In vivo MRI assessment of a novel GdIII-based contrast agent designed for high magnetic field applications

Mitochondriotropic lanthanide nanorods: implications for multimodal imaging

Two-photon active mitochondriotropic lanthanide nanorods for high resolution fluorescence imaging. The presence of Gd in the nanorods also enabled us to utilize this material as a T₁-T₂ dual-mode contrast reagent for recording magnetic resonance images of the mouse brain.

Comparison of Solution Properties of Polymethylated DOTA-like Lanthanide Complexes with Opposite Chirality of the Pendant Arms

Polymethylated lanthanide 4S4R-M4DOTMA complexes, bearing the ring methyl groups oriented in the SSSS position and the arm methyl groups in the RRRR configuration, exist exclusively as the SAP [Λ(δδδδ)] isomer in solution throughout the lanthanide series. This observation is in contrast to Ln-8S-M4DOTMA, which was recently reported to adopt the SAP [Λ(δδδδ)] isomer in the early lanthanides, while the late lanthanides adopt the TSAP [Δ(δδδδ)] isomer. The methyl groups on the ring and the arm are both oriented in the SSSS configuration for Ln-8S-M4DOTMA ( Dalton Trans. 2016 , 45 , 4673 - 4687 , DOI: 10.1039/C5DT03210E ). Quantum chemical calculations for Pr- and Yb-4S4R-M4DOTMA indicate that the SAP isomer is significantly more stable. The luminescence profiles of Eu-8S-M4DOTMA and Eu-4S4R-M4DOTMA showed similar profiles signifying identical coordination environments. The hydration state, q, of the Eu(III) complexes was q = 0.91-0.95, while Tb-8S-M4DOTMA had q = 0.86. A much lower q value was obtained for Tb-4S4R-M4DOTMA (q = 0.67), which indicates an elongation of the Ln-Ow bond. At 400 MHz, the relaxivity of Gd-8S-M4DOTMA is 5.1 ± 0.1 mM^-1 s^-1 and 3.9 ± 0.1 mM^-1 s^-1 at 25 and 37 °C, respectively, whereas the relaxivity of Gd-4S4R-M4DOTMA is 4.6 ± 0.1 mM^-1 s^-1 at 25 °C and 3.6 ± 0.1 mM^-1 s^-1 at 37 °C. At 45 MHz, the relaxivity of Gd-8S-M4DOTMA is 5.4 ± 0.1 mM^-1 s^-1, and the relaxivity of Gd-4S4R-M4DOTMA is 4.5 ± 0.1 mM^-1 s^-1 at 25 °C. The temperature dependence of the ¹⁷O NMR transverse relaxation rate of the Gd complexes revealed a 7-fold increase in the bound water residence lifetime of Gd-8S-M4DOTMA (1/k_ex = τ_M = 9.0 ± 0.5 ns and 1/k_ex = τ_M = 60 ± 3 ns). The Pr(III) complex of 8S-M4DOTMA crystallized as TSAP isomer with an apical water. The presence of the apical water for the TSAP of Pr-8S-M4DOTMA was further confirmed with the observation that the fluoride ion replaces the bound water from the TSAP isomer of Pr-8S-M4DOTMA. This was shown by the disappearance of the TSAP peaks and appearance of a new set of less shifted resonances, which exchange with the SAP isomer as confirmed by NMR exchange spectroscopy (EXSY).

An advanced focused ultrasound protocol improves the blood-brain barrier permeability and doxorubicin delivery into the rat brain

