Cell Labeling via Membrane-Anchored Lipophilic MR Contrast Agents

Cell tracking <i>in vivo</i> with MR imaging requires the development of contrast agents with increased sensitivity that effectively label and are <i>retained</i> 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 <i>in vitro</i>. Two of the agents were synthesized with a multiplexing strategy to contain three Gd­(III) chelates (<b>1</b> and <b>2</b>) while the third contains a single Gd­(III) chelate (<b>3</b>). 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 (<b>4</b> and Prohance)

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 <i>T</i>₁- and <i>T</i>₂-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

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 (<i>r</i>₁) and transverse (<i>r</i>₂) relaxivity are modulated between 12–85 mM^–1 s^–1 and 24–115 mM^–1 s^–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 (<i>q</i>), water residence time (τ_M), molecular tumbling rate (τ_R), and local motion (τ_fast) 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