Magnetic resonance imaging studies on gadonanotube-reinforced biodegradable polymer nanocomposites

A Review on the Design of Carbon-Based Nanomaterials as MRI Contrast Agents

The administration of magnetic resonance imaging (MRI) contrast agents (CAs) has been conducted since 1988 by clinicians to enhance the clarity and interpretability of MR images. CAs based on gadolinium chelates are the clinical standard used worldwide for the diagnosis of various pathologies, such as the detection of brain lesions, the visualization of blood vessels, and the assessment of soft tissue disorders. However, due to ongoing concerns associated with the safety of gadolinium-based contrast agents, considerable efforts have been directed towards developing contrast agents with better relaxivities, reduced toxicity, and eventually combined therapeutic modalities. In this context, grafting (or encapsulating) paramagnetic metals or chelates onto (within) carbon-based nanoparticles is a straightforward approach enabling the production of contrast agents with high relaxivities while providing extensive tuneability regarding the functionalization of the nanoparticles. Here, we provide an overview of the parameters defining the efficacy of lanthanide-based contrast agents and the subsequent developments in the field of nanoparticular-based contrast agents incorporating paramagnetic species.

Carbon Nano-Allotrope/Magnetic Nanoparticle Hybrid Nanomaterials as T2 Contrast Agents for Magnetic Resonance Imaging Applications

Magnetic resonance imaging (MRI) is the most powerful tool for deep penetration and high-quality 3D imaging of tissues with anatomical details. However, the sensitivity of the MRI technique is not as good as that of the radioactive or optical imaging methods. Carbon-based nanomaterials have attracted significant attention in biomaterial research in recent decades due to their unique physical properties, versatile functionalization chemistry, as well as excellent biological compatibility. Researchers have employed various carbon nano-allotropes to develop hybrid MRI contrast agents for improved sensitivity. This review summarizes the new research progresses in carbon-based hybrid MRI contrast agents, especially those reported in the past five years. The review will only focus on T2-weighted MRI agents and will be categorized by the different carbon allotrope types and magnetic components. Considering the strong trend in recent bio-nanotechnology research towards multifunctional diagnosis and therapy, carbon-based MRI contrast agents integrated with other imaging modalities or therapeutic functions are also covered.

Amalgamation of complex iron(III) ions and iron nanoclusters with MWCNTs as a route to potential T2 MRI contrast agents

Iron-filled multiwall carbon nanotubes (Fe@MWCNTs) were functionalized toward a variety of potential magnetic resonance imaging contrast agents. Oxidized Fe@MWNCTs were covered with PEG5000 via direct esterification or using acyl chloride derivatives. Alternatively, the latter were functionalized with an aminophenol ligand (Fe@O-MWCNT-L). Moreover, pristine Fe@MWCNTs were functionalized with N-phenylaziridine groups (Fe@f-MWCNT) via [2+1] cycloaddition of nitrene. All of these chemically modified nanotubes served as a vehicle for anchoring Fe³⁺ ions. The new hybrids – Fe(III)/Fe@(f-/O-)MWCNTs – containing 6%–14% of the “tethered” Fe³⁺ions were studied in terms of the acceleration of relaxation of water protons in nuclear magnetic resonance. The highest transverse relaxivity r₂=63.9±0.9 mL mg⁻¹ s⁻¹ was recorded for Fe(III)/Fe@O-MWCNT-L, while for Fe(III)/Fe@f-MWCNT, with r₂=57.9±2.9 mL mg⁻¹ s⁻¹, the highest impact of the anchored Fe(III) ions was observed. The T₁/T₂ ratio of 30–100 found for all of the nanotube hybrids presented in this work is a very important factor for their potential application as T₂ contrast agents. Increased stability of the hybrids was confirmed by ultraviolet–visible spectrophotometry.

Multimodal ultrasound-photoacoustic imaging of tissue engineering scaffolds and blood oxygen saturation in and around the scaffolds

Preclinical, noninvasive imaging of tissue engineering polymeric scaffold structure and/or the physiological processes such as blood oxygenation remains a challenge. In vitro or ex vivo, the widely used scaffold characterization modalities such as porosimetry, electron or optical microscopy, and X-ray microcomputed tomography have limitations or disadvantages-some are invasive or destructive, others have limited tissue penetration (few hundred micrometers) and/or show poor contrast under physiological conditions. Postmortem histological analysis, the most robust technique for the evaluation of neovascularization is obviously not appropriate for acquiring physiological or longitudinal data. In this study, we have explored the potential of ultrasound (US)-coregistered photoacoustic (PA) imaging as a noninvasive multimodal imaging modality to overcome some of the above challenges and/or provide complementary information. US-PA imaging was employed to characterize poly(lactic-co-glycolic acid) (PLGA) polymer scaffolds or single-walled carbon nanotube (SWCNT)-incorporated PLGA (SWCNT-PLGA) polymer scaffolds as well as blood oxygen saturation within and around the scaffolds. Ex vivo, PLGA and SWCNT-PLGA scaffolds were placed at 0.5, 2, and 6 mm depths in chicken breast tissues. PLGA scaffolds could be localized with US imaging, but generate no PA signal (excitation wavelengths 680 and 780 nm). SWCNT-PLGA scaffolds generated strong PA signals at both wavelengths due to the presence of the SWCNTs and could be localized with both US and PA imaging depths between 0.5-6 mm (lateral resolution = 90 μm, axial resolution = 40 μm). In vivo, PLGA and SWCNT-PLGA scaffolds were implanted in subcutaneous pockets at 2 mm depth in rats, and imaged at 7 and 14 days postsurgery. The anatomical position of both the scaffolds could be determined from the US images. Only SWCNT-PLGA scaffolds could be easily detected in the US-PA images. SWCNT-PLGA scaffolds had significant four times higher PA signal intensity compared with the surrounding tissue and PLGA scaffolds. In vivo blood oxygen saturation maps around and within the PLGA scaffolds could be obtained by PA imaging. There was no significant difference in oxygen saturation for the PLGA scaffolds at the two time points. The blood oxygen saturation maps complemented the histological analysis of neovascularization of the PLGA scaffolds.

