Discovery of Balovaptan, a Vasopressin 1a Receptor Antagonist for the Treatment of Autism Spectrum Disorder

We recently reported the discovery of a potent, selective, and brain-penetrant V1a receptor antagonist, which was not suitable for full development. Nevertheless, this compound was found to improve surrogates of social behavior in adults with autism spectrum disorder in an exploratory proof-of-mechanism study. Here we describe scaffold hopping that gave rise to triazolobenzodiazepines with improved pharmacokinetic properties. The key to balancing potency and selectivity while minimizing P-gp mediated efflux was fine-tuning of hydrogen bond acceptor basicity. Ascertaining a V1a antagonist specific brain activity pattern by pharmacological magnetic resonance imaging in the rat played a seminal role in guiding optimization efforts, culminating in the discovery of balovaptan (RG7314, RO5285119) 1. In a 12-week clinical phase 2 study in adults with autism spectrum disorder balovaptan demonstrated improvements in Vineland-II Adaptive Behavior Scales, a secondary end point comprising communication, socialization, and daily living skills. Balovaptan entered phase 3 clinical development in August 2018.

Identification of a Corticohabenular Circuit Regulating Socially Directed Behavior

BACKGROUND

The prefrontal cortex (PFC) has been implicated in the pathophysiology of social dysfunction, but the specific circuit partners mediating PFC function in health and disease are unclear.

METHODS

The excitatory designer receptor exclusively activated by designer drugs (DREADD) hM3Dq was used to induce PFC activation during social behavior measured in the three-chamber sociability assay (rats/mice). Functional magnetic resonance imaging was combined with hM3Dq-mediated PFC activation to identify novel nodes in the "social brain" in a hypothesis-free manner. In multiplexed DREADD experiments, hM3Dq and the inhibitory KORDi were used to bidirectionally modulate PFC activity and measure social behavior and global functional magnetic resonance imaging signature. To characterize the functional role of specific nodes identified in this functional magnetic resonance imaging screen, we used anterograde and retrograde tracers, optogenetic and DREADD-assisted circuit mapping, and circuit behavioral experiments.

RESULTS

PFC activation suppressed social behavior and modulated activity in a number of regions involved in emotional behavior. Bidirectional modulation of PFC activity further refined this subset of brain regions and identified the habenula as a node robustly correlated with PFC activity. Furthermore, we showed that the lateral habenula (LHb) receives direct synaptic input from the PFC and that activation of LHb neurons or the PFC inputs to the LHb suppresses social preference. Finally, we demonstrated that LHb inhibition can prevent the social deficits induced by PFC activation.

CONCLUSIONS

The LHb is thought to provide reward-related contextual information to the mesolimbic reward system known to be involved in social behavior. Thus, PFC projections to the LHb may represent an important part of descending PFC pathways that control social behavior.

Temporal profile of brain response to alprazolam in patients with generalized anxiety disorder

This study investigated the temporal pattern of brain response to emotional stimuli during 28 days of alprazolam treatment among patients with generalized anxiety disorder (GAD) randomized 2:1 to drug or placebo in a double-blind design. Functional magnetic resonance imaging scans obtained during an emotion face matching task (EFMT) and an affective stimulus expectancy task (STIMEX) were performed at baseline, one hour after initial drug administration and 28 days later. Alprazolam significantly reduced scores on the Hamilton Anxiety Scale and the Penn State Worry Questionnaire after one week and 28 days of treatment. Brain activation in the amygdala during the EFMT and in the insula during the STIMEX was reduced one hour after alprazolam administration but returned to baseline levels at Day 28. Exploratory analyses revealed significant treatment differences in brain activity during the STIMEX on Day 28 in frontal lobe, caudate nucleus, middle temporal gyrus, secondary visual cortex, and supramarginal gyrus. These results are consistent with the notion that the neural mechanisms supporting sustained treatment effects of benzodiazepines in GAD differ from those underlying their acute effects.

