Sensitive targeted methods for brain metabolomic studies in microdialysis samples

Unravelling the brain metabolome: A review of liquid chromatography - mass spectrometry strategies for extracellular brain metabolomics

The analysis of the brain extracellular metabolome is of interest for numerous subdomains within neuroscience. Not only does it provide information about normal physiological functions, it is even more of interest for biomarker discovery and target discovery in disease. The extracellular analysis of the brain is particularly interesting as it provides information about the release of mediators in the brain extracellular fluid to look at cellular signaling and metabolic pathways through the release, diffusion and re-uptake of neurochemicals. In vivo samples are obtained through microdialysis, cerebral open-flow microperfusion or solid-phase microextraction. The analytes of potential interest are typically low in concentration and can have a wide range of physicochemical properties. Liquid chromatography coupled to mass spectrometry has proven its usefulness in brain metabolomics. It allows sensitive and specific analysis of low sample volumes, obtained through different approaches. Several strategies for the analysis of the extracellular fluid have been proposed. The most widely used approaches apply sample derivatization, specific stationary phases and/or hydrophilic interaction liquid chromatography. Miniaturization of these methods allows an even higher sensitivity. The development of chiral metabolomics is indispensable, as it allows to compare the enantiomeric ratio of compounds and provides even more challenges. Some limitations continue to exist for the previously developed methods and the development of new, more sensitive methods remains needed. This review provides an overview of the methods developed for sampling and liquid chromatography-mass spectrometry analysis of the extracellular metabolome.

Tutorial review for peptide assays: An ounce of pre-analytics is worth a pound of cure

The recent increase in peptidomimetic-based medications and the growing interest in peptide hormones has brought new attention to the quantification of peptides for diagnostic purposes. Indeed, the circulating concentrations of peptide hormones in the blood provide a snapshot of the state of the body and could eventually lead to detecting a particular health condition. Although extremely useful, the quantification of such molecules, preferably by liquid chromatography coupled to mass spectrometry, might be quite tricky. First, peptides are subjected to hydrolysis, oxidation, and other post-translational modifications, and, most importantly, they are substrates of specific and nonspecific proteases in biological matrixes. All these events might continue after sampling, changing the peptide hormone concentrations. Second, because they include positively and negatively charged groups and hydrophilic and hydrophobic residues, they interact with their environment; these interactions might lead to a local change in the measured concentrations. A phenomenon such as nonspecific adsorption to lab glassware or materials has often a tremendous effect on the concentration and needs to be controlled with particular care. Finally, the circulating levels of peptides might be low (pico- or femtomolar range), increasing the impact of the aforementioned effects and inducing the need for highly sensitive instruments and well-optimized methods. Thus, despite the extreme diversity of these peptides and their matrixes, there is a common challenge for all the assays: the need to keep concentrations unchanged from sampling to analysis. While significant efforts are often placed on optimizing the analysis, few studies consider in depth the impact of pre-analytical steps on the results. By working through practical examples, this solution-oriented tutorial review addresses typical pre-analytical challenges encountered during the development of a peptide assay from the standpoint of a clinical laboratory. We provide tips and tricks to avoid pitfalls as well as strategies to guide all new developments. Our ultimate goal is to increase pre-analytical awareness to ensure that newly developed peptide assays produce robust and accurate results.

Behavioral metabolomics: how behavioral data can guide metabolomics research on neuropsychiatric disorders

INTRODUCTION

Metabolomics produces vast quantities of data but determining which metabolites are the most relevant to the disease or disorder of interest can be challenging.

OBJECTIVES

This study sought to demonstrate how behavioral models of psychiatric disorders can be combined with metabolomics research to overcome this limitation.

METHODS

We designed a preclinical, untargeted metabolomics procedure, that focuses on the determination of central metabolites relevant to substance use disorders that are (a) associated with changes in behavior produced by acute drug exposure and (b) impacted by repeated drug exposure. Untargeted metabolomics analysis was carried out on liquid chromatography-mass spectrometry data obtained from 336 microdialysis samples. Samples were collected from the medial striatum of male Sprague-Dawley (N = 21) rats whilst behavioral data were simultaneously collected as part of a (±)-3,4-methylenedioxymethamphetamine (MDMA)-induced behavioral sensitization experiment. Analysis was conducted by orthogonal partial least squares, where the Y variable was the behavioral data, and the X variables were the relative concentrations of the 737 detected features.