Despite the recent development of a focused ultrasound (FUS) technique for disrupting the blood-brain barrier (BBB) and enabling the delivery of drugs into the targeted brain region, different sonication protocols have not been fully explored. In this study, we suggest a simple and cost-effective protocol that improves the BBB permeability and drug delivery without damaging the tissue. In this protocol, called "FUS+BBBD protocol", an additional FUS stimulation without microbubbles ("FUS protocol"; 0.5, 1.0, or 2.0MPa acoustic pressure, 10ms tone burst, 1Hz pulse repetition frequency, 120s total duration) is applied prior to the conventional BBB disruption with microbubbles ("BBBD protocol"; 0.6∼0.72MPa acoustic pressure, 10ms tone burst, 1Hz pulse repetition frequency, 120s total duration). With the "FUS+BBBD protocol", the magnetic resonance signal intensity and doxorubicin delivery at the targeted brain region were increased by 1.59 and 1.75 times at an FUS intensity of 1.0MPa, respectively, compared to the conventional BBBD. Other conditions also increase the drug delivery, but the increase was smaller than that at 1.0MPa (1.15 times for 0.5MPa and 1.60 times for 2.0MPa). The H&E histopathological analysis of the sonicated brain region using the proposed "FUS+BBBD protocol" showed no significant brain tissue damage at a FUS intensity of 0.5 and 1.0MPa. However, region cavities due to the damage were observed after an FUS intensity of 2.0MPa. These results suggest that the 1.0MPa "FUS+BBBD protocol" increases the BBB permeability and enhances the drug delivery efficiency without noticeable brain tissue damage, compared with the conventional BBBD. Although further studies are needed to determine the underlying mechanism of this effect, drugs that have been reported to be effective in the treatment of brain disease but had limited use due to severe systemic side effects will benefit from the enhanced drug delivery of "FUS+BBBD protocol". Furthermore, the suggested protocol may facilitate the development of new strategies in clinical trials to treat brain disorders with improved drug delivery and safety.

Gadolinium Complexes of Highly Rigid, Open-Chain Ligands Containing a Cyclobutane Ring in the Backbone: Decreasing Ligand Denticity Might Enhance Kinetic Inertness

In an effort to explore novel ligand scaffolds for stable and inert lanthanide complexation in magnetic resonance imaging contrast agent research, three chiral ligands containing a highly rigid (1S,2S)-1,2-cyclobutanediamine spacer and different number of acetate and picolinate groups were efficiently synthesized. Potentiometric studies show comparable thermodynamic stability for the Gd³⁺ complexes formed with either the octadentate (L3)^4- bearing two acetate or two picolinate groups or the heptadentate (L2)^4- analogue bearing one picolinate and three acetate groups (log K_GdL = 17.41 and 18.00 for [Gd(L2)]^- and [Gd(L3)]^-, respectively). In contrast, their dissociation kinetics is revealed to be very different: the monohydrated [Gd(L3)]^- is considerably more labile, as a result of the significant kinetic activity of the protonated picolinate function, as compared to the bishydrated [Gd(L2)]^-. This constitutes an uncommon example in which lowering ligand denticity results in a remarkable increase in kinetic inertness. Another interesting observation is that the rigid ligand backbone induces an unusually strong contribution of the spontaneous dissociation to the overall decomplexation process. Thanks to the presence of two inner-sphere water molecules, [Gd(L2)]^- is endowed with high relaxivity (r₁ = 7.9 mM^-1 s^-1 at 20 MHz, 25 °C), which is retained in the presence of large excess of endogenous anions, excluding ternary complex formation. The water exchange rate is similar for [Gd(L3)]^- and [Gd(L2)]^-, while it is 1 order of magnitude higher for the trishydrated tetraacetate analogue [Gd(L1)]^- (k_ex²⁹⁸ = 8.1, 10, and 127 × 10⁶ s^-1, respectively). A structural analysis via density functional theory calculations suggests that the large bite angle imposed by the rigid (1S,2S)-1,2-cyclobutanediamine spacer could allow the design of ligands based on this scaffold with suitable properties for the coordination of larger metal ions with biomedical applications.

