Paramagnetic complexes of manganese(II), iron(III), and gadolinium(III) as contrast agents for magnetic resonance imaging. The influence of stability constants on the biodistribution of radioactive aminopolycarboxylate complexes

Low-Molecular-Weight Fe(III) Complexes for MRI Contrast Agents

Fe(III) complexes have again attracted much attention for application as MRI contrast agents in recent years due to their high thermodynamic stability, low long-term toxicity, and large relaxivity at a higher magnetic field. This mini-review covers the recent progress on low-molecular-weight Fe(III) complexes, which have been considered as one of the promising alternatives to clinically used Gd(III)-based contrast agents. Two kinds of complexes including mononuclear Fe(III) complexes and multinuclear Fe(III) complexes are summarized in sequence, with a specific highlight of the structural relationships between the complexes and their relaxivity and thermodynamic stability. In additional, the future perspectives for the design of low-molecular-weight Fe(III) complexes for MRI contrast agents are suggested.

Biocompatible FePO4 Nanoparticles: Drug Delivery, RNA Stabilization, and Functional Activity

FePO4 NPs are of special interest in food fortification and biomedical imaging because of their biocompatibility, high bioavailability, magnetic property, and superior sensory performance that do not cause adverse organoleptic effects. These characteristics are desirable in drug delivery as well. Here, we explored the FePO4 nanoparticles as a delivery vehicle for the anticancer drug, doxorubicin, with an optimum drug loading of 26.81% ± 1.0%. This loading further enforces the formation of Fe3+ doxorubicin complex resulting in the formation of FePO4-DOX nanoparticles. FePO4-DOX nanoparticles showed a good size homogeneity and concentration-dependent biocompatibility, with over 70% biocompatibility up to 80 µg/mL concentration. Importantly, cytotoxicity analysis showed that Fe3+ complexation with DOX in FePO4-DOX NPs enhanced the cytotoxicity by around 10 times than free DOX and improved the selectivity toward cancer cells. Furthermore, FePO4 NPs temperature-stabilize RNA and support mRNA translation activity showing promises for RNA stabilizing agents. The results show the biocompatibility of iron-based inorganic nanoparticles, their drug and RNA loading, stabilization, and delivery activity with potential ramifications for food fortification and drug/RNA delivery.

Iron(III)-tCDTA derivatives as MRI contrast agents: Increased T1 relaxivities at higher magnetic field strength and pH sensing

PURPOSE

Low molecular weight iron(III) complex-based contrast agents (IBCA) including iron(III) trans-cyclohexane diamine tetraacetic acid [Fe(tCDTA)]^- could serve as alternatives to gadolinium-based contrast agents in MRI. In search for IBCA with enhanced properties, we synthesized derivatives of [Fe(tCDTA)]^- and compared their contrast effects.

METHODS

Trans-cyclohexane diamine tetraacetic acid (tCDTA) was chemically modified in 2 steps: first the monoanhydride of Trans-cyclohexane diamine tetraacetic acid was generated, and then it was coupled to amines in the second step. After purification, the chelators were analyzed by high-performance liquid chromatography, mass spectrometry, and NMR spectrometry. The chelators were complexed with iron(III), and the relaxivities of the complexes were measured at 0.94, 1.5, 3, and 7 Tesla. Kinetic stabilities of the complexes were analyzed spectrophotometrically and the redox properties by cyclic voltammetry.

RESULTS

Using ethylenediamine (en) and trans-1,4-diaminocyclohexane, we generated monomers and dimers of tCDTA: en-tCDTA, en-tCDTA-dimer, trans-1,4-diaminocyclohexane-tCDTA, and trans-1,4-diaminocyclohexane-tCDTA-dimer. The iron(III) complexes of these derivatives had similarly high stabilities as [Fe(tCDTA)]^- . The iron(III) complexes of the trans-1,4-diaminocyclohexane derivatives had higher T₁ relaxivities than [Fe(tCDTA)]^- that increased with increasing magnetic field strengths and were highest at 6.8 L·mmol^-1 ·s^-1 per molecule for the dimer. Remarkably, the relaxivity of [Fe(en-tCDTA)]⁺ had a threefold increase from neutral pH toward pH6.

CONCLUSION

Four iron(III) complexes with similar stability in comparison to [Fe(tCDTA)]^- were synthesized. The relaxivities of trans-1,4-diaminocyclohexane-tCDTA and trans-1,4-diaminocyclohexane-tCDTA-dimer complexes were in the same range as gadolinium-based contrast agents at 3 Tesla. The [Fe(en-tCDTA)]⁺ complex is a pH sensor at weakly acidic pH levels, which are typical for various cancer types.

Iron(iii) chelated paramagnetic polymeric nanoparticle formulation as a next-generation T1-weighted MRI contrast agent

In pursuit of safer alternatives to Gd-based MRI contrast agents due to its toxicity and organ deposition, herein, we developed a safer and efficient clinically relevant iron(iii) chelated polymeric nanoparticle as a T1-weighted MRI contrast agent.

Applications for Transition-Metal Chemistry in Contrast-Enhanced Magnetic Resonance Imaging

Contrast-enhanced magnetic resonance imaging (MRI) is an indispensable tool for diagnostic medicine. However, safety concerns related to gadolinium in commercial MRI contrast agents have emerged in recent years. For patients suffering from severe renal impairment, there is an important unmet medical need to perform contrast-enhanced MRI without gadolinium. There are also concerns over the long-term effects of retained gadolinium within the general patient population. Demand for gadolinium-free MRI contrast agents is driving a new wave of inorganic chemistry innovation as researchers explore paramagnetic transition-metal complexes as potential alternatives. Furthermore, advances in personalized care making use of molecular-level information have motivated inorganic chemists to develop MRI contrast agents that can detect pathologic changes at the molecular level. Recent studies have highlighted how reaction-based modulation of transition-metal paramagnetism offers a highly effective mechanism to achieve MRI contrast enhancement that is specific to biochemical processes. This Viewpoint highlights how recent advances in transition-metal chemistry are leading the way for a new generation of MRI contrast agents.

