High- and low-affinity transport of D-glucose from blood to brain

L-Arginine and asymmetric dimethylarginine (ADMA) transport across the mouse blood-brain and blood-CSF barriers: Evidence of saturable transport at both interfaces and CNS to blood efflux

L-Arginine is the physiological substrate for the nitric oxide synthase (NOS) family, which synthesises nitric oxide (NO) in endothelial and neuronal cells. NO synthesis can be inhibited by endogenous asymmetric dimethylarginine (ADMA). NO has explicit roles in cellular signalling and vasodilation. Impaired NO bioavailability represents the central feature of endothelial dysfunction associated with vascular diseases. Interestingly, dietary supplementation with L-arginine has been shown to alleviate endothelial dysfunctions caused by impaired NO synthesis. In this study the transport kinetics of [3H]-arginine and [3H]-ADMA into the central nervous system (CNS) were investigated using physicochemical assessment and the in situ brain/choroid plexus perfusion technique in anesthetized mice. Results indicated that L-arginine and ADMA are tripolar cationic amino acids and have a gross charge at pH 7.4 of 0.981. L-Arginine (0.00149±0.00016) has a lower lipophilicity than ADMA (0.00226±0.00006) as measured using octanol-saline partition coefficients. The in situ perfusion studies revealed that [3H]-arginine and [3H]-ADMA can cross the blood-brain barrier (BBB) and the blood-CSF barrier. [3H]-Arginine (11.6nM) and [3H]-ADMA (62.5nM) having unidirectional transfer constants (Kin) into the frontal cortex of 5.84±0.86 and 2.49±0.35 μl.min-1.g-1, respectively, and into the CSF of 1.08±0.24 and 2.70±0.90 μl.min-1.g-1, respectively. In addition, multiple-time uptake studies revealed the presence of CNS-to-blood efflux of ADMA. Self- and cross-inhibition studies indicated the presence of transporters at the BBB and the blood-CSF barriers for both amino acids, which were shared to some degree. Importantly, these results are the first to demonstrate: (i) saturable transport of [3H]-ADMA at the blood-CSF barrier (choroid plexus) and (ii) a significant CNS to blood efflux of [3H]-ADMA. Our results suggest that the arginine paradox, in other words the clinical observation that NO-deficient patients respond well to oral supplementation with L-arginine even though the plasma concentration is sufficient to saturate endothelial NOS, could be related to altered ADMA transport (efflux).

BrainPepPass: A Framework Based on Supervised Dimensionality Reduction for Predicting Blood-Brain Barrier-Penetrating Peptides

Peptides that pass through the blood-brain barrier (BBB) not only are implicated in brain-related pathologies but also are promising therapeutic tools for treating brain diseases, e.g., as shuttles carrying active medicines across the BBB. Computational prediction of BBB-penetrating peptides (B3PPs) has emerged as an interesting approach because of its ability to screen large peptide libraries in a cost-effective manner. In this study, we present BrainPepPass, a machine learning (ML) framework that utilizes supervised manifold dimensionality reduction and extreme gradient boosting (XGB) algorithms to predict natural and chemically modified B3PPs. The results indicate that the proposed tool outperforms other classifiers, with average accuracies exceeding 94% and 98% in 10-fold cross-validation and leave-one-out cross-validation (LOOCV), respectively. In addition, accuracy values ranging from 45% to 97.05% were achieved in the independent tests. The BrainPepPass tool is available in a public repository for academic use (https://github.com/ewerton-cristhian/BrainPepPass).

Comparison of parametric imaging and SUV imaging with [68 Ga]Ga-PSMA-11 using dynamic total-body PET/CT in prostate cancer

PURPOSE

Standardized uptake value (SUV) has been prevalently used to measure [68 Ga]Ga-PSMA-11 activity in prostate cancer, but it is susceptible to multiple factors. Parametric imaging allows for absolute quantification of tracer uptake and provides a better diagnostic accuracy that is crucial for lesion detection. However, the clinical significance of total-body parametric imaging of [68 Ga]Ga-PSMA-11 remains to be fully assessed. Therefore, the aim of our study is to delve into the diagnostic implications of total-body parametric imaging of [68 Ga]Ga-PSMA-11 PET/CT for patients with prostate cancer.

METHODS

Twenty prostate cancer patients were included and underwent a dynamic total-body [68 Ga]Ga-PSMA-11 PET/CT scan. An irreversible two-tissue compartment model (2T3k) was fitted for each tissue time-to-activity curve, and the net influx rate (Ki) was obtained. The image quality and semi-quantitative analysis of lesion-to-background ratio (LBR), signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) were compared between parametric images and SUV images.

RESULTS

Kinetic modeling using 2T3k demonstrated favorable model fitting in both normal organs and lesions. All of the lesions detected on SUV images (55-60 min) could be detected on Ki images. The correlation between Ki, SUVmean, and SUVmax in both normal organs and pathological lesions was found to be positive and statistically significant. Conversely, a moderate positive correlations were found between Ki and K1 (R = 0.69, P < 0.001; R = 0.61, P < 0.001) and Ki and k3 (R = 0.69, P < 0.001; R = 0.62, P < 0.001), in normal organs and pathological lesions, respectively. Visual assessment in Ki images showed less image noise and higher lesions conspicuity compared to SUV images. Ki image-derived LBR, SNR, and CBR of pathological lesions including primary tumors (PTs), lymph node metastases (LNMs) and bone metastases (BMs), exhibited remarkably higher folds (1.4-3.6 folds) compared to those derived from SUV of corresponding lesions.

CONCLUSIONS

Total-body parametric imaging of [68 Ga]Ga-PSMA-11 enhanced lesion contrast and improved lesion detectability compared to SUV images. This may potentially serve as an imaging biomarker and theranostic tool for precise diagnosis and treatment evaluation in prostate cancer patients.

Four decades of mapping and quantifying neuroreceptors at work in vivo by positron emission tomography

Decryption of brain images is the basis for the necessary translation of the findings from imaging to information required to meet the demands of clinical intervention. Tools of brain imaging, therefore, must satisfy the conditions dictated by the needs for interpretation in terms of diagnosis and prognosis. In addition, the applications must serve as fundamental research tools that enable the understanding of new therapeutic drugs, including compounds as diverse as antipsychotics, antidepressants, anxiolytics, and drugs serving the relief of symptoms from neurochemical disorders as unrelated as multiple sclerosis, stroke, and dementia. Here we review and explain the kinetics of methods that enable researchers to describe the brain’s work and functions. We focus on methods invented by neurokineticists and expanded upon by practitioners during decades of experimental work and on the methods that are particularly useful to predict possible future approaches to the treatment of neurochemical disorders. We provide an overall description of the basic elements of kinetics and the underlying quantification methods, as well as the mathematics of modeling the recorded brain dynamics embedded in the images we obtain in vivo. The complex presentation to follow is necessary to justify the contribution of modeling to the development of methods and to support the specifications dictated by the proposed use in clinical settings. The quantification and kinetic modeling processes are equally essential to image reconstruction and labeling of brain regions of structural or functional interest. The procedures presented here are essential tools of scientific approaches to all conventional and novel forms of brain imaging. The foundations of the kinetic and quantitative methods are keys to the satisfaction of clinicians that actively engage in treating the neurochemical disorders of mammalian brains in the fields of neurology, neurosurgery, and neuropsychiatry.