RESULTS

MDMA and its derivatives, serotonin, and several dopamine/norepinephrine metabolites were the greatest predictors of acute MDMA-produced behavior. Subsequent univariate analyses showed that repeated MDMA exposure produced significant changes in MDMA metabolism, which may contribute to the increased abuse liability of the drug as a function of repeated exposure.

CONCLUSION

These findings highlight how the inclusion of behavioral data can guide metabolomics data analysis and increase the relevance of the results to the phenotype of interest.

A study on puerarin in situ gel eye drops: Formulation optimization and pharmacokinetics on rabbits by microdialysis

Puerarin (PUE), an isoflavonoid isolated from Pueraria lobata (Willd) Ohwi root, is a β-adrenergic receptor inhibitor used in treating glaucoma. The concentration range of gellan gum was determined based on the formulation viscosity and gelling capacity. PVP-K30 and gellan gum were used as variables, with the viscosity of formulation: STF = 40: 21, the 4 h permeation rate of rabbit isolated sclera, and 2 h in vitro release rate as response values. The JMP software was used to optimize the results, presenting that gellan gum was the main factor influencing viscosity. The in vitro release and permeation rate were primarily influenced by PVP-K30. The optimal prescription was 0.45% gellan gum and 6.0% PVP-K30. The in vitro release and permeation characteristics of puerarin in situ gel (PUE-ISG) were investigated using PUE solution as a control. The dialysis bag method results indicated that the release of the solution group leveled off after 4 h, while the PUE-ISG group had been continuously releasing. However, the cumulative release rates of the two were no longer significantly different at 10 h. The cumulative permeation rates of the ISG and solution groups were not significantly different (P > 0.05) in the rabbit isolated sclera. The apparent permeability Papp and steady-state flux Jss of PUE-ISG were 0.950 ± 0.059 cm·h^-1 and 9.504 ± 0.587 mg·cm^-2·h^-1, respectively. A sensitive and stable HPLC-MS/MS analytical method for quantifying aqueous humor concentrations of PUE was validated. A microdialysis technique was successfully used in the aqueous humor pharmacokinetics study to sample aqueous humor from rabbit eye continuously. The results revealed that PUE-ISG significantly increased the drug concentration in the aqueous humor, with C_max and AUC_(0-t) 3.77 and 4.40 times higher than those of the solution group, respectively. T_max was also significantly prolonged, indicating good prospects for clinical application. The developed PUE-ISG preparation has the characteristics of rapid drug release and sustained permeation, and increase the drug concentration in aqueous humor, with all inactive ingredients remaining within the maximum allowable limits recommended by the FDA guideline.

Development of Pseudo-targeted Profiling of Isotopic Metabolomics using Combined Platform of High Resolution Mass Spectrometry and Triple Quadrupole Mass Spectrometry with Application of 13C6-Glucose Tracing in HepG2 Cells

Isotope tracing assisted metabolic analysis is becoming a unique tool to understand metabolic regulation in cell biology and biomedical research. Targeted mass spectrometry analysis based on selected reaction monitoring (SRM) has been widely applied in isotope tracing experiment with the advantages of high sensitivity and broad linearity. However, its application for new pathway discovery is largely restrained by molecular coverage. To overcome this limitation, we describe a strategy called pseudo-targeted profiling of isotopic metabolomics (PtPIM) to expand the analysis of isotope labeled metabolites beyond the limit of known pathways and chemical standards. Pseudo-targeted metabolomics was first established with ion transitions and retention times transformed from high resolution (orbitrap) mass spectrometry. Isotope labeled MRM transitions were then generated according to chemical formulas of fragments, which were derived from accurate ion masses acquired by HRMS. An in-house software "PseudoIsoMRM" was developed to simulate isotope labeled ion transitions in batch mode and correct the interference of natural isotopologues. This PtPIM strategy was successfully applied to study ¹³C₆-glucose traced HepG2 cells. As 313 molecules determined as analysis targets, a total of 4104 ion transitions were simulated to monitor ¹³C labeled metabolites in positive-negative switching mode of QQQ mass spectrometer with minimum dwell time of 0.3 ms achieved. A total of 68 metabolites covering glycolysis, TCA cycle, nucleotide biosynthesis, one-carbon metabolism and related derivatives were found to be labeled (> 2%) in HepG2 cells. Active pentose phosphate pathway was observed with diverse labeling status of glycolysis intermediates. Meanwhile, our PtPIM strategy revealed that rotenone severely suppressed mitochondrial function e.g. oxidative phosphorylation and fatty acid beta-oxidation. In this case, anaerobic respiration became the major source of energy metabolism by producing abundant lactate. Conclusively, the simulation based PtPIM method demonstrates a strategy to broaden metabolite coverage in isotope tracing analysis independent of standard chemicals.