Heteronuclear d-d and d-f Ru(ii)/M complexes [M = Gd(iii), Yb(iii), Nd(iii), Zn(ii) or Mn(ii)] of ligands combining phenanthroline and aminocarboxylate binding sites: combined relaxivity, cell imaging and photophysical studies

A ligand skeleton combining a 1,10-phenanthroline (phen) binding site and one or two heptadentate N3O4 aminocarboxylate binding sites, connected via alkyne spacers to the phen C3 or C3/C8 positions, has been used to prepare a range of heteronuclear Ru·M and Ru·M2 complexes which have been evaluated for their cell imaging, relaxivity, and photophysical properties. In all cases the phen unit is bound to a {Ru(bipy)2}2+ unit to give a phosphorescent {Ru(bipy)2(phen)}2+ luminophore, and the pendant aminocarboxylate sites are occupied by a secondary metal ion M which is either a lanthanide [Gd(iii), Nd(iii), Yb(iii)] or another d-block ion [Zn(ii), Mn(ii)]. When M = Gd(iii) or Mn(ii) these ions provide the complexes with a high relaxivity for water; in the case of Ru·Gd and Ru·Gd2 the combination of high water relaxivity and 3MLCT phosphorescence from the Ru(ii) unit provides the possibility of two different types of imaging modality in a single molecular probe. In the case of Ru·Mn and Ru·Mn2 the Ru(ii)-based phosphorescence is substantially reduced compared to the control complexes Ru·Zn and Ru·Zn2 due to the quenching effect of the Mn(ii) centres. Ultrafast transient absorption spectroscopy studies on Ru·Mn (and Ru·Zn as a non-quenched control) reveal the occurrence of fast (<1 ns) PET in Ru·Mn, from the Mn(ii) ion to the Ru(ii)-based 3MLCT state, i.e. MnII-(phen˙-)-RuIII → MnIII-(phen˙-)-RuII; the resulting MnIII-(phen˙-) state decays with τ ≈ 5 ns and is non-luminescent. This occurs in conformers when an ET pathway is facilitated by a planar, conjugated bridging ligand conformation connecting the two units across the alkyne bridge but does not occur in conformers where the two units are electronically decoupled by a twisted conformation of the bridging ligand. Computational studies (DFT) on Ru·Mn confirmed both the occurrence of the PET quenching pathway and its dependence on molecular conformation. In the complexes Ru·Ln and Ru·Ln2 (Ln = Nd, Yb), sensitised near-infrared luminescence from Nd(iii) or Yb(iii) is observed following photoinduced energy-transfer from the Ru(ii) core, with Ru → Nd energy-transfer being faster than Ru → Yb energy-transfer due to the higher density of energy-accepting states on Nd(iii).

Reversibility of neuroimaging markers influenced by lifetime occupational manganese exposure

Manganese (Mn) is a neurotoxicant that many workers are exposed to daily. There is limited knowledge about how changes in exposure levels impact measures in magnetic resonance imaging (MRI). We hypothesized that changes in Mn exposure would be reflected by changes in the MRI relaxation rate R1 and thalamic γ-aminobutyric acid (GABAThal). As part of a prospective cohort study, 17 welders were recruited and imaged on two separate occasions approximately two years apart. MRI relaxometry was used to assess changes of Mn accumulation in the brain. Additionally, GABA was measured using magnetic resonance spectroscopy (MRS) in the thalamic and striatal regions of the brain. Air Mn exposure ([Mn]Air) and cumulative exposure indexes of Mn (Mn-CEI) for the past three months (Mn-CEI3M), past year (Mn-CEI12M), and lifetime (Mn-CEILife) were calculated using personal air sampling and a comprehensive work history, while toenails were collected for analysis of internal Mn body burden. Finally, welders' motor function was examined using the Unified Parkinson's Disease Rating Scale (UPDRS). Median exposure decreased for all exposure measures between the first and second scan. ΔGABAThal was significantly correlated with ΔMn-CEI3M (ρ = 0.66, adjusted p = 0.02), ΔMn-CEI12M (ρ = 0.70, adjusted p = 0.006) , and Δ[Mn]Air (ρ = 0.77, adjusted p = 0.002). ΔGABAThal significantly decreased linearly with ΔMn-CEI3M (quantile regression, β = 15.22, p = 0.02) as well as Δ[Mn]Air (β = 1.27, p = 0.04). Finally, Mn-CEILife interacted with Δ[Mn]Air in the substantia nigra where higher Mn-CEILife lessened the ΔR1 per Δ[Mn]Air (F-test, p = 0.005). While R1 and GABA changed with Mn exposure, UPDRS was unaffected. In conclusion, our study shows that effects from changes in Mn exposure are reflected in thalamic GABA levels and brain Mn levels, as measured by R1, in most brain regions.

Effect of early experience on neuronal and behavioral responses to con- and heterospecific odors in closely related Mus taxa: epigenetic contribution in formation of precopulatory isolation

BACKGROUND

The most effective learning occurs during sensitive periods. Olfactory plasticity to main social olfactory cues is limited to a critical period to a large degree. The objective was to evaluate the influence of early olfactory experience on the behavioral and neuronal responses of males to con- and heterospecific odors of receptive females in two species, M. musculus (subspecies musculus, wagneri) and M. spicilegus, and thus to determine the potential role of epigenetic contribution in the formation of precopulatory isolation.

RESULTS

Males were reciprocally cross-fostered shortly after the birth and were tested for response to con- and heterospecific urine odors of estrus females using two-choice tests at 70-85 days of age. Neuronal activity of non- and cross-fostered males was evaluated at 90-110 days of age in the MOB and AOB to con- and heterospecific female odor using fMRI (MEMRI). Non-cross-fostered males of three taxa demonstrated a strong preference for odor of conspecific females compared to odor of heterospecific ones. Spicilegus-nursed musculus preferred odor of heterospecific females. Wagneri-nursed spicilegus and spicilegus-nursed wagneri did not demonstrate significant choice of con - or heterospecific female odor. The level of MRI signal obtained from the evaluation of manganese accumulation in AOB neurons was significantly higher when the odor of conspecific estrus females was exposed, compared to urine exposure of heterospecific females. The response pattern changed to the opposite in males raised by heterospecific females. Response patterns of neuronal activity in the MOB to con- and heterospecific female odors were different in cross-fostered and control males.

CONCLUSION

The maternal environment, including odor, had a greater effect on the level of MRI signal in the AOB than the genetic relationships of the recipient and the donor of the odor stimulus. Behavioral and neuronal responses to con- and heterospecific odors changed in closely related Mus taxa as a result of early experience. We demonstrated the importance of early learning in mate choice in adulthood in mice and the possibility of epigenetic contribution in the formation of precopulatory reproductive isolation.

Manganese-Enhanced Magnetic Resonance Imaging: Overview and Central Nervous System Applications With a Focus on Neurodegeneration

Manganese-enhanced magnetic resonance imaging (MEMRI) rose to prominence in the 1990s as a sensitive approach to high contrast imaging. Following the discovery of manganese conductance through calcium-permeable channels, MEMRI applications expanded to include functional imaging in the central nervous system (CNS) and other body systems. MEMRI has since been employed in the investigation of physiology in many animal models and in humans. Here, we review historical perspectives that follow the evolution of applied MRI research into MEMRI with particular focus on its potential toxicity. Furthermore, we discuss the more current in vivo investigative uses of MEMRI in CNS investigations and the brief but decorated clinical usage of chelated manganese compound mangafodipir in humans.