Shedding Light on the Blood-Brain Barrier Transport with Two-Photon Microscopy In Vivo

Treatment of brain disorders relies on efficient delivery of therapeutics to the brain, which is hindered by the blood-brain barrier (BBB). The work of Prof. Margareta Hammarlund-Udenaes was instrumental in understanding the principles of drug delivery to the brain and developing new tools to study it. Here, we show how some of the concepts developed in her research can be translated to in vivo 2-photon microscopy (2PM) studies of the BBB. We primarily focus on the methods developed in our laboratory to characterize the paracellular diffusion, adsorptive-mediated transcytosis, and receptor-mediated transcytosis of drug nanocarriers at the microscale, illustrating how 2PM can deepen our understanding of the mechanisms of drug delivery to the brain.

Dopaminergic Activity in Antipsychotic-Naïve Patients Assessed With Positron Emission Tomography Before and After Partial Dopamine D2 Receptor Agonist Treatment: Association With Psychotic Symptoms and Treatment Response

BACKGROUND

Dopamine activity has been associated with the response to antipsychotic treatment. Our study used a four-parameter model to test the association between the striatal decarboxylation rate of ¹⁸F-DOPA to ¹⁸F-dopamine (k₃) and the effect of treatment on psychotic symptoms in antipsychotic-naïve patients with first-episode psychosis. We further explored the effect of treatment with a partial dopamine D₂ receptor agonist (aripiprazole) on k₃ and dopamine synthesis capacity (DSC) determined by the four-parameter model and by the conventional tissue reference method.

METHODS

Sixty-two individuals (31 patients and 31 control subjects) underwent ¹⁸F-DOPA positron emission tomography at baseline, and 15 patients were re-examined after 6 weeks. Clinical re-examinations were completed after 6 weeks (n = 28) and 6 months (n = 15). Symptoms were evaluated with the Positive and Negative Syndrome Scale.

RESULTS

High baseline decarboxylation rates (k₃) were associated with more positive symptoms at baseline (p < .001) and with symptom improvement after 6 weeks (p = .006). Subregion analyses showed that baseline k₃ for the putamen (p = .003) and nucleus accumbens (p = .013) and DSC values for the nucleus accumbens (p = .003) were associated with psychotic symptoms. The tissue reference method yielded no associations between DSC and symptoms or symptom improvement. Neither method revealed any effects of group or treatment on average magnitudes of k₃ or DSC, whereas changes in dopamine synthesis were correlated with higher baseline values, implying a potential effect of treatment.

CONCLUSIONS

Striatal decarboxylation rate at baseline was associated with psychotic symptoms and treatment response. The strong association between k₃ and treatment effect potentially implicate on new treatment strategies.

Multiparametric cardiac18F-FDG PET in humans: pilot comparison of FDG delivery rate with82Rb myocardial blood flow

Myocardial blood flow (MBF) and flow reserve are usually quantified in the clinic with positron emission tomography (PET) using a perfusion-specific radiotracer (e.g.⁸²Rb-chloride). However, the clinical accessibility of existing perfusion tracers remains limited. Meanwhile,¹⁸F-fluorodeoxyglucose (FDG) is a commonly used radiotracer for PET metabolic imaging without similar limitations. In this paper, we explore the potential of¹⁸F-FDG for myocardial perfusion imaging by comparing the myocardial FDG delivery rateK₁with MBF as determined by dynamic⁸²Rb PET in fourteen human subjects with heart disease. Two sets of FDGK₁were derived from one-hour dynamic FDG scans. One was the original FDGK₁estimates and the other was the correspondingK₁values that were linearly normalized for blood glucose levels. A generalized Renkin-Crone model was used to fit FDGK₁with Rb MBF, which then allowed for a nonlinear extraction fraction correction for converting FDGK₁to MBF. The linear correlation between FDG-derived MBF and Rb MBF was moderate (r= 0.79) before the glucose normalization and became much improved (r> 0.9) after glucose normalization. The extraction fraction of FDG was also similar to that of Rb-chloride in the myocardium. The results from this pilot study suggest that dynamic cardiac FDG-PET with tracer kinetic modeling has the potential to provide MBF in addition to its conventional use for metabolic imaging.

Concepts for design and analysis of receptor radiopharmaceuticals: The Receptor-Binding Radiotracers series of meetings provided the foundation

A symposium at George Washington University on Receptor-Binding Radiotracers in 1980 and three follow-up meetings held at University of California, San Diego provided a forum for debating the critical concepts involved in the new field of designing and evaluating radiotracers for imaging receptors and transporters. This review is intended to educate young investigators who may be relatively new to receptor radiopharmaceutical development. Our anticipated audience includes researchers in basic pharmacology, radiochemistry, imaging technology and kinetic data analysis and how these disciplines have worked together to build our understanding of the human biology of transporters and receptor signaling in health and disease. We have chosen to focus on radiochemical design of a useful imaging agent and how design is coupled to analysis of data collected from dynamic imaging with that agent. Some pharmacology may be required for designing the imaging agent and some imaging physics may be important in optimizing the quality of data that is collected. However, the key to a successful imaging agent is matching the radiotracer to the target receptor and to analysis of the time-course data that is used to parse delivery from specific binding and subsequent metabolism or degradation. Properly designed imaging agents are providing critical information about human biology in health and disease as well as pharmacodynamic response to drug interventions. The review emphasizes some of the ideas that were controversial at the 1980 conference and chronicles with literature examples how they have resolved over the four decades of using radiotracers to study transporters and receptors in human subjects. These examples show that there are situations where a very small KD, i.e. high affinity, has the potential to yield an image that reflects blood flow more than receptor density. The examples also show that by combining two studies, one with high specific activity and a second with low specific activity injections one can unravel the pseudo-first order rate B'max into the true second-order rate constant, k3, and the unoccupied receptor density. The final section describes how mathematical methods first presented to the receptor-imaging community in 1980 are now being used to provide confidence in the analysis of kinetic biodistribution studies. Our hope is that by bringing these concepts together in a single review, the next generation of scientists developing receptor imaging agents can be much more efficient than their pioneers in developing useful imaging methods.

Blood-brain barrier transport kinetics of NOTA-modified proteins: the somatropin case

BACKGROUND

Chemical modifications such as PEG, polyamine and radiolabeling on proteins can alter their pharmacokinetic behavior and their blood-brain barrier (BBB) transport characteristics. NOTA, i.e. 1,4,7-triazacyclononane-1,4,7-triacetic acid, is a bifunctional chelating agent that has attracted the interest of the scientific community for its high complexation constant with metals like gallium. Until now, the comparative BBB transport characteristics of NOTA-modified proteins versus unmodified proteins are not yet described.

METHODS

Somatropin (i.e. recombinant human growth hormone), NOTA-conjugated somatropin and gallium-labelled NOTA-conjugated somatropin were investigated for their brain penetration characteristics (multiple time regression and capillary depletion [CD]) in an in vivo mice model to determine the blood-brain transfer properties.

RESULTS

The three compounds showed comparable initial brain influx, with Kin=0.38±0.14 µL/(g×min), 0.36±0.16 µL/(g×min) and 0.28±0.18 µL/(g×min), respectively. CD indicated that more than 80% of the influxed compounds reached the brain parenchyma. All three compounds were in vivo stable in serum and brain during the time frame of the experiments.

CONCLUSIONS

Our results show that modification of NOTA as well as gallium chelation onto proteins, in casu somatropin, does not lead to a significantly changed pharmacokinetic profile at the blood-brain barrier.

Dynamic whole-body PET imaging: principles, potentials and applications

PURPOSE

In this article, we discuss dynamic whole-body (DWB) positron emission tomography (PET) as an imaging tool with significant clinical potential, in relation to conventional standard uptake value (SUV) imaging.

