A spiked tissue-based approach for quantification of phosphatidylcholines in brain section by MALDI mass spectrometry imaging

Tracking Drugs and Lipids: Quantitative Mass Spectrometry Imaging of Liposomal Doxorubicin Delivery and Bilayer Fate in Three-Dimensional Tumor Models

Targeted therapy to the tumor would greatly advance precision medicine. Many drug delivery vehicles have emerged, but liposomes are cited as the most successful to date. Recent efforts to develop liposomal drug delivery systems focus on drug distribution in tissues and ignore liposomal fate. In this study, we developed a novel method to elucidate both drug and liposomal bilayer distribution in a three-dimensional cell culture model using quantitative matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI qMSI) alongside fluorescence microscopy. Imaging liposomal distribution in a cell culture model is challenging, as lipids forming the bilayer are endogenous to the model system. To resolve this issue, we functionalized the bilayer by chemically cross-linking a fluorescent tag to the alkyne-containing lipid hexynoyl phosphoethanolamine (HPE). We synthesized liposomes incorporating the tagged HPE lipid and encapsulated within them doxorubicin, yielding a theranostic liposome capable of both drug delivery and monitoring liposomal uptake. We employed an "in-tissue" MALDI qMSI approach to generate a calibration curve with R2 = 0.9687, allowing for quantification of doxorubicin within spheroid sections at multiple time points. After 72 h of treatment with the theranostic liposomes, full doxorubicin penetration was observed. The metabolites doxorubicinone and 7-deoxydoxorubicinone were also detected after 48 h. Modification of the bilayer allowed for fluorescence microscopy tracking of liposomes, while MALDI MSI simultaneously permitted the imaging of drugs and metabolites. While we demonstrated the utility of our method with doxorubicin, this system could be applied to examine the uptake, release, and metabolism of many other liposome-encapsulated drugs.

Quantitative mass spectrometry imaging: therapeutics & biomolecules

Mass spectrometry imaging (MSI) has become increasingly utilized in the analysis of biological molecules. MSI grants the ability to spatially map thousands of molecules within one experimental run in a label-free manner. While MSI is considered by most to be a qualitative method, recent advancements in instrumentation, sample preparation, and development of standards has made quantitative MSI (qMSI) more common. In this feature article, we present a tailored review of recent advancements in qMSI of therapeutics and biomolecules such as lipids and peptides/proteins. We also provide detailed experimental considerations for conducting qMSI studies on biological samples, aiming to advance the methodology.

Toward Omics-Scale Quantitative Mass Spectrometry Imaging of Lipids in Brain Tissue Using a Multiclass Internal Standard Mixture

Mass spectrometry imaging (MSI) has accelerated our understanding of lipid metabolism and spatial distribution in tissues and cells. However, few MSI studies have approached lipid imaging quantitatively and those that have focused on a single lipid class. We overcome this limitation by using a multiclass internal standard (IS) mixture sprayed homogeneously over the tissue surface with concentrations that reflect those of endogenous lipids. This enabled quantitative MSI (Q-MSI) of 13 lipid classes and subclasses representing almost 200 sum-composition lipid species using both MALDI (negative ion mode) and MALDI-2 (positive ion mode) and pixel-wise normalization of each lipid species in a manner analogous to that widely used in shotgun lipidomics. The Q-MSI approach covered 3 orders of magnitude in dynamic range (lipid concentrations reported in pmol/mm2) and revealed subtle changes in distribution compared to data without normalization. The robustness of the method was evaluated by repeating experiments in two laboratories using both timsTOF and Orbitrap mass spectrometers with an ∼4-fold difference in mass resolution power. There was a strong overall correlation in the Q-MSI results obtained by using the two approaches. Outliers were mostly rationalized by isobaric interferences or the higher sensitivity of one instrument for a particular lipid species. These data provide insight into how the mass resolving power can affect Q-MSI data. This approach opens up the possibility of performing large-scale Q-MSI studies across numerous lipid classes and subclasses and revealing how absolute lipid concentrations vary throughout and between biological tissues.

An Alternative Method for Quantitative Mass Spectrometry Imaging (q-MSI) of Dopamine Utilizing Fragments of Animal Tissue

Mass spectrometry imaging (MSI) is a well-known method for the ionization of molecules on tissue sections and the visualization of their localization. Recently, different sample preparation methods and new instruments have been used for MSI, and different molecules are becoming visible. On the other hand, although several quantification methods (q-MSI) have been proposed, there is still room for the development of a simplified procedure. Here, we have attempted to develop a reproducible and reliable quantification method using a calibration curve prepared from tissue debris of a frozen section of a sample when we trim the frozen blocks. We discuss the reproducibility of this method across different sample lots and the effect of the biological matrix (ion suppression) on our results. The quantitative performance was evaluated in terms of accuracy and relative standard deviation, and the reliability of the quantitative values obtained by matrix-assisted laser desorption/ionization-MSI was further evaluated by enzyme-linked immunosorbent assay (ELISA). Our q-MSI method for the quantification of dopamine in mouse brain tissue was found to be highly linear, accurate, and precise. The quantitative values obtained by MSI were found to be highly comparable (>85% similarity) to the results obtained by ELISA from the same tissue extracts.