Automated Computational Processing of 3-D MR Images of Mouse Brain for Phenotyping of Living Animals

Magnetic resonance (MR) imaging provides a method to obtain anatomical information from the brain in vivo that is not typically available by optical imaging because of this organ's opacity. MR is nondestructive and obtains deep tissue contrast with 100-µm³ voxel resolution or better. Manganese-enhanced MRI (MEMRI) may be used to observe axonal transport and localized neural activity in the living rodent and avian brain. Such enhancement enables researchers to investigate differences in functional circuitry or neuronal activity in images of brains of different animals. Moreover, once MR images of a number of animals are aligned into a single matrix, statistical analysis can be done comparing MR intensities between different multi-animal cohorts comprising individuals from different mouse strains or different transgenic animals, or at different time points after an experimental manipulation. Although preprocessing steps for such comparisons (including skull stripping and alignment) are automated for human imaging, no such automated processing has previously been readily available for mouse or other widely used experimental animals, and most investigators use in-house custom processing. This protocol describes a stepwise method to perform such preprocessing for mouse. © 2017 by John Wiley & Sons, Inc.

Multi-system state shifts and cognitive deficits induced by chronic morphine during abstinence

Chronic morphine administration induces neural plasticity followed by withdraw. And clinic observation indicates that obvious cognitive deficits are found during withdrawal. However, current neural substrates that regulate dysfunction in withdrawal are unknown. In our studies, chronic morphine administration was used to induce the spontaneous withdrawal model in rats. A series of cognitive abilities was tested to explore brain function. To further evaluate the neural substrates of dysfunction, Manganese-enhanced MRI(MEMRI) was used to map the dysfunctional regions in vivo.We observed that chronic morphine administration could induce obvious withdrawal behaviors in abstinence followed by cognitive impairments, such as impairments in working memory, reward, interaction and enhancement of anxiety. Our in-vivo MEMRI data using the voxel-wise comparisons showed that the manganese-enhanced signal intensity (VMI) within morphine withdrawal groups was increased in cingulate cortex (Cg), secondary motor cortex (M2), CA3 subfield of hippocampus, dorsal striatum (D-striatum), retrosplenial cortex (RS), shell subregion of NAc (AcbSh), core subregion of NAc (AcbC), central nucleus of amygdala (CeC), basolateral amygdaloid nucleus (BLA), central amygdaloid nucleus (CeM), anterior hypothalamic area, central (AHC), ventral tegmental area (VTA) and scaphoid thalamic nucleus (SC).However, decreasing of VMI was found in the ventrolateral striatum (V-striatum) and lateral posterior thalamic nucleus (LP) compared to the control group. These brain regions were beleived to be components of the memory, executive, limbic and regulatory systems. Therefore, our present studies indicate that withdrawal induced by chronic morphine adiministration could disturb brain function leading to multi-systems state shifts and cognitive deficits in abstinence.

Radiolabelling with isotopic mixtures of (52g/55)Mn(II) as a straight route to stable manganese complexes for bimodal PET/MR imaging

Radiolabelling using isotopic mixtures of (52g/55)Mn(ii) offers fast and easy access to new small molecule PET/MR tracers, composed of chemically identical reporting units. trans-1,2-Diaminocyclohexane-N,N,N',N'-tetraacetic acid (CDTA) was radiolabelled with carrier-added (52g)Mn(ii) in >99% radiochemical yield, producing the first manganese-based bimodal PET/MR probe. The Mn-CDTA chelate was shown to be very stable to air oxidation and sufficiently inert to decomplexation in blood serum. These data sparked our interest in functionalized CDTA ligands for the design of optimized PET/MR tracers.

Thermoactivatable polymer-grafted liposomes for low-invasive image-guided chemotherapy

The objective of this study was to develop a thermotriggered, polymer-based liposomal drug carrier with an activatable magnetic resonance imaging (MRI) contrast property for monitoring the release of substances and for localized tumor therapy. The multimodal thermoactivatable polymer-grafted liposomes (MTPLs) were tested to investigate whether the accumulation of MTPLs in colon-26 grafted tumors could be visualized in vivo using MRI and optical imaging, whether MTPLs induce signal enhancement, reflecting the release of their contents, after triggering by short-term heating (42.5°C for 10 minutes) 9 hours after MTPL administration (late-phase triggering), and whether MTPLs can provide a sufficient antitumor effect. The imaging and therapeutic properties of MTPLs were tested both in vitro and in vivo (BALB/c nude mice: heated group with MTPLs (n = 5), nonheated group with MTPLs (n = 5), heated group with doxorubicin-free MTPLs (n = 5), nonheated group with manganese-free MTPLs (n = 5), and kinetics observation group (n = 3); N = 23). Through in vivo MRI and fluorescent imaging, the MTPLs were shown to have significantly accumulated in the grafted colon-26 tumors 8 hours after administration. Delayed thermotriggering (9 hours after administration) caused MR signal enhancement, reflecting the release of their contents, after a short exposure to tolerable heat. In addition, significant antitumor effects were observed after treatment. The proposed polymer-based activatable MTPLs with a "delayed thermotrigger" provide a promising technology for cancer theranostics that allows minimal adverse effects and rapid interactive therapy.

Manganese-Enhanced MRI Reflects Both Activity-Independent and Activity-Dependent Uptake within the Rat Habenulomesencephalic Pathway

Manganese-enhanced magnetic resonance imaging (MEMRI) is a powerful technique for assessing the functional connectivity of neurons within the central nervous system. Despite the widely held proposition that MEMRI signal is dependent on neuronal activity, few studies have directly tested this implicit hypothesis. In the present series of experiments, MnCl2 was injected into the habenula of urethane-anesthetized rats alone or in combination with drugs known to alter neuronal activity by modulating specific voltage- and/or ligand-gated ion channels. Continuous quantitative T1 mapping was used to measure Mn2+ accumulation in the interpeduncular nucleus, a midline structure in which efferents from the medial habenula terminate. Microinjection of MnCl2 into the habenular complex using a protocol that maintained spontaneous neuronal activity resulted in a time-dependent increase in MEMRI signal intensity in the interpeduncular nucleus consistent with fast axonal transport of Mn2+ between these structures. Co-injection of the excitatory amino-acid agonist AMPA, increased the Mn2+-enhanced signal intensity within the interpeduncular nucleus. AMPA-induced increases in MEMRI signal were attenuated by co-injection of either the sodium channel blocker, TTX, or broad-spectrum Ca2+ channel blocker, Ni2+, and were occluded in the presence of both channel blockers. However, neither Ni2+ nor TTX, alone or in combination, attenuated the increase in signal intensity following injection of Mn2+ into the habenula. These results support the premise that changes in neuronal excitability are reflected by corresponding changes in MEMRI signal intensity. However, they also suggest that basal rates of Mn2+ uptake by neurons in the medial habenula may also occur via activity-independent mechanisms.

