Spatially Resolved Quantitation of Drug in Skin Equivalents Using Mass Spectrometry Imaging (MSI).

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) has seen a growing interest as a leading technique in the pharmaceutical industry for mapping label-free exogenous and endogenous species in biological tissues. However, the use of MALDI-MSI to perform spatially resolved absolute quantitation of species directly in tissues is still challenging, and robust quantitative mass spectrometry imaging (QMSI) methods need to be developed. In this study, we describe the microspotting technique for analytical and internal standard deposition, matrix sublimation, powerful QMSI software, and mass spectrometry imaging setup to obtain absolute quantitation of drug distribution in 3D skin models.

The Adaptation of the QV600 LLI Milli-Fluidics System to House Ex Vivo Gastrointestinal Tissue Suitable for Drug Absorption and Permeation Studies, Utilizing MALDI MSI and LC-MS/MS.

Careful formulation of pharmaceuticals for oral delivery is essential to ensure that the optimal amount of the active ingredient reaches its intended site of action. This chapter demonstrates how mass spectrometry can be used in conjunction with ex vivo tissue and an adapted milli-fluidics system to carry out a drug absorption study. MALDI MSI is used to visualize the drug within the small intestine tissue from the absorption experimentation. LC-MS/MS is used to complete a mass balance of the experiment and quantify the amount of drug that has permeated through the tissue.

Perspective: Mass Spectrometry Imaging - The Next 5 Years.

In order to achieve even more widespread adoption over the next 5 years, a number of issues in mass spectrometry imaging need to be addressed. These are non-observation of compounds (due to ionization suppression), sample throughput, imaging of low-abundant species, and how to extract information from the large volumes of data generated. In this article, how current research indicates that these issues will be resolved along with potential application areas that MSI could look to exploit is discussed.

Adaptation of the Kirkstall QV600 LLI Microfluidics System for the Study of Gastrointestinal Absorption by Mass Spectrometry Imaging and LC-MS/MS.

Absorption studies on oral drugs can be difficult due to the challenge of replicating the complex structure and environment of the GI tract. Drug absorption studies can be conducted using in vivo and ex vivo animal tissue or animal-free techniques. These studies typically involve the use of Caco-2 cells. However, Caco-2 cells do not incorporate all the cell types found in intestinal tissue and lack P450 metabolizing enzymes. The QV600 LLI system is a microfluidics system designed for use with cell culture. Here, it has been adapted to house appropriate sections of ex vivo porcine tissue to act as a system that models the duodenum section of the small intestine. A pH regulated solution of Atorvastatin was flowed over the apical layer of the GI tissue and a nutrient solution flowed over the basal layer of the tissue to maintain tissue viability. The tissue samples were snap-frozen, cryosectioned, and imaged using MALDI Mass Spectrometry Imaging (MSI). A proof-of-concept study on the effect of excipients on absorption was conducted. Different concentrations of the solubilizing agent were added to the donor circuit of the QV600 LLI. The amount of Atorvastatin in the acceptor circuit was determined to study the effect of the excipient on the amount of drug that had permeated through the tissue. Using these data, Papp, pig values were calculated and compared with the literature.

Elemental Mapping of Human Malignant Mesothelioma Tissue Samples Using High-Speed LA-ICP-TOFMS Imaging.