Size-tunable MRI-visible nitroxide-based magnetic mixed micelles: preparation, stability, and theranostic application

Metal-free magnetic mixed micelles (mean diameter: 16 nm) composed of biocompatible surfactant Tween 80 and hydrophobic pyrrolidine-N-oxyl radical were prepared by mixing them in phosphate-buffered saline. The magnetic mixed micelles were characterized by dynamic light scattering and small angle neutron scattering measurements. The stability of the micelles is found to depend on the length of alkyl side chain in the nitroxide compounds and degree of unsaturation in the hydrophobic chain in the surfactant. The size of the mixed micelle can be tuned by changing the molar ratio of Tween 80 and nitroxyl radical. In view of theranostic application of the micelle, the cytotoxicity and stability in a physiological environment was investigated; the mixed micelle exhibited no cytotoxicity, high colloidal stability and high resistance towards reduction by large excess ascorbic acid. The in vitro and in vivo magnetic resonance imaging (MRI) revealed sufficient contrast enhancement in the proton longitudinal relaxation time (T ₁) weighted images. In addition, hydrophobic fluorophores and an anticancer drug are stably encapsulated in the mixed micelles and showed fluorescence (FL) upon reduction by ascorbic acid and cytotoxicity to cancer cells, respectively. For example, the paclitaxel-loaded mixed micelles efficiently suppressed cancer cell growth. Furthermore, they were found to give higher MRI contrast (higher r ₁ value) in vitro than the micelles without paclitaxel. The magnetic mixed micelles presented here are promising theranostic agents in nanomedicine due to their high biocompatibility and high resistivity towards reduction as well as functioning as a drug carrier in therapy and MR or FL imaging probe in diagnosis.

Magnetic Mixed Micelles Composed of a Non-Ionic Surfactant and Nitroxide Radicals Containing a D-Glucosamine Unit: Preparation, Stability, and Biomedical Application

Metal-free magnetic mixed micelles (mean diameter: < 20 nm) were prepared by mixing the biocompatible non-ionic surfactant Tween 80 and the non-toxic, hydrophobic pyrrolidine-N-oxyl radicals bearing a D-glucosamine unit in pH 7.4 phosphate-buffered saline (PBS). The time-course stability and in vitro magnetic resonance imaging (MRI) contrast ability of the mixed micelles was found to depend on the length of the alkyl chain in the nitroxide radicals. It was also confirmed that the mixed micelles exhibited no toxicity in vivo and in vitro and high stability in the presence of a large excess of ascorbic acid. The in vivo MRI experiment revealed that one of these mixed micelles showed much higher contrast enhancement in the proton longitudinal relaxation time (T₁) weighted images than other magnetic mixed micelles that we have reported previously. Thus, the magnetic mixed micelles presented here are expected to serve as a promising contrast agent for theranostic nanomedicines, such as MRI-visible targeted drug delivery carriers.

Cerebrospinal and interstitial fluid transport via the glymphatic pathway modeled by optimal mass transport

The glymphatic pathway is a system which facilitates continuous cerebrospinal fluid (CSF) and interstitial fluid (ISF) exchange and plays a key role in removing waste products from the rodent brain. Dysfunction of the glymphatic pathway may be implicated in the pathophysiology of Alzheimer's disease. Intriguingly, the glymphatic system is most active during deep wave sleep general anesthesia. By using paramagnetic tracers administered into CSF of rodents, we previously showed the utility of MRI in characterizing a macroscopic whole brain view of glymphatic transport but we have yet to define and visualize the specific flow patterns. Here we have applied an alternative mathematical analysis approach to a dynamic time series of MRI images acquired every 4min over ∼3h in anesthetized rats, following administration of a small molecular weight paramagnetic tracer into the CSF reservoir of the cisterna magna. We use Optimal Mass Transport (OMT) to model the glymphatic flow vector field, and then analyze the flow to find the network of CSF-ISF flow channels. We use 3D visualization computational tools to visualize the OMT defined network of CSF-ISF flow channels in relation to anatomical and vascular key landmarks from the live rodent brain. The resulting OMT model of the glymphatic transport network agrees largely with the current understanding of the glymphatic transport patterns defined by dynamic contrast-enhanced MRI revealing key CSF transport pathways along the ventral surface of the brain with a trajectory towards the pineal gland, cerebellum, hypothalamus and olfactory bulb. In addition, the OMT analysis also revealed some interesting previously unnoticed behaviors regarding CSF transport involving parenchymal streamlines moving from ventral reservoirs towards the surface of the brain, olfactory bulb and large central veins.