Manganese-enhanced MRI optic nerve tracking: effect of intravitreal manganese dose on retinal toxicity

The aim of this study was to provide data on the dose dependence of manganese-enhanced MRI (MEMRI) in the visual pathway of experimental rats and to study the toxicity of MnCl₂ to the retina. Sprague-Dawley rats were intravitreally injected with 2 μL of 0, 10, 25, 50, 75, 100, 150 and 300 mM MnCl₂, respectively. The contrast-to-noise ratio (CNR) of MEMRI for optic nerve enhancement was measured at different concentrations of MnCl₂. Simultaneously, the toxicity of manganese was evaluated by counting retinal ganglion cells and by retinal histological examination using light microscopy and transmission electron microscopy. The CNR increased with increasing concentration of MnCl₂ up to 75 mM. Retinal ganglion cell densities were reduced significantly when the concentration of MnCl₂ in the intravitreal injection was equal to or greater than 75 mM. Increasing numbers of ribosomes in retinal ganglion cells were first detected at 25 mM of MnCl₂. The retinal toxicity of MnCl₂ at higher concentration also included mitochondrial pathology and cell disruption of retinal ganglion cells, as well as abnormalities of photoreceptor and retinal pigment epithelium cells. It can be concluded that intravitreal injection of MnCl₂ induces retinal cell damage that appears to start from 25 mM. The concentration of MnCl₂ should not exceed 25 mm through intravitreal injection for visual pathway MEMRI in the rat.

Using manganese-enhanced MRI to understand BOLD

The 1990s were designated "The Decade of the Brain" by U.S. Congress, perhaps in great anticipation of the impact that functional neuroimaging techniques would have on advancing our understanding of how the brain is functionally organized. While it is impossible to overestimate the impact of functional MRI in neuroscience, many aspects of the blood oxygenation level-dependent (BOLD) contrast remain poorly understood, in great part due to the complex relationship between neural activity and hemodynamic changes. To better understand such relationship, it is important to probe neural activity independently. Manganese-enhanced MRI (MEMRI), when used to monitor neural activity, is a technique that uses the divalent manganese ion, Mn(2+), as a surrogate measure of calcium influx. A major advantage of using Mn(2+) as a functional marker is that the contrast obtained is directly related to the accumulation of the ion in excitable cells in an activity dependent manner. As such, the contrast in MEMRI is more directly related to neural activity then hemodynamic-based fMRI techniques. In the present work, the early conceptualization of MEMRI is reviewed, and the comparative experiments that have helped provide a better understanding of the spatial specificity of BOLD signal changes in the cortex is discussed.

Manganese enhanced MRI (MEMRI): neurophysiological applications

Manganese ion (Mn(2+)) is a calcium (Ca(2+)) analog that can enter neurons and other excitable cells through voltage gated Ca(2+) channels. Mn(2+) is also a paramagnetic that shortens the spin-lattice relaxation time constant (T(1)) of tissues where it has accumulated, resulting in positive contrast enhancement. Mn(2+) was first investigated as a magnetic resonance imaging (MRI) contrast agent approximately 20 years ago to assess the toxicity of the metal in rats. In the late 1990s, Alan Koretsky and colleagues pioneered the use of manganese enhanced MRI (MEMRI) towards studying brain activity, tract tracing and enhancing anatomical detail. This review will describe the methodologies and applications of MEMRI in the following areas: monitoring brain activity in animal models, in vivo neuronal tract tracing and using MEMRI to assess in vivo axonal transport rates.

Manganese-enhanced magnetic resonance imaging (MEMRI)

The use of manganese ions (Mn(2+)) as an MRI contrast agent was introduced over 20 years ago in studies of Mn(2+) toxicity in anesthetized rats (1). Manganese-enhanced MRI (MEMRI) evolved in the late nineties when Koretsky and associates pioneered the use of MEMRI for brain activity measurements (2) as well as neuronal tract tracing (3). Currently, MEMRI has three primary applications in biological systems: (1) contrast enhancement for anatomical detail, (2) activity-dependent assessment and (3) tracing of neuronal connections or tract tracing. MEMRI relies upon the following three main properties of Mn(2+): (1) it is a paramagnetic ion that shortens the spin lattice relaxation time constant (T(1)) of tissues, where it accumulates and hence functions as an excellent T(1) contrast agent; (2) it is a calcium (Ca(2+)) analog that can enter excitable cells, such as neurons and cardiac cells via voltage-gated Ca(2+) channels; and (3) once in the cells Mn(2+) can be transported along axons by microtubule-dependent axonal transport and can also cross synapses trans-synaptically to neighboring neurons. This chapter will emphasize the methodological approaches towards the use of MEMRI in biological systems.

Convergence of presenilin- and tau-mediated pathways on axonal trafficking and neuronal function

Alzheimer's disease (AD) is a significant and growing health problem in the aging population. Although definitive mechanisms of pathogenesis remain elusive, genetic and histological clues have implicated the proteins presenilin (PS) and tau as key players in AD development. PS mutations lead to familial AD, and although tau is not mutated in AD, tau pathology is a hallmark of the disease. Axonal transport deficits are a common feature of several neurodegenerative disorders and may represent a point of intersection of PS and tau function. To investigate the contribution of wild-type, as opposed to mutant, tau to axonal transport defects in the context of presenilin loss, we used a mouse model postnatally deficient for PS (PS cDKO) and expressing wild-type human tau (WtTau). The resulting PS cDKO;WtTau mice exhibited early tau pathology and axonal transport deficits that preceded development of these phenotypes in WtTau or PS cDKO mice. These deficits were associated with reduced neurotrophin signaling, defective learning and memory and impaired synaptic plasticity. The combination of these effects accelerated neurodegeneration in PS cDKO;WtTau mice. Our results strongly support a convergent role for PS and tau in axonal transport and neuronal survival and function and implicate their misregulation as a contributor to AD pathogenesis.