This is the first report of the use of laser ablation-inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOFMS) to analyze human malignant pleural mesothelioma (MPM) samples at the cellular level. MPM is an aggressive, incurable cancer associated with asbestos exposure, with a long latency and poor overall survival. Following careful optimization of the laser fluence, the simultaneous ablation of soft biological tissue and hard mineral fibers was possible, allowing the spatial detection of elements such as Si, Mg, Ca, and Fe, which are also present in the glass substrate. A low-dispersion LA setup was employed, which provided the high spatial resolution necessary to identify the asbestos fibers and fiber fragments in the tissue and to characterize the metallome at the cellular level (a pixel size of 2 µm), with a high speed (at 250 Hz). The multielement LA-ICP-TOFMS imaging approach enabled (i) the detection of asbestos fibers/mineral impurities within the MPM tissue samples of patients, (ii) the visualization of the tissue structure with the endogenous elemental pattern at high spatial resolution, and (iii) obtaining insights into the metallome of MPM patients with different pathologies in a single analysis run. Asbestos and other mineral fibers were detected in the lung and pleura tissue of MPM patients, respectively, based on their multielement pattern (Si, Mg, Ca, Fe, and Sr). Interestingly, strontium was detected in asbestos fibers, suggesting a link between this potential toxic element and MPM pathogenesis. Furthermore, monitoring the metallome around the talc deposit regions (characterized by elevated levels of Al, Mg, and Si) revealed significant tissue damage and inflammation caused by talc pleurodesis. LA-ICP-TOFMS results correlated to Perls' Prussian blue and histological staining of the corresponding serial sections. Ultimately, the ultra-high-speed and high-spatial-resolution capabilities of this novel LA-ICP-TOFMS setup may become an important clinical tool for simultaneous asbestos detection, metallome monitoring, and biomarker identification.

Comparison of Osteosarcoma Aggregated Tumour Models with Human Tissue by Multimodal Mass Spectrometry Imaging.

Osteosarcoma (OS) is the most common primary bone malignancy and largely effects adolescents and young adults, with 60% of patients under the age of 25. There are multiple cell models of OS described in vitro that express the specific genetic alterations of the sarcoma. In the work reported here, multiple mass spectrometry imaging (MSI) modalities were employed to characterise two aggregated cellular models of OS models formed using the MG63 and SAOS-2 cell lines. Phenotyping of the metabolite activity within the two OS aggregoid models was achieved and a comparison of the metabolite data with OS human tissue samples revealed relevant fatty acid and phospholipid markers. Although, annotations of these species require MS/MS analysis for confident identification of the metabolites. From the putative assignments however, it was suggested that the MG63 aggregoids are an aggressive tumour model that exhibited metastatic-like potential. Alternatively, the SAOS-2 aggregoids are more mature osteoblast-like phenotype that expressed characteristics of cellular differentiation and bone development. It was determined the two OS aggregoid models shared similarities of metabolic behaviour with different regions of OS human tissues, specifically of the higher metastatic grade.

Multi-Modal Mass Spectrometric Imaging of Uveal Melanoma.

Matrix assisted laser desorption ionisation mass spectrometry imaging (MALDI-MSI), was used to obtain images of lipids and metabolite distribution in formalin fixed and embedded in paraffin (FFPE) whole eye sections containing primary uveal melanomas (UM). Using this technique, it was possible to obtain images of lysophosphatidylcholine (LPC) type lipid distribution that highlighted the tumour regions. Laser ablation inductively coupled plasma mass spectrometry images (LA-ICP-MS) performed on UM sections showed increases in copper within the tumour periphery and intratumoural zinc in tissue from patients with poor prognosis. These preliminary data indicate that multi-modal MSI has the potential to provide insights into the role of trace metals and cancer metastasis.

Visualizing Cholesterol in the Brain by On-Tissue Derivatization and Quantitative Mass Spectrometry Imaging.

Despite being a critical molecule in the brain, mass spectrometry imaging (MSI) of cholesterol has been under-reported compared to other lipids due to the difficulty in ionizing the sterol molecule. In the present work, we have employed an on-tissue enzyme-assisted derivatization strategy to improve detection of cholesterol in brain tissue sections. We report distribution and levels of cholesterol across specific structures of the mouse brain, in a model of Niemann-Pick type C1 disease, and during brain development. MSI revealed that in the adult mouse, cholesterol is the highest in the pons and medulla and how its distribution changes during development. Cholesterol was significantly reduced in the corpus callosum and other brain regions in the Npc1 null mouse, confirming hypomyelination at the molecular level. Our study demonstrates the potential of MSI to the study of sterols in neuroscience.