BACKGROUND

DWB PET involves dynamic data acquisition over an extended axial range, capturing tracer kinetic information that is not available with conventional static acquisition protocols. The method can be performed within reasonable clinical imaging times, and enables generation of multiple types of PET images with complementary information in a single imaging session. Importantly, DWB PET can be used to produce multi-parametric images of (i) Patlak slope (influx rate) and (ii) intercept (referred to sometimes as "distribution volume"), while also providing (iii) a conventional 'SUV-equivalent' image for certain protocols.

RESULTS

We provide an overview of ongoing efforts (primarily focused on FDG PET) and discuss potential clinically relevant applications.

CONCLUSION

Overall, the framework of DWB imaging [applicable to both PET/CT(computed tomography) and PET/MRI (magnetic resonance imaging)] generates quantitative measures that may add significant value to conventional SUV image-derived measures, with limited pitfalls as we also discuss in this work.

Contributions of the glycocalyx, endothelium, and extravascular compartment to the blood-brain barrier

The endothelial cells that form the blood-brain barrier (BBB) are coated with glycocalyx, on the luminal side, and with the basement membrane and astrocyte endfeet, on the abluminal side. However, it is unclear how exactly the glycocalyx and extravascular structures contribute to BBB properties. We used two-photon microscopy in anesthetized mice to record passive transport of four different-sized molecules-sodium fluorescein (376 Da), Alexa Fluor (643 Da), 40-kDa dextran, and 150-kDa dextran-from blood to brain, at the level of single cortical capillaries. Both fluorescein and Alexa penetrated nearly the entire glycocalyx volume, but the dextrans penetrated less than 60% of the volume. This suggested that the glycocalyx was a barrier for large but not small molecules. The estimated permeability of the endothelium was the same for fluorescein and Alexa but several-fold lower for the larger dextrans. In the extravascular compartment, co-localized with astrocyte endfeet, diffusion coefficients of the dyes were an order of magnitude lower than in the brain parenchyma. This suggested that the astrocyte endfeet and basement membrane also contributed to BBB properties. In conclusion, the passive transport of small and large hydrophilic molecules through the BBB was determined by three separate barriers: the glycocalyx, the endothelium, and the extravascular compartment. All three barriers must be taken into account in drug delivery studies and when considering BBB dysfunction in disease states.

A new non-invasive graphical method for quantification of cerebral blood flow with[[Formula: see text]] IMP

Objective

[\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{123}\hbox {I}$$\end{document}123I] N-isopropyl-p-iodoamphetamine (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{123}\hbox {I}$$\end{document}123I-IMP) is an ideal perfusion tracer for single photon emission computed tomography, which shows good linearity between cerebral blood flow (CBF) and accumulation. However, quantification of CBF using \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{123}\hbox {I}$$\end{document}123I-IMP without arterial blood sampling has been challenging, with previous methods requiring empirically obtained regression formulae to estimate CBF. Furthermore, the CBF value obtained via some of the previous methods would be affected by the clearance rate of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{123}\hbox {I}$$\end{document}123I-IMP from the lungs. This paper introduces a new non-invasive quantification method for CBF using \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{123}\hbox {I}$$\end{document}123I-IMP and dynamic planar images.

Methods

We have developed a theory based on Microsphere model. This method does not involve regression formulae for estimation and allows for direct measurement of CBF, considering the clearance rate of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{123}\hbox {I}$$\end{document}123I-IMP from the lungs. The study method is executed as easily as conventional Graph-Plot method. We compared the CBF values obtained by our study method and the established autoradiograph (ARG) method.

Results

CBF values obtained using the new method demonstrated significant correlation with values determined using ARG method.

Conclusions

The novel method described presents a reliable and more simple way of determining CBF when compared to current methods.

Steady-state relationship between average glucose, HbA1c and RBC lifespan

HbA1c is used to estimate average glucose. Previous studies showed linear relationship between average glucose and HbA1c. We made a new theoretical relationship using recently proposed Γ-like function model of erythrocyte lifespan. We showed the relationship between average glucose and HbA1c; we approximated it into a simple hyperbolic function: HbA1c=M_RBCk_gAG/(1+(2/3)M_RBCk_gAG), whose inverse function is easily obtained. Apparent linear relationship is an approximation of the curved relationship. Hyperbolic function would provide a more accurate approximation than a linear equation. Physicians should keep in mind the curved relationship and be aware that extremely high HbA1c indicates acceleratingly high glucose level.

Effects of hepatic ischemia-reperfusion injury on the blood-brain barrier permeability to [14C] and [13C]sucrose

Hepatic encephalopathy that is associated with severe liver failure may compromise the blood-brain barrier (BBB) integrity. However, the effects of less severe liver diseases, in the absence of overt encephalopathy, on the BBB are not well understood. The goal of the current study was to investigate the effects of hepatic ischemia-reperfusion (IR) injury on the BBB tight junction permeability to small, hydrophilic molecules using the widely used [¹⁴C]sucrose and recently-proposed alternative [¹³C]sucrose as markers. Rats were subjected to 20 min of hepatic ischemia or sham surgery, followed by 8 h of reperfusion before administration of a single bolus dose of [¹⁴C] or [¹³C]sucrose and collection of serial (0-30 min) blood and plasma and terminal brain samples. The concentrations of [¹⁴C] and [¹³C]sucrose in the samples were determined by measurement of total radioactivity (nonspecific) and LC-MS/MS (specific), respectively. IR injury significantly increased the blood, plasma, and brain concentrations of both [¹⁴C] and [¹³C]sucrose. However, when the brain concentrations were corrected for their respective area under the blood concentration-time curve, only [¹⁴C]sucrose showed significantly higher (30%) BBB permeability values in the IR animals. Because [¹³C]sucrose is a more specific BBB permeability marker, these data indicate that our animal model of hepatic IR injury does not affect the BBB tight junction permeability to small, hydrophilic molecules. Methodological differences among studies of the effects of liver diseases on the BBB permeability may confound the conclusions of such studies.

Pseudo-extravasation rate constant of dynamic susceptibility contrast-MRI determined from pharmacokinetic first principles

Dynamic susceptibility contrast-magnetic resonance imaging (DSC-MRI) is widely used to obtain informative perfusion imaging biomarkers, such as the relative cerebral blood volume (rCBV). The related post-processing software packages for DSC-MRI are available from major MRI instrument manufacturers and third-party vendors. One unique aspect of DSC-MRI with low-molecular-weight gadolinium (Gd)-based contrast reagent (CR) is that CR molecules leak into the interstitium space and therefore confound the DSC signal detected. Several approaches to correct this leakage effect have been proposed throughout the years. Amongst the most popular is the Boxerman-Schmainda-Weisskoff (BSW) K₂ leakage correction approach, in which the K₂ pseudo-first-order rate constant quantifies the leakage. In this work, we propose a new method for the BSW leakage correction approach. Based on the pharmacokinetic interpretation of the data, the commonly adopted R₂ * expression accounting for contributions from both intravascular and extravasating CR components is transformed using a method mathematically similar to Gjedde-Patlak linearization. Then, the leakage rate constant (K_L ) can be determined as the slope of the linear portion of a plot of the transformed data. Using the DSC data of high-molecular-weight (~750 kDa), iron-based, intravascular Ferumoxytol (FeO), the pharmacokinetic interpretation of the new paradigm is empirically validated. The primary objective of this work is to empirically demonstrate that a linear portion often exists in the graph of the transformed data. This linear portion provides a clear definition of the Gd CR pseudo-leakage rate constant, which equals the slope derived from the linear segment. A secondary objective is to demonstrate that transformed points from the initial transient period during the CR wash-in often deviate from the linear trend of the linearized graph. The inclusion of these points will have a negative impact on the accuracy of the leakage rate constant, and even make it time dependent.