Manganese enhancement in non-CNS organs

Manganese-enhanced magnetic resonance imaging (MEMRI) is a novel imaging technique capable of monitoring calcium influx, in vivo. Manganese (Mn2+) ions, similar to calcium ions (Ca2+), are taken up by activated cells where their paramagnetic properties afford signal enhancement in T(1)-weighted MRI methodologies. In this study we have assessed Mn2+ distribution in mice using magnetization-prepared rapid gradient echo (MP-RAGE) based MRI, by measuring changes in T(1)-effective relaxation times (T(1)-eff), effective R(1)-relaxation rates (R(1)-eff) and signal intensity (SI) profiles over time. The manganese concentration in the tissue was also determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). Our results show a strong positive correlation between infused dose of MnCl2 and the tissue manganese concentration. Furthermore, we demonstrate a linear relationship between R(1)-eff and tissue manganese concentration and tissue-specific Mn2+ distribution in murine tissues following dose-dependent Mn2+ administration. This data provides an optimized MnCl2 dose regimen for an MP-RAGE based sequence protocol for specific target organs and presents a potential 3D MRI technique for in vivo imaging of Ca2+ entry during Ca2+-dependent processes in a wide range of tissues.

Determination of proton relaxivities of Mn(II), Cu(II) and Cr(III) added to solutions of serum proteins

Relaxometric studies are still of scientific interest due to their use in medicine and biology. In this study, proton T₁ and T₂ relaxivities of Mn(II), Cu(II) and Cr(III) in water were determined in the presence and absence of various proteins (albumin, α-globulin, γ-globulin, lysozyme, fibrinogen). The 1/T₁ and 1/T₂ in all solutions are linearly proportional to the concentration of the paramagnetic ions. Mn(II) has the great influence to alter relaxations in all protein solutions, while Cu(II) and Cr(III) have a poor influence on the relaxations. In addition, Mn(II) and Cu(II) are bound to each protein, but Cr(III) is not bound to any protein.

Magnetic Resonance Imaging of Functional Anatomy: Use for Small Animal Epilepsy Models

Neuroimaging has greatly assisted the diagnosis and treatment of epilepsy. Volumetric analysis, diffusion-weighted imaging, and other magnetic resonance imaging (MRI) modalities provide a clear picture of altered anatomical structures in both focal and nonfocal disease. More recently, advances in novel imaging methodologies have provided unique insights into this disease. Two examples include manganese-enhanced MRI (MEMRI) and diffusion tensor imaging (DTI). MEMRI involves injection of MnCl(2) to evaluate neuronal activity where it is actively transported. Areas of neuronal hyperactivity are expected to have altered uptake and transport. Mapping of activation along preferential uptake pathways can be confirmed by T(1)-weighted imaging. DTI uses the intrinsic preferential mobility of water movement along axonal pathways to map anatomical regions. DTI has been used to investigate white matter disease and is now being applied to clinical and, to a lesser extent, animal investigations of seizure disorders. These two diverse MRI methods can be applied to animal models to provide important information about the functional status of anatomical regions that may be altered by epilepsy.

In vivo axonal transport rates decrease in a mouse model of Alzheimer's disease

Axonopathy is a pronounced attribute of many neurodegenerative diseases. In Alzheimer's disease (AD), axonal swellings and degeneration are prevalent and may contribute to the symptoms of AD senile dementia. Current limitations in identifying the contribution of axonal damage to AD include the inability to detect when this damage occurs in relation to other identifiers of AD because of the invasiveness of existing methods. To overcome this, we further developed the MRI methodology Manganese Enhanced MRI (MEMRI) to assess in vivo axonal transport rates. Prior to amyloid-beta (Abeta) deposition, the axonal transport rates in the Tg2576 mouse model of AD were normal. As Abeta levels increased and before plaque formation, we observed a significant decrease in axonal transport rates of the Tg2576 mice compared to controls. After plaque formation, the decline in the transport rate in the Tg2576 mice became even more pronounced. These data indicate that in vivo axonal transport rates decrease prior to plaque formation in the Tg2576 mouse model of AD.

Magnetic resonance imaging of temperature-sensitive liposome release: drug dose painting and antitumor effects

BACKGROUND

In preclinical studies, lysolipid-based temperature-sensitive liposomes (LTSLs) containing chemotherapy drugs administered in combination with local hyperthermia have been found to increase tumor drug concentrations and improve antitumor efficacy of the drugs. We used a novel magnetic resonance imaging (MRI) method to measure the temporal and spatial patterns of drug delivery in a rat fibrosarcoma model during treatment with LTSLs containing doxorubicin and an MRI contrast agent (manganese) (Dox/Mn-LTSLs) administered at different times with respect to hyperthermia.

METHODS

Rats bearing 10- to 12-mm fibrosarcomas (n = 6-7 per group) were treated with Dox/Mn-LTSLs (at a dose of 5 mg doxorubicin/kg body weight) before and/or during 60 minutes of local tumor hyperthermia administered via a catheter inserted at the center of the tumor. Drug distribution was monitored continuously via MRI. Magnetic resonance changes were used to calculate intratumoral doxorubicin concentrations throughout treatment. Tumors were monitored until they reached five times their volume on the day of treatment or 60 days. Doxorubicin concentrations and times for tumors to reach five times their volume on the day of treatment were analyzed using the Kruskal-Wallis test and the Kaplan-Meier product-limit method, respectively. All statistical tests were two-sided.

RESULTS

Administration of Dox/Mn-LTSLs before, during, and both before and during hyperthermia yielded central, peripheral, and uniform drug distributions, respectively. Doxorubicin accumulated more quickly and reached higher concentrations in the tumor when Dox/Mn-LTSLs were administered during hyperthermia than when administered before hyperthermia (rate: 9.8 versus 1.8 microg/min, difference = 8.0 microg/min, 95% confidence interval [CI] = 6.8 to 12.8 microg/min, P = .003; concentration: 15.1 versus 8.0 ng/mg, difference = 7.1 ng/mg, 95% CI = 3.6 to 10.6 ng/mg, P = .028). LTSL administered during hyperthermia also yielded the greatest antitumor effect, with a median time for tumors to reach five times their volume on the day of treatment of 34 days (95% CI = 30 days to infinity) compared with 18.5 days (95% CI = 16 to 23 days) for LTSL before hyperthermia and 22.5 days (95% CI = 15 to 25 days) for LTSL before and during hyperthermia.

CONCLUSIONS

In this rat fibrosarcoma model, LTSLs were most effective when delivered during hyperthermia, which resulted in a peripheral drug distribution.