"Domain gauges": A reference system for multivariate profiling of brain fMRI activation patterns induced by psychoactive drugs in rats

Pharmacological magnetic resonance imaging (phMRI) of the brain has become a widely used tool in both preclinical and clinical drug research. One of its challenges is to condense the observed complex drug-induced brain-activation patterns into semantically meaningful metrics that can then serve as a basis for informed decision making. To aid interpretation of spatially distributed activation patterns, we propose here a set of multivariate metrics termed "domain gauges", which have been calibrated based on different classes of marketed or validated reference drugs. Each class represents a particular "domain" of interest, i.e., a specific therapeutic indication or mode of action. The drug class is empirically characterized by the unique activation pattern it evokes in the brain-the "domain profile". A domain gauge provides, for any tested intervention, a "classifier" as a measure of response strength with respect to the domain in question, and a "differentiator" as a measure of deviation from the domain profile, both along with error ranges. Capitalizing on our in-house database with an unprecedented wealth of standardized perfusion-based phMRI data obtained from rats subjected to various validated treatments, we exemplarily focused on 3 domains based on therapeutic indications: an antipsychotic, an antidepressant and an anxiolytic domain. The domain profiles identified as part of the gauge definition process, as well as the outputs of the gauges when applied to both reference and validation data, were evaluated for their reconcilability with prior biological knowledge and for their performance in drug characterization. The domain profiles provided quantitative activation patterns with high biological plausibility. The antipsychotic profile, for instance, comprised key areas (e.g., cingulate cortex, nucleus accumbens, ventral tegmental area, substantia nigra) which are believed to be strongly involved in mediating an antipsychotic effect, and which are in line with network-level dysfunctions observed in schizophrenia. The domain gauges plausibly positioned the vast majority of the pharmacological and even non-pharmacological treatments. The results also suggest the segregation of sub-domains based on, e.g., the mode of action. Upon judicious selection of domains and careful calibration of the gauges, our approach represents a valuable analytical tool for biological interpretation and decision making in drug discovery.

"Domain gauges": A reference system for multivariate profiling of brain fMRI activation patterns induced by psychoactive drugs in rats

Pharmacological magnetic resonance imaging (phMRI) of the brain has become a widely used tool in both preclinical and clinical drug research. One of its challenges is to condense the observed complex drug-induced brain-activation patterns into semantically meaningful metrics that can then serve as a basis for informed decision making. To aid interpretation of spatially distributed activation patterns, we propose here a set of multivariate metrics termed "domain gauges", which have been calibrated based on different classes of marketed or validated reference drugs. Each class represents a particular "domain" of interest, i.e., a specific therapeutic indication or mode of action. The drug class is empirically characterized by the unique activation pattern it evokes in the brain-the "domain profile". A domain gauge provides, for any tested intervention, a "classifier" as a measure of response strength with respect to the domain in question, and a "differentiator" as a measure of deviation from the domain profile, both along with error ranges. Capitalizing on our in-house database with an unprecedented wealth of standardized perfusion-based phMRI data obtained from rats subjected to various validated treatments, we exemplarily focused on 3 domains based on therapeutic indications: an antipsychotic, an antidepressant and an anxiolytic domain. The domain profiles identified as part of the gauge definition process, as well as the outputs of the gauges when applied to both reference and validation data, were evaluated for their reconcilability with prior biological knowledge and for their performance in drug characterization. The domain profiles provided quantitative activation patterns with high biological plausibility. The antipsychotic profile, for instance, comprised key areas (e.g., cingulate cortex, nucleus accumbens, ventral tegmental area, substantia nigra) which are believed to be strongly involved in mediating an antipsychotic effect, and which are in line with network-level dysfunctions observed in schizophrenia. The domain gauges plausibly positioned the vast majority of the pharmacological and even non-pharmacological treatments. The results also suggest the segregation of sub-domains based on, e.g., the mode of action. Upon judicious selection of domains and careful calibration of the gauges, our approach represents a valuable analytical tool for biological interpretation and decision making in drug discovery.