Desorption Kinetics Evaluation for the Development of Validated Desorption Electrospray Ionization-Mass Spectrometric Assays for Drug Quantification in Tissue Sections

The development of desorption/ionization (DI) mass spectrometric (MS) assays for drug quantification in tissue sections and their validation according to regulatory guidelines would enable their universalization for applications in (clinical) pharmacology. Recently, new enhancements in desorption electrospray ionization (DESI) have highlighted the reliability of this ion source for the development of targeted quantification methods that meet requirements for method validation. However, it is necessary to consider subtle parameters leading to the success of such method developments, such as the morphology of desorption spots, the analytical time, and sample surface, to cite but a few. Here, we provide additional experimental data highlighting an additional important parameter, based on the unique advantage of DESI-MS on continuous extraction during analysis. We demonstrate that considering desorption kinetics during DESI analyses would largely help (i) reducing analytical time during profiling analyses, (ii) verifying solvent-based drug extraction using the selected sample preparation method for profiling and imaging modes, and (iii) predicting the feasibility of imaging assays using samples in a given expected concentration range of the targeted drug. These observations will likely serve as precious guidance for the development of validated DESI-profiling and imaging methods in the future.

Matrix-assisted laser desorption/ionization-Fourier-transform ion cyclotron resonance-mass spectrometry analysis of exosomal lipids from human serum

RATIONALE

Exosomes contain biomarkers such as proteins and lipids that help in understanding normal physiology and diseases. Lipids, in particular, are infrequently studied using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) for biomarker discovery. In this study, MALDI was equipped with a high-resolution MS to investigate exosomal lipids from human serum.

METHODS

Exosomal lipids were profiled using MALDI with Fourier-transform ion cyclotron resonance (FTICR)-MS. Four matrices (i.e., α-cyano-4-hydroxycinnamic acid [CHCA], 2,5-dihydroxybenzoic acid, sinapinic acid, and graphene oxide [GO]) and three sample preparation methods (i.e., dried droplet, thin layer, and two layer) were compared for the number of lipid species detected and the relative abundance of each lipid from human serum and human serum exosomes.

RESULTS

In sum, 172 and 89 lipid species were identified from human serum and human serum exosomes, respectively, using all the methods. The highest number of exosome lipid species, 69, was detected using the CHCA matrix, whereas only 8 exosome lipid species were identified using the GO matrix. Among the identified lipid species, phosphatidylcholine was identified most frequently, probably due to the use of a positive ion mode.

CONCLUSIONS

Exosomes and human serum showed comparable lipid profiles as determined using MALDI-FTICR-MS. These findings provide a new perspective on exosomal lipidomics analysis and may serve as a foundation for future lipidomics-based biomarker research using MALDI-FTICR-MS.

Evaluation of Quantitative Platforms for Single Target Mass Spectrometry Imaging

(1) Imaging of pharmaceutical compounds in tissue is an increasingly important subsection of Mass Spectrometry Imaging (MSI). Identifying proper target engagement requires MS platforms with high sensitivity and spatial resolution. Three prominent categories of drugs are small molecule drugs, antibody-drug conjugate payloads, and protein degraders. (2) We tested six common MSI platforms for their limit of detection (LoD) on a representative compound for each category: a Matrix-Assisted Laser Desorption/Ionization (MALDI) Fourier Transform Ion Cyclotron, a MALDI-2 Time-of-Flight (ToF), a MALDI-2 Trapped Ion Mobility Spectrometry ToF, a Desorption Electrospray Ionization Orbitrap, and 2 Atmospheric Pressure-MALDI Triple Quadrupoles. Samples were homogenized tissue mimetic models of rat liver spiked with known concentrations of analytes. (3) We found that the AP-MALDI-QQQ platform outperformed all 4 competing platforms by a minimum of 2- to 52-fold increase in LoD for representative compounds from each category of pharmaceutical. (4) AP-MALDI-QQQ platforms are effective, cost-efficient mass spectrometers for the identification of targeted analytes of interest.