The use of magnetic resonance imaging (MRI) in the study of manganese neurotoxicity

Manganese (Mn), an element found in many foods, is an important and essential nutrient for proper health and maintenance. It is toxic in high doses, however, and exposure to excessive levels can result in the onset of a neurological disorder similar to, but distinct from, Parkinson's disease. Historically, Mn neurotoxicity was most commonly associated with various occupations, such as Mn mining, welding and steel production. More recently, increases in both blood and brain Mn levels have been observed in persons with liver disease or those receiving prolonged parenteral nutrition. Additionally, rodent data suggest that iron deficiency and anemia may be risk factors for Mn neurotoxicity. Clinically, brain Mn accumulation can be monitored in vivo using non-invasive magnetic resonance imaging (MRI) due to the paramagnetic nature of this element. Indeed, MRI has been used in a variety of settings to evaluate the brain Mn deposition in various populations. This review focuses on the use of MRI technology in studies related specifically to Mn neurotoxicity. Thus, we will examine reports using MRI to confirm brain Mn accumulation in human populations, and conclude with data from non-human primate and rodent models of Mn neurotoxicity.

BRAIN MAGNETIC RESONANCE IMAGING CHARACTERISTICS IN DOGS AND CATS WITH CONGENITAL PORTOSYSTEMIC SHUNTS

Animals with a portosystemic shunt (PSS) often have neurologic abnormalities. Diagnostic imaging, including brain magnetic resonance (MR) imaging, is not performed routinely in these animals. In this study, brain MR images were obtained in 13 dogs and three cats with a PSS, and in 15 dogs and five cats that were neurologically normal and used as controls. All animals with a PSS had widened sulci. In addition, 10 out of 13 dogs with a PSS and one out of three cats with a PSS had hyperintense focal areas in the lentiform nuclei on T1-weighted (T1W) images, which did not enhance after intravenous gadolinium. Following surgical correction of the PSS, MR imaging examinations were repeated in one dog and one cat. The hyperintensity of the lentiform nuclei had decreased. This study indicates that MR imaging findings of widened sulci and hyperintensity of the lentiform nuclei on T1W images may be found in dogs and cats with a PSS.

Magnetic resonance imaging at 9.4 T as a tool for studying neural anatomy in non-vertebrates

This report describes magnetic resonance imaging (MRI) methods we have developed at 9.4 T for observing internal organs and the nervous system of an invertebrate organism, the crayfish, Cherax destructor. We have compared results acquired using two different pulse sequences, and have tested manganese (Mn(2+)) as an agent to enhance contrast of neural tissues in this organism. These techniques serve as a foundation for further development of functional MRI and neural tract-tracing methods in non-vertebrate systems.

Manganese-enhanced magnetic resonance imaging (MEMRI) of brain activity and applications to early detection of brain ischemia

Divalent manganese ion (Mn2+) has been reported to be a useful contrast agent for functional MRI, through a technique named activity-induced manganese-dependent MRI (AIM). In AIM, signal enhancement is related to functional increases in calcium influx, and therefore AIM is, thus far, the only MRI method able to map brain activation in vivo independently of the surrogate hemodynamic changes used in functional MRI. Because of its high signal-to-noise ratio (SNR) and high sensitivity, AIM allows the use of multi-slice or three-dimensional MRI techniques to map functional activity at high spatial resolution. In the present review, we define AIM as a functional MRI tool based on the administration of divalent ionized manganese through an open or disrupted blood-brain barrier (BBB). The adequacy and efficacy of AIM in detecting neural activation is described in light of supporting experiments on inhibition of calcium channels, FOS expression, and on direct comparison to BOLD- and perfusion-based functional MRI. Two main applications of AIM, mapping brain activation in rat somatosensory cortex, as well stroke research based on the well-established middle cerebral artery occlusion model, are described in detail. Methodological problems associated with a strong dependence on anesthetic conditions, potential corruption due to disruption of the BBB, and unspecific increase of the baseline signal due to acoustical noise are discussed. Finally, recommended preparation methods and experimental protocols for AIM are introduced.

In vivo, trans-synaptic tract-tracing utilizing manganese-enhanced magnetic resonance imaging (MEMRI)

It is well established that manganese ion (Mn2+) can access neurons through voltage-gated calcium (Ca2+) channels. Based upon this fundamental principle, Mn2+ has long been used in biomedical research as an indicator of Ca2+ influx in conjunction with fluorescent microscopy. Additionally, after entry into neurons, Mn2+ is transported down axons via microtubule based fast axonal transport. Furthermore, Mn2+ is paramagnetic, resulting in a shortening of the spin-lattice relaxation time-constant, T1, which yields positive contrast enhancement in T1-weighted MRI images, specific to tissues where the ion has accumulated. Manganese-enhanced MRI (MEMRI) utilizes a combination of these properties of Mn2+ to trace neuronal pathways in an MRI-detectable manner. The focus of this review will detail some of the current MEMRI tract-tracing methodologies in mice and non-human primates as well as biological applications of MEMRI tract-tracing.

Manganese-enhanced magnetic resonance imaging (MEMRI): methodological and practical considerations

Manganese-enhanced MRI (MEMRI) is being increasingly used for MRI in animals due to the unique T1 contrast that is sensitive to a number of biological processes. Three specific uses of MEMRI have been demonstrated: to visualize activity in the brain and the heart; to trace neuronal specific connections in the brain; and to enhance the brain cytoarchitecture after a systemic dose. Based on an ever-growing number of applications, MEMRI is proving useful as a new molecular imaging method to visualize functional neural circuits and anatomy as well as function in the brain in vivo. Paramount to the successful application of MEMRI is the ability to deliver Mn2+ to the site of interest at an appropriate dose and in a time-efficient manner. A major drawback to the use of Mn2+ as a contrast agent is its cellular toxicity. Therefore, it is critical to use as low a dose as possible. In the present work the different approaches to MEMRI are reviewed from a practical standpoint. Emphasis is given to the experimental methodology of how to achieve significant, yet safe, amounts of Mn2+ to the target areas of interest.

The year(s) of the contrast agent - micro-MRI in the new millennium

The beginning of the new millennium has been a dynamic time for the field of magnetic resonance imaging (MRI). Exciting recent advances have been made at all levels of imaging, ranging from the visualization of single cells to rodents, birds and the human brain. Many of these techniques employ contrast agents to visualize the movement or activity of cells or organs in vivo; examples of this include the observation of stem cell migration, the tracking of labeled T cells, and the visualization of the events of gastrulation in developing Xenopus embryos. Other advances include improved techniques for elucidating white matter tracts in brain by either monitoring the diffusion of water along the tracts or tracing active neuronal tracts in vivo with Mn(2+) ions. Imaging of the immune system presents two dramatically different challenges: imaging most if not all of the body to follow cell trafficking, and imaging at cellular resolution to follow key intercellular and intracellular events.