Evaluation of [14C] and [13C]Sucrose as Blood-Brain Barrier Permeability Markers

Nonspecific quantitation of [¹⁴C]sucrose in blood and brain has been routinely used as a quantitative measure of the in vivo blood-brain barrier (BBB) integrity. However, the reported apparent brain uptake clearance (K_in) of the marker varies widely (∼100-fold). We investigated the accuracy of the use of the marker in comparison with a stable isotope of sucrose ([¹³C]sucrose) measured by a specific liquid chromatography-tandem mass spectrometry method. Rats received single doses of each marker, and the K_in values were determined. Surprisingly, the K_in value of [¹³C]sucrose was 6- to 7-fold lower than that of [¹⁴C]sucrose. Chromatographic fractionation after in vivo administration of [¹⁴C]sucrose indicated that the majority of the brain content of radioactivity belonged to compounds other than the intact [¹⁴C]sucrose. However, mechanistic studies failed to reveal any substantial metabolism of the marker. The octanol:water partition coefficient of [¹⁴C]sucrose was >2-fold higher than that of [¹³C]sucrose, indicating the presence of lipid-soluble impurities in the [¹⁴C]sucrose solution. Our data indicate that [¹⁴C]sucrose overestimates the true BBB permeability to sucrose. We suggest that specific quantitation of the stable isotope (¹³C) of sucrose is a more accurate alternative to the current widespread use of the radioactive sucrose as a BBB marker.

Identification and characterization of the glucose dual-affinity transport system in Neurospora crassa: pleiotropic roles in nutrient transport, signaling, and carbon catabolite repression

BACKGROUND

The glucose dual-affinity transport system (low- and high-affinity) is a conserved strategy used by microorganisms to cope with natural fluctuations in nutrient availability in the environment. The glucose-sensing and uptake processes are believed to be tightly associated with cellulase expression regulation in cellulolytic fungi. However, both the identities and functions of the major molecular components of this evolutionarily conserved system in filamentous fungi remain elusive. Here, we systematically identified and characterized the components of the glucose dual-affinity transport system in the model fungus Neurospora crassa.

RESULTS

Using RNA sequencing coupled with functional transport analyses, we assigned GLT-1 (K_m = 18.42 ± 3.38 mM) and HGT-1/-2 (K_m = 16.13 ± 0.95 and 98.97 ± 22.02 µM) to the low- and high-affinity glucose transport systems, respectively. The high-affinity transporters hgt-1/-2 complemented a moderate growth defect under high glucose when glt-1 was deleted. Simultaneous deletion of hgt-1/-2 led to extensive derepression of genes for plant cell wall deconstruction in cells grown on cellulose. The suppression by HGT-1/-2 was connected to both carbon catabolite repression (CCR) and the cyclic adenosine monophosphate-protein kinase A pathway. Alteration of a residue conserved across taxa in hexose transporters resulted in a loss of glucose-transporting function, whereas CCR signal transduction was retained, indicating dual functions for HGT-1/-2 as "transceptors."

CONCLUSIONS

In this study, GLT-1 and HGT-1/-2 were identified as the key components of the glucose dual-affinity transport system, which plays diverse roles in glucose transport and carbon metabolism. Given the wide conservation of the glucose dual-affinity transport system across fungal species, the identification of its components and their pleiotropic roles in this study shed important new light on the molecular basis of nutrient transport, signaling, and plant cell wall degradation in fungi.

Noninvasive bi-graphical analysis for the quantification of slowly reversible radioligand binding

In this paper, we presented a novel reference-region-based (noninvasive) bi-graphical analysis for the quantification of a reversible radiotracer binding that may be too slow to reach relative equilibrium (RE) state during positron emission tomography (PET) scans. The proposed method indirectly implements the noninvasive Logan plot, through arithmetic combination of the parameters of two other noninvasive methods and the apparent tissue-to-plasma efflux rate constant for the reference region ([Formula: see text]). We investigated its validity and statistical properties, by performing a simulation study with various noise levels and [Formula: see text] values, and also evaluated its feasibility for [¹⁸F]FP-CIT PET in human brain. The results revealed that the proposed approach provides distribution volume ratio estimation comparable to the Logan plot at low noise levels while improving underestimation caused by non-RE state differently depending on [Formula: see text]. Furthermore, the proposed method was able to avoid noise-induced bias of the Logan plot, and the variability of its results was less dependent on [Formula: see text] than the Logan plot. Therefore, this approach, without issues related to arterial blood sampling given a pre-estimate of [Formula: see text] (e.g. population-based), could be useful in parametric image generation for slow kinetic tracers staying in a non-RE state within a PET scan.

Comparison of the Accumulation Kinetics of L-(Methyl-11C)-Methionine and D-(Methyl-11C)-Methionine in Brain Tumors Studied with Positron Emission Tomography

Five patients with glioma were examined with positron emission tomography after the administration of 11C-L-methionine and at a following day with 11C-D-methionine. The rates of accumulation of the tracers were determined in the tumor and in the normal brain tissue according to a graphical technique of Patlak et coll. (24). The accumulation rates for L-methionine were on the average 2.4 times higher than those of D-methionine in the tumors. The corresponding ratio for normal brain tissue was 2.3. It is concluded that in this group of tumors without obvious blood-tumor-barrier breakdown, a stereospecific process with similar properties as in the normal brain tissue, is responsible for the accumulation of the labelled methionine.

Application of a Kinetic Model on the Methionine Accumulation in Intracranial Tumours Studied with Positron Emission Tomography

Eleven patients were studied with positron emission tomography (PET) using 11C-methionine. They all had low-grade astrocytomas (Kernohan grade II). The PET studies were analyzed with a metabolic model to obtain values for the influx, the accumulation rate and the partition coefficient of methionine in normal and tumourous tissue. Seven of the tumours showed an increased accumulation of methionine as compared with normal tissue on the static PET scans and also had higher values as to the kinetic parameters. Four tumours had a methionine accumulation equal to or lower than that of normal tissue and the kinetic parameters were also lower. Application of the kinetic model did not aid significantly in the delineation of the tumours. There was a correlation between the three parameters indicating an adaption of the transport of methionine to the regional metabolic demand. The accumulation rate for normal cortical tissue was 0.49 nmol/g/min, the influx 0.97 nmol/ml and the partition coefficient 0.45 ml/g. These values are considerably higher than those previously reported. The differences might be attributed to differences in the corrections introduced for i.a. the occurrence of labelled metabolites in serum. With the use of a kinetic model, more information about the tracer is utilized and gained compared with the previously used graphic approach.

Quantitative In Vitro and In Vivo Evaluation of Intestinal and Blood-Brain Barrier Transport Kinetics of the Plant N-Alkylamide Pellitorine

Objective. To evaluate the gut mucosa and blood-brain barrier (BBB) pharmacokinetic permeability properties of the plant N-alkylamide pellitorine. Methods. Pure pellitorine and an Anacyclus pyrethrum extract were used to investigate the permeation of pellitorine through (1) a Caco-2 cell monolayer, (2) the rat gut after oral administration, and (3) the BBB in mice after intravenous and intracerebroventricular administration. A validated bioanalytical UPLC-MS(2) method was used to quantify pellitorine. Results. Pellitorine was able to cross the Caco-2 cell monolayer from the apical-to-basolateral and from the basolateral-to-apical side with apparent permeability coefficients between 0.6 · 10(-5) and 4.8 · 10(-5) cm/h and between 0.3 · 10(-5) and 5.8 · 10(-5) cm/h, respectively. In rats, a serum elimination rate constant of 0.3 h(-1) was obtained. Intravenous injection of pellitorine in mice resulted in a rapid and high permeation of pellitorine through the BBB with a unidirectional influx rate constant of 153 μL/(g·min). In particular, 97% of pellitorine reached the brain tissue, while only 3% remained in the brain capillaries. An efflux transfer constant of 0.05 min(-1) was obtained. Conclusion. Pellitorine shows a good gut permeation and rapidly permeates the BBB once in the blood, indicating a possible role in the treatment of central nervous system diseases.

