Analysis of erlotinib and its metabolites in rat tissue sections by MALDI quadrupole time-of-flight mass spectrometry

Absorption and translocation of selected pharmaceuticals in Pistia stratiotes: Spatial distribution analysis using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Phytoremediation can eliminate pharmaceuticals from aquatic environments through absorption; however, understanding of absorption and transport processes in plants remains limited. In this study, a matrix-assisted laser desorption/ionization time-of-flight mass spectrometry imaging (MALDI-MSI) method was developed to explore the absorption and translocation mechanisms of seven common pharmaceuticals in Pistia stratiotes. Results showed that 2,3-dicyanohydroquinone, an infrequently used matrix, exhibited outstanding performance in MALDI-MSI analysis, producing the highest signal intensity for four of the seven pharmaceuticals. Region of Interest (ROI) analysis revealed that charge speciation of pharmaceuticals significantly influenced their ability to enter vascular bundle. Neutral and positively charged pharmaceuticals easily entered vascular bundle, while negatively charged pharmaceuticals faced difficulty. ROI results for neutral and negatively charged pharmaceuticals exhibited positive correlation with their transfer factor values, indicating that their translocation ability from root to shoot was related to their capacity to enter vascular bundle. However, no correlation was observed for positively charged pharmaceuticals, suggesting that these compounds, upon entering vascular bundle, encountered difficulties in upward translocation through the xylem. This study introduces an innovative approach and offers novel insights into the retention and migration of pharmaceuticals in plant tissues, aiming to enhance the understanding of pharmaceutical accumulation in plants. ENVIRONMENTAL IMPLICATION: Pharmaceuticals in aquatic environment can inflict detrimental effects on both human health and ecosystem. Phytoremediation can remove pharmaceuticals from aquatic environments through absorption. However, our understanding of absorption and transportation of pharmaceuticals in plants remains limited. This study developed a matrix-assisted laser desorption/ionization time-of-flight mass spectrometry imaging (MALDI-MSI) method for pharmaceuticals in plant roots, and to explore the absorption and translocation mechanisms of pharmaceuticals. The study offers direct evidence of differences in accumulation behavior of pharmaceuticals in plants, providing valuable insights for targeted and effective strategies in using plants for remediating the aquatic ecosystem from pharmaceuticals.

Quantitative mass spectrometry imaging (qMSI): A tutorial

Mass spectrometry imaging (MSI) is an analytical technique that enables the simultaneous detection of hundreds to thousands of chemical species while retaining their spatial information; usually, MSI is applied to biological tissues. Combining these elements can create ion images, which allows for the identification and localization of multiple chemical species within the sample. Being able to produce molecular images of biological tissues has already impacted the study of health and disease; however, the next logical step is being able to combine MSI with quantitative mass spectrometry methods to both quantify and determine the localization of disease progression or drug action. In this tutorial, we will detail the main factors to consider when designing a qMSI experiment and highlight the methods that have been developed to overcome these added complexities, specifically for those newer to the field of MSI.

Quantification of pharmaceutical compounds in tissue and plasma samples using selective ion accumulation with multiple mass isolation windows

Quantification of pharmaceutical compounds using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) is an alternative to traditional liquid chromatography (LC)-MS techniques. Benefits of MALDI-based approaches include rapid analysis times for liquid samples and imaging mass spectrometry capabilities for tissue samples. As in most quantification experiments, the use of internal standards can compensate for spot-to-spot and shot-to-shot variability associated with MALDI sampling. However, the lack of chromatographic separation in traditional MALDI analyses results in diminished peak capacity due to the chemical noise background, which can be detrimental to the dynamic range and limit of detection of these approaches. These issues can be mitigated by using a hybrid mass spectrometer equipped with a quadrupole mass filter (QMF) that can be used to fractionate ions based on their mass-to-charge ratios. When the masses of the analytes and internal standards are sufficiently disparate in mass, it can be beneficial to effect multiple narrow mass isolation windows using the QMF, as opposed to a single wide mass isolation window, to minimize chemical noise while allowing for internal standard normalization. Herein, we demonstrate a MALDI MS quantification workflow incorporating multiple sequential mass isolation windows enabled on a QMF, which divides the total number of MALDI laser shots into multiple segments (i.e., one segment for each mass isolation window). This approach is illustrated through the quantitative analysis of the pharmaceutical compound enalapril in human plasma samples as well as the simultaneous quantification of three pharmaceutical compounds (enalapril, ramipril, and verapamil). Results show a decrease in the limit of detection, relative standard deviations below 10%, and accuracy above 85% for drug quantification using multiple mass isolation windows. This approach has also been applied to the quantification of enalapril in brain tissue from a rat dosed in vitro. The average concentration of enalapril determined by imaging mass spectrometry is in agreement with the concentration determined by LC-MS, giving an accuracy of 104%.