Harnessing cerebrospinal fluid circulation for drug delivery to brain tissues

Initially thought to be useful only to reach tissues in the immediate vicinity of the CSF circulatory system, CSF circulation is now increasingly viewed as a viable pathway to deliver certain therapeutics deeper into brain tissues. There is emerging evidence that this goal is achievable in the case of large therapeutic proteins, provided conditions are met that are described herein. We show how fluid dynamic modeling helps predict infusion rate and duration to overcome high CSF turnover. We posit that despite model limitations and controversies, fluid dynamic models, pharmacokinetic models, preclinical testing, and a qualitative understanding of the glymphatic system circulation can be used to estimate drug penetration in brain tissues. Lastly, in addition to highlighting landmark scientific and medical literature, we provide practical advice on formulation development, device selection, and pharmacokinetic modeling. Our review of clinical studies suggests a growing interest for intra-CSF delivery, particularly for targeted proteins.

Spatially Resolved Analytical Chemistry in Intact, Living Tissues

Tissues are an exciting frontier for bioanalytical chemistry, one in which spatial distribution is just as important as total content. Intact tissue preserves the native cellular and molecular organization and the cell-cell contacts found in vivo. Live tissue, in particular, offers the potential to analyze dynamic events in a spatially resolved manner, leading to fundamental biological insights and translational discoveries. In this Perspective, we provide a tutorial on the four fundamental challenges for the bioanalytical chemist working in living tissue samples as well as best practices for mitigating them. The challenges include (i) the complexity of the sample matrix, which contributes myriad interfering species and causes nonspecific binding of reagents; (ii) hindered delivery and mixing; (iii) the need to maintain physiological conditions; and (iv) tissue reactivity. This framework is relevant to a variety of methods for spatially resolved chemical analysis, including optical imaging, inserted sensors and probes such as electrodes, and surface analyses such as sensing arrays. The discussion focuses primarily on ex vivo tissues, though many considerations are relevant in vivo as well. Our goal is to convey the exciting potential of analytical chemistry to contribute to understanding the functions of live, intact tissues.

Isoflurane anesthesia disrupts the cortical metabolome

Identifying similarities and differences in the brain metabolome during different states of consciousness has broad relevance for neuroscience and state-dependent autonomic function. This study focused on the prefrontal cortex (PFC) as a brain region known to modulate states of consciousness. Anesthesia was used as a tool to eliminate wakefulness. Untargeted metabolomic analyses were performed on microdialysis samples obtained from mouse PFC during wakefulness and during isoflurane anesthesia. Analyses detected 2,153 molecules, 91 of which could be identified. Analytes were grouped as detected during both wakefulness and anesthesia (n = 61) and as unique to wakefulness (n = 23) or anesthesia (n = 7). Data were analyzed using univariate and multivariate approaches. Relative to wakefulness, during anesthesia there was a significant (q < 0.0001) fourfold change in 21 metabolites. During anesthesia 11 of these 21 molecules decreased and 10 increased. The Kyoto Encyclopedia of Genes and Genomes database was used to relate behavioral state-specific changes in the metabolome to metabolic pathways. Relative to wakefulness, most of the amino acids and analogs measured were significantly decreased during isoflurane anesthesia. Nucleosides and analogs were significantly increased during anesthesia. Molecules associated with carbohydrate metabolism, maintenance of lipid membranes, and normal cell functions were significantly decreased during anesthesia. Significant state-specific changes were also discovered among molecules comprising lipids and fatty acids, monosaccharides, and organic acids. Considered together, these molecules regulate point-to-point transmission, volume conduction, and cellular metabolism. The results identify a novel ensemble of candidate molecules in PFC as putative modulators of wakefulness and the loss of wakefulness.

NEW & NOTEWORTHY The loss of wakefulness caused by a single concentration of isoflurane significantly altered levels of interrelated metabolites in the prefrontal cortex. The results support the interpretation that states of consciousness reflect dynamic interactions among cortical neuronal networks involving a humbling number of molecules that comprise the brain metabolome.