What are we imaging? Software tools and experimental strategies for annotation and identification of small molecules in mass spectrometry imaging

Mass spectrometry imaging (MSI) has become a widespread analytical technique to perform nonlabeled spatial molecular identification. The Achilles' heel of MSI is the annotation and identification of molecular species due to intrinsic limitations of the technique (lack of chromatographic separation and the difficulty to apply tandem MS). Successful strategies to perform annotation and identification combine extra analytical steps, like using orthogonal analytical techniques to identify compounds; with algorithms that integrate the spectral and spatial information. In this review, we discuss different experimental strategies and bioinformatics tools to annotate and identify compounds in MSI experiments. We target strategies and tools for small molecule applications, such as lipidomics and metabolomics. First, we explain how sample preparation and the acquisition process influences annotation and identification, from sample preservation to the use of orthogonal techniques. Then, we review twelve software tools for annotation and identification in MSI. Finally, we offer perspectives on two current needs of the MSI community: the adaptation of guidelines for communicating confidence levels in identifications; and the creation of a standard format to store and exchange annotations and identifications in MSI.

Cell and Tissue Imaging by TOF-SIMS and MALDI-TOF: An Overview for Biological and Pharmaceutical Analysis

The potential of mass spectrometry imaging (MSI) has been demonstrated in cell and tissue research since 1970. MSI can reveal the spatial distribution of a wide range of atomic and molecular ions detected from biological sample surfaces, it is a powerful and valuable technique used to monitor and detect diverse chemical and biological compounds, such as drugs, lipids, proteins, and DNA. MSI techniques, notably matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) and time of flight secondary ion mass spectrometry (TOF-SIMS), witnessed a dramatic upsurge in studying and investigating biological samples especially, cells and tissue sections. This advancement is attributed to the submicron lateral resolution, the high sensitivity, the good precision, and the accurate chemical specificity, which make these techniques suitable for decoding and understanding complex mechanisms of certain diseases, as well as monitoring the spatial distribution of specific elements, and compounds. While the application of both techniques for the analysis of cells and tissues is thoroughly discussed, a briefing of MALDI-TOF and TOF-SIMS basis and the adequate sampling before analysis are briefly covered. The importance of MALDI-TOF and TOF-SIMS as diagnostic tools and robust analytical techniques in the medicinal, pharmaceutical, and toxicology fields is highlighted through representative published studies.

Mass spectrometry in the lipid study of cancer

Introduction: Cancer is a heterogeneous disease that exploits various metabolic pathways to meet the demand for increased energy and structural components. Lipids are biomolecules that play essential roles as high energy sources, mediators, and structural components of biological membranes. Accumulating evidence has established that altered lipid metabolism is a hallmark of cancer.Areas covered: Mass spectrometry (MS) is a label-free analytical tool that can simultaneously identify and quantify hundreds of analytes. To date, comprehensive lipid studies exclusively rely on this technique. Here, we reviewed the use of MS in the study of lipids in various cancers and discuss its instrumental limitations and challenges.Expert opinion: MS and MS imaging have significantly contributed to revealing altered lipid metabolism in a variety of cancers. Currently, a single MS approach cannot profile the entire lipidome because of its lack of sensitivity and specificity for all lipid classes. For the metabolic pathway investigation, lipid study requires the integration of MS with other molecular approaches. Future developments regarding the high spatial resolution, mass resolution, and sensitivity of MS instruments are warranted.

Desorption/Ionization-MS Methods for Drug Quantification in Biological Matrices and Their Validation Following Regulatory Guidance

Desorption/ionization (DI) methods play an important role among the panel of mass spectrometric (MS) approaches for the rapid and sensitive quantification of drugs from the surface of solid samples. The possibility to implement these approaches for pharmacokinetic/pharmacodynamic investigations in early phase clinical trials depends on the ability to validate quantification assays according to regulatory guidelines (e.g., US Food and Drug Administration and European Medicines Agency) for bioanalytical method validation. However, these guidelines were designed for the validation of liquid chromatography-MS (LC-MS) methods and ligand binding assays. To apply the validation parameters to DI-MS methods (also referred here as on-surface MS) for drug quantification, it is important to consider the particularities of DI approaches compared to LC-MS methods. In this Perspective, we summarize the various applications of on-surface MS methods for drug quantification with their advantages over other MS methods, and provide our point of view regarding future proper method development and validation.

Applications of stable isotopes in MALDI imaging: current approaches and an eye on the future

Matrix-assisted laser desorption/ionisation-imaging mass spectrometry (MALDI-IMS) is now an established imaging modality with particular utility in the study of biological, biomedical and pathological processes. In the first instance, the use of stable isotopically labelled (SIL) compounds in MALDI-IMS has addressed technical barriers to increase the accuracy and versatility of this technique. This has undoubtedly enhanced our ability to interpret the two-dimensional ion intensity distributions produced from biological tissue sections. Furthermore, studies using delivery of SIL compounds to live tissues have begun to decipher cell, tissue and inter-tissue metabolism while maintaining spatial resolution. Here, we review both the technical and biological applications of SIL compounds in MALDI-IMS, before using the uptake and metabolism of glucose in bovine ocular lens tissue to illustrate the current limitations of SIL compound use in MALDI-IMS. Finally, we highlight recent instrumentation advances that may further enhance our ability to use SIL compounds in MALDI-IMS to understand biological and pathological processes. Graphical Abstract.