Federated optimisation of kinetic analysis problems

Positron Emission Tomography (PET) data is intrinsically dynamic, and kinetic analysis of dynamic PET data can substantially augment the information provided by static PET reconstructions. Yet despite the insights into disease that kinetic analysis offers, it is not used clinically and seldom used in research beyond the preclinical stage. The utility of PET kinetic analysis is hampered by several factors including spatial inconsistency within regions of homogeneous tissue and relative computational expense when fitting complex models to individual voxels. Even with sophisticated algorithms inconsistencies can arise because local optima frequently have narrow basins of convergence, are surrounded by relatively flat (uninformative) regions, have relatively low-gradient valley floors, or combinations thereof. Based on the observation that cost functions for individual voxels frequently bear some resemblance to each-other, this paper proposes the federated optimisation of the individual kinetic analysis problems within a given image. This approach shares parameters proposed during optimisation with other, similar voxels. Federated optimisation exploits the redundancy typical of large medical images to improve the optimisation residuals, computational efficiency and, to a limited extent, image consistency. This is achieved without restricting the formulation of the kinetic model, resorting to an explicit regularisation parameter, or limiting the resolution at which parameters are computed.

Mucosal and blood-brain barrier transport kinetics of the plant N-alkylamide spilanthol using in vitro and in vivo models

Background

N-alkylamides (NAAs) are a large group of secondary metabolites occurring in more than 25 plant families which are often used in traditional medicine. A prominent active NAA is spilanthol. The general goal was to quantitatively investigate the gut mucosa and blood-brain barrier (BBB) permeability pharmacokinetic properties of spilanthol.

Methods

Spilanthes acmella (L.) L. extracts, as well as purified spilanthol were used to investigate (1) the permeation of spilanthol through a Caco-2 cell monolayer in vitro, (2) the absorption from the intestinal lumen after oral administration to rats, and (3) the permeation through the BBB in mice after intravenous injection. Quantification of spilanthol was performed using a validated bio-analytical UPLC-MS² method.

Results

Spilanthol was able to cross the Caco-2 cell monolayer in vitro from the apical-to-basolateral side and from the basolateral-to-apical side with apparent permeability coefficients P_app between 5.2 · 10⁻⁵ and 10.2 · 10⁻⁵ cm/h. This in vitro permeability was confirmed by the in vivo intestinal absorption in rats after oral administration, where an elimination rate constant k_e of 0.6 h⁻¹ was obtained. Furthermore, once present in the systemic circulation, spilanthol rapidly penetrated the blood-brain barrier: a highly significant influx of spilanthol into the brains was observed with a unidirectional influx rate constant K₁ of 796 μl/(g · min).

Conclusions

Spilanthol shows a high intestinal absorption from the gut into the systemic circulation, as well as a high BBB permeation rate from the blood into the brain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12906-016-1159-0) contains supplementary material, which is available to authorized users.

Blood-brain barrier transport kinetics of the neuromedin peptides NMU, NMN, NMB and NT

The neuromedin peptides are peripherally and centrally produced, but until now, it is generally believed that they only function as locally acting compounds without any quantitative knowledge about their blood-brain barrier (BBB) passage. Here, we characterize the transport kinetics of four neuromedins (NMU, NMN, NMB and NT) across the BBB, as well as their metabolization profile, and evaluate if they can act as endocrine hormones. Using the in vivo mouse model, multiple time regression (MTR), capillary depletion (CD) and brain efflux studies were performed. Data was fitted using linear (NMU, NT and NMB) or biphasic modeling (NMU and NMN). Three of the four investigated peptides, i.e. NMU, NT and NMN, showed a significant influx into the brain with unidirectional influx rate constants of 1.31 and 0.75 μL/(g × min) for NMU and NT respectively and initial influx constants (K1) of 72.14 and 7.55 μL/(g × min) and net influx constants (K) of 1.28 and 1.36 × 10(-16) μL/(g×min) for NMU and NMN respectively. The influx of NMB was negligible. Only NMN and NT showed a significant efflux out of the brain with an efflux constant (kout) of 0.042 min(-1) and 0.053 min(-1) respectively. Our results indicate that locally produced neuromedin peptides and/or fragments can be transported through the whole body, including passing the BBB, and taken up by different organs/tissues, supporting the idea that the neuromedins could have a much bigger role in the regulation of biological processes than currently assumed.

Biodistribution of the cyclotide MCoTI-II, a cyclic disulfide-rich peptide drug scaffold

Disulfide-rich macrocyclic peptides are promising templates for drug design because of their unique topology and remarkable stability. However, little is known about their pharmacokinetics. In this study, we characterize the biodistribution in mice of Momordica cochinchinensis trypsin inhibitor-II (MCoTI-II), a cyclic three-disulfide-containing peptide that has been used in a number of studies as a drug scaffold. The distribution of MCoTI-II was compared with that of chlorotoxin, which is a four-disulfide-containing peptide that has been used to develop brain tumor imaging agents; dermorphin, which is a disulfide-less peptide; and bovine serum albumin, a large protein. Both MCoTI-II and chlorotoxin distributed predominantly to the serum and kidneys, confirming that they are stable in serum and suggesting that they are eliminated from the blood through renal clearance. Although cell-penetrating peptides have been reported to be able to transport across the blood-brain barrier, MCoTI-II, which is a cell-penetrating peptide, showed no uptake into the brain. The uptake of chlorotoxin was higher than that of MCoTI-II but lower than that of dermorphin, which is considered to have low uptake into the brain. This study provides insight into the behavior of disulfide-rich peptides in vivo. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

Quorum Sensing Peptides Selectively Penetrate the Blood-Brain Barrier

Bacteria communicate with each other by the use of signaling molecules, a process called 'quorum sensing'. One group of quorum sensing molecules includes the oligopeptides, which are mainly produced by Gram-positive bacteria. Recently, these quorum sensing peptides were found to biologically influence mammalian cells, promoting i.a. metastasis of cancer cells. Moreover, it was found that bacteria can influence different central nervous system related disorders as well, e.g. anxiety, depression and autism. Research currently focuses on the role of bacterial metabolites in this bacteria-brain interaction, with the role of the quorum sensing peptides not yet known. Here, three chemically diverse quorum sensing peptides were investigated for their brain influx (multiple time regression technique) and efflux properties in an in vivo mouse model (ICR-CD-1) to determine blood-brain transfer properties: PhrCACET1 demonstrated comparatively a very high initial influx into the mouse brain (Kin = 20.87 μl/(g×min)), while brain penetrabilities of BIP-2 and PhrANTH2 were found to be low (Kin = 2.68 μl/(g×min)) and very low (Kin = 0.18 μl/(g×min)), respectively. All three quorum sensing peptides were metabolically stable in plasma (in vitro) during the experimental time frame and no significant brain efflux was observed. Initial tissue distribution data showed remarkably high liver accumulation of BIP-2 as well. Our results thus support the potential role of some quorum sensing peptides in different neurological disorders, thereby enlarging our knowledge about the microbiome-brain axis.