Monitoring dexamethasone skin biodistribution with ex vivo MALDI-TOF mass spectrometry imaging and confocal Raman microscopy

Two of the most promising techniques in terms of ex vivo skin imaging and quantifying are confocal Raman microscopy and MALDI-TOF mass spectrometry imaging (MALDI-TOF MSI). Both techniques were set up, and the semiquantitative skin biodistribution of previously developed dexamethasone (DEX) loaded lipomers was compared using Benzalkonium chloride (BAK) as a tracer of the nanoparticles. In MALDI-TOF MSI, DEX was derivatised with GirT (DEX-GirT) and the semiquantitative biodistribution of both DEX-GirT and BAK was successfully obtained. The amount of DEX measured by confocal Raman microscopy was higher than that measured by MALDI-TOF MSI, but MALDI-TOF MSI proved to be a more suitable technique for tracing BAK. An absorption-promoting tendency of DEX loaded in lipomers versus a free-DEX solution was observed in confocal Raman microscopy. The higher spatial resolution of confocal Raman microscopy (350 nm) with respect to MALDI-TOF MSI (50 μm) allowed to observe specific skin structures like hair follicles. Nevertheless, the faster sampling rate of MALDI-TOF-MSI, permitted the analysis of larger tissue regions. In conclusion, both techniques allowed to simultaneously analyze semiquantitative data together with qualitative images of biodistribution, which is a very helpful tool when designing nanoparticles that accumulate in specific anatomical regions.

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.

The standard addition method and its validation in forensic toxicology

Purpose

In the quantitative forensic toxicological analyses using instruments, major methods to be employed are conventional matrix-matched calibration method (MMCM). However, nowadays, the needs for using the standard addition methods (SAM) are increasing. In spite of this situation, there are no reports of the guidelines for the validations of SAM. In this review, the principle, how to perform it, advantages, disadvantages, reported application data, and the details of validation procedures for the SAM are described.

Methods

Various databases such as SciFinder, Google and Google Scholar were utilized to collect relevant reports referring to the SAM. The long experiences of our research group on the SAM were also included in this review.

Results

Although the experimental procedures for the SAM are much more laborious than those of the MMCM, the SAM is essential to quantify target xenobiotic(s) in special matrices such as human solid tissues or biles, which remarkably interfere with the usual quantitative analyses. The validation methods for the SAM have been also proposed for the cases in the absence of the blank matrices.

Conclusions

To our knowledge, this is the first presentation of detailed SAM procedure and its validation, which will facilitate the use of the SAM in forensic toxicology. Especially for its validation, new simple methods have been proposed.

Bioanalysis of erlotinib, its O-demethylated metabolites OSI-413 and OSI-420, and other metabolites by liquid chromatography-tandem mass spectrometry with additional ion mobility identification

Erlotinib is a first-generation epithelial growth factor receptor inhibitor used in the treatment of non-small cellular lung cancers. Our previously published method on a Thermo TSQ Quantum Ultra triple quadrupole mass spectrometer for the quantitation of erlotinib, OSI-420, and OSI-413 and some other kinase inhibitors was transferred to a more sensitive Sciex QTRAP5500 system. Both methods showed comparable performance in the previous range (5-5000 and 1-1000 ng/mL for erlotinib and OSI-420) with comparable accuracies and precisions (98.9-106.2 vs 98.7.0-104.0, and 3.7-13.4 vs 4.6-13.2), and a high level of agreement between the methods (R² = 0.9984 and 0.9951) for the quality control samples. The new system however was also capable of quantifying lower concentrations of both erlotinib and OSI-420 (0.5 and 0.1 ng/mL) with sufficient accuracy and precision. Along with the increased sensitivity we included the semi-quantitative determination of additional erlotinib metabolites M2, M3, M5, M6, M7, M8, M9, M10, M11, M12, M16 (hydroxy-erlotinib), M17, M18, M19, M20, M21 in a 0.1-1000 ng/mL range to the method. With a simple crash, dilute, and shoot sample preparation with acetonitrile and a 4.5 min analytical run time the method outperformed most other published methods in speed and simplicity and was suitable for TDM. Further, enhancement of the understanding of the pharmacokinetics of erlotinib and its metabolites was demonstrated.