Dynamic activity-induced manganese-dependent contrast magnetic resonance imaging (DAIM MRI)

Activity-induced manganese-dependent contrast (AIM) MRI is a hemodynamic-independent functional MRI method that used manganese ion as an MR-detectable contrast agent. In AIM, MnCl(2) is infused intra-arterially after the blood-brain barrier (BBB) is opened with a hyperosmolar agent. Upon functional stimulation of the brain, Mn(2+) accumulates in the active region(s) by entering active cells through voltage-gated Ca(2+) channels, causing local signal increases in T(1)-weighted images. The contrast of AIM MRI depends strongly on the depth of anesthesia, and the low levels used in somatosensory stimulation studies can lead to significant nonspecific accumulation of manganese ion throughout the brain. The purpose of this study was to produce an AIM functional map of somatosensory stimulation, which separates the stimulation-specific signal increase from the nonspecific activation due to light anesthesia. A dynamic AIM (DAIM) paradigm was developed, which used sequential MR scans during MnCl(2) infusion, prior to and following functional stimulation of the brain. Stimulation-specific functional maps were produced using time-course analysis. The new method was tested during glutamate administration and electric stimulation of the rat forepaw. It was shown that DAIM maps are better confined to the specific region of brain activated by somatosensory stimulation as compared to AIM MRI.

Tracing odor-induced activation in the olfactory bulbs of mice using manganese-enhanced magnetic resonance imaging

Ithas previously been demonstrated that it is possible to map active regions of the brain using MRI relying on the fact that Mn(2+) ion enters excitable cells through voltage-gated calcium channels and is an excellent relaxation agent. In addition, Mn(2+) has been shown to trace neuronal connections in the mouse olfactory and visual systems, enabling MRI neuronal tract tracing. The purpose of the present studies was to determine if these two properties could be combined to trace Mn(2+) from sites of activation in the olfactory epithelium to the olfactory bulb thereby localizing regions within the olfactory bulb that respond to a particular odor. Mice were exposed to an aerosolized solution containing either a high pheromone content odor (male mouse urine) or amyl acetate plus MnCl(2). In both cases the odors caused a localized T(1) MRI enhancement in the olfactory epithelium and bulb that was dependent upon the presence of Mn(2+). The high pheromone containing solution caused enhancement in the anatomically correct location of the accessory olfactory bulb. Amyl acetate also caused T(1)-weighted MRI enhancement in specific regions of the olfactory bulb. These areas showing activation agree well with previous 2-deoxyglucose and BOLD fMRI results in the rat. Using manganese-enhanced MRI (MEMRI) it should be possible to rapidly map a variety of odors. Furthermore, since the effects of activation are imaged after the activation protocol it should be possible to take the time to obtain very high resolution images and make MEMRI maps from awake behaving animals.

Noninvasive detection of radiation-induced optic neuropathy by manganese-enhanced MRI

Available imaging techniques have a limited ability to detect radiation-induced injury of the normal brain. In particular, there is no noninvasive method available for detection of structural or functional neuronal damage induced by radiation. This study was designed to determine whether MRI enhanced using the neuronal track tracer MnCl(2) can detect radiation-induced optic neuropathy. A single dose of radiation (35 Gy) was delivered to produce optic neuropathy in Fischer 344 rats by using a stereotactic method with a 6-mm dorsoventral secondary collimator. At 6 months after irradiation, MRI was performed in 1-mm sections using a 7-T magnetic field with the neuronal tracer MnCl(2) injected into the vitreous of the eye 24 h prior to imaging. The rats were then killed humanely for a histological study with hematoxylin and eosin, glial fibrillary acidic protein (Gfap) for the detection of astrocytic activity, Luxol Fast Blue/Periodic Acid Schiff (LFB/PAS) for the detection of myelinization status, and Bielschowski silver stain for axon status. In nonirradiated control animals, T1-weighted MRI with manganese vitreous injection revealed an optic nerve track that was brightly enhanced from the orbit to the optic chiasm. In the irradiated animals, there was clear evidence of the damage at the optic chiasm and optic nerves, with loss of axon and demyelinization within the site of irradiation upon histological examination. T1-weighted MRI with manganese vitreous injection showed an enhancing optic nerve posterior to the orbit. However, this enhancement disappeared at the site of irradiation. The area of loss of manganese contrast on the MRI scan correlated well with the area of histological abnormality showing axonal degeneration and demyelinization. Radiation-induced optic neuropathy was thus detected noninvasively by MRI with the antegrade neuronal tracer manganese, which exhibited negative contrast enhancement by causing loss of signal. This study represents the first demonstration of MR imaging of radiation-induced neuronal damage and could provide a means to explore the biological and functional integrity of neuronal pathways.

In vivo neuronal tract tracing using manganese-enhanced magnetic resonance imaging

Development of efficient imaging techniques to trace neuronal connections would be very useful. Manganese ion (Mn2+) is an excellent T1 contrast agent for magnetic resonance imaging (MRI). Four reports utilizing radioactive Mn2+ in fish and rat brain indicate that Mn2+ may be useful for tracing neuronal connections. Therefore, the purpose of this work was to determine if Mn2+ can be used as an in vivo MRI neuronal tract tracer. The results indicate that topical administration of MnCI2 solution to the naris of mice as well as to the retinal ganglion cells via intravitreal injection leads to enhancement of contrast along the respective pathways. Therefore, application of Mn2+ to neurons allows the use of MRI to visualize neuronal connections.

Manganese ion enhances T1-weighted MRI during brain activation: an approach to direct imaging of brain function

Present techniques for functional MRI rely on detecting changes in hemodynamics that result as a consequence of brain activation. It would be useful if MRI techniques could be developed that enable imaging of a parameter directly related to neuronal activity. Influx of calcium into neurons is necessary for release of neurotransmitters. Divalent manganese ions (Mn2+) can enter cells through voltage-gated calcium channels and Mn2+ is paramagnetic. Mn2+ accumulation in brain due to activation should alter relaxation times offering an approach to sensitize MRI to calcium influx in the brain. To test this idea, T1-weighted MRI was obtained from the rat brain in the presence of a continuous intravenous infusion of 3.6 mumol/min MnCl2. In the anesthetized rat brain, signal enhancement was detected in regions corresponding to ventricles. Activation of the brain with glutamate led to increase in MRI signal intensity in the brain to 238 +/- 23% of the original. This increase in signal was dependent on the presence of MnCl2 and was not due to changes in blood flow. It was necessary to break the blood brain barrier with mannitol to make Mn2+ accessible to the active sites for efficient detection. Enhancement of MRI signal in the brain was also detected with decreasing anesthesia and with somatosensory stimulation. Due to the slow clearance of Mn2+ from the stimulated region of the brain, MRI enhancement could also be detected after stimulation that occurred on awake, behaving rats outside the magnet. These data indicate that MnCl2 shows potential as a MRI contrast agent that is directly sensitive to brain activation.