Preserved modular network organization in the sedated rat brain

Translation of resting-state functional connectivity (FC) magnetic resonance imaging (rs-fMRI) applications from human to rodents has experienced growing interest, and bears a great potential in pre-clinical imaging as it enables assessing non-invasively the topological organization of complex FC networks (FCNs) in rodent models under normal and various pathophysiological conditions. However, to date, little is known about the organizational architecture of FCNs in rodents in a mentally healthy state, although an understanding of the same is of paramount importance before investigating networks under compromised states. In this study, we characterized the properties of resting-state FCN in an extensive number of Sprague-Dawley rats (n = 40) under medetomidine sedation by evaluating its modular organization and centrality of brain regions and tested for reproducibility. Fully-connected large-scale complex networks of positively and negatively weighted connections were constructed based on Pearson partial correlation analysis between the time courses of 36 brain regions encompassing almost the entire brain. Applying recently proposed complex network analysis measures, we show that the rat FCN exhibits a modular architecture, comprising six modules with a high between subject reproducibility. In addition, we identified network hubs with strong connections to diverse brain regions. Overall our results obtained under a straight medetomidine protocol show for the first time that the community structure of the rat brain is preserved under pharmacologically induced sedation with a network modularity contrasting from the one reported for deep anesthesia but closely resembles the organization described for the rat in conscious state.

Neuropharmacological and neurobiological relevance of in vivo ¹H-MRS of GABA and glutamate for preclinical drug discovery in mental disorders

Proton magnetic resonance spectroscopy ((1)H-magnetic resonance spectroscopy (MRS)) is a translational modality with great appeal for neuroscience since the two major excitatory and inhibitory neurotransmitters, glutamate, and GABA, can be noninvasively quantified in vivo and have served to explore disease state and effects of drug treatment. Yet, if (1)H-MRS shall serve for decision making in preclinical pharmaceutical drug discovery, it has to meet stringent requirements. In particular, (1)H-MRS needs to reliably report neurobiologically relevant but rather small changes in neurometabolite levels upon pharmacological interventions and to faithfully appraise target engagement in the associated molecular pathways at pharmacologically relevant doses. Here, we thoroughly addressed these matters with a three-pronged approach. Firstly, we determined the sensitivity and reproducibility of (1)H-MRS in rat at 9.4 Tesla for detecting changes in GABA and glutamate levels in the striatum and the prefrontal cortex, respectively. Secondly, we evaluated the neuropharmacological and neurobiological relevance of the MRS readouts by pharmacological interventions with five well-characterized drugs (vigabatrin, 3-mercaptopropionate, tiagabine, methionine sulfoximine, and riluzole), which target key nodes in GABAergic and glutamatergic neurotransmission. Finally, we corroborated the MRS findings with ex vivo biochemical analyses of drug exposure and neurometabolite concentrations. For all five interventions tested, (1)H-MRS provided distinct drug dose-effect relationships in GABA and glutamate over preclinically relevant dose ranges and changes as low as 6% in glutamate and 12% in GABA were reliably detected from 16 mm(3) volumes-of-interest. Taken together, these findings demonstrate the value and limitation of quantitative (1)H-MRS of glutamate and GABA for preclinical pharmaceutical research in mental disorders.

Chronic metabotropic glutamate receptor 5 inhibition corrects local alterations of brain activity and improves cognitive performance in fragile X mice

BACKGROUND

Fragile X syndrome (FXS) is the most common genetic cause for intellectual disability. Fmr1 knockout (KO) mice are an established model of FXS. Chronic pharmacological inhibition of metabotropic glutamate receptor 5 (mGlu5) in these mice corrects multiple molecular, physiological, and behavioral phenotypes related to patients' symptoms. To better understand the pathophysiology of FXS and the effect of treatment, brain activity was analyzed using functional magnetic resonance imaging in relation to learning and memory performance.