Multilabel Per-Pixel Quantitation in Mass Spectrometry Imaging

In quantitative mass spectrometry imaging (MSI), the gold standard adds a single structural homologue of the target compound at a known concentration to the sample. This internal standard enables to map the detected intensity of the target molecule against an external calibration curve. This approach, however, ignores local noise levels and disproportional ion suppression effects, which might depend on the concentration of the target compound. To overcome these issues, we propose a novel approach that applies several isotopically labeled versions, each at a different concentration, to the sample. This allows creating individual internal calibration curves for every MSI pixel. As proof of principle, we have quantified an endogenous peptide of histone H4 by matrix-assisted laser desorption/ionization-Q-MSI (MALDI-Q-MSI), using a mixture of three isotopically labeled versions. The usage of a fourth label allowed us to compare the gold standard to our multilabel approach. We observed substantial heterogeneity in ion suppression across the tissue, which disclosed itself as varying slopes in the per-pixel regression analyses. These slopes were histology-dependent and differed from each other by up to a factor of 4. The results were validated by liquid chromatography-mass spectrometry (LC-MS), exhibiting a high agreement between LC-MS and MALDI-Q-MSI (Pearson correlation r = 0.87). A comparison between the multilabel and single-label approaches revealed a higher accuracy for the multilabel method when the local target compound concentration differed too much from the concentration of the single label. In conclusion, we show that the multilabel approach provides superior quantitation compared to a single-label approach, in case the target compound is inhomogeneously distributed at a wide concentration range in the tissue.

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.

Homospermidine Lipids: A Compound Class Specifically Formed during Fruiting Body Formation of Myxococcus xanthus DK1622

The fascinating ability of myxobacteria to form multicellular spore filled fruiting bodies under starvation conditions was widely studied as a model for cooperative microbial behavior. The potential of a life cycle induced change of secondary metabolism, as a means to discover novel natural products, remains largely underexplored. We therefore studied the model organism Myxococcus xanthus DK1622 under submersed and solid cultivation conditions to find putatively life-cycle related compounds by applying statistical analysis on analytical data. Utilizing the advantageous characteristics of LC-MS, LC-MS/MS, and MALDI-MSI allowed the identification of compounds unambiguously associated with myxobacterial fruiting bodies. Our screening effort resulted in the purification and structure elucidation of a novel compound, the homospermidine lipid, from cultures that had undergone the fruiting process. A combination of molecular networking and targeted LC-MS/MS in conjunction with our in-house metabolomics database subsequently revealed alternative producers of the respective compound as well as a number of compounds belonging to the same structural class. Three further members of this compound class were isolated from an alternative producer and structurally elucidated by NMR. Insights into the biosynthesis of this novel compound class was gained by feeding of isotopically labeled substrates and in silico analysis.

Nanostructured surfaces for analysis of anticancer drug and cell diagnosis based on electrochemical and SERS tools

Discovering new anticancer drugs and screening their efficacy requires a huge amount of resources and time-consuming processes. The development of fast, sensitive, and nondestructive methods for the in vitro and in vivo detection of anticancer drugs' effects and action mechanisms have been done to reduce the time and resources required to discover new anticancer drugs. For the in vitro and in vivo detection of the efficiency, distribution, and action mechanism of anticancer drugs, the applications of electrochemical techniques such as electrochemical cell chips and optical techniques such as surface-enhanced Raman spectroscopy (SERS) have been developed based on the nanostructured surface. Research focused on electrochemical cell chips and the SERS technique have been reviewed here; electrochemical cell chips based on nanostructured surfaces have been developed for the in vitro detection of cell viability and the evaluation of the effects of anticancer drugs, which showed the high capability to evaluate the cytotoxic effects of several chemicals at low concentrations. SERS technique based on the nanostructured surface have been used as label-free, simple, and nondestructive techniques for the in vitro and in vivo monitoring of the distribution, mechanism, and metabolism of different anticancer drugs at the cellular level. The use of electrochemical cell chips and the SERS technique based on the nanostructured surface should be good tools to detect the effects and action mechanisms of anticancer drugs.