Cerebral Microdialysis as a Tool for Assessing the Delivery of Chemotherapy in Brain Tumor Patients

The development of curative treatment for glioblastoma has been extremely challenging. Chemotherapeutic agents that have seemed promising have failed in clinical trials. Drugs that can successfully target cancer cells within the brain must first traverse the brain interstitial fluid. Cerebral microdialysis (CMD) is an invasive technique in which interstitial fluid can be directly sampled. CMD has primarily been used clinically in the setting of head trauma and subarachnoid hemorrhage. Our goal was to review the techniques, principles, and new data pertaining to CMD to highlight its use in neuro-oncology. We conducted a literature search using the PubMed database and selected studies in which the investigators had used CMD in either animal brain tumor models or clinical trials. The references were reviewed for additional information. Studies of CMD have shown its importance as a neurosurgical technique. CMD allows for the collection of pharmacokinetic data on drug penetrance across the blood-brain barrier and metabolic data to characterize the response to chemotherapy. Although no complications have been reported, the current CMD technique (as with any procedure) has risks and limitations, which we have described in the present report. Animal CMD experiments have been used to exclude central nervous system drug candidates from progressing to clinical trials. At present, patients undergoing CMD have been monitored in the intensive care unit, owing to the requisite tethering to the apparatus. This can be expected to change soon because of advances in microminiaturization. CMD is an extremely valuable, yet underused, technique. Future CMD applications will have central importance in assessing drug delivery to tumor cells in vivo, allowing a pathway to successful therapy for malignant brain tumors.

Solid-phase microextraction: An appealing alternative for the determination of endogenous substances - A review

The determination of endogenous substances is of great significance for obtaining important biotic information such as biological components, metabolic pathways and disease biomarkers in different living organisms (e.g. plants, insects, animals and humans). However, due to the complex matrix and the trace concentrations of target analytes, the sample preparation procedure is an essential step before the analytes of interest are introduced into a detection instrument. Solid-phase microextraction (SPME), an emerging sample preparation technique that integrates sampling, extraction, concentration, and sample introduction into one step, has gained wide acceptance in various research fields, including in the determination of endogenous compounds. In this review, recent developments and applications of SPME for the determination of endogenous substances over the past five years are summarized. Several aspects, including the design of SPME devices (sampling configuration and coating), applications (in vitro and in vivo sampling), and coupling with emerging instruments (comprehensive two-dimensional gas chromatography (GC × GC), ambient mass spectrometry (AMS) and surface enhanced Raman scattering (SERS)) are involved. Finally, the challenges and opportunities of SPME methods in endogenous substances analysis are also discussed.

A novel microemulsion-based isotonic perfusate modulated by Ringer's solution for improved microdialysis recovery of liposoluble substances

Background

Microdialysis is promising technique for dynamic microbiochemical sampling from tissues. However, the application of typical aqueous perfusates to liposoluble substances is limited. In this study, a novel microemulsion (ME)-based isotonic perfusate (RS-ME) was prepared to improve the recovery of liposoluble components using microdialysis probes.

Results

Based on pseudo-ternary phase diagrams and comparisons of the ME area, Kolliphor^® EL and Transcutol^® P were selected as the surfactant and co-surfactant, respectively, with a weight ratio (Km) of 2:1 and ethyl oleate as the oil phase. The ME was mixed with Ringer’s solution at a 1:6 ratio (v/v) to obtain the isotonic RS-ME. The droplet size distribution of the ME in RS-ME was 78.3 ± 9.2 nm, with a zeta potential of − 3.5 ± 0.3 mV. By microdialysis perfusion, RS-ME achieved higher recovery rates of the poorly water-soluble compounds evodiamine (EVO) and ruthenium (RUT), i.e., 58.36 ± 0.57% and 49.40 ± 0.57%, respectively, than those of 20% (v/v) PEG 400 Ringer's solution (RS-PEG) and 10% (v/v) ethanol Ringer’s solution (RS-EtOH). In vivo microdialysis experiments confirmed that RS-ME captured EVO and RUT molecules around the dialysis membrane more efficiently and exhibited less spreading than RS-PEG and RS-EtOH.

Conclusions

Owing to the nanosized droplets formed by lipid components in the RS-ME and the limited dispersion out of the dialysis membrane, we obtained good biocompatibility and reliable dialysis results, without affecting the tissue microenvironment. As a novel perfusate, RS-ME provides an easy and reliable approach to the microdialysis sampling of fat-soluble components.