METHODS

Wild-type (WT) and Fmr1 KO animals receiving chronic treatment with the mGlu5 inhibitor CTEP or vehicle were evaluated consecutively for 1) learning and memory performance in the inhibitory avoidance and extinction test, and 2) for the levels of brain activity using continuous arterial spin labeling based functional magnetic resonance imaging. Neural activity patterns were correlated with cognitive performance using a multivariate regression analysis. Furthermore, mGlu5 receptor expression in brains of untreated mice was analyzed by autoradiography and saturation analysis using [(3)H]-ABP688.

RESULTS

Chronic CTEP treatment corrected the learning deficit observed in Fmr1 KO mice in the inhibitory avoidance and extinction test and prevented memory extinction in WT and Fmr1 KO animals. Chronic CTEP treatment normalized perfusion in the amygdala and the lateral hypothalamus in Fmr1 KO mice and furthermore decreased perfusion in the hippocampus and increased perfusion in primary sensorimotor cortical areas. No significant differences in mGlu5 receptor expression levels between Fmr1 WT and KO mice were detected.

CONCLUSIONS

Chronic mGlu5 inhibition corrected the learning deficits and partially normalized the altered brain activity pattern in Fmr1 KO mice.

Quantitative Body Composition Analysis in Awake Mice and Rats by Magnetic Resonance Relaxometry

OBJECTIVE

Magnetic resonance (MR) relaxometry has recently been introduced for noninvasive body composition analysis in awake mice. The purpose of the present study was to extend the method to rats and to introduce calibration procedures that render MR relaxometry fully quantitative.

RESEARCH METHODS AND PROCEDURES

Proton T(2) MR relaxometry at 4.7 Tesla was used for body composition analyses in 700 awake mice and 400 rats of different strains and conditions. Relaxograms calculated from the signal decays observed with multi-spin-echo acquisition provided well-separated contributions of tissue water and fat. Analysis of fat composition was carried out in vivo using (13)C-MR spectroscopy. Evolution of body composition in rats was assessed during drug treatment.

RESULTS

MR relaxometry for noninvasive body composition analysis in laboratory rodents was implemented on a standard MR scanner, and a throughput of >30 animals per hour was achieved. Excellent linearity and reproducibility with coefficients of variance as low as 2.5% and 1.7% were obtained in mice and rats, respectively. The lean mass-to-water ratio (mice, 1.35 +/- 0.03; rats, 1.39 +/- 0.04) and the proton density of fat (mice, 8.1 +/- 0.2; rats, 8.9 +/- 0.2 g/mol) were determined from cross-sectional data. Fat composition analysis by (13)C-MR spectroscopy corroborated these findings and yielded information on the average acyl chain length (16.3 +/- 1.6) and contributions of saturated (27 +/- 3%), monounsaturated (22 +/- 2%), and polyunsaturated (51 +/- 3%) fatty acids. Longitudinal assessments in rats treated with sibutramine and dexfenfluramine showed dose-related changes in body composition.

DISCUSSION

T(2) MR relaxometry backed by solid calibration provides a powerful means for rapid quantitative body composition analysis in awake mice and rats that is suitable for serial investigations in pharmaceutical research.

Imaging trait anxiety in high anxiety F344 rats: Focus on the dorsomedial prefrontal cortex

Functional magnetic resonance imaging (fMRI) has become an important method in clinical psychiatry research whereas there are still only few comparable preclinical investigations. Herein, we report that fMRI in rats can provide key information regarding brain areas underlying anxiety behavior. Perfusion as surrogate for neuronal activity was measured by means of arterial spin labeling-based fMRI in various brain areas of high anxiety F344 rats and control Sprague-Dawley rats. In one of these areas, the dorsomedial prefrontal cortex (dmPFC), c-Fos labeling was compared between these two strains with immunolabeling. The effects of a neurotoxic ibotenic acid lesion of the dmPFC in F344 rats were examined in a social approach-avoidance anxiety procedure and fMRI. Regional brain activity of high anxiety F344 rats was different in selective cortical and subcortical areas as compared to that of low anxiety Sprague-Dawley rats; the largest difference (i.e. hyperactivity) was measured in the dmPFC. Independently, c-Fos labeling confirmed that F344 rats show increased dmPFC activity. The functional role was confirmed by neurotoxic lesion of the dmPFC that reversed the high anxiety-like behavior and partially normalized the brain activity pattern of F344 rats. The current findings may have translational value as increased activity is reported in an equivalent cortical area in patients with social anxiety, suggesting that pharmacological or functional inhibition of activity in this brain area should be explored to alleviate social anxiety in patients.