Cell-Penetrating Peptides Selectively Cross the Blood-Brain Barrier In Vivo

Cell-penetrating peptides (CPPs) are a group of peptides, which have the ability to cross cell membrane bilayers. CPPs themselves can exert biological activity and can be formed endogenously. Fragmentary studies demonstrate their ability to enhance transport of different cargoes across the blood-brain barrier (BBB). However, comparative, quantitative data on the BBB permeability of different CPPs are currently lacking. Therefore, the in vivo BBB transport characteristics of five chemically diverse CPPs, i.e. pVEC, SynB3, Tat 47-57, transportan 10 (TP10) and TP10-2, were determined. The results of the multiple time regression (MTR) analysis revealed that CPPs show divergent BBB influx properties: Tat 47-57, SynB3, and especially pVEC showed very high unidirectional influx rates of 4.73 μl/(g × min), 5.63 μl/(g × min) and 6.02 μl/(g × min), respectively, while the transportan analogs showed a negligible to low brain influx. Using capillary depletion, it was found that 80% of the influxed peptides effectively reached the brain parenchyma. Except for pVEC, all peptides showed a significant efflux out of the brain. Co-injection of pVEC with radioiodinated bovine serum albumin (BSA) did not enhance the brain influx of radiodionated BSA, indicating that pVEC does not itself significantly alter the BBB properties. A saturable mechanism could not be demonstrated by co-injecting an excess dose of non-radiolabeled CPP. No significant regional differences in brain influx were observed, with the exception for pVEC, for which the regional variations were only marginal. The observed BBB influx transport properties cannot be correlated with their cell-penetrating ability, and therefore, good CPP properties do not imply efficient brain influx.

Bioavailability of dietary polyphenols: Factors contributing to their clinical application in CNS diseases

The anatomical location of the central nervous system (CNS) renders it immunologically and pharmacologically privileged due to the blood brain barrier (BBB). Although this limits the transport of unfavorable molecules to the CNS, the ensuing privilege could be disadvantageous for therapeutic compounds. Hence, the greatest challenge in the pharmacotherapy of CNS diseases is to ensure efficient brain targeting and drug delivery. Research evidences indicate that dietary polyphenols have neuroprotective potential against CNS diseases. However, their selective permeability across BBB, poor absorption, rapid metabolism and systemic elimination limit their bioavailability and therapeutic efficacy. Consequently, the beneficial effects of these orally administered agents in the CNS still remain a subject of debate. This has also limited its clinical application either as independent or adjunctive therapy. Improving the in vivo bioavailability by novel methods could improve the therapeutic feasibility of polyphenols and assist in evolving novel drugs and their derivatives with improved efficacy in vivo. Here we review the mechanistic and pharmacological issues related to the bioavailability of polyphenols with therapeutic implications for CNS diseases. We surmise that improving the bioavailability of polyphenols entails efficient in vivo transport across BBB, biochemical stability, improved half-life and persistent neuroprotection in the CNS.

Blood-brain transfer and antinociception of linear and cyclic N-methyl-guanidine and thiourea-enkephalins

Enkephalins are active in regulation of nociception in the body and are key in development of new synthetic peptide analogs that target centrally located opioid receptors. In this study, we investigated the in vivo blood-brain barrier (BBB) penetration behavior and antinociceptive activity of two cyclic enkephalin analogs with a thiourea (CycS) or a N-methyl-guanidine bridge (CycNMe), and their linear counterparts (LinS and LinNMe) in mice, as well as their in vitro metabolic stability. (125)I-LinS had the highest blood-brain clearance (K1=3.46μL/gmin), followed by (125)I-LinNMe, (125)I-CycNMe, and (125)I-CycS (K1=1.64, 0.31, and 0.11μL/gmin, respectively). Also, these peptides had a high metabolic stability (t1/2>1h) in mouse serum and brain homogenate, and half-inhibition constant (Ki) values in the nanomolar range with predominantly μ-opioid receptor selectivity. The positively charged NMe-enkephalins showed a higher antinociceptive activity (LinNMe: 298% and CycNMe: 205%), expressed as molar-dose normalized area under the curve (AUC) relative to morphine, than the neutral S-enkephalins (CycS: 122% and LinS: 130%).

Recent advances in parametric neuroreceptor mapping with dynamic PET: basic concepts and graphical analyses

Tracer kinetic modeling in dynamic positron emission tomography (PET) has been widely used to investigate the characteristic distribution patterns or dysfunctions of neuroreceptors in brain diseases. Its practical goal has progressed from regional data quantification to parametric mapping that produces images of kinetic-model parameters by fully exploiting the spatiotemporal information in dynamic PET data. Graphical analysis (GA) is a major parametric mapping technique that is independent on any compartmental model configuration, robust to noise, and computationally efficient. In this paper, we provide an overview of recent advances in the parametric mapping of neuroreceptor binding based on GA methods. The associated basic concepts in tracer kinetic modeling are presented, including commonly-used compartment models and major parameters of interest. Technical details of GA approaches for reversible and irreversible radioligands are described, considering both plasma input and reference tissue input models. Their statistical properties are discussed in view of parametric imaging.

Development of (18)F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

Positron emission tomography (PET) is an in vivo molecular imaging tool which is widely used in nuclear medicine for early diagnosis and treatment follow-up of many brain diseases. PET uses biomolecules as probes which are labeled with radionuclides of short half-lives, synthesized prior to the imaging studies. These probes are called radiotracers. Fluorine-18 is a radionuclide routinely used in the radiolabeling of neuroreceptor ligands for PET because of its favorable half-life of 109.8 min. The delivery of such radiotracers into the brain provides images of transport, metabolic, and neurotransmission processes on the molecular level. After a short introduction into the principles of PET, this review mainly focuses on the strategy of radiotracer development bridging from basic science to biomedical application. Successful radiotracer design as described here provides molecular probes which not only are useful for imaging of human brain diseases, but also allow molecular neuroreceptor imaging studies in various small-animal models of disease, including genetically-engineered animals. Furthermore, they provide a powerful tool for in vivo pharmacology during the process of pre-clinical drug development to identify new drug targets, to investigate pathophysiology, to discover potential drug candidates, and to evaluate the pharmacokinetics and pharmacodynamics of drugs in vivo.

Influence of O-methylated metabolite penetrating the blood-brain barrier to estimation of dopamine synthesis capacity in human L-[β-(11)C]DOPA PET

O-methyl metabolite (L-[β-(11)C]OMD) of (11)C-labeled L-3,4-dihydroxyphenylalanine (L-[β-(11)C]DOPA) can penetrate into brain tissue through the blood-brain barrier, and can complicate the estimation of dopamine synthesis capacity by positron emission tomography (PET) study with L-[β-(11)C]DOPA. We evaluated the impact of L-[β-(11)C]OMD on the estimation of the dopamine synthesis capacity in a human L-[β-(11)C]DOPA PET study. The metabolite correction with mathematical modeling of L-[β-(11)C]OMD kinetics in a reference region without decarboxylation and further metabolism, proposed by a previous [(18)F]FDOPA PET study, were implemented to estimate radioactivity of tissue L-[β-(11)C]OMD in 10 normal volunteers. The component of L-[β-(11)C]OMD in tissue time-activity curves (TACs) in 10 regions were subtracted by the estimated radioactivity of L-[β-(11)C]OMD. To evaluate the influence of omitting blood sampling and metabolite correction, relative dopamine synthesis rate (kref) was estimated by Gjedde-Patlak analysis with reference tissue input function, as well as the net dopamine synthesis rate (Ki) by Gjedde-Patlak analysis with the arterial input function and TAC without and with metabolite correction. Overestimation of Ki was observed without metabolite correction. However, the kref and Ki with metabolite correction were significantly correlated. These data suggest that the influence of L-[β-(11)C]OMD is minimal for the estimation of kref as dopamine synthesis capacity.