MALDI mass spectrometry imaging of erlotinib administered in combination with bevacizumab in xenograft mice bearing B901L, EGFR-mutated NSCLC cells

Combination therapy of erlotinib plus bevacizumab improves progression-free survival of patients with epidermal growth factor receptor–mutated (EGFR-mutated) advanced non–small-cell lung cancer (NSCLC) compared with erlotinib alone. Although improved delivery and distribution of erlotinib to tumours as a result of the normalization of microvessels by bevacizumab is thought to be one of the underlying mechanisms, there is insufficient supporting evidence. B901L cells derived from EGFR-mutated NSCLC were subcutaneously implanted into mice, and mice were treated with bevacizumab or human IgG followed by treatment with erlotinib. The distribution of erlotinib in their tumours at different times after erlotinib administration was analysed by matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI). We also analysed the distribution of erlotinib metabolites and the distribution of erlotinib in tumours refractory to erlotinib, which were established by long-term treatment with erlotinib. We found that erlotinib was broadly diffused in the tumours from B901L-implanted xenograft mice, independently of bevacizumab treatment. We also found that erlotinib metabolites were co-localized with erlotinib and that erlotinib in erlotinib-refractory tumours was broadly distributed throughout the tumour tissue. Multivariate imaging approaches using MALDI MSI as applied in this study are of great value for pharmacokinetic studies in drug development.

Alterations in Pharmacokinetics of Gemcitabine and Erlotinib by Concurrent Administration of Hyangsayukgunja-Tang, a Gastroprotective Herbal Medicine

Gemcitabine and erlotinib are the chemotherapeutic agents used in the treatment of various cancers and their combination is being accepted as a first-line treatment of advanced pancreatic cancer. Hyangsayukgunja-tang (HYT) is a traditional oriental medicine used in various digestive disorders and potentially helpful to treat gastrointestinal adverse effects related to chemotherapy. The present study was aimed to evaluate the effect of HYT on the pharmacokinetics of gemcitabine and erlotinib given simultaneously in rats. Rats were pretreated with HYT at an oral dose of 1200 mg/kg/day once daily for a single day or 14 consecutive days. Immediately after pretreatment with HYT, gemcitabine and erlotinib were administered by intravenous injection (10 mg/kg) and oral administration (20 mg/kg), respectively. The effects of HYT on pharmacokinetics of the two drugs were estimated by non-compartmental analysis and pharmacokinetic modeling. The pharmacokinetics of gemcitabine and erlotinib were not altered by single dose HYT pretreatment. However, the plasma levels of OSI-420 and OSI-413, active metabolites of erlotinib, were significantly decreased in the multiple dose HYT pretreatment group. The pharmacokinetic model estimated increased systemic clearances of OSI-420 and OSI-413 by multiple doses of HYT. These data suggest that HYT may affect the elimination of OSI-420 and OSI-413.

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.

MALDI-MS drug analysis in biological samples: opportunities and challenges

Drug analysis represents a large field in different disciplines. Plasma is commonly considered to be the biosample of choice for that purpose. However, concentrations often do not represent the levels present within deeper compartments and therefore cannot sufficiently explain efficacy or toxicology of drugs. MALDI-MS in drug analysis is of great interest for high-throughput quantification and particularly spatially resolved tissue imaging. The current perspective article will deal with challenges and opportunities of MALDI-MS drug analysis in different biological samples. A particular focus will be on hair samples. Recent applications were included, reviewed for their instrumental setup and sample preparation and pros and cons as well as future perspectives are critically discussed.

Novel insights in drug metabolism by MS imaging

During the last decade, lateral and temporal localization of drug compounds and their metabolites have been demonstrated and dynamically developed using MS imaging. The pharmaceutical industry has recognized the potential of the technology that provides simultaneous distribution and quantitative data. In this review, we present the latest technological achievements and summarize applications of drug imaging focusing on studies about metabolites by MALDI-MS imaging. We also introduce potential areas with pharmaceutical applications that are currently under exploration, including pharmacological, toxicological characterizations and metabolic enzyme localization in comparison with drug and metabolite distribution.

Mass spectrometry imaging of therapeutics from animal models to three-dimensional cell cultures

Mass spectrometry imaging (MSI) is a powerful label-free technique for the investigation of the spatial distribution of molecules at complex surfaces and has been widely used in the pharmaceutical sciences to understand the distribution of different drugs and their metabolites in various biological samples, ranging from cell-based models to tissues. Here, we review the current applications of MSI for drug studies in animal models, followed by a discussion of the novel advances of MSI in three-dimensional (3D) cell cultures for accurate, efficient, and high-throughput analyses to evaluate therapeutics.

Drug localizations in tissue by mass spectrometry imaging

Advances in development of mass spectrometry (MS) are successfully utilized for spatial localization of pharmaceutical compounds in these tissue sections. Today MS instruments can be used in imaging mode when the datasets are generated from the surface of the tissue over an array of acquisition positions. This review is focused on the technological developments of matrix-assisted laser desorption/ionization MS imaging (MALDI-MSI) and related sample preparation procedures. MALDI-MSI provides a sensitive and label-free approach for imaging of drugs and their metabolites. Due to these features, MALDI-MSI is expected to become a standard technique in pharmaceutical development providing complementary information to current methods.