fMRI fingerprint of unconditioned fear-like behavior in rats exposed to trimethylthiazoline

Unconditioned fear plays an important yet poorly understood role in anxiety disorders, and only few neuroimaging studies have focused on evaluating the underlying neuronal mechanisms. In rodents the predator odor trimethylthiazoline (TMT), a synthetic component of fox feces, is commonly used to induce states of unconditioned fear. In this study, arterial spin labeling-based functional magnetic resonance imaging (fMRI) was applied to detect TMT-induced regional modulations of neuronal activity in Wistar rats. During TMT exposure the rats displayed increased freezing behavior and reduced exploration in the odor-associated area. Neuronal activity was selectively increased in the dorsal periaqueductal gray, superior colliculus and medial thalamus and reduced in the median raphe, locus coeruleus, nucleus accumbens shell, ventral tegmental area, ventral pallidum and entorhinal piriform cortex. This fMRI fingerprint involving distinct neuronal pathways was used to describe a schematic model of fear processing. Key brain areas known to underlie fear and anxiety-related autonomic and behavioral responses as well as centers of motivational processing were identified as being part of this functional circuitry of innate fear. Thus, preclinical fMRI studies based on unconditioned fear methods may provide a valuable translational approach to better characterize etiological and pathological processes underlying anxiety disorders.

Phosphorylation of Tau at S422 is enhanced by Abeta in TauPS2APP triple transgenic mice

Amyloid beta peptides and microtubule-associated protein Tau are misfolded and form aggregates in brains of Alzheimer's disease patients. To examine their specific roles in the pathogenesis of Alzheimer's disease and their relevance in neurodegenerative processes, we have created TauPS2APP triple transgenic mice that express human mutated Amyloid Precursor Protein, presenilin 2 and Tau. We present a cross-sectional analysis of these mice at 4, 8, 12 and 16 months of age. By comparing with single transgenic Tau mice, we demonstrate that accumulation of Abeta in TauPS2APP triple transgenic mice impacts on Tau pathology by increasing the phosphorylation of Tau at serine 422, as determined by a novel immunodetection method that is able to reliably measure phospho-Tau species in transgenic mouse brains. The TauPS2APP triple transgenic mouse model will be very useful for studying the effect of new therapeutic paradigms on amyloid deposition and downstream neurofibrillary tangle development.

Cortical hypoperfusion in the B6.PS2APP mouse model for Alzheimer's disease: comprehensive phenotyping of vascular and tissular parameters by MRI

Function and morphology of the cerebral vasculature were studied in the amyloid (Abeta) plaque-containing double-transgenic (TG) B6.PS2APP Alzheimer's disease (AD) mouse model with MRI at an age range of 10 to 17 months. Perfusion, blood volume, and average vessel geometry were assessed in the brain and compared to age-matched controls (wild-type [WT] C57Bl/6). Additionally, the MR relaxation times T(1), T(2), and T(2)* were measured to detect potential pathological changes that might be associated with Abeta plaque depositions. Both decreased perfusion and decreased blood volume were observed in the occipital cortex in B6.PS2APP mice as compared to controls. A significant decrease in T(1) and T(2) was found in the frontal cortex and in the subiculum/parasubiculum. Immunohistochemistry confirmed plaque depositions in the cortex and in the subiculum/parasubiculum. In summary, our data indicate a reduced blood supply of B6.PS2APP mice in the occipital cortex that parallels the findings in cortical regions of patients with AD.