Cytotoxicity, cytocompatibility, cell-labeling efficiency, and in vitro cellular magnetic resonance imaging of gadolinium-catalyzed single-walled carbon nanotubes

Cell tracking by magnetic resonance imaging (MRI) is an emerging technique that typically requires the use of MRI contrast agents (CAs). A MRI CA for cellular imaging should label cells efficiently at potentially safe concentrations, have high relaxivity, and not affect the cellular machinery. In this article, we report the cytotoxicity, cytocompatibility, and cell labeling efficiency in NIH/3T3 fibroblasts of novel, single-walled carbon nanotubes synthesized using gadolinium nanoparticles as catalysts (Gd-SWCNTs). Cells incubated with the Gd-SWCNT showed a dose- (50-100 µg/mL nanotube concentration) and time- (12-48 h) dependent decrease in viability. 30% cell death was observed for cells incubated with Gd-SWCNTs at the maximum dose of 100 µg/mL for 48 h. Cells incubated with the Gd-SWCNTs at concentrations between 1-10 μg/mL for 48 h showed no change in viability or proliferation compared to untreated controls. Additionally, at these potentially safe concentrations, up to 48 h, the cells showed no phosphatidyl serine externalization (pre-apoptotic condition), caspase-3 activity (point of no return for apoptosis), genetic damage, or changes in their division cycle. Localization of Gd-SWCNTs within the cells was confirmed by transmission electron microscopy (TEM) and Raman microscopy, and these results show 100% cell labeling efficiency. Elemental analysis also indicates significant uptake of Gd-SWCNTs by the cells (10(8) -10(9) Gd(3+) ions per cell). Finally, T1 -weighted MRI at 3 T of Gd-SWCNT-labelled cells show up to a four-fold increase in MR signal intensities as compared to untreated cells. These results indicate that Gd-SWCNTs label cells efficiently at potentially safe concentrations, and enhance MRI contrast without any structural damage to the cells.

The magnetic, relaxometric, and optical properties of gadolinium-catalyzed single walled carbon nanotubes

We report the magnetic behavior, relaxometry, phantom magnetic resonance imaging (MRI), and near-infrared (NIR) photoluminescence spectroscopy of gadolinium (Gd) catalyzed single-walled carbon nanotubes (Gd-SWCNTs). Gd-SWCNTs are paramagnetic with an effective magnetic moment of 7.29 μ_B . Gd-SWCNT solutions show high r₁ and r₂ relaxivities at very low (0.01 MHz) to clinically relevant (61 MHz) magnetic fields (r₁ ≥ 130 mM^-1 s^-1, r₂ ≥ 160 mM^-1 s^-1). Analysis of nuclear magnetic resonance dispersion profiles using Solomon, Bloembergen, and Morgan equations suggests that multiple structural and dynamic parameters such as rotational correlation time [Formula: see text], rate of water exchange [Formula: see text], and the number of fast-exchanging water molecules within the inner sphere q may be responsible for the increase in r₁ and r₂ relaxivity. The T₁ weighted MRI signal intensity (gradient echo sequence; repetition time (TR) = 66 ms, echo time (TE) = 3 ms, flop angle = 108°) of Gd-SWCNT phantom solution is 14 times greater than the Gd-based clinical MRI contrast agent Magnevist. Additionally, these nanotubes exhibit near infrared fluorescence with distinct E₁₁ transitions of several semiconducting SWCNTs. Taken together, these results demonstrate that Gd-SWCNTs have potential as a novel, highly efficacious, multimodal MRI-NIR optical imaging contrast agent.