Monitoring the three-dimensional distribution of endogenous species in the lungs by matrix-assisted laser desorption/ionization mass spectrometry imaging.

RATIONALE: Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) is routinely employed to monitor the distribution of compounds in tissue sections and generate two-dimensional (2D) images. Whilst informative the images do not represent the distribution of the analyte of interest through the entire organ. The generation of 3D images is an exciting field that can provide a deeper view of the analyte of interest throughout an entire organ. METHODS: Serial sections of mouse and rat lung tissue were obtained at 120 µm depth intervals and imaged individually. Homogenate registration markers were incorporated in order to aid the final 3D image construction. Using freely available software packages, the images were stacked together to generate a 3D image that showed the distribution of endogenous species throughout the lungs. RESULTS: Preliminary tests were performed on 16 serial tissue sections of mouse lungs. A 3D model showing the distribution of phosphocholine at m/z 184.09 was constructed, which defined the external structure of the lungs and trachea. Later, a second experiment was performed using 24 serial tissue sections of the left lung of a rat. Two molecular markers, identified as [PC (32:1) + K]+ at m/z 770.51 and [PC (36:4) + K]+ at m/z 820.52, were used to generate 3D models of the parenchyma and airways, respectively. CONCLUSIONS: A straightforward method to generate 3D MALDI-MS images of selected molecules in lung tissue has been presented. Using freely available imaging software, the 3D distributions of molecules related to different anatomical features were determined.

Quantitative MALDI mass spectrometry imaging for exploring cutaneous drug delivery of tofacitinib in human skin.

In skin penetration studies, HPLC-MS/MS analysis on extracts of heat-separated epidermis and dermis provides an estimate of the amount of drug penetrated. In this study, MALDI-MSI enabled qualitative skin distribution analysis of endogenous molecules and the drug molecule, tofacitinib and quantitative analysis of the amount of tofacitinib in the epidermis. The delivery of tofacitinib to the skin was investigated in a Franz diffusion cell using three different formulations (two oil-in-water creams, C1 and C2 and an aqueous gel). Further, in vitro release testing (IVRT) was performed and resulted in the fastest release of tofacitinib from the aqueous gel and the lowest from C2. In the ex vivo skin penetration and permeation study, C1 showed the largest skin retention of tofacitinib, whereas, lower retention and higher permeation were observed for the gel and C2. The quantitative MALDI-MSI analysis showed that the content of tofacitinib in the epidermis for the C1 treated samples was comparable to HPLC-MS/MS analysis, whereas, the samples treated with C2 and the aqueous gel were below LOQ. The study demonstrates that MALDI-MSI can be used for the quantitative determination of drug penetration in epidermis, as well as, to provide valuable information on qualitative skin distribution of tofacitinib.

Label-Free Quantitative Proteomics and Substrate-Based Mass Spectrometry Imaging of Xenobiotic Metabolizing Enzymes in Ex Vivo Human Skin and a Human Living Skin Equivalent Model.

We report for the first time label-free quantification of xenobiotic metabolizing enzymes (XME), transporters, redox enzymes, proteases, and nucleases in six human skin explants and a three-dimensional living skin equivalent model from LabSkin. We aimed to evaluate the suitability of LabSkin as an alternative to animal testing for the development of topical formulations. More than 2000 proteins were identified and quantified from total cellular protein. Alcohol dehydrogenase 1C, the most abundant phase I XME in human skin, and glutathione S-transferase pi 1, the most abundant phase II XME in human skin, were present in similar abundance in LabSkin. Several esterases were quantified and esterase activity was confirmed in LabSkin using substrate-based mass spectrometry imaging. No cytochrome P450 (P450) activity was observed for the substrates tested, in agreement with the proteomics data, where the cognate P450s were absent in both human skin and LabSkin. Label-free protein quantification allowed insights into other related processes such as redox homeostasis and proteolysis. For example, the most abundant antioxidant enzymes were thioredoxin and peroxiredoxin-1. This systematic determination of functional equivalence between human skin and LabSkin is a key step toward the construction of a representative human in vitro skin model, which can be used as an alternative to current animal-based tests for chemical safety and for predicting dosage of topically administered drugs. SIGNIFICANCE STATEMENT: The use of label-free quantitative mass spectrometry to elucidate the abundance of xenobiotic metabolizing enzymes, transporters, redox enzymes, proteases, and nucleases in human skin enhance our understanding of the skin physiology and biotransformation of topical drugs and cosmetics. This will help to develop mathematical models to predict drug metabolism in human skin and to develop more robust in vitro engineered human skin tissue as alternatives to animal testing.