Non-invasive analysis of gallbladder bile composition in cynomolgus monkeys using in vivo 1H magnetic resonance spectroscopy

The purpose of the present study was (i) to establish a modality for non-invasively probing bile composition in cynomolgus monkeys and (ii) to ascertain the variability in biliary metabolism by repeatedly assessing gallbladder bile in situ. Localised in vivo (1)H magnetic resonance spectroscopy (MRS) provided high-resolution spectra of gallbladder bile that allowed for the first time different species of bile acids, their taurine and glycine conjugates, and phospholipids to be identified and quantified in situ. A combined cross-sectional and longitudinal study of bile composition was conducted over 4 weeks in monkeys kept under standardised nutritional conditions. All biles were composed of the same major constituents. Bile acids contributed 267+/-47 micromol/ml whereof cholate, deoxycholate and chenodeoxycholate were the most abundant primary bile acids. Bile acid conjugation reached an extent of 100%. However, the actual quantitative contributions of different bile constituents varied distinctly. Correlation analysis revealed that intra-individual variability (r=0.77+/-0.03) was significantly (p<0.01) smaller than inter-individual variability (r=0.68+/-0.01), thus purporting the notion that bile composition is a hallmark of individual metabolism. Extension of quantitative bile analysis by in vivo (1)H-MRS to pathological states will provide a rapid and non-invasive modality for monitoring an important, yet elusive compartment of cholesterol and lipid metabolism.

Age and gender dependence of human cardiac phosphorus metabolites determined by SLOOP 31P MR spectroscopy

The aim of this study was to apply (31)P magnetic resonance spectroscopy (MRS) using spatial localization with optimal point spread function (SLOOP) to investigate possible age and gender dependencies of the energy metabolite concentrations in the human heart. Thirty healthy volunteers (18 males and 12 females, 21-67 years old, mean = 40.7 years) were examined with the use of (31)P-MRS on a 1.5 T scanner. Intra- and interobserver variability measures (determined in eight of the volunteers) were both 3.8% for phosphocreatine (PCr), and 4.7% and 8.3%, respectively, for adenosine triphosphate (ATP). High-energy phosphate (HEP) concentrations in mmol/kg wet weight were 9.7 +/- 2.4 (age < 40 years, N = 16) and 7.7 +/- 2.5 (age >or= 40 years, N = 14) for PCr, and 5.1 +/- 1.0 (age < 40 years) and 4.1 +/- 0.8 (age >or= 40 years) for ATP, respectively. Separated by gender, PCr concentrations of 9.2 +/- 2.4 (men, N = 18) and 8.0 +/- 2.8 (women, N = 12) and ATP concentrations of 4.9 +/- 1.0 (men) and 4.2 +/- 0.9 (women) were measured. A significant decrease of PCr and ATP was found for volunteers older than 40 years (P < 0.05), but the differences in metabolic concentrations between both sexes were not significant. In conclusion, age has a minor but still significant impact on cardiac energy metabolism, and no significant gender differences were detected.

Fast perfusion measurements in rat skeletal muscle at rest and during exercise with single-voxel FAIR (flow-sensitive alternating inversion recovery)

Non-invasive measurement of perfusion in skeletal muscle by in vivo magnetic resonance remains a challenge due to its low level and the correspondingly low signal-to-noise ratio. To enable accurate, quantitative, and time-resolved perfusion measurements in the leg muscle, a technique with a high sensitivity is required. By combining a flow-sensitive alternating inversion recovery (FAIR)-sequence with a single-voxel readout, we have developed a new technique to measure the perfusion in the rat gastrocnemius muscle at rest, yielding an average value of 19.4 +/- 4.8 mL/100 g/min (n = 22). In additional experiments, perfusion changes were elicited by acute ischemia and reperfusion or by exercise induced by electrical, noninvasive muscle stimulation with varying duration and intensity. The perfusion time courses during these manipulations were measured with a temporal resolution of 2.2 min, showing increases in perfusion of a factor of up to 2.5. In a direct comparison, the results agreed closely with values found with microsphere measurements in the same animals. The quantitative and noninvasive method can significantly facilitate the investigation of atherosclerotic diseases and the examination of drug efficacy.