Prediction of methotrexate CNS distribution in different species - influence of disease conditions

Children and adults with malignant diseases have a high risk of prevalence of the tumor in the central nervous system (CNS). As prophylaxis treatment methotrexate is often given. In order to monitor methotrexate exposure in the CNS, cerebrospinal fluid (CSF) concentrations are often measured. However, the question is in how far we can rely on CSF concentrations of methotrexate as appropriate surrogate for brain target site concentrations, especially under disease conditions. In this study, we have investigated the spatial distribution of unbound methotrexate in healthy rat brain by parallel microdialysis, with or without inhibition of Mrp/Oat/Oatp-mediated active transport processes by a co-administration of probenecid. Specifically, we have focused on the relationship between brain extracellular fluid (brainECF) and CSF concentrations. The data were used to develop a systems-based pharmacokinetic (SBPK) brain distribution model for methotrexate. This model was subsequently applied on literature data on methotrexate brain distribution in other healthy and diseased rats (brainECF), healthy dogs (CSF) and diseased children (CSF) and adults (brainECF and CSF). Important differences between brainECF and CSF kinetics were found, but we have found that inhibition of Mrp/Oat/Oatp-mediated active transport processes does not significantly influence the relationship between brainECF and CSF fluid methotrexate concentrations. It is concluded that in parallel obtained data on unbound brainECF, CSF and plasma concentrations, under dynamic conditions, combined with advanced mathematical modeling is a most valid approach to develop SBPK models that allow for revealing the mechanisms underlying the relationship between brainECF and CSF concentrations in health and disease.

Influence of GLP-1 on myocardial glucose metabolism in healthy men during normo- or hypoglycemia

Background and Aims

Glucagon-like peptide-1 (GLP-1) may provide beneficial cardiovascular effects, possibly due to enhanced myocardial energetic efficiency by increasing myocardial glucose uptake (MGU). We assessed the effects of GLP-1 on MGU in healthy subjects during normo- and hypoglycemia.

Materials and Methods

We included eighteen healthy men in two randomized, double-blinded, placebo-controlled cross-over studies. MGU was assessed with GLP-1 or saline infusion during pituitary-pancreatic normo- (plasma glucose (PG): 4.5 mM, n = 10) and hypoglycemic clamps (PG: 3.0 mM, n = 8) by positron emission tomography with ¹⁸fluoro-deoxy-glucose (¹⁸F-FDG) as tracer.

Results

In the normoglycemia study mean (± SD) age was 25±3 years, and BMI was 22.6±0.6 kg/m² and in the hypoglycemia study the mean age was 23±2 years with a mean body mass index of 23±2 kg/m². GLP-1 did not change MGU during normoglycemia (mean (+/− SD) 0.15+/−0.04 and 0.16+/−0.03 µmol/g/min, P = 0.46) or during hypoglycemia (0.16+/−0.03 and 0.13+/−0.04 µmol/g/min, P = 0.14). However, the effect of GLP-1 on MGU was negatively correlated to baseline MGU both during normo- and hypoglycemia, (P = 0.006, r² = 0.64 and P = 0.018, r² = 0.64, respectively) and changes in MGU correlated positively with the level of insulin resistance (HOMA 2IR) during hypoglycemia, P = 0.04, r² = 0.54. GLP-1 mediated an increase in circulating glucagon levels at PG levels below 3.5 mM and increased glucose infusion rates during the hypoglycemia study. No differences in other circulating hormones or metabolites were found.

Conclusions

While GLP-1 does not affect overall MGU, GLP-1 induces changes in MGU dependent on baseline MGU such that GLP-1 increases MGU in subjects with low baseline MGU and decreases MGU in subjects with high baseline MGU. GLP-1 preserves MGU during hypoglycemia in insulin resistant subjects.

ClinicalTrials.gov registration numbers: NCT00418288: (hypoglycemia) and NCT00256256: (normoglycemia).

Blood-brain transfer of Pittsburgh compound B in humans

In the labeled form, the Pittsburgh compound B (2-(4′-{N-methyl-[¹¹C]}methyl-aminophenyl)-6-hydroxy-benzothiazole, [¹¹C]PiB), is used as a biomarker for positron emission tomography (PET) of brain β-amyloid deposition in Alzheimer's disease (AD). The permeability of [¹¹C]PiB in the blood-brain barrier is held to be high but the permeability-surface area product and extraction fractions in patients or healthy volunteers are not known. We used PET to determine the clearance associated with the unidrectional blood-brain transfer of [¹¹C]PiB and the corresponding cerebral blood flow rates in frontal lobe, whole cerebral cortex, and cerebellum of patients with Alzheimer's disease and healthy volunteers. Regional cerebral blood flow rates differed significantly between the two groups. Thus, regional and whole-brain permeability-surface area products were identical, in agreement with the observation that numerically, but insignificantly, unidirectional blood-brain clearances are lower and extraction fractions higher in the patients. The evidence of unchanged permeability-surface area products in the patients implies that blood flow changes can be deduced from the unidirectional blood-brain clearances of [¹¹C]PiB in the patients.

Measurement of blood-brain barrier permeability in acute ischemic stroke using standard first-pass perfusion CT data

Background and purpose

Increased blood–brain barrier permeability is believed to be associated with complications following acute ischemic stroke and with infarct expansion. Measurement of blood–brain barrier permeability requires a delayed image acquisition methodology, which prolongs examination time, increasing the likelihood of movement artefacts and radiation dose. Existing quantitative methods overestimate blood–brain barrier permeability when early phase CT perfusion data are used. The purpose of this study is to develop a method that yields the correct blood–brain barrier permeability value using first-pass perfusion CT data.

Methods

We acquired 43 CT perfusion datasets, comprising experimental (n = 30) and validation subject groups (n = 13). The Gjedde–Patlak method was used to estimate blood–brain barrier permeability using first-pass (30–60 s after contrast administration) and delayed phase (30–200 s) data. In the experimental group, linear regression was used to obtain a function predicting first-pass blood–brain barrier permeability estimates from delayed phase estimates in each stroke compartment. The reliability of prediction with this function was then tested using data from the validation group.

Results

The predicted delayed phase blood–brain barrier permeability was strongly correlated with the measured delayed phase value (r = 0.67 and 0.6 for experimental and validation group respectively; p < 0.01). Predicted and measured delayed phase blood–brain barrier permeability in each stroke compartment were not significantly different in both experimental and validation groups.

Conclusion

We have developed a method of estimating blood–brain barrier permeability using first-pass perfusion CT data. This predictive method allows reliable blood–brain barrier permeability estimation within standard acquisition time, minimizing the likelihood of motion artefacts thereby improving image quality and reducing radiation dose.

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Delayed phase BBBP can be predicted from first-pass perfusion CT data.

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Predicted BBBP was not significantly different from delayed phase measurements.

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Prediction model allows reliable BBBP estimation within the standard acquisition time.

Glucagon-like peptide-1 (GLP-1) raises blood-brain glucose transfer capacity and hexokinase activity in human brain

In hyperglycemia, glucagon-like peptide-1 (GLP-1) lowers brain glucose concentration together with increased net blood-brain clearance and brain metabolism, but it is not known whether this effect depends on the prevailing plasma glucose (PG) concentration. In hypoglycemia, glucose depletion potentially impairs brain function. Here, we test the hypothesis that GLP-1 exacerbates the effect of hypoglycemia. To test the hypothesis, we determined glucose transport and consumption rates in seven healthy men in a randomized, double-blinded placebo-controlled cross-over experimental design. The acute effect of GLP-1 on glucose transfer in the brain was measured by positron emission tomography (PET) during a hypoglycemic clamp (3 mM plasma glucose) with (18)F-fluoro-2-deoxy-glucose (FDG) as tracer of glucose. In addition, we jointly analyzed cerebrometabolic effects of GLP-1 from the present hypoglycemia study and our previous hyperglycemia study to estimate the Michaelis-Menten constants of glucose transport and metabolism. The GLP-1 treatment lowered the vascular volume of brain tissue. Loading data from hypo- to hyperglycemia into the Michaelis-Menten equation, we found increased maximum phosphorylation velocity (V max) in the gray matter regions of cerebral cortex, thalamus, and cerebellum, as well as increased blood-brain glucose transport capacity (T max) in gray matter, white matter, cortex, thalamus, and cerebellum. In hypoglycemia, GLP-1 had no effects on net glucose metabolism, brain glucose concentration, or blood-brain glucose transport. Neither hexokinase nor transporter affinities varied significantly with treatment in any region. We conclude that GLP-1 changes blood-brain glucose transfer and brain glucose metabolic rates in a PG concentration-dependent manner. One consequence is that hypoglycemia eliminates these effects of GLP-1 on brain glucose homeostasis.

