdAAZTA (AAZTA = 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid) complex represents a platform of great interest for the design of innovative MRI probes due to its remarkable magnetic properties, thermodynamic stability, kinetic inertness, and high chemical versatility. Here, we detail the synthesis and characterization of new derivatives functionalized with four amino acids with different molecular weights and charges: l-serine, l-cysteine, l-lysine, and l-glutamic acid. The main reason for conjugating these moieties to the ligand AAZTA is the in-depth study of the chemical properties in aqueous solution of model compounds that mimic complex structures based on polypeptide fragments used in molecular imaging applications. The analysis of the ¹H NMR spectra of the corresponding Eu(III)-complexes indicates the presence of a single isomeric species in solution, and measurements of the luminescence lifetimes show that functionalization with amino acid residues maintains the hydration state of the parent complex unaltered (q = 2). The relaxometric properties of the Gd(III) chelates were analyzed by multinuclear and multifrequency NMR techniques to evaluate the molecular parameters that determine their performance as MRI probes. The relaxivity values of all of the novel chelates are higher than that of GdAAZTA over the entire range of applied magnetic fields because of the slower rotational dynamics. Data obtained in reconstituted human serum indicate the occurrence of weak interactions with the proteins, which result in larger relaxivity values at the typical imaging fields. Finally, all of the new complexes are characterized by excellent chemical stability in biological matrices over time, by the absence of transmetallation processes, or the formation of ternary complexes with oxyanions of biological relevance. In particular, the kinetic stability of the new complexes, measured by monitoring the release of Gd³⁺ in the presence of a large excess of Zn²⁺, is ca. two orders of magnitude higher than that of the clinical MRI contrast agent GdDTPA.

Novel GdAAZTA derivatives conjugated to four amino acids were synthesized and characterized through a multi-technique approach. The complexes maintained the favorable thermodynamic and kinetic properties of the parent compound and showed higher relaxivity values in clinical fields. Therefore, they represent a useful model of more complex bio-conjugated structures used in molecular imaging applications.

The critical role of ligand topology: strikingly different properties of Gd(iii) complexes with regioisomeric AAZTA derivatives

Two regioisomeric Gd(III) complexes with heptadentate AAZTA-like ligands show different hydration state (q = 1 and 2) and astonishingly different thermodynamic stability and dissociation kinetics.

1H NMR Relaxometric Study of Chitosan-Based Nanogels Containing Mono- and Bis-Hydrated Gd(III) Chelates: Clues for MRI Probes of Improved Sensitivity

Hydrogel nanoparticles composed of chitosan and hyaluronate and incorporating Gd-based MRI contrast agents with different hydration number (e.g., [Gd(DOTA)(H₂O)]^- and [Gd(AAZTA)(H₂O)₂]^-) were prepared and fully characterized. In particular, ¹H NMR relaxometric data, acquired as a function of temperature and applied magnetic field strength, were for the first time thoroughly analyzed using a theoretical model that includes the effects of a static zero-field splitting and an anisotropic molecular tumbling. The paramagnetic nanoparticles show excellent stability in aqueous solution for over 150 h and do not release the load of Gd(III) chelates. These nanoparticles exhibit enhanced efficacy (relaxivity) as relaxation agents, over 6 times that of the free complexes, thanks to the combination of a restricted molecular dynamics in the presence of a fast exchange of metal-bound water molecule(s) and between the water inside the nanogel and the bulk water. The knowledge of the molecular parameters that control the effectiveness of these MRI nanoprobes and those that limit their further increase will be crucial for the development of optimized systems with high sensitivity and stability.

Supramolecular assemblies based on amphiphilic Mn2+-complexes as high relaxivity MRI probes

The Mn²⁺ complexes of amphiphilic derivatives of EDTA and 1,4-DO2A ligands show a strong increase in relaxivity upon micellar aggregation and human serum albumin binding.

Tuning magnetic relaxation properties of "hard cores" in core-shell colloids by modification of "soft shell"

The present work introduces an impact of polyelectrolyte-based hydrophilic shell on magnetic relaxivity and luminescence of hard cores built from isostructural complexes of Tb(III) and Gd(III) in the core-shell aqueous colloids. Microscopic and scattering techniques reveal "plum pudding" morphology of the colloids, where polyelectrolyte-coated ultrasmall (<5nm) hard cores form aggregates in aqueous solutions. Interaction of bovine serum albumin (BSA) with the colloids provides a tool to modify the polyelectrolyte-based shell, which is the reason for the improvement in both aggregation behavior of the colloids and their relaxivity. The modification of the hydrophilic polyelectrolyte-based shell enables to tune the longitudinal relaxivity from 5.9 to 23.3mM^-1s^-1 at 0.47T. This tendency is the reason for significant improvement of contrasting effect of the colloids in T₁- and T₂-weighted images obtained by whole body scanner at 1.5T. High contrasting effect of the colloids, together with low cytotoxicity towards Wi-38 diploid human cells makes them promising MRI contrast agents.

Mannose-coated gadolinium liposomes for improved magnetic resonance imaging in acute pancreatitis

Background

Acute pancreatitis (AP) is an acute inflammatory condition of the pancreas. The symptoms, treatment, and prognosis of mild and severe AP are different, and severe AP is a potentially life-threatening disease with a high incidence of complications and high mortality rate. Thus, it is urgent to develop an effective approach to reliably discriminate between mild and severe AP.

Methods

We have developed novel gadolinium-diethylenetriaminepentaacetic (Gd-DTPA)-loaded mannosylated liposomes (named thereafter M-Gd-NL) that preferably target macrophages in AP. The targeting ability of M-Gd-NL toward macrophages in AP and its ability to discriminate between mild and severe AP were evaluated.

Results

The liposomes were of desired particle size (~100 nm), Gd-DTPA encapsulation efficiency (~85%), and stability. M-Gd-NL and non-targeted Gd-DTPA-loaded liposomes (Gd-NL) exhibited increased relaxivity compared with Gd-DTPA. Compared with Gd-NL and Gd-DTPA, M-Gd-NL showed increased uptake in macrophages, resulting in increased T₁ imaging ability both in vitro (macrophage cell line) and in vivo (severe AP model). Importantly, M-Gd-NL had the ability to discriminate between mild and severe AP, as reflected by a significantly higher T₁ magnetic resonance imaging signal in severe AP than in mild AP. M-Gd-NL did not show severe organ toxicity in rats.

Conclusion

Our data suggest that M-Gd-NL had enhanced magnetic resonance imaging ability by targeting macrophages in AP and good ability to discriminate between mild and severe AP. We believe that M-Gd-NL could shed new light on the diagnosis of AP in the near future.