Parallel Comparative Studies on Mouse Toxicity of Oxide Nanoparticle- and Gadolinium-Based T1 MRI Contrast Agents

Magnetic resonance imaging (MRI) contrast agents with high relaxivity are highly desirable because they can significantly increase the accuracy of diagnosis. However, they can be potentially toxic to the patients. In this study, using a mouse model, we investigate the toxic effects and subsequent tissue damage induced by three T1 MRI contrast agents: gadopentetate dimeglumine injection (GDI), a clinically used gadolinium (Gd)-based contrast agent (GBCAs), and oxide nanoparticle (NP)-based contrast agents, extremely small-sized iron oxide NPs (ESIONs) and manganese oxide (MnO) NPs. Biodistribution, hematological and histopathological changes, inflammation, and the endoplasmic reticulum (ER) stress responses are evaluated for 24 h after intravenous injection. These thorough assessments of the toxic and stress responses of these agents provide a panoramic description of safety concerns and underlying mechanisms of the toxicity of contrast agents in the body. We demonstrate that ESIONs exhibit fewer adverse effects than the MnO NPs and the clinically used GDI GBCAs, providing useful information on future applications of ESIONs as potentially safe MRI contrast agents.

Lumazine Synthase Protein Nanoparticle-Gd(III)-DOTA Conjugate as a T1 contrast agent for high-field MRI

With the applications of magnetic resonance imaging (MRI) at higher magnetic fields increasing, there is demand for MRI contrast agents with improved relaxivity at higher magnetic fields. Macromolecule-based contrast agents, such as protein-based ones, are known to yield significantly higher r1 relaxivity at low fields, but tend to lose this merit when used as T1 contrast agents (r1/r2 = 0.5 ~ 1), with their r1 decreasing and r2 increasing as magnetic field strength increases. Here, we developed and characterized an in vivo applicable magnetic resonance (MR) positive contrast agent by conjugating Gd(III)-chelating agent complexes to lumazine synthase isolated from Aquifex aeolicus (AaLS). The r1 relaxivity of Gd(III)-DOTA-AaLS-R108C was 16.49 mM(-1)s(-1) and its r1/r2 ratio was 0.52 at the magnetic field strength of 7 T. The results of 3D MR angiography demonstrated the feasibility of vasculature imaging within 2 h of intravenous injection of the agent and a significant reduction in T1 values were observed in the tumor region 7 h post-injection in the SCC-7 flank tumor model. Our findings suggest that Gd(III)-DOTA-AaLS-R108C could serve as a potential theranostic nanoplatform at high magnetic field strength.

Complementary strategies for developing Gd-free high-field T₁ MRI contrast agents based on Mn(III) porphyrins

Mn(III) porphyrin (MnP) holds the promise of addressing the emerging challenges associated with Gd-based clinical MRI contrast agents (CAs), namely, Gd-related adverse effect and decreasing sensitivity at high clinical magnetic fields. Two complementary strategies for developing new MnPs as Gd-free CAs with optimized biocompatibility were established to improve relaxivity or clearance rate. MnPs with distinct and tunable pharmacokinetic properties can consequently be constructed for different in vivo applications at clinical field of 3 T.

Synthesis and characterization of a novel gadolinium-based contrast agent for magnetic resonance imaging of myelination

Myelin is a membrane system that fosters nervous impulse conduction in the vertebrate nervous system. Myelin sheath disruption is a common characteristic of several neurodegenerative diseases such as multiple sclerosis (MS) and various leukodystrophies. To date, the diagnosis of MS is obtained using a set of criteria in which MRI observations play a central role. However, because of the lack of specificity for myelin integrity, the use of MRI as the primary diagnostic tool has not yet been accepted. In order to improve MR specificity, we began developing MR probes targeted toward myelin. In this work we describe a new myelin-targeted MR contrast agent, Gd-DODAS, based on a stilbene binding moiety and demonstrate its ability to specifically bind to myelin in vitro and in vivo. We also present evidence that Gd-DODAS generates MR contrast in vivo in T1-weighed images and in T1 maps that correlates to the myelin content.