Invited review: biophysical properties and clinical applications of magnetic resonance imaging contrast agents

Contrast enhanced magnetic resonance imaging (MRI) is a very versatile and effective technique for detecting and characterizing lesions, for identifying a variety of patho-physiological abnormalities, and for providing perfusion and functional information. The application of contrast enhanced MRI to many clinical and research indications has emerged because of the rapid evolution in imaging techniques, improved methodology, and the development of efficient and specific contrast agents. Problems related to optimizing parameters and dosage have been due to complex interplay of relaxation times, biophysical mechanisms and acquisition parameters. A knowledge of basic biophysical aspects is therefore essential for a full understanding of the results obtained for different organs under different conditions, and for optimizing the image parameters and dosage of contrast agents. This article underlines the biophysical basis of the effects of contrast agents in MRI, identifies the problems involved in optimizing the parameters for maximum efficiency, and presents a general overview of the clinical studies and research applications in the central nervous system, perfusion abnormalities, hepatobiliary system, musculoskeletal system and the gastrointestinal tract. The section on perfusion studies includes a discussion of quantitative analysis and kinetic models describing the effects of contrast agents. Finally, a critical evaluation of the scope and limitations of contrast enhanced MRI is presented.

Dose-dependent biodistribution of [153Gd]Gd(acetate)n in mice

[153Gd]Gd(acetate)n was administered i.v. to mice to study the effect of dose on the distribution of free Gd. Distribution from blood was slow with the majority of the Gd distributing in the liver. Gd saturated in bone. Heart, lungs, kidneys, brain and skeletal muscle exhibited time-dependent decreases in Gd concentration. Gd that washed out of heart, lungs, kidneys and/or muscle redistributed in liver, spleen and femur. These results indicate a complex dose- and time-dependent tissue distribution for Gd and emphasize the importance of eliminating unchelated free Gd as a contaminant in Gd-chelates before testing in biodistribution experiments. The long-term residual accumulation of Gd suggests the need to minimize Gd-chelate dissociation in vivo.

Dissociation of gadolinium chelates in mice: relationship to chemical characteristics

Tissue distributions of seven 153Gd-labeled Gd chelates were determined at five residence intervals (5 min to 14 days) following intravenous administration of 0.4 mmol/kg to mice. Relationships were sought among physicochemical parameters: thermodynamic and conditional (pH 7.4) equilibrium stability constants (log K and log K'), acid dissociation rate constants (k(obs)), lipophilicity (log P), overall charge, and size (molecular weight). Size and lipophilicity did not correlate with tissue distributions. There were possible correlations between anionic charge and rapid, early renal excretion and between stability constants and long-term residual Gd deposition. Strong correlations (r greater than 0.99) were found between acid dissociation rates and long-term deposition of Gd in the whole body, liver, and femur. This is attributed to dissociation of Gd from the chelates in vivo. Acid dissociation rates may be useful in predicting dissociation of Gd from chelates in vivo.

Reaction of gadolinium chelates with endogenously available ions

The extent of reaction of 153Gd-radiolabeled Gd(L) chelates with 25 mM CO23- (25 mF), PO34-, Zn2+ and Cu2+ at pH 7 was determined for L = EDTA, DTPA, DOTA, HP-DO3A, and DO3A. Gd(EDTA)- and Gd(DTPA)2- reacted (greater than 20% in 10 min) with Cu2+ and Zn2+ in the presence of PO34-. These double replacement reactions yielded precipitated GdPO4 and chelated Cu(L). Gd(HP-DO3A), Gd(DO3A) and Gd(DOTA)- were inert to reaction with all four ions at room temperature (less than or equal to 1% reaction detected). The thermodynamic binding constants of the ligands for Gd3+ and Cu2+ were found to be equal (10(20) M-1) for DO3A, while DOTA and HP-DO3A favored Gd3+ over Cu2+ by greater than or equal to 10(2) M-1. The low order of reactivity of Gd(DOTA)- and Gd(HP-DO3A) was anticipated by the binding constants, but the lack of reactivity of Gd(DO3A) is attributed to kinetic inertia. This latter property, desirable in MRI contrast agents, is promoted by the conformational stability of the tetraazacyclododecane macrocycle, which forms the backbone of the ligand. It is concluded that this class of chelates is exceptionally inert in solutions of endogenously available ions, and that thermodynamics alone is an insufficient predictor of the reactivity of the highly inert Gd complexes based on the tetraazamacrocycle.

NMR and ESR study of liposome delivery of Mn2+ to murine liver

The mechanism of tissue relaxation of liposome-delivered Mn2+ as a contrast agent for magnetic resonance imaging (MRI) was examined using magnetic resonance and electron spin resonance (ESR) techniques. It is known that liposomes of the size and composition used in this study are taken up by fixed liver macrophages (Kupffer cells). It was determined that Mn2+ must be released from the liposomes in order to affect the water proton relaxation rate in the liver. As long as the Mn2+ was confined to the Kupffer cells, no substantial changes in the relaxation of the majority of the liver water were observed. Unlike other contrast agents delivered to the Kupffer cells (for example, Gd-starch microspheres or magnetite), once the Mn2+ is delivered and released into the Kupffer cells, it can diffuse from the Kupffer cells and be rapidly taken up by the hepatocytes. This seems to be the mechanism for selective relaxation enhancement in the liver. A consequence of this behavior is that the time at which maximum contrast enhancement occurs for MRI can be varied by the choice of liposome phospholipid composition. ESR techniques were used to directly determine the state of Mn2+ and the integrity of liposomes in various stages of processing.

The relationship between thermodynamics and the toxicity of gadolinium complexes

The suitability of gadolinium complexes as magnetic resonance imaging contrast agents depends on a number of factors. A thermodynamic relationship to toxicity exists if one assumes that the chemotoxicity of the intact complex is minimal but that the toxicity of the components of the complex (free metal and uncomplexed ligands) is substantial. Release of Gd3+ from the complex is responsible for the toxicity associated with gadolinium complexes; this release appears to be a consequence of Zn2+, Cu2+, and Ca2+ transmetallation in vivo. This hypothesis is supported by acute toxicity experiments, which demonstrate that despite a 50-fold range of LD50 values for four Gd complexes, all become lethally toxic when they release precisely the same quantity of Gd3+, and by subchronic rodent toxicity experiments, which demonstrate a set of gross and microscopic findings similar to those known to be caused by Zn2+ deficiency. Finally, this hypothesis predicts that subtle changes in formulation can further enhance the intrinsic safety of these complexes.