Toward single-walled carbon nanotube-gadolinium complex as advanced MRI contrast agents: pharmacodynamics and global genomic response in small animals

Gadolinium nanoparticle-catalyzed single-walled carbon nanotubes (Gd-SWCNTs) have recently shown potential in vitro as high-performance T1 magnetic resonance imaging (MRI) contrast agents (CAs). Their preclinical safety assessment at nontoxic dosages is essential for MRI applications. Herein, the in vivo (in rats) pharmacodynamics of Gd-SWCNTs (water solubilized with the amphiphilic polymer PEG-DSPE) at the organ, tissue, molecular, and genetic level is reported. Gd-SWCNT, commercially available iron catalyzed SWCNTs (Fe-SWCNTs, control 1) and PEG-DSPE (control 2) solutions were intravenously injected at a potential nontoxic therapeutic dose (0.5 mg/kg body weight, single bolus). Postinjection, bright-field optical microscopy showed their macroscale distribution in lung, liver, kidney, brain, and spleen up to 5 days. Raman and transmission electron microscopy (TEM) showed their presence at the nanoscale within hepatocytes. Their effects on the host organ tissue, molecular, and genetic level were analyzed after 1, 5, 10, 20, and 30 days by histology, biomolecular [lipid peroxidation, plasma tumor necrosis factor TNF-α assay, microarrays] assays. The results indicate that Gd-SWCNTs neither cause any inflammation, nor damage to the above organs, nor any significant change in the lipid peroxidation or plasma proinflammatory cytokine (TNF-α) levels for all the groups at all time points. Global gene expression profile of liver (main organ for the metabolism) after day 1 treatment with Gd-SWCNTs shows that the gene regulation is directed toward maintaining normal homeostasis. The results taken together indicate that PEG-DSPE water-solubilized Gd-SWCNTs at potentially nontoxic dosages have pharmacodynamics similar to other commercially available Fe-SWCNTs and are suitable for future preclinical development as in vivo MRI CAs.

Combination of small size and carboxyl functionalisation causes cytotoxicity of short carbon nanotubes

The use of carbon nanotubes (CNTs) could improve medical diagnosis and treatment provided they show no adverse effects in the organism. In this study, short CNTs with different diameters with and without carboxyl surface functionalisation were assessed. After physicochemical characterisation, cytotoxicity in phagocytic and non-phagocytic cells was determined. The role of oxidative stress was evaluated according to the intracellular glutathione levels and protection by N-acetyl cysteine (NAC). In addition to this, the mode of cell death was also investigated. CNTs <8 nm acted more cytotoxic than CNTs ≥20 nm and carboxylated CNTs more than pristine CNTs. Protection by NAC was maximal for large diameter pristine CNTs and minimal for small diameter carboxylated CNTs. Thin (<8 nm) CNTs acted mainly by disruption of membrane integrity and CNTs with larger diameter induced mainly apoptotic changes. It is concluded that cytotoxicity of small carboxylated CNTs occurs by necrosis and cannot be prevented by antioxidants.

Hybrid magnetic nanostructures (MNS) for magnetic resonance imaging applications

The development of MRI contrast agents has experienced its version of the gilded age over the past decade, thanks largely to the rapid advances in nanotechnology. In addition to progress in single mode contrast agents, which ushered in unprecedented R(1) or R(2) sensitivities, there has also been a boon in the development of agents covering more than one mode of detection. These include T(1)-PET, T(2)-PET T(1)-optical, T(2)-optical, T(1)-T(2) agents and many others. In this review, we describe four areas which we feel have experienced particular growth due to nanotechnology, specifically T(2) magnetic nanostructure development, T(1)/T(2)-optical dual mode agents, and most recently the T(1)-T(2) hybrid imaging systems. In each of these systems, we describe applications including in vitro, in vivo usage and assay development. In all, while the benefits and drawbacks of most MRI contrast agents depend on the application at hand, the recent development in multimodal nanohybrids may curtail the shortcomings of single mode agents in diagnostic and clinical settings by synergistically incorporating functionality. It is hoped that as nanotechnology advances over the next decade, it will produce agents with increased diagnostics and assay relevant capabilities in streamlined packages that can meaningfully improve patient care and prognostics. In this review article, we focus on T(2) materials, its surface functionalization and coupling with optical and/or T(1) agents.

RGD-targeted paramagnetic liposomes for early detection of tumor: in vitro and in vivo studies

Magnetic resonance molecular imaging has emerged as a potential approach for tumor diagnosis in the last few decades. This approach consists of the delivery of MR contrast agents to the tumor by specific targeted carriers. For this purpose, a lipopeptide was constructed by using a cyclic RGD peptide headgroup coupled to palmitic acid anchors via a KGG tripeptide spacer. Targeted paramagnetic liposomes were then prepared by the incorporation of RGD-coupled-lipopeptides into lipid bilayers for specific bounding to tumor. In vitro, study demonstrated that RGD-targeted liposomes exhibited a better binding affinity to targeted cells than non-targeted liposomes. MR imaging of mice bearing A549 tumors with the RGD-targeted paramagnetic liposomes also resulted in a greater signal enhancement of tumor compared to non-targeted liposomes and pure contrast agents groups. In addition, biodistribution study also showed specific tumor targeting of RGD-targeted paramagnetic liposomes in vivo. Therefore, RGD-targeted paramagnetic liposomes prepared in the present study may be a more promising method for early tumor diagnosis.