A procedure for method development and protein binding ratio as the indicator of sensitivity with anticancer agents on MALDI mass spectrometry imaging

The concentration and distribution of a drug and its metabolites in tissues are key factors for elucidating both drug efficacy and toxicity in drug development. In this study we developed a pharamaco-imaging procedure for 12 agents and investigated the relationship between the properties of target compounds and the sensitivities of detection in matrix-assisted laser desorption/ionization-mass spectrometer imaging (MALDI-MSI). We prepared mock samples with mouse liver homogenates diluted with gelatin solution, limit of detection concentrations of each compound was confirmed. The correlation was evaluated between the intensities of mass signals obtained in MALDI-MSI with each test compound (the intensities of the test compounds) at a consistent concentration and the properties of each test compound. The liver homogenate diluted with gelatin solution showed easier handling and lower coefficients of variation than did liver homogenate only, and can be used as a good surrogate matrix. Based on the analysis of 12 agents, the protein binding ratios showed significant correlation to the detection sensitivities. We presented a procedure for standardization of pharmaco-imaging method development with an in-tissue method using MALDI-MS. Our results indicated the correlation between test compound's sensitivity and their protein binding ratios in plasma or serum.

Approaching sites of action of drugs in clinical pharmacology: New analytical options and their challenges

Clinical pharmacology is an important discipline for drug development aiming to define pharmacokinetics (PK), pharmacodynamics (PD) and optimum exposure to drugs, i.e. the concentration-response relationship and its modulators. For this purpose, information on drug concentrations at the anatomical, cellular and molecular sites of action is particularly valuable. In pharmacological assays, the limited accessibility of target cells in readily available samples (i.e. blood) often hampers mass spectrometry-based monitoring of the absolute quantity of a compound and the determination of its molecular action at the cellular level. Recently, new sample collection methods have been developed for the specific capture of rare circulating cells, especially for the diagnosis of circulating tumour cells. In parallel, new advances and developments in mass spectrometric instrumentation now allow analyses to be scaled down to the cellular level. Together, these developments may permit the monitoring of minute drug quantities and show their effect at the cellular level. In turn, such PK/PD associations on a cellular level would not only enrich our pharmacological knowledge of a given compound but also expand the basis for PK/PD simulations. In this review, we describe novel concepts supporting clinical pharmacology at the anatomical, cellular and molecular sites of action, and highlight the new challenges in mass spectrometry-based monitoring. Moreover, we present methods to tackle these challenges and define future needs.

Considerations for MALDI-Based Quantitative Mass Spectrometry Imaging Studies

Significant advances in mass spectrometry imaging (MSI) have pushed the boundaries in obtaining spatial information and quantification in biological samples. Quantitative MSI (qMSI) has typically been challenging to achieve because of matrix and tissue heterogeneity, inefficient analyte extraction, and ion suppression effects, but recent studies have demonstrated approaches to obtain highly robust methods and reproducible results. In this perspective, we share our insights into sample preparation, how the choice of matrix influences sensitivity, construction of calibration curves, signal normalization, and visualization of MSI data. We hope that by articulating these guidelines that qMSI can be routinely conducted while retaining the analytical merits of other mass spectrometry modalities.

Validation of MALDI-MS imaging data of selected membrane lipids in murine brain with and without laser postionization by quantitative nano-HPLC-MS using laser microdissection

MALDI mass spectrometry imaging (MALDI-MSI) is a widely used technique to map the spatial distribution of molecules in sectioned tissue. The technique is based on the systematic generation and analysis of ions from small sample volumes, each representing a single pixel of the investigated sample surface. Subsequently, mass spectrometric images for any recorded ion species can be generated by displaying the signal intensity at the coordinate of origin for each of these pixels. Although easily equalized, these recorded signal intensities, however, are not necessarily a good measure for the underlying amount of analyte and care has to be taken in the interpretation of MALDI-MSI data. Physical and chemical properties that define the analyte molecules’ adjacencies in the tissue largely influence the local extraction and ionization efficiencies, possibly leading to strong variations in signal intensity response. Here, we inspect the validity of signal intensity distributions recorded from murine cerebellum as a measure for the underlying molar distributions. Based on segmentation derived from MALDI-MSI measurements, laser microdissection (LMD) was used to cut out regions of interest with a homogenous signal intensity. The molar concentration of six exemplary selected membrane lipids from different lipid classes in these tissue regions was determined using quantitative nano-HPLC-ESI-MS. Comparison of molar concentrations and signal intensity revealed strong deviations between underlying concentration and the distribution suggested by MSI data. Determined signal intensity response factors strongly depend on tissue type and lipid species.

