Nanodiamond-Gadolinium(III) Aggregates for Tracking Cancer Growth In Vivo at High Field.

The ability to track labeled cancer cells in vivo would allow researchers to study their distribution, growth, and metastatic potential within the intact organism. Magnetic resonance (MR) imaging is invaluable for tracking cancer cells in vivo as it benefits from high spatial resolution and the absence of ionizing radiation. However, many MR contrast agents (CAs) required to label cells either do not significantly accumulate in cells or are not biologically compatible for translational studies. We have developed carbon-based nanodiamond-gadolinium(III) aggregates (NDG) for MR imaging that demonstrated remarkable properties for cell tracking in vivo. First, NDG had high relaxivity independent of field strength, a finding unprecedented for gadolinium(III) [Gd(III)]-nanoparticle conjugates. Second, NDG demonstrated a 300-fold increase in the cellular delivery of Gd(III) compared to that of clinical Gd(III) chelates without sacrificing biocompatibility. Further, we were able to monitor the tumor growth of NDG-labeled flank tumors by T1- and T2-weighted MR imaging for 26 days in vivo, longer than was reported for other MR CAs or nuclear agents. Finally, by utilizing quantitative maps of relaxation times, we were able to describe tumor morphology and heterogeneity (corroborated by histological analysis), which would not be possible with competing molecular imaging modalities.

Cell labeling via membrane-anchored lipophilic MR contrast agents.

Cell tracking in vivo with MR imaging requires the development of contrast agents with increased sensitivity that effectively label and are retained by cells. Most clinically approved Gd(III)-based contrast agents require high incubation concentrations and prolonged incubation times for cellular internalization. Strategies to increase contrast agent permeability have included conjugating Gd(III) complexes to cell penetrating peptides, nanoparticles, and small molecules which have greatly improved cell labeling but have not resulted in improved cellular retention. To overcome these challenges, we have synthesized a series of lipophilic Gd(III)-based MR contrast agents that label cell membranes in vitro. Two of the agents were synthesized with a multiplexing strategy to contain three Gd(III) chelates (1 and 2) while the third contains a single Gd(III) chelate (3). These new agents exhibit significantly enhanced labeling and retention in HeLa and MDA-MB-231-mcherry cells compared to agents that are internalized by cells (4 and Prohance).

Mechanisms of Gadographene-Mediated Proton Spin Relaxation.

Gd(III) associated with carbon nanomaterials relaxes water proton spins at an effectiveness that approaches or exceeds the theoretical limit for a single bound water molecule. These Gd(III)-labeled materials represent a potential breakthrough in sensitivity for Gd(III)-based contrast agents used for magnetic resonance imaging (MRI). However, their mechanism of action remains unclear. A gadographene library encompassing GdCl3, two different Gd(III)-complexes, graphene oxide (GO), and graphene suspended by two different surfactants and subjected to varying degrees of sonication was prepared and characterized for their relaxometric properties. Gadographene was found to perform comparably to other Gd(III)-carbon nanomaterials; its longitudinal (r1) and transverse (r2) relaxivity is modulated between 12-85 mM-1s-1 and 24-115 mM-1s-1, respectively, depending on the Gd(III)-carbon backbone combination. The unusually large relaxivity and its variance can be understood under the modified Florence model incorporating the Lipari-Szabo approach. Changes in hydration number (q), water residence time (τM), molecular tumbling rate (τR), and local motion (τfast) sufficiently explain most of the measured relaxivities. Furthermore, results implicated the coupling between graphene and Gd(III) as a minor contributor to proton spin relaxation.

pH-Responsive Theranostic Polymer-Caged Nanobins (PCNs): Enhanced Cytotoxicity and T1 MRI Contrast by Her2-Targeting.

A PCN theranostic platform comprises a doxorubicin (DXR)-loaded liposomal core and an acid-sensitive polymer shell that is functionalized with Herceptin and GdIII-based MRI contrast agents. In vitro testing reveals a 14-fold increase in DXR-based cytotoxicity versus a non-targeted analogue and an 120-fold improvement in cellular GdIII-uptake in comparison with clinically approved DOTA-GdIII, leading to significant T1 MRI contrast enhancement.

Reporter protein-targeted probes for magnetic resonance imaging.

Contrast agents for magnetic resonance imaging are frequently employed as experimental and clinical probes. Drawbacks include low signal sensitivity, fast clearance, and nonspecificity that limit efficacy in experimental imaging. In order to create a bioresponsive MR contrast agent, a series of four Gd(III) complexes targeted to the HaloTag reporter were designed and synthesized. HaloTag is unique among reporter proteins for its specificity, versatility, and the covalent interaction between substrate and protein. In similar systems, these properties produce prolonged in vivo lifetimes and extended imaging opportunities for contrast agents, longer rotational correlation times, and increases in relaxivity (r(1)) upon binding to the HaloTag protein. In this work we report a new MR contrast probe, 2CHTGd, which forms a covalent bond with its target protein and results in a dramatic increase in sensitivity. A 6-fold increase in r(1), from 3.8 to 22 mM(-1) s(-1), is observed upon 2CHTGd binding to the target protein. This probe was designed for use with the HaloTag protein system which allows for a variety of substrates (specific for MRI, florescence, or protein purification applications) to be used with the same reporter.

A modular system for the synthesis of multiplexed magnetic resonance probes.

We have developed a modular architecture for preparing high-relaxivity multiplexed probes utilizing click chemistry. Our system incorporates azide bearing Gd(III) chelates and a trialkyne scaffold with a functional group for subsequent modification. In optimizing the relaxivity of this new complex, we undertook a study of the linker length between a chelate and the scaffold to determine its effect on relaxivity. The results show a strong dependence on flexibility between the individual chelates and the scaffold with decreasing linker length leading to significant increases in relaxivity. Nuclear magnetic resonance dispersion (NMRD) spectra were obtained to confirm a 10-fold increase in the rotational correlation time from 0.049 to 0.60 ns at 310 K. We have additionally obtained a crystal structure demonstrating that modification with an azide does not impact the coordination of the lanthanide. The resulting multinuclear center has a 500% increase in per Gd (or ionic) relaxivity at 1.41 T versus small molecule contrast agents and a 170% increase in relaxivity at 9.4 T.

Novel aerobic oxidation of primary sulfones to carboxylic acids.

Primary alkyl aryl sulfones are converted to the corresponding carboxylic acids in fair to excellent yield through double deprotonation and exposure to atmospheric oxygen. The methodology allows for the convenient synthesis of (13)C labeled carboxylic acids.

Gd(III)-nanodiamond conjugates for MRI contrast enhancement.

A Gd(III)-nanodiamond conjugate [Gd(III)-ND] was prepared and characterized, enabling detection of nanodiamonds by MR imaging. The Gd(III)-ND particles significantly reduced the T(1) of water protons with a per-Gd(III) relaxivity of 58.82 +/- 1.18 mM(-1) s(-1) at 1.5 T (60 MHz). This represents a 10-fold increase compared to the monomer Gd(III) complex (r(1) = 5.42 +/- 0.20 mM(-1) s(-1)) and is among the highest per-Gd(III) relaxivities reported.