Thermal- and pH-Dependent Size Variable Radical Nanoparticles and Its Water Proton Relaxivity for Metal-Free MRI Functional Contrast Agents

For development of the metal-free MRI contrast agents, we prepared the supra-molecular organic radical, <b>TEMPO-UBD</b>, carrying TEMPO radical, as well as the urea, alkyl group, and phenyl ring, which demonstrate self-assembly behaviors using noncovalent bonds in an aqueous solution. In addition, <b>TEMPO-UBD</b> has the tertiary amine and the oligoethylene glycol chains (OEGs) for the function of pH and thermal responsiveness. By dynamic light scattering and transmission electron microscopy imaging, the resulting self-assembly was seen to form the spherical nanoparticles 10–150 nm in size. On heating, interestingly, the nanoparticles showed a lower critical solution temperature (LCST) behavior having two-step variation. This double-LCST behavior is the first such example among the supra-molecules. To evaluate of the ability as MRI contrast agents, the values of proton (¹H) longitudinal relaxivity (<i>r</i>₁) were determined using MRI apparatus. In conditions below and above CAC at pH 7.0, the distinguishable <i>r</i>₁ values were estimated to be 0.17 and 0.21 mM^–1 s¹, indicating the suppression of fast tumbling motion of TEMPO moiety in a nanoparticle. Furthermore, <i>r</i>₁ values became larger in the order of pH 7.0 > 9.0 > 5.0. Those thermal and pH dependencies indicated the possibility of metal-fee MRI functional contrast agents in the future

Self-Assembly Behavior of Emissive Urea Benzene Derivatives Enables Heat-Induced Accumulation in Tumor Tissue

In this study we describe the construction of a system composed of thermally responsive molecules that can be induced to accumulate in tumor tissues by heating. EgX molecules consisting of an urea–benzene framework and oligoethylene glycol (OEG) functional groups with an emissive aminoquinoline formed nanoparticles (NPs) ∼10 nm in size at 23 °C with a fluorescence quantum yield of 7–10%. At higher temperatures, additional self-assembly occurred as a result of OEG dehydration, and the NPs grew to over 1000 nm in size; this was accompanied by low critical solution temperature behavior. EgXs accumulated in tumor tissues of mice at a body temperature of around 33–35 °C, an effect that was accelerated by external heating around the tumor to approximately 40 °C as a result of increased particle size and enhanced retention in tissue. These EgX NPs can serve as a tool for in vivo monitoring of tumor progression and response to treatment

Giant Vesicles Containing Superparamagnetic Iron Oxide as Biodegradable Cell-Tracking MRI Probes

A major breakthrough in <i>in vivo</i> cellular imaging has been the clinical/preclinical use of magnetic resonance imaging (MRI) with contrast agent. Superparamagnetic iron oxide (SPIO) is a promising candidate for the development of smart MRI probes for cell-tracking. In the present study, we describe biodegradable probes made of giant vesicles (GVs; closed lipid membranes with diameters >1 μm) that encapsulate SPIO for use as an MRI contrast agent. These SPIO-containing GVs (SPIO-GVs) exhibited excellent contrast enhancement in the single cell of medaka fish (<i>Oryzias latipes</i>) embryos immediately after their microinjection, and this enhancement disappeared when the GV membranes were destroyed. Our results demonstrate that SPIO-GVs are useful MRI probes for single cell-tracking that have minimum cytotoxicity and will greatly improve clinical/preclinical <i>in vivo</i> cellular imaging techniques

Magnetic Resonance Imaging of Mitochondrial Dysfunction and Metabolic Activity, Accompanied by Overproduction of Superoxide

This study shows that a mitochondria-penetrating nitroxide probe (mito-TEMPO) allows detection of superoxide and visualization of mitochondrial dysfunction in living cells due to the effect of <i>T</i>₁ shortening in MRI. Mitochondrial dysfunction was induced by treatment of cells with rotenone and 2-methoxyestradiol (2-ME/Rot). The MRI measurements were performed on 7T MRI. The 2-ME/Rot-treated cells were characterized by overproduction of superoxide, which was confirmed by a conventional dihydroethidium test. In the presence of mito-TEMPO, the intensity of MRI signal in 2-ME/Rot-treated cells was ∼30–40% higher, in comparison with that in untreated cells or culture media. In model (cell-free) systems, we observed that superoxide, but not hydrogen peroxide, increased the intensity of <i>T</i>₁-weighted MRI signal of mito-TEMPO. Moreover, the superoxide restores the <i>T</i>₁-weighted MRI contrast of mito-TEMPOH, a noncontrast (diamagnetic) analogue of mito-TEMPO. This was also confirmed by using EPR spectroscopy. The results demonstrate that superoxide radical is involved in the enhancement of <i>T</i>₁-weighted MRI contrast in living cells, in the absence and presence of mito-TEMPO. This report gives a direction for discovering new opportunities for functional MRI, for detection of metabolic activity, accompanied by overproduction of superoxide, as well as by disturbance of the balance between superoxide and hydrogen peroxide, a very important approach to clarify the fine molecular mechanisms in the regulation of many pathologies. The visualization of mitochondrial activity in real-time can be crucial to clarify the molecular mechanism of the functional MRI in its commonly accepted definition, as a method for detection of neurovascular coupling