Development of a Mass Spectrometry Imaging Method for Detecting and Mapping Graphene Oxide Nanoparticles in Rodent Tissues

Graphene-based nanoparticles are continuously being developed for biomedical applications, and their use raises concerns about their environmental and biological impact. In the literature, some imaging techniques based on fluorescence and radioimaging have been used to explore their fate in vivo. Here, we report on the use of label-free mass spectrometry and mass spectrometry imaging (MSI) for graphene oxide (GO) and reduced graphene oxide (rGO) analyses in rodent tissues. Thereby, we extend previous work by focusing on practical questions to obtain reliable and meaningful images. Specific radical anionic carbon clusters ranging from C₂^-• to C₉^-• were observed for both GO and rGO species, with a base peak at m/z 72 under negative laser desorption ionization mass spectrometry (LDI-MS) conditions. Extension to an LDI-MSI method was then performed, thus enabling the efficient detection of GO nanoparticles in lung tissue sections of previously exposed mice. The possibility of quantifying those nanoparticles on tissue sections has also been investigated. Two different ways of building calibration curves (i.e., GO suspensions spotted on tissue sections, or added to lung tissue homogenates) were evaluated and returned similar results, with linear dynamic concentration ranges over at least 2 orders of magnitude. Moreover, intra- and inter-day precision studies have been assessed, with relative standard deviation below 25% for each concentration point of a calibration curve. In conclusion, our study confirms that LDI-MSI is a relevant approach for biodistribution studies of carbon-based nanoparticles, as quantification can be achieved, provided that nanoparticle suspension and manufacturing are carefully controlled.

Quantification and assessment of detection capability in imaging mass spectrometry using a revised mimetic tissue model

Aim: A revised method of preparing the mimetic tissue model for quantitative imaging mass spectrometry (IMS) is evaluated. Concepts of assessing detection capability are adapted from other imaging or mass spectrometry (MS)-based technologies to improve upon the reliability of IMS quantification. Materials & methods: The mimetic tissue model is prepared by serially freezing spiked-tissue homogenates into a cylindrical mold to create a plug of tissue with a stepped concentration gradient of matrix-matched standards. Weighted least squares (WLS) linear regression is applied due to the heteroscedastisity (change in variance with intensity) of most MS data. Results & conclusions: Imaging poses several caveats for quantification which are unique compared with other MS-based methods. Aspects of the design, construction, application, and evaluation of the matrix-matched standard curve for the mimetic tissue model are discussed. In addition, the criticality of the ion distribution in the design of a purposeful liquid chromatography coupled to mass spectrometry (LC-MS) validation is reviewed.

Quantitative Imaging of Proteins in Tissue by Stable Isotope Labeled Mimetic Liquid Extraction Surface Analysis Mass Spectrometry

Absolute quantification of proteins in tissue is important for numerous fields of study. Liquid chromatography–mass spectrometry (LC–MS) methods are the norm but typically involve lengthy sample preparation including tissue homogenization, which results in the loss of information relating to spatial distribution. Here, we propose liquid extraction surface analysis (LESA) mass spectrometry (MS) of stable isotope labeled mimetic tissue models for the spatially resolved quantification of intact ubiquitin in rat and mouse brain tissue. Measured ubiquitin concentrations are in agreement with values found in the literature. Images of rat and mouse brain tissue demonstrate spatial variation in the concentration of ubiquitin and demonstrate the utility of spatially resolved quantitative measurement of proteins in tissue. Although we have focused on ubiquitin, the method has the potential for broader application to the absolute quantitation of any endogenous protein or protein-based drug in tissue.

Fabrication of homogenous three-dimensional biomimetic tissue for mass spectrometry imaging

Reference samples are essential for mass spectrometric method optimization, data quality control, and target analyte quantitation. However, it is highly challenging to prepare an ideal homogeneous, standard-spiked tissue sample for mass spectrometry imaging (MSI) research. Herein, we present a standard-spiked 3D biomimetic tissue model fabricated with native cells, homogenate matrix, and biocompatible polymer. Unlike traditional homogenized tissue surrogates or those constructed with "on-tissue" or "under-tissue" micropipetting strategies, this simulated tissue shares both structural integrity of cells and homogeneous properties of matrix. As a result, analyte standards could undergo more in-depth incorporation and has a more comparable native status with a real tissue. Series of tissue sections made from the 3D tissue model were proven to be feasible and useful for the parameter optimization, analyte quantitation, and calibration curve fitting for the air-flow assisted desorption electrospray ionization MSI. Additionally, by analyzing the quality control model sections, we proposed a median principal component score calibration and demonstrated that this method can normalize instrumental fluctuations to stable levels in a large-scale untargeted MSI experiments for the reliable metabolomic biomarker discovery. Thus, these results indicated that the standard-spiked 3D biomimetic tissue has convincing significance in MSI analysis.