Functional magnetic resonance imaging reveals similar brain activity changes in two different animal models of schizophrenia

RATIONALE AND OBJECTIVES

In schizophrenia research, most of the functional imaging studies have been performed in psychotic patients, but little is known about brain areas involved in the expression of psychotic-like symptoms in animal models. The objective of this study was to visualize and compare brain activity abnormalities in a neurodevelopmental and a pharmacological animal model of schizophrenia.

METHODS

Blood perfusion of specific brain areas, taken as indirect measure of brain activity, was investigated in adult rats following either neonatal ventral hippocampal lesion or acute administration of phencyclidine. Quantitative perfusion magnetic resonance imaging was performed on five frontal brain slices using the continuous arterial spin labeling technique. The mean perfusion was calculated in several brain structures, which were identified on anatomical images.

RESULTS

Lesioned animals exhibiting deficits in prepulse inhibition of the startle reflex showed a significant blood perfusion increase in the nucleus accumbens, basolateral amygdala, ventral pallidum, entorhinal-piriform cortex, orbital prefrontal cortex, and in the bed nucleus of the stria terminalis, and a decrease of perfusion in the temporal cortex. Similar effects were seen following acute phencyclidine administration in naïve animals.

CONCLUSION

Our data point out specific cortical and subcortical brain areas involved in the development of psychotic-like symptoms in two different animal models of schizophrenia. The observed brain activity abnormalities are reminiscent of classical neuroimaging findings described in schizophrenic patients.

Altered metabolic profile in the frontal cortex of PS2APP transgenic mice, monitored throughout their life span

The transgenic mouse line PS2APP (PS2N141I x APP(swe)) develops an age-related cognitive decline associated with severe amyloidosis, mimicking the pathophysiologic processes in Alzheimer disease (AD). In the quest for biomarkers to monitor, noninvasively, the progression of the disease, we used magnetic resonance imaging and 1H-spectroscopy to characterize PS2APP mice throughout their life span. Morphometric measurements revealed only small size differences to controls. The metabolic profile, however, showed clear indicators of hypometabolism with age in the PS2APP mice: both N-acetyl-aspartate and glutamate were significantly reduced in the older animals. These spectroscopic measures in vivo correlated well with the plaque load in the frontal cortex. A diagnostic test, based on these measures, reached 92% sensitivity and 82% specificity at age 20 months. These noninvasive biomarkers can be exploited in preclinical pharmaceutical research to cope with the high variability in transgenic animal models and to enhance the power of drug efficacy studies.

Improved cardiac sodium MR imaging by density-weighted phase-encoding

PURPOSE

To show that density-weighted (DW) k-space sampling improves the quality of human cardiac sodium imaging, a novel method was implemented that combines the high signal-to-noise efficiency of three-dimensional phase-encoding with the advantageous localization performance of nonuniform sampling. A simulation demonstrates substantially reduced blood contamination in the myocardium.

MATERIALS AND METHODS

At 2.0 T, DW cardiac "fast" sodium images with a voxel size of 844 microL in seven minutes and "high-resolution" scans in 30 minutes with a voxel size of 570 microL were acquired. For comparison, conventional gradient-echo imaging was also performed.

RESULTS

In the DW images, a myocardial signal-to-noise ratio (SNR) of 16.0 in the left ventricle and 8.5 in the septum (N = 4) was measured. With longer experimental duration (about 30 minutes; N = 3), the image quality and the SNR could be further improved (voxel size: 570 microL; SNR: blood 16.1, septum 10.6). Compared to the gradient-echo images, the image quality was substantially improved.

CONCLUSION

This new method for human cardiac sodium imaging provides high image quality combined with optimal sensitivity and thus may improve the clinical applicability of 23Na cardiac MRI.

