MRI Contrast Agent Studies of Compartmental Differentiation, Dose-dependence, and Tumor Characterization in the Brain

Magnetic resonance imaging (MRI) has increasingly become the preferred imaging modality in modern day research to study disease. MRI presents an imaging technique that is practically non-invasive and without any ionizing radiation. This dissertation presents the use of contrast agents in MRI studies to differentiate compartments, to study dose dependence of relaxation times, and to characterize tumors using signal amplifying enzymes in the brain. Differentiating compartments in the brain can be useful in diffusion studies to detect stroke at an early stage. Diffusion-weighted NMR techniques have established that the apparent diffusion coefficient (ADC) of cerebral tissue water decreases during ischemia. However, it is unclear whether the ADC change occurs due to changes in the intracellular (IC) space, extracellular (EC) space, or both. To better understand the mechanism of water ADC changes in response to ischemic injury, making IC and EC compartment specific measurements of water diffusion is essential. The first study was done where manganese (Mn2+) was used as an IC contrast agent. Mn2+ uptake by cells causes shortening of the T1 relaxation time of IC water. The relative difference in T1 relaxation times between the IC and EC compartments can be used to discriminate between the MR signals arising from water in the respective compartments. Mn2+ is also widely used in manganese-enhanced MRI (MEMRI) studies to visualize functional neural tracts and anatomy in the brain in vivo. In animal studies, the goal is to use a dose of Mn2+ that will maximize the contrast while minimizing its toxic effects. The goal of dose study was to investigate the MRI dose response of Mn2+ in rat brain following SC administration of Mn2+. The dose dependence and temporal dynamics of Mn2+ after SC injection can prove useful for longitudinal in vivo studies that require brain enhancement to persist for a long period of time to visualize neuroarchitecture like in neurodegenerative disease studies. Contrast agents, in addition to their use in compartmental differentiation and dose studies, can be used for imaging tumors. The last study in this dissertation focuses on imaging EGF receptors in brain tumors. We tested a novel pretargeting imaging approach that includes the administration of humanized monoclonal antibody (anti-EGFR mAb, EMD72000) linked to enzymes with complementing activities that use a low-molecular weight paramagnetic molecule (diTyr-GdDTPA) as a reducing substrate administered following the mAb conjugates. We analyzed the differential MR tumor signal decay in vivo using orthotopic models of human glioma. The patterns of MR signal change following substrate administration revealed differences in elimination patterns that allowed distinguishing between non-specific and specific modes of MR signal decay

Infarct evolution in a large animal model of middle cerebral artery occlusion

Mechanical thrombectomy for the treatment of ischemic stroke shows high rates of recanalization; however, some patients still have a poor clinical outcome. A proposed reason for this relates to the fact that the ischemic infarct growth differs significantly between patients. While some patients demonstrate rapid evolution of their infarct core (fast evolvers), others have substantial potentially salvageable penumbral tissue even hours after initial vessel occlusion (slow evolvers). We show that the dog middle cerebral artery occlusion model recapitulates this key aspect of human stroke rendering it a highly desirable model to develop novel multimodal treatments to improve clinical outcomes. Moreover, this model is well suited to develop novel image analysis techniques that allow for improved lesion evolution prediction; we provide proof-of-concept that MRI perfusion-based time-to-peak maps can be utilized to predict the rate of infarct growth as validated by apparent diffusion coefficient-derived lesion maps allowing reliable classification of dogs into fast versus slow evolvers enabling more robust study design for interventional research

MRI Simulation Study Investigating Effects of Vessel Topology, Diffusion, and Susceptibility on Transverse Relaxation Rates Using a Cylinder Fork Model

Brain vasculature is conventionally represented as straight cylinders when simulating blood oxygenation level dependent (BOLD) contrast effects in functional magnetic resonance imaging (fMRI). In reality, the vasculature is more complicated with branching and coiling especially in tumors. Diffusion and susceptibility changes can also introduce variations in the relaxation mechanisms within tumors. This study introduces a simple cylinder fork model (CFM) and investigates the effects of vessel topology, diffusion, and susceptibility on the transverse relaxation rates R2* and R2. Simulations using Monte Carlo methods were performed to quantify R2* and R2 by manipulating the CFM at different orientations, bifurcation angles, and rotation angles. Other parameters of the CFM were chosen based on physiologically relevant values: vessel diameters (~2‒10 µm), diffusion rates (1 × 10−11‒1 × 10−9 m2/s), and susceptibility values (3 × 10−8–4 × 10−7 cgs units). R2* and R2 measurements showed a significant dependence on the bifurcation and rotation angles in several scenarios using different vessel diameters, orientations, diffusion rates, and susceptibility values. The angular dependence of R2* and R2 using the CFM could potentially be exploited as a tool to differentiate between normal and tumor vessels. The CFM can also serve as the elementary building block to simulate a capillary network reflecting realistic topological features