Toward Higher Sensitivity in Quantitative MALDI Imaging Mass Spectrometry of CNS Drugs Using a Nonpolar Matrix

Tissue-specific ion suppression is an unavoidable matrix effect in MALDI mass spectrometry imaging (MALDI-MSI), the negative impact of which on precision and accuracy in quantitative MALDI-MSI can be reduced to some extent by applying isotope internal standards for normalization and matrix-matched calibration routines. The detection sensitivity still suffers, however, often resulting in significant loss of signal for the investigated analytes. An MSI application considerably affected by this phenomenon is the quantitative spatial analysis of central nervous system (CNS) drugs. Most of these drugs are low molecular weight, lipophilic compounds, which exhibit inefficient desorption and ionization during MALDI using conventional polar acidic matrices (CHCA, DHB). Here, we present the application of the (2-[(2 E)-3-(4- tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile) matrix for high sensitivity imaging of CNS drugs in mouse brain sections. Since DCTB is usually described as an electron-transfer matrix, we provide a rationale (i.e., computational calculations of gas-phase proton affinity and ionization energy) for an additional proton-transfer ionization mechanism with this matrix. Furthermore, we compare the extent of signal suppression for five different CNS drugs when employing DCTB versus CHCA matrices. The results showed that the signal suppression was not only several times lower with DCTB than with CHCA but also depended on the specific tissue investigated. Finally, we present the application of DCTB and ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry to quantitative MALDI imaging of the anesthetic drug xylazine in mouse brain sections based on a linear matrix-matched calibration curve. DCTB afforded up to 100-fold signal intensity improvement over CHCA when comparing representative single MSI pixels and >440-fold improvement for the averaged mass spectrum of the adjacent tissue sections.

Lactate-Induced Glucose Output Is Unchanged by Metformin at a Therapeutic Concentration - A Mass Spectrometry Imaging Study of the Perfused Rat Liver

Metformin is the first line drug for type 2 diabetes but its molecular mechanisms remain unclear. Here, we have studied the acute effect of a therapeutically relevant intrahepatic concentration of metformin on glucose production from lactate. We selected the perfused rat liver as experimental system since it enables the complete control of drug dosage. We used MALDI (matrix-assisted laser desorption/ionization) mass spectrometry imaging to estimate the concentration of metformin in the livers and we measured the concentration of glucose in the effluent medium under basal conditions and following lactate addition. MALDI mass spectra of thin-sections of freeze-clamped rat liver perfused with metformin showed a peak at 130.16 m/z which was unambiguously assigned to metformin. The mass spectrometric detection limit was at a tissue concentration of about 250 nM, and uptake of metformin from the perfusion medium to the liver occurred with a K_m of 0.44 mM. Metformin was evenly distributed in the liver irrespective of its concentration in the perfusion medium and the duration of a perfusion. At a parenchymal concentration of 30 μM, metformin did not induce any significant suppression of the basal or lactate-induced glucose release from the liver. These results show that matrix-assisted laser desorption/ionization mass spectrometry imaging can be applied to estimate the tissue concentration and distribution of metformin in a therapeutically relevant micromolar concentration range. Our findings challenge the view that metformin causes an inhibition of glucose release from the liver by an acute inhibition of mitochondrial glycerol 3-phosphate dehydrogenase (EC 1.1.5.3).

Absolute Quantification of Rifampicin by MALDI Imaging Mass Spectrometry Using Multiple TOF/TOF Events in a Single Laser Shot

Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) allows for the visualization of molecular distributions within tissue sections. While providing excellent molecular specificity and spatial information, absolute quantification by MALDI IMS remains challenging. Especially in the low molecular weight region of the spectrum, analysis is complicated by matrix interferences and ionization suppression. Though tandem mass spectrometry (MS/MS) can be used to ensure chemical specificity and improve sensitivity by eliminating chemical noise, typical MALDI MS/MS modalities only scan for a single MS/MS event per laser shot. Herein, we describe TOF/TOF instrumentation that enables multiple fragmentation events to be performed in a single laser shot, allowing the intensity of the analyte to be referenced to the intensity of the internal standard in each laser shot while maintaining the benefits of MS/MS. This approach is illustrated by the quantitative analyses of rifampicin (RIF), an antibiotic used to treat tuberculosis, in pooled human plasma using rifapentine (RPT) as an internal standard. The results show greater than 4-fold improvements in relative standard deviation as well as improved coefficients of determination (R2) and accuracy (>93% quality controls, <9% relative errors). This technology is used as an imaging modality to measure absolute RIF concentrations in liver tissue from an animal dosed in vivo. Each microspot in the quantitative image measures the local RIF concentration in the tissue section, providing absolute pixel-to-pixel quantification from different tissue microenvironments. The average concentration determined by IMS is in agreement with the concentration determined by HPLC-MS/MS, showing a percent difference of 10.6%. Graphical Abstract ᅟ.