MALDI Imaging mass spectrometry: current frontiers and perspectives in pathology research and practice

MALDI Imaging mass spectrometry has entered the field of tissue-based research by providing unique advantages for analyzing tissue specimen in an unprecedented detail. A broad spectrum of analytes ranging from proteins, peptides, protein modification over small molecules, drugs and their metabolites as well as pharmaceutical components, endogenous cell metabolites, lipids, and other analytes are made accessible by this in situ technique in tissue. Some of them were even not accessible in tissues within the histological context before. Thereby, the great advantage of MALDI Imaging is the correlation of molecular information with traditional histology by keeping the spatial localization information of the analytes after mass spectrometric measurement. This method is label-free and allows multiplex analysis of hundreds to thousands of molecules in the very same tissue section simultaneously. Imaging mass spectrometry brings a new quality of molecular data and links the expert discipline of pathology and deep molecular mass spectrometric analysis to tissue-based research. This review will focus on state-of-the-art of MALDI Imaging mass spectrometry, its recent applications by analyzing tissue specimen and the contributions in understanding the biology of disease as well as its perspectives for pathology research and practice.

Assessing drug and metabolite detection in liver tissue by UV-MALDI and IR-MALDESI mass spectrometry imaging coupled to FT-ICR MS

Determining the distribution of a drug and its metabolites within tissue is a key facet of evaluating drug candidates. Drug distribution can have a significant implication in appraising drug efficacy and potential toxicity. The specificity and sensitivity of mass spectrometry imaging (MSI) make it a perfect complement to the analysis of drug distributions in tissue. The detection of lapatinib as well as several of its metabolites in liver tissue was determined by MSI using infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) coupled to high resolving power Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers. IR-MALDESI required minimal sample preparation while maintaining high sensitivity. The effect of the electrospray solvent composition on IR-MALDESI MSI signal from tissue analysis was investigated and an empirical comparison of IR-MALDESI and UV-MALDI for MSI analysis is also presented.

Advances in tissue section preparation for MALDI imaging MS

Enriched by a decade of remarkable developments, matrix-assisted laser desorption ionization imaging mass spectrometry (MALDI IMS) has witnessed a phenomenal expansion. Initially introduced for the mapping of peptides and intact proteins from mammalian tissue sections, MALDI IMS applications now extend to a wide range of molecules including peptides, lipids, metabolites and xenobiotics. Technology and methodology are quickly evolving to push the limits of the technique forward. Within a short period of time, numerous protocols and concepts have been developed and introduced in tissue section preparation, nonexhaustively including in situ tissue chemistries and solvent-free matrix depositions. Considering the past progress and current capabilities, this Review aims to cover the different aspects and challenges of tissue section preparation for MALDI IMS.

Quantification in MALDI-MS imaging: what can we learn from MALDI-selected reaction monitoring and what can we expect for imaging?

Quantification by mass spectrometry imaging (Q-MSI) is one of the hottest topics of the current discussions among the experts of the MS imaging community. If MSI is established as a powerful qualitative tool in drug and biomarker discovery, its reliability for absolute and accurate quantification (QUAN) is still controversial. Indeed, Q-MSI has to deal with several fundamental aspects that are difficult to control, and to account for absolute quantification. The first objective of this manuscript is to review the state-of-the-art of Q-MSI and the current strategies developed for absolute quantification by direct surface sampling from tissue sections. This includes comments on the quest for the perfect matrix-matched standards and signal normalization approaches. Furthermore, this work investigates quantification at a pixel level to determine how many pixels must be considered for accurate quantification by ultraviolet matrix-assisted laser desorption/ionization (MALDI), the most widely used technique for MSI. Particularly, this study focuses on the MALDI-selected reaction monitoring (SRM) in rastering mode, previously demonstrated as a quantitative and robust approach for small analyte and peptide-targeted analyses. The importance of designing experiments of good quality and the use of a labeled compound for signal normalization is emphasized to minimize the signal variability. This is exemplified by measuring the signal for cocaine and a tryptic peptide (i.e., obtained after digestion of a monoclonal antibody) upon different experimental conditions, such as sample stage velocity, laser power and frequency, or distance between two raster lines. Our findings show that accurate quantification cannot be performed on a single pixel but requires averaging of at least 4-5 pixels. The present work demonstrates that MALDI-SRM/MSI is quantitative with precision better than 10-15 %, which meets the requirements of most guidelines (i.e., in bioanalysis or toxicology) for quantification of drugs or peptides from tissue homogenates.