Pre-validation of a MALDI MS proteomics-based method for the reliable detection of blood and blood provenance.

The reliable identification of blood, as well as the determination of its origin (human or animal) is of great importance in a forensic investigation. Whilst presumptive tests are rapid and deployed in situ, their very nature requires confirmatory tests to be performed remotely. However, only serological tests can determine blood provenance. The present study improves on a previously devised Matrix Assisted Laser Desorption Ionisation Mass Spectrometry (MALDI MS)-proteomics based method for the reliable detection of blood by enabling the determination of blood provenance. The overall protocol was developed to be more specific than presumptive tests and faster/easier than the gold standard liquid chromatography (LC) MS/MS analysis. This is considered a pre-validation study that has investigated stains and fingermarks made in blood, other biofluids and substances that can elicit a false-positive response to colorimetric or presumptive tests, in a blind fashion. Stains and marks were either untreated or enhanced with a range of presumptive tests. Human and animal blood were correctly discriminated from other biofluids and non-biofluid related matrices; animal species determination was also possible within the system investigated. The procedure is compatible with the prior application of presumptive tests. The refined strategy resulting from iterative improvements through a trial and error study of 56 samples was applied to a final set of 13 blind samples. This final study yielded 12/13 correct identifications with the 13th sample being correctly identified as animal blood but with no species attribution. This body of work will contribute towards the validation of MALDI MS based methods and deployment in violent crimes involving bloodshed.

Characterization of an Aggregated Three-Dimensional Cell Culture Model by Multimodal Mass Spectrometry Imaging.

Mass spectrometry imaging (MSI) is an established analytical tool capable of defining and understanding complex tissues by determining the spatial distribution of biological molecules. Three-dimensional (3D) cell culture models mimic the pathophysiological environment of in vivo tumors and are rapidly emerging as a valuable research tool. Here, multimodal MSI techniques were employed to characterize a novel aggregated 3D lung adenocarcinoma model, developed by the group to mimic the in vivo tissue. Regions of tumor heterogeneity and the hypoxic microenvironment were observed based on the spatial distribution of a variety of endogenous molecules. Desorption electrospray ionization (DESI)-MSI defined regions of a hypoxic core and a proliferative outer layer from metabolite distribution. Targeted metabolites (e.g., lactate, glutamine, and citrate) were mapped to pathways of glycolysis and the TCA cycle demonstrating tumor metabolic behavior. The first application of imaging mass cytometry (IMC) with 3D cell culture enabled single-cell phenotyping at 1 µm spatial resolution. Protein markers of proliferation (Ki-67) and hypoxia (glucose transporter 1) defined metabolic signaling in the aggregoid model, which complemented the metabolite data. Laser ablation inductively coupled plasma (LA-ICP)-MSI analysis localized endogenous elements including magnesium and copper, further differentiating the hypoxia gradient and validating the protein expression. Obtaining a large amount of molecular information on a complementary nature enabled an in-depth understanding of the biological processes within the novel tumor model. Combining powerful imaging techniques to characterize the aggregated 3D culture highlighted a future methodology with potential applications in cancer research and drug development.