Myelin imaging compound (MIC) enhanced magnetic resonance imaging of myelination

The vertebrate nervous system is characterized by myelination, a fundamental biological process that protects the axons and facilitates electric pulse transduction. Damage to myelin is considered a major effect of autoimmune diseases such as multiple sclerosis (MS). Currently, therapeutic interventions are focused on protecting myelin integrity and promoting myelin repair. These efforts need to be accompanied by an effective imaging tool that correlates the disease progression with the extent of myelination. To date, magnetic resonance imaging (MRI) is the primary imaging technique to detect brain lesions in MS. However, conventional MRI cannot differentiate demyelinated lesions from other inflammatory lesions and therefore cannot predict disease progression in MS. To address this problem, we have prepared a Gd-based contrast agent, termed MIC (myelin imaging compound), which binds to myelin with high specificity. In this work, we demonstrate that MIC exhibits a high kinetic stability toward transmetalation with promising relaxometric properties. MIC was used for in vivo imaging of myelination following intracerebroventricular infusion in the rat brain. MIC was found to distribute preferentially in highly myelinated regions and was able to detect regions of focally induced demyelination.

Optimization of gadolinium-based MRI contrast agents for high magnetic-field applications

The search for higher spatial resolution and better sensitivity stimulates the development of high-field (3 T) and ultrahigh field (>3 T) MRI scanners. Gadolinium-based MRI contrast agents, commercial ones used in clinics, as well as recently developed more efficient ones, become less and less effective as the magnetic field is increased above 3 T and, therefore, special contrast agents for ultrahigh-field MRI have to be developed. As the relaxivity, defined as relaxation enhancement per Gd-ion, is rather limited, marked boosts in performance can only be achieved by creating systems transporting many paramagnetic centers to the desired site. To obtain maximum efficiency gadolinium chelates with more than one water molecule in the first coordination sphere must be used. The rotational correlation time should be in the range of 0.5-1 ns and the residence time of first sphere water molecules should be short (<10 ns).

Molecular imaging of in vivo gene expression

Molecular imaging provides spatial and temporal information on cellular changes that occur during development and in disease. MRI and optical imaging of reporter genes allows for the visualization of promoter activity, protein-protein interactions, protein stability and the tracking of individual proteins and cells. Reporter genes can be genetically encoded in transgenic animals or detected through the administration of an exogenous contrast agent. Advances in molecular imaging of reporter genes have led to the development of imaging probes that detect changes in endogenous cellular changes. The ability to use contrast agents coupled with functional information on cellular events will allow for sensitive assessment of individual patient therapies, leading to an accurately tailored treatment regimen.

Influence of molecular parameters and increasing magnetic field strength on relaxivity of gadolinium- and manganese-based T1 contrast agents

Simulations were performed to understand the relative contributions of molecular parameters to longitudinal (r(1)) and transverse (r(2)) relaxivity as a function of applied field, and to obtain theoretical relaxivity maxima over a range of fields to appreciate what relaxivities can be achieved experimentally. The field-dependent relaxivities of a panel of gadolinium and manganese complexes with different molecular parameters, water exchange rates, rotational correlation times, hydration state, etc. were measured to confirm that measured relaxivities were consistent with theory. The design tenets previously stressed for optimizing r(1) at low fields (very slow rotational motion; chelate immobilized by protein binding; optimized water exchange rate) do not apply at higher fields. At 1.5 T and higher fields, an intermediate rotational correlation time is desired (0.5-4 ns), while water exchange rate is not as critical to achieving a high r(1). For targeted applications it is recommended to tether a multimer of metal chelates to a protein-targeting group via a long flexible linker to decouple the slow motion of the protein from the water(s) bound to the metal ions. Per ion relaxivities of 80, 45, and 18 mM(-1) s(-1) at 1.5, 3 and 9.4 T, respectively, are feasible for Gd(3+) and Mn(2+) complexes.