Mapping antiretroviral drugs in tissue by IR-MALDESI MSI coupled to the Q Exactive and comparison with LC-MS/MS SRM assay

This work describes the coupling of the IR-MALDESI imaging source with the Q Exactive mass spectrometer. IR-MALDESI MSI was used to elucidate the spatial distribution of several HIV drugs in cervical tissues that had been incubated in either a low or high concentration. Serial sections of those analyzed by IR-MALDESI MSI were homogenized and analyzed by LC-MS/MS to quantify the amount of each drug present in the tissue. By comparing the two techniques, an agreement between the average intensities from the imaging experiment and the absolute quantities for each drug was observed. This correlation between these two techniques serves as a prerequisite to quantitative IR-MALDESI MSI. In addition, a targeted MS(2) imaging experiment was also conducted to demonstrate the capabilities of the Q Exactive and to highlight the added selectivity that can be obtained with SRM or MRM imaging experiments.

The development and assessment of high-throughput mass spectrometry-based methods for the quantification of a nanoparticle drug delivery agent in cellular lysate

The safe use of lipid-based drug delivery agents requires fast and sensitive qualitative and quantitative assessment of their cellular interactions. Many mass spectrometry (MS) based analytical platforms can achieve such task with varying capabilities. Therefore, four novel high-throughput MS-based quantitative methods were evaluated for the analysis of a small organic gene delivery agent: N,N-bis(dimethylhexadecyl)-1,3-propane-diammonium dibromide (G16-3). Analysis utilized MS instruments that detect analytes using low-resolution tandem MS (MS/MS) analysis (i.e. QTRAP or linear ion trap in this work) or high-resolution MS analysis (i.e. time of flight (ToF) or Orbitrap). Our results indicate that the validated fast chromatography (FC)-QTRAP-MS/MS, FC- LTQ-Orbitrap-MS, desorption electrospray ionization-collision-induced dissociation (CID)-MS/MS and matrix assisted laser desorption ionization-ToF/ToF-MS MS methods were superior in the area of method development and sample analysis time to a previously developed liquid chromatography (LC)-CID-MS/MS. To our knowledge, this is the first evaluation of the abilities of five MS-based quantitative methods that target a single pharmaceutical analyte. Our findings indicate that, in comparison to conventional LC-CID-MS/MS, the new MS-based methods resulted in a (1) substantial reduction in the analysis time, (2) reduction in the time required for method development and (3) production of either superior or comparable quantitative data. The four new high-throughput MS methods, therefore, were faster, more efficient and less expensive than a conventional LC-CID-MS/MS for the quantification of the G16-3 analyte within tissue culture. When applied to cellular lysate, no significant change in the concentration of G16-3 gemini surfactant within PAM212 cells was observed between 5 and 53 h, suggesting the absence of any metabolism/excretion from PAM212 cells.

Quantitative mass spectrometry imaging of emtricitabine in cervical tissue model using infrared matrix-assisted laser desorption electrospray ionization

A quantitative mass spectrometry imaging (QMSI) technique using infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) is demonstrated for the antiretroviral (ARV) drug emtricitabine in incubated human cervical tissue. Method development of the QMSI technique leads to a gain in sensitivity and removal of interferences for several ARV drugs. Analyte response was significantly improved by a detailed evaluation of several cationization agents. Increased sensitivity and removal of an isobaric interference was demonstrated with sodium chloride in the electrospray solvent. Voxel-to-voxel variability was improved for the MSI experiments by normalizing analyte abundance to a uniformly applied compound with similar characteristics to the drug of interest. Finally, emtricitabine was quantified in tissue with a calibration curve generated from the stable isotope-labeled analog of emtricitabine followed by cross-validation using liquid chromatography tandem mass spectrometry (LC-MS/MS). The quantitative IR-MALDESI analysis proved to be reproducible with an emtricitabine concentration of 17.2 ± 1.8 μg/gtissue. This amount corresponds to the detection of 7 fmol/voxel in the IR-MALDESI QMSI experiment. Adjacent tissue slices were analyzed using LC-MS/MS which resulted in an emtricitabine concentration of 28.4 ± 2.8 μg/gtissue.