Laser ablation inductively coupled plasma mass spectrometry as a novel clinical imaging tool to detect asbestos fibres in malignant mesothelioma.

RATIONALE: Malignant pleural mesothelioma is an extremely aggressive and incurable malignancy associated with prior exposure to asbestos fibres. Difficulties remain in relation to early diagnosis, notably due to impeded identification of asbestos in lung tissue. This study describes a novel laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) imaging approach to identify asbestos within mesothelioma models with clinical significance. METHODS: Human mesothelioma cells were exposed to different types of asbestos fibres and prepared on plastic slides for LA-ICP-MS analysis. No further sample preparation was required prior to analysis, which was performed using an NWR Image 266 nm laser ablation system coupled to an Element XR sector-field ICP mass spectrometer, with a lateral resolution of 2 µm. Data was processed using LA-ICP-MS ImageTool v1.7 with the final graphic production made using DPlot software. RESULTS: Four different mineral fibres were successfully identified within the mesothelioma samples based on some of the most abundant elements that make up these fibres (Si, Mg and Fe). Using LA-ICP-MS as an imaging tool provided information on the spatial distribution of the fibres at cellular level, which is essential in asbestos detection within tissue samples. Based on the metal counts generated by the different types of asbestos, different fibres can be identified based on shape, size, and elemental composition. Detection of Ca was attempted but requires further optimisation. CONCLUSIONS: Detection of asbestos fibres in lung tissues is very useful, if not necessary, to complete the pathological dt9iagnosis of asbestos-related malignancies in the medicolegal field. For the first time, this study demonstrates the successful application of LA-ICP-MS imaging to identify asbestos fibres and other mineral fibres within mesothelioma samples. Ultimately, high-resolution, fast-speed LA-ICP-MS analysis has the potential to be integrated into clinical workflow to aid earlier detection and stratification of mesothelioma patient samples.

Localization of sterols and oxysterols in mouse brain reveals distinct spatial cholesterol metabolism.

Dysregulated cholesterol metabolism is implicated in a number of neurological disorders. Many sterols, including cholesterol and its precursors and metabolites, are biologically active and important for proper brain function. However, spatial cholesterol metabolism in brain and the resulting sterol distributions are poorly defined. To better understand cholesterol metabolism in situ across the complex functional regions of brain, we have developed on-tissue enzyme-assisted derivatization in combination with microliquid extraction for surface analysis and liquid chromatography-mass spectrometry to locate sterols in tissue slices (10 µm) of mouse brain. The method provides sterolomic analysis at 400-µm spot diameter with a limit of quantification of 0.01 ng/mm2 It overcomes the limitations of previous mass spectrometry imaging techniques in analysis of low-abundance and difficult-to-ionize sterol molecules, allowing isomer differentiation and structure identification. Here we demonstrate the spatial distribution and quantification of multiple sterols involved in cholesterol metabolic pathways in wild-type and cholesterol 24S-hydroxylase knockout mouse brain. The technology described provides a powerful tool for future studies of spatial cholesterol metabolism in healthy and diseased tissues.

Localization and Composition of Fructans in Stem and Rhizome of Agave tequilana Weber var. azul.

Methodology combining mass spectrometry imaging (MSI) with ion mobility separation (IMS) has emerged as a biological imaging technique due to its versatility, sensitivity and label-free approach. This technique has been shown to separate isomeric compounds such as lipids, amino acids, carboxylic acids and carbohydrates. This report describes mass spectrometry imaging in combination with traveling-wave ion mobility separation and matrix-assisted laser desorption/ionization (MALDI). Positive ionization mode was used to locate fructans on tissue printed sections of Agave rhizome and stem tissue and distinguished fructan isoforms. Here we show the location of fructans ranging from DP3 to DP17 to be differentially abundant across the stem tissue and for the first time, experimental collision cross sections of endogenous fructan structures have been collected, revealing at least two isoforms for fructans of DP4