Quantification of low molecular weight compounds by MALDI imaging mass spectrometry - A tutorial review

Matrix-assisted laser desorption/ionization (MALDI)-mass spectrometry imaging (MSI) permits label-free in situ analysis of chemical compounds directly from the surface of two-dimensional biological tissue slices. It links qualitative molecular information of compounds to their spatial coordinates and distribution within the investigated tissue. MALDI-MSI can also provide the quantitative amounts of target compounds in the tissue, if proper calibration techniques are performed. Obviously, as the target molecules are embedded within the biological tissue environment and analysis must be performed at their precise locations, there is no possibility for extensive sample clean-up routines or chromatographic separations as usually performed with homogenized biological materials; ion suppression phenomena therefore become a critical side effect of MALDI-MSI. Absolute quantification by MALDI-MSI should provide an accurate value of the concentration/amount of the compound of interest in relatively small, well-defined region of interest of the examined tissue, ideally in a single pixel. This goal is extremely challenging and will not only depend on the technical possibilities and limitations of the MSI instrument hardware, but equally on the chosen calibration/standardization strategy. These strategies are the main focus of this article and are discussed and contrasted in detail in this tutorial review. This article is part of a Special Issue entitled: MALDI Imaging, edited by Dr. Corinna Henkel and Prof. Peter Hoffmann.

Compound Property Optimization in Drug Discovery Using Quantitative Surface Sampling Micro Liquid Chromatography with Tandem Mass Spectrometry

Surface sampling micro liquid chromatography tandem mass spectrometry (SSμLC-MS/MS) was explored as a quantitative tissue distribution technique for probing compound properties in drug discovery. A method was developed for creating standard curves using surrogate tissue sections from blank tissue homogenate spiked with compounds. The resulting standard curves showed good linearity and high sensitivity. The accuracy and precision of standards met acceptance criteria of ±30%. A new approach was proposed based on an experimental and mathematical method for tissue extraction efficiency evaluation by means of consecutively sampling a location on tissue twice by SSμLC-MS/MS. The observed extraction efficiency ranged from 69% to 82% with acceptable variation for the test compounds. Good agreement in extraction efficiency was observed between surrogate tissue sections and incurred tissue sections. This method was successfully applied to two case studies in which tissue distribution was instrumental in advancing project teams' understanding of compound properties.

An Improved Molecular Histology Method for Ion Suppression Monitoring and Quantification of Phosphatidyl Cholines During MALDI MSI Lipidomics Analyses

Tissue lipidomics is one of the latest omics approaches for biomarker discovery in pharmacology, pathology, and the life sciences at large. In this context, matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) is the most versatile tool to map compounds within tissue sections. However, ion suppression events occurring during MALDI MSI analyses make it impossible to use this method for quantitative investigations without additional validation steps. This is especially true for lipidomics, since different lipid classes are responsible for important ion suppression events. We propose here an improved lipidomics method to assess local ion suppression of phospatidylcholines in tissues. Serial tissue sections were spiked with different amounts of PC(16:0 d31/18:1) using a nebulization device. Settings for standard nebulization were strictly controlled for a detection similar to when using spiked tissue homogenates. The sections were simultaneously analyzed by MALDI MSI using a Fourier transform ion cyclotron resonance analyzer. Such a spray-based approach allows taking into account the biochemical heterogeneity of the tissue for the detection of PC(16:0 d31/18:1). Thus, here we present the perspective to use this method for quantification purposes. The linear regression lines are considered as calibration curves and we calculate PC(16:0/18:1) quantification values for different ROIs. Although those values need to be validated by a using a different independent approach, the workflow offers an insight into new quantitative mass spectrometry imaging (q-MSI) methods. This approach of ion suppression monitoring of phosphocholines in tissues may be highly interesting for a large range of applications in MALDI MSI, particularly for pathology using translational science workflows.

MALDI mass spectrometry imaging: A cutting-edge tool for fundamental and clinical histopathology

Histopathological diagnoses have been done in the last century based on hematoxylin and eosin staining. These methods were complemented by histochemistry, electron microscopy, immunohistochemistry (IHC), and molecular techniques. Mass spectrometry (MS) methods allow the thorough examination of various biocompounds in extracts and tissue sections. Today, mass spectrometry imaging (MSI), and especially matrix-assisted laser desorption ionization (MALDI) imaging links classical histology and molecular analyses. Direct mapping is a major advantage of the combination of molecular profiling and imaging. MSI can be considered as a cutting edge approach for molecular detection of proteins, peptides, carbohydrates, lipids, and small molecules in tissues. This review covers the detection of various biomolecules in histopathological sections by MSI. Proteomic methods will be introduced into clinical histopathology within the next few years.