Alternative two-step matrix application method for imaging mass spectrometry to avoid tissue shrinkage and improve ionization efficiency

Mass spectrometry (MS) was used to measure the concentrations of drug and biological compounds in plasma and tissues. Matrix-assisted laser desorption/ionization (MALDI) imaging MS (IMS) has recently been applied to the analysis of localized drugs on biological tissue surfaces. In MALDI-IMS, matrix application process is crucial for successful results. However, it is difficult to obtain homogeneous matrix crystals on the tissue surface due to endogenous salts and tissue surface heterogeneity. Consequently, the non-uniform crystals degrade the quality of the spectrum and likely cause surface imaging artifacts. Furthermore, the direct application of matrix solution can cause tissue shrinkage due to the organic solvents. Here, we report an alternative two-step matrix application protocol which combines the vacuum deposition of matrix crystals and the spraying of matrix solution to produce a homogeneous matrix layer on the tissue surface. Our proposed technique can also prevent cracking or shrinking of the tissue samples and improve the ionization efficiency of the distributed exogenous material.

A mimetic tissue model for the quantification of drug distributions by MALDI imaging mass spectrometry

The full potential of imaging mass spectrometry (IMS) as a tool in drug development will not be realized until reliable quantitative information can be integrated with the molecular distributions. Here we report a novel method for the quantification of drugs in tissue sections using matrix-assisted laser desorption/ionization (MALDI) IMS. This method uses a mimetic tissue model consisting of a set of tissue homogenates spiked with a range of different drug concentrations that have been frozen into a polymer support mold. The goal of this model is to mimic a dosed tissue in its effects on analyte extraction and ion suppression. Parallel preparation and analysis of sections from the tissue model and the dosed tissues allow for the quantification of a drug's distribution. Here we detail the steps involved in constructing the model and provide proof of concept data to highlight the potential of this approach. Several figures of merit are evaluated including linearity of response, variability, and section-to-section reproducibility. Finally, the tissue model is used to quantify two different drugs, lapatinib and nevirapine, in dosed tissues from nonclinical species and the results are compared with those generated by LC-MS quantification.

Molecular imaging of drug transit through the blood-brain barrier with MALDI mass spectrometry imaging

Drug transit through the blood-brain barrier (BBB) is essential for therapeutic responses in malignant glioma. Conventional methods for assessment of BBB penetrance require synthesis of isotopically labeled drug derivatives. Here, we report a new methodology using matrix assisted laser desorption ionization mass spectrometry imaging (MALDI MSI) to visualize drug penetration in brain tissue without molecular labeling. In studies summarized here, we first validate heme as a simple and robust MALDI MSI marker for the lumen of blood vessels in the brain. We go on to provide three examples of how MALDI MSI can provide chemical and biological insights into BBB penetrance and metabolism of small molecule signal transduction inhibitors in the brain - insights that would be difficult or impossible to extract by use of radiolabeled compounds.

Qualitative and quantitative mass spectrometry imaging of drugs and metabolites in tissue at therapeutic levels

Mass spectrometry imaging (MSI) is a rapidly evolving technology that yields qualitative and quantitative distribution maps of small pharmaceutical-active molecules and their metabolites in tissue sections in situ. The simplicity, high sensitivity and ability to provide comprehensive spatial distribution maps of different classes of biomolecules make MSI a valuable tool to complement histopathology for diagnostics and biomarker discovery. In this review, qualitative and quantitative MSI of drugs and metabolites in tissue at therapeutic levels are discussed and the impact of this technique in drug discovery and clinical research is highlighted.

Recent Advances in Mass Spectrometry for the Identification of Neuro-chemicals and their Metabolites in Biofluids

Recently, mass spectrometric related techniques have been widely applied for the identification and quantification of neurochemicals and their metabolites in biofluids. This article presents an overview of mass spectrometric techniques applied in the detection of neurological substances and their metabolites from biological samples. In addition, the advances of chromatographic methods (LC, GC and CE) coupled with mass spectrometric techniques for analysis of neurochemicals in pharmaceutical and biological samples are also discussed.

Comprehensive identification of proteins from MALDI imaging

Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) is a powerful tool for the visualization of proteins in tissues and has demonstrated considerable diagnostic and prognostic value. One main challenge is that the molecular identity of such potential biomarkers mostly remains unknown. We introduce a generic method that removes this issue by systematically identifying the proteins embedded in the MALDI matrix using a combination of bottom-up and top-down proteomics. The analyses of ten human tissues lead to the identification of 1400 abundant and soluble proteins constituting the set of proteins detectable by MALDI IMS including >90% of all IMS biomarkers reported in the literature. Top-down analysis of the matrix proteome identified 124 mostly N- and C-terminally fragmented proteins indicating considerable protein processing activity in tissues. All protein identification data from this study as well as the IMS literature has been deposited into MaTisse, a new publically available database, which we anticipate will become a valuable resource for the IMS community.