Variation of the net charge, lipophilicity, and side chain flexibility in Dmt(1)-DALDA: Effect on Opioid Activity and Biodistribution

The influence of the side chain charges of the second and fourth amino acid residues in the peptidic μ opioid lead agonist Dmt-d-Arg-Phe-Lys-NH(2) ([Dmt(1)]-DALDA) was examined. Additionally, to increase the overall lipophilicity of [Dmt(1)]-DALDA and to investigate the Phe(3) side chain flexibility, the final amide bond was N-methylated and Phe(3) was replaced by a constrained aminobenzazepine analogue. The in vitro receptor binding and activity of the peptides, as well as their in vivo transport (brain in- and efflux and tissue biodistribution) and antinociceptive properties after peripheral administration (ip and sc) in mice were determined. The structural modifications result in significant shifts of receptor binding, activity, and transport properties. Strikingly, while [Dmt(1)]-DALDA and its N-methyl analogue, Dmt-d-Arg-Phe-NMeLys-NH(2), showed a long-lasting antinociceptive effect (>7 h), the peptides with d-Cit(2) generate potent antinociception more rapidly (maximal effect at 1h postinjection) but also lose their analgesic activity faster when compared to [Dmt(1)]-DALDA and [Dmt(1),NMeLys(4)]-DALDA.

Region-specific causal mechanism in the effects of ammonia on cerebral glucose metabolism in the rat brain

Ammonia, which is considered to be the main agent responsible for hepatic encephalopathy, inhibits oxidative glucose metabolism in the brain. However, the effects of ammonia on cerebral glucose metabolism in different brain regions remains unclear. To clarify this issue, we added ammonia directly to fresh rat brain slices and measured its effects on glucose metabolism. Dynamic positron autoradiography with [(18)F]2-fluoro-2-deoxy-D-glucose and 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium (WST-1) colorimetric assay revealed that ammonia significantly increased the cerebral glucose metabolic rate and depressed mitochondrial function, as compared to the unloaded control in each of the brain regions examined (cerebral cortex, striatum, and cerebellum), reflecting increased glycolysis that compensates for the decrease in aerobic metabolism. Pre-treatment with (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), a N-methyl-D-aspartate (NMDA) receptor antagonist, significantly attenuated these changes induced by ammonia in cerebellum, but not in cerebral cortex or striatum. The addition of ammonia induced an increase in cyclic guanosine monophosphate (cGMP) levels in cerebellum, but not in cerebral cortex or striatum, reflecting the activation of the NMDA receptor-nitric oxide-cGMP pathway. These results suggested that NMDA receptor activation is responsible for the impairment of glucose metabolism induced by ammonia specifically in cerebellum.

Api88 is a novel antibacterial designer peptide to treat systemic infections with multidrug-resistant Gram-negative pathogens

The emergence of multiple-drug-resistant (MDR) bacterial pathogens in hospitals (nosocomial infections) presents a global threat of growing importance, especially for Gram-negative bacteria with extended spectrum β-lactamase (ESBL) or the novel New Delhi metallo-β-lactamase 1 (NDM-1) resistance. Starting from the antibacterial peptide apidaecin 1b, we have optimized the sequence to treat systemic infections with the most threatening human pathogens, such as Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii. The lead compound Api88 enters bacteria without lytic effects at the membrane and inhibits chaperone DnaK at the substrate binding domain with a K(D) of 5 μmol/L. The Api88-DnaK crystal structure revealed that Api88 binds with a seven residue long sequence (PVYIPRP), in two different modes. Mice did not show any sign of toxicity when Api88 was injected four times intraperitoneally at a dose of 40 mg/kg body weight (BW) within 24 h, whereas three injections of 1.25 mg/kg BW and 5 mg/kg BW were sufficient to rescue all animals in lethal sepsis models using pathogenic E. coli strains ATCC 25922 and Neumann, respectively. Radioactive labeling showed that Api88 enters all organs investigated including the brain and is cleared through both the liver and kidneys at similar rates. In conclusion, Api88 is a novel, highly promising, 18-residue peptide lead compound with favorable in vitro and in vivo properties including a promising safety margin.

Brainpeps: the blood-brain barrier peptide database

Peptides are able to cross the blood-brain barrier (BBB) through various mechanisms, opening new diagnostic and therapeutic avenues. However, their BBB transport data are scattered in the literature over different disciplines, using different methodologies reporting different influx or efflux aspects. Therefore, a comprehensive BBB peptide database (Brainpeps) was constructed to collect the BBB data available in the literature. Brainpeps currently contains BBB transport information with positive as well as negative results. The database is a useful tool to prioritize peptide choices for evaluating different BBB responses or studying quantitative structure-property (BBB behaviour) relationships of peptides. Because a multitude of methods have been used to assess the BBB behaviour of compounds, we classified these methods and their responses. Moreover, the relationships between the different BBB transport methods have been clarified and visualized.

Characterization of [11C]RO5013853, a novel PET tracer for the glycine transporter type 1 (GlyT1) in humans

We characterize a novel radioligand for the glycine transporter type 1 (GlyT1), [(11)C]RO5013853, in humans. Ten healthy male volunteers, 23-60 years of age, were enrolled in this PET study; seven subjects participated in the evaluation of test-retest reliability and three subjects in whole body dosimetry. Subjects were administered intravenous bolus injections of approximately 1100 MBq (30 mCi) [(11)C]RO5013853 with a high specific activity of about 481 GBq (13 Ci)/μmol. Standard compartmental model analysis with arterial plasma input function, and an alternative noninvasive analysis method which was evaluated and validated by occupancy studies in both baboons and humans, were performed. Mean parameter estimates of the volumes of distribution (VT) obtained by a 2-tissue 5-parameter model were higher in the cerebellum, pons, and thalamus (1.99 to 2.59 mL/mL), and lower in the putamen, caudate, and cortical areas (0.86 to 1.13 mL/mL), with estimates showing less than 10% difference between test and retest scans. Tracer retention was effectively blocked by the specific glycine reuptake inhibitor (GRI), bitopertin (RG1678). [(11)C]RO5013853 was safe and well tolerated. Human dosimetry studies showed that the effective dose was approximately 0.0033 mSv/MBq, with the liver receiving the highest absorbed dose. In conclusion, quantitative dynamic PET of the human brain after intravenous injection of [(11)C]RO5013853 attains reliable measurements of GlyT1 binding in accordance with the expected transporter distribution in the human brain. [(11)C]RO5013853 is a radioligand suitable for further clinical PET studies. Full characterization of a novel radiotracer for GlyT1 in humans is provided. The tracer has subsequently been used to assess receptor occupancy in healthy volunteers and to estimate occupancy at doses associated with best efficacy in a clinical trial with schizophrenic patients with predominantly negative symptoms.