Applications of Fourier Transform Ion Cyclotron Resonance (FT-ICR) and Orbitrap Based High Resolution Mass Spectrometry in Metabolomics and Lipidomics

Metabolomics, along with other "omics" approaches, is rapidly becoming one of the major approaches aimed at understanding the organization and dynamics of metabolic networks. Mass spectrometry is often a technique of choice for metabolomics studies due to its high sensitivity, reproducibility and wide dynamic range. High resolution mass spectrometry (HRMS) is a widely practiced technique in analytical and bioanalytical sciences. It offers exceptionally high resolution and the highest degree of structural confirmation. Many metabolomics studies have been conducted using HRMS over the past decade. In this review, we will explore the latest developments in Fourier transform mass spectrometry (FTMS) and Orbitrap based metabolomics technology, its advantages and drawbacks for using in metabolomics and lipidomics studies, and development of novel approaches for processing HRMS data.

Imaging Mass Spectrometry Reveals Acyl-Chain- and Region-Specific Sphingolipid Metabolism in the Kidneys of Sphingomyelin Synthase 2-Deficient Mice

Obesity was reported to cause kidney injury by excessive accumulation of sphingolipids such as sphingomyelin and ceramide. Sphingomyelin synthase 2 (SMS2) is an important enzyme for hepatic sphingolipid homeostasis and its dysfunction is considered to result in fatty liver disease. The expression of SMS2 is also high in the kidneys. However, the contribution of SMS2 on renal sphingolipid metabolism remains unclear. Imaging mass spectrometry is a powerful tool to visualize the distribution and provide quantitative data on lipids in tissue sections. Thus, in this study, we analyzed the effects of SMS2 deficiency on the distribution and concentration of sphingomyelins in the liver and kidneys of mice fed with a normal-diet or a high-fat-diet using imaging mass spectrometry and liquid chromatography/electrospray ionization-tandem mass spectrometry. Our study revealed that high-fat-diet increased C18–C22 sphingomyelins, but decreased C24-sphingomyelins, in the liver and kidneys of wild-type mice. By contrast, SMS2 deficiency decreased C18–C24 sphingomyelins in the liver. Although a similar trend was observed in the whole-kidneys, the effects were minor. Interestingly, imaging mass spectrometry revealed that sphingomyelin localization was specific to each acyl-chain length in the kidneys. Further, SMS2 deficiency mainly decreased C22-sphingomyelin in the renal medulla and C24-sphingomyelins in the renal cortex. Thus, imaging mass spectrometry can provide visual assessment of the contribution of SMS2 on acyl-chain- and region-specific sphingomyelin metabolism in the kidneys.

Absolute Quantitative MALDI Imaging Mass Spectrometry: A Case of Rifampicin in Liver Tissues

Matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) elucidates molecular distributions in thin tissue sections. Absolute pixel-to-pixel quantitation has remained a challenge, primarily lacking validation of the appropriate analytical methods. In the present work, isotopically labeled internal standards are applied to tissue sections to maximize quantitative reproducibility and yield accurate quantitative results. We have developed a tissue model for rifampicin (RIF), an antibiotic used to treat tuberculosis, and have tested different methods of applying an isotopically labeled internal standard for MALDI IMS analysis. The application of the standard and subsequently the matrix onto tissue sections resulted in quantitation that was not statistically significantly different from results obtained using HPLC-MS/MS of tissue extracts. Quantitative IMS experiments were performed on liver tissue from an animal dosed in vivo. Each microspot in the quantitative images measures the local concentration of RIF in the thin tissue section. Lower concentrations were detected from the blood vessels and around the portal tracts. The quantitative values obtained from these measurements were comparable (>90% similarity) to HPLC-MS/MS results obtained from extracts of the same tissue.

The biochemistry of blister fluid from pediatric burn injuries: proteomics and metabolomics aspects

Burn injury is a prevalent and traumatic event for pediatric patients. At present, the diagnosis of burn injury severity is subjective and lacks a clinically relevant quantitative measure. This is due in part to a lack of knowledge surrounding the biochemistry of burn injuries and that of blister fluid. A more complete understanding of the blister fluid biochemistry may open new avenues for diagnostic and prognostic development. Burn insult induces a highly complex network of signaling processes and numerous changes within various biochemical systems, which can ultimately be examined using proteome and metabolome measurements. This review reports on the current understanding of burn wound biochemistry and outlines a technical approach for 'omics' profiling of blister fluid from burn wounds of differing severity.

MALDI mass spectrometric imaging meets “omics”: recent advances in the fruitful marriage

Matrix-assisted laser desorption/ionization mass spectrometric imaging (MALDI MSI) is a method that allows the investigation of the molecular content of surfaces, in particular, tissues, within its morphological context.