Imaging mass spectrometry: a new tool for kidney disease investigations

Matrix-assisted laser desorption ionization (MALDI)-profiling and imaging mass spectrometry are promising technologies for measuring hundreds of different molecules directly on tissues. For instance, small molecules, drugs and their metabolites, endogenous lipids, carbohydrates and complex peptides/proteins can be measured at the same time without significant disruption of sample integrity. In this review, the potential of MALDI-profiling/imaging technologies in disease proteomics, drug action and studies of cellular processes in the context of kidney tissue is described. Spatial and sequence information obtained in tissue MALDI-profiling/imaging studies can be correlated with other mass spectrometry-based techniques, auxiliary imaging technologies and routine (immuno) histochemical staining.

Impact of drug administration route on drug delivery and distribution into the lung: an imaging mass spectrometry approach

During the last decade, significant technological improvements in mass spectrometry have had a great impact on drug discovery. The development of matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) has set a new frontier for the study of the distribution of endogenous and exogenous molecules present within a tissue. MALDI-IMS is a surface sampling technique that allows not only the detection of multiple analytes but also gives the spatial distribution of those analytes. Active compounds for pulmonary disease need an optimal and well-studied delivery into the lungs, in order to assure distribution with greater penetration into the peripheral or the alveolar region of the lung to maximize the therapeutic effects. IMS is very useful in the field of drug discovery, showing drug delivery and distribution in the body and organs. In this study, we present a comparison between two different ways of carrying out pulmonary drug administration: inhalation of a nebulized aerosol of aqueous drug solutions and intratracheal administration, which is much simpler, not expensive and commonly used during in vivo screening. Tiotropium bromide is a long-acting anticholinergic medicine used for maintenance treatment of chronic obstructive pulmonary disease. In the present work, tiotropium was administered by nebulization and by intratracheal instillation to guinea pigs at doses able to induce significant anti-bronchoconstrictive activity. Lung samples were dissected, frozen, cryosectioned and coated with matrix (α-hydroxy-cinnamic acid). IMS analyses were performed using a MALDI-LTQ-Orbitrap XL. Using this technique we were able to compare different distributions of the drug depending on the method of administration.

Identifying tissue-specific signal variation in MALDI mass spectrometric imaging by use of an internal standard

Generating analyte-specific distribution maps of compounds in a tissue sample by matrix-assisted laser desorption/ionization (MALDI) mass spectrometric imaging (MSI) has become a useful tool in numerous areas across the biological sciences. Direct analysis of the tissue sample provides MS images of an analyte's distribution with minimal sample pretreatment. The technique, however, suffers from the inability to account for tissue-specific variations in ion signal. The variation in the makeup of different tissue types can result in significant differences in analyte extraction, cocrystallization, and ionization across a sample. In this study, a deuterated internal standard was used to account for these signal variations. Initial experiments were performed using pure standards and optimal cutting temperature compound (OCT) to generate known areas of ion suppression. By monitoring the analyte-to-internal-standard ratio, differences in ion signal were taken into account, resulting in images that better represented the analyte concentration. These experiments were then replicated using multiple tissue types in which the analyte's MS signal was monitored. In certain tissues, including liver and kidney, the analyte signal was attenuated by up to 90%; however, when the analyte-to-internal-standard ratio was monitored, these differences were taken into account. These experiments further exemplify the need for an internal standard in the MSI workflow.

Development and validation of a spectrofluorimetric method for the determination of erlotinib in spiked human plasma

A rapid and sensitive spectrofluorimetric method was developed and validated for the determination of erlotinib (ETB), a potent anticancer drug, in spiked human plasma without any derivatization. The described method was validated and the analytical parameters of linearity, accuracy, precision (intra- and inter-day), limit of detection (LOD), and limit of quantification (LOQ) were evaluated. The relation between the fluorescence intensity and concentration was found to be linear (r(2) 0.9998) over the range 125 to 1000 ng/mL with the detection limit of 15 ng/mL. A simple liquid-liquid extraction method was followed in order to extract the drug from spiked plasma. The mean absolute recoveries of ETB were 85.59 % (±0.57), 86.91 % (±1.77) and 89.31 % (±3.01) at spiked plasma ETB concentration of 5000, 3750 and 2500 ng/mL, respectively. The spectrofluorimetric method presented here is a rapid, simple, specific, and reproducible method and can be used to characterize the plasma pharmacokinetics of ETB.