Liposome-encapsulated superoxide dismutase mimetic: theranostic potential of an MR detectable and neuroprotective agent

Endogenous manganese based superoxide dismutase (Mn-SOD) provides the primary defense against excess production of potentially toxic superoxide anion (O2 (-) ). M40401 is a synthetic enzyme mimetic that has a catalytic activity rate exceeding that of the native SOD enzymes. The presence of a paramagnetic Mn(II) cation in M40401 suggests that the delivery and spatial distribution of this enzyme mimetic in vivo may be directly detectible using magnetic resonance imaging (MRI); however, the cardiotoxicity of Mn(II) severely limits the use of free M40401 in living systems. To deliver M40401 in vivo in amounts sufficient for MRI detection and to limit potential cardiotoxicity, we encapsulated M40401 into 170 nm liposomes composed of phosphatidylcholine and PEGylated phosphatidylethanolamine to achieve extended circulation in the bloodstream. The obtained liposomes efficiently catalyzed superoxide dismutation in vitro. Using 3 T MRI we investigated the biokinetics of liposome-encapsulated M40401 in mice and found that, in addition to catalyzing superoxide dismutation in vitro, M40401 caused differential and region-specific enhancement of mouse brain after systemic administration. Thus, liposome encapsulated M40401 is an ideal candidate for development as a theranostic compound useful for simultaneous MRI-mediated tracking of delivery as well as for neuroprotective treatment of ischemic brain

MR signal amplification for imaging of the mutant EGF receptor in orthotopic human glioma model

PURPOSE: To investigate the potential of targeted MR signal amplification strategy for imaging of EGF receptor variant III (EGFRvIII) overexpression associated with the infiltrating margin of aggressive orthotopic brain tumors. PROCEDURES: F(ab\u27)2 fragments of humanized anti-EGFRvIII monoclonal antibody (EMD72000) were linked to deglycosylated horseradish peroxidase (HRP) and glucose oxidase (GOX). Detection of the F(ab\u27)2 conjugate pair colocalization in vivo was enabled by a subsequent IV injection of a low molecular weight paramagnetic substrate of HRP, diTyr-GdDTPA. RESULTS: The delivery of the targeted fragments to the tumor was validated using SPECT/CT imaging of radiolabeled anti-EGFRvIII F(ab\u27)2 conjugates. Further, by using 3 T MRI, we observed time-dependent differences in tumor signal intensity and signal retention at the endpoint depending on whether or not the animals were pre-injected with the anti-EGFRvIII F(ab\u27)2 conjugates. CONCLUSIONS: Imaging of EGFRvIII expression in vivo was enabled by consecutive administration of targeted F(ab\u27)2 conjugates and a paramagnetic substrate resulting in a tumor-specific receptor detection with high specificity and resolution

Additional file 1 of Cumulative effect of simvastatin, l-arginine, and tetrahydrobiopterin on cerebral blood flow and cognitive function in Alzheimer’s disease

Additional file 1: eTable 1. Psychometric assessments. eTable 2. Individual ADAS-cog 13 scores and group assignment. eFigure 1. Regions of interest (ROIs) for cerebral perfusion assessment and brain volumetric measurements

In Vivo Optical Imaging of Myelination Events in a Myelin Basic Protein Promoter-Driven Luciferase Transgenic Mouse Model

The compact myelin sheath is important for axonal function, and its loss can lead to neuronal cell death and irreversible functional deficits. Myelin is vulnerable to a variety of metabolic, toxic, and autoimmune insults. In diseases like multiple sclerosis, there is currently no therapy to stop myelin loss, underscoring the need for neuroprotective and remyelinating therapies. Noninvasive, robust techniques are also needed to confirm the effect of such therapies in animal models. This article describes the generation, characterization, and potential uses for a myelin basic protein-luciferase (MBP-luci) transgenic mouse model, in which the firefly luciferase reporter gene is selectively controlled by the MBP promoter. In vivo bioluminescence imaging can be used to visualize and quantify demyelination and remyelination at the transcriptional level, noninvasively, and in real time. Transgenic mice were assessed in the cuprizone-induced model of demyelination, and luciferase activity highly correlated with demyelination and remyelination events as confirmed by both magnetic resonance imaging and postmortem histological analysis. Furthermore, MBP-luci mice demonstrated enhanced luciferase signal and remyelination in the cuprizone model after treatment with a peroxisome proliferator activated receptor-delta selective agonist and quetiapine. Imaging sensitivity was further enhanced by using CycLuc 1, a luciferase substrate, which has greater blood–brain barrier penetration. We demonstrated the utility of MBP-luci model in tracking myelin changes in real time and supporting target and therapeutic validation efforts