Water-Proton Relaxivities of Radical Nanoparticles Self-Assembled via Hydration or Dehydration Processes

Nanoparticles capable of accumulating in tumor tissues are promising materials for tumor imaging and therapy. In this study, two radical nanoparticles (RNPs), denoted as <b>1</b> and <b>2</b>, composed of self-assembled ureabenzene derivatives possessing one or two amphiphilic side chains were demonstrated to be candidates for metal-free functional magnetic resonance imaging (MRI) contrast agents (CAs). Because of the self-assembly behavior of <b>1</b> and <b>2</b> in a saline solution, spherical RNPs of sizes ∼50–90 and ∼30–100 nm were detected. In a highly concentrated solution, <b>RNP 1</b> showed considerably small water-proton relaxivity values (<i>r</i>₁ and <i>r</i>₂), whereas <b>RNP 2</b> showed an <i>r</i>₁ value that was around 5 times larger than that of <b>RNP 1</b>. These distinct <i>r</i>₁ values might be caused by differences in the self-assembly behavior by a hydration or dehydration process. In vivo studies with <b>RNP 2</b> demonstrated a slightly enhanced <i>T</i>₁-weighted image in mice, suggesting that the RNPs can potentially be used as metal-free functional MRI CAs for <i>T</i>₁-weighted imaging

Comparison of the enhanced volume in T₁-weighted MRI due to intact and dead Mn-labeled MNCs.

<p>The shaded and white bars indicate the enhanced volume (normalized by the volume measured at day 0) of the intact and dead Mn-labeled MNCs, respectively. The enhanced volume of the intact MNCs shrank slower than that for the dead MNCs (bar-graph). The ratio of intact-to-dead MNCs is also superimposed as a line-graph (black circles, right vertical axis). An intact/dead ratio of “1” means that the enhanced volumes on the T₁-weighted MRI are the same for the Mn-labeled intact and dead MNCs. The intact/dead ratio tended to increase with time. * Significant difference (P<0.05) between the enhanced volumes of the intact and dead MNCs, two-way ANOVA with Bonferroni post-hoc test (n = 6). # Significant difference (P<0.05) vs. intact MNCs (day 1) § Significant difference (P<0.05) vs. dead MNCs (day 0) † Significant difference (P<0.05) vs. dead MNCs (day 1)</p

Chronic evaluation of angiogenesis using MRI angiography and Laser Doppler perfusion imaging.

<p>Representative MRI angiographs (A and B) and laser Doppler perfusion images (C) are presented. The MRI angiographic images were obtained at 0 (A) and 21 (B) days after muscle ischemia and MNC transplantation. Recovery of arterial blood flow was observed in the hindpaw for both the intact and dead MNC administered legs at 21 days. The intact MNC transplanted side showed a similar number of arteries to the contralateral side containing dead MNCs. The laser-Doppler perfusion image was acquired 43 days after the ischemia and transplantation (C). The muscle perfusion measured with laser-Doppler in the left legs (injected with intact MNCs) was 20±9% (n = 6) higher than that in the right legs (dead MNCs).</p

Relaxation times and contrast of Mn-labeled MNCs.

<p>(A) R₁ and R₂ calculated for the <i>in vitro</i> samples are presented. The pelleted 0.1 mM Mn-labeled MNCs showed larger R₁ and R₂ in comparison to the saline control. It was not possible to calculate the R₁ and R₂ values for the MNCs with 0.25 mM Mn-labeling and over (N/A) because the T₂ was too short due to the large susceptibility effect. The R₁s of the MnCl₂ solutions were approximately linearly correlated with the Mn concentration. The R₁ and R₂ for the supernatant fluid were approximately independent of the initial Mn labeling concentration. (B) T₁-weighted sagittal images of pelleted MNCs after suspension in 0, 0.1, 0.25, and 0.5 mM Mn solutions. The pelleted MNCs at 0.1 mM MnCl₂-labeling showed partial signal enhancement in the T₁-weighted 7 T MRI (arrow). The pelleted MNCs with MnCl₂-labeling over 0.25 mM lost signal in comparison with the unlabeled (0 mM) control due to the very short T₂ and the T₂* susceptibility effect at 7 T. In the 0.2 T MRI, the the 0.1, 0.25 and 0.5 mM MnCl₂-labeled MNCs showed positive contrast, although there may have been some signal loss at the base of the 0.5 mM sample.</p

Immunohistochemistry for CD31.

<p>Both intact and dead Mn-MNC implanted sites were stained with CD31 at 2 and 43 days. Necrotic myofibers and neutrophilic infiltration induced by hindlimb ischemia were observed in both the intact and dead Mn-MNCs implanted sites at day 2. The bar graph indicates the fraction of the total area covered by CD31-positive cells (brown) of the intact (shaded) and dead (white) Mn-labeled transplanted MNCs. The areas coevered by CD31-positive cells in the intact Mn-MNCs implanted site at 43 days was larger than that at 2 days or in the dead Mn-MNCs implanted site at 43 days.</p

Evaluation of MR angiography after transplantation of manganese-labeled mononuclear cells.

<p>Values are mean ± SE.</p><p>Significant difference vs. Day 1 group, level of significance P<0.05</p><p>(*) for paired T test.</p