Efficientk-space sampling by density-weighted phase-encoding

Acquisition-weighting improves the localization of MRI experiments. An approach to acquisition-weighting in a purely phase-encoded experiment is presented that is based on a variation of the sampling density in k-space. In contrast to conventional imaging or to accumulation-weighting, where k-space is sampled with uniform increments, density-weighting varies the distance between neighboring sampling points Deltak to approximate a given radial weighting function. A fast, noniterative algorithm has been developed to calculate the sampling matrix in one, two, and three dimensions from a radial weighting function w(k), the desired number of scans NA(tot) and the nominal spatial resolution Deltax(nom). Density-weighted phase-encoding combines the improved shape of the spatial response function and the high SNR of acquisition-weighting with an extended field of view. The artifact energy that results from aliasing due to a small field of view is substantially reduced. The properties of density-weighting are compared to uniform and to accumulation-weighted phase-encoding in simulations and experiments. Density-weighted (31)P 3D chemical shift imaging of the human heart is shown which demonstrates the superior performance of density-weighted metabolic imaging.

Absolute concentrations of high-energy phosphate metabolites in normal, hypertrophied, and failing human myocardium measured noninvasively with (31)P-SLOOP magnetic resonance spectroscopy

OBJECTIVE

The purpose of the present study was to measure absolute concentrations of phosphocreatine (PCr) and adenosine triphosphate (ATP) in normal, hypertrophied, and failing human heart.

BACKGROUND

Conflicting evidence exists on the extent of changes of high-energy phosphate metabolites in hypertrophied and failing human heart. Previous reports using phosphorus-31 magnetic resonance spectroscopy ((31)P-MRS) have quantified metabolites in relative terms only. However, this analysis cannot detect simultaneous reductions.

METHODS

Four groups of subjects (n = 10 each), were studied: volunteers and patients with hypertensive heart disease (HHD), aortic stenosis, and dilated cardiomyopathy (DCM). Left ventricular (LV) function and mass were measured by cine magnetic resonance imaging. Absolute and relative concentrations of PCr and ATP were determined by (31)P-MRS with spatial localization with optimum point spread function.

RESULTS

Left ventricular ejection fraction remained normal in HHD and aortic stenosis, but was severely reduced to 18% in DCM; LV mass was increased by 55%, 79%, and 68% respectively. In volunteers, PCr and ATP concentrations were 8.82 +/- 1.30 mmol/kg wet weight and 5.69 +/- 1.02 mmol/kg wet weight, and the PCr/ATP ratio was 1.59 +/- 0.33. High-energy phosphate levels were unaltered in HHD. In aortic stenosis, PCr was decreased by 28%, whereas ATP remained constant. In DCM, PCr was reduced by 51%, ATP by 35%, and reduction of the PCr/ATP ratio by 25% was of borderline significance (p = 0.06). Significant correlations were observed among energetic and functional variables, with the closest relations for PCr.

CONCLUSIONS

In human heart failure due to DCM, both PCr and ATP are significantly reduced. Ratios of PCr to ATP underestimate changes of high-energy phosphate levels.

Imaging of intracellular sodium with shift reagent aided (23)Na CSI in isolated rat hearts

23Na chemical shift imaging (CSI) in conjunction with shift reagents was used to obtain images of intracellular (Na(i)) and extracellular sodium (Na(e)) in isolated rat hearts. It was demonstrated that the increase of Na(i) concentration in ischemic myocardium can be detected with this technique. 3D acquisition-weighted (23)Na CSI datasets with a nominal spatial resolution of 1.7 x 1.7 x 2.9 mm were acquired in 30 min in normoxic hearts and in globally or locally ischemic hearts. The shift reagent Tm(DOTP)(5-) was used to discriminate Na(i) and Na(e) signals. Na(i) maps could be generated in ischemic hearts, but not in normoxic hearts as the signal-to-noise ratio is too low. The Na(i) signal increased by more than 100% and the Na(e) signal decreased by more than 50% in myocardium of globally ischemic hearts (n = 3) compared to normoxic hearts (n = 3). In hearts with an acute occlusion of the left anterior descending coronary artery (n = 3), there was a local Na(i) signal increase in the anterior wall in the range of 60-110% compared to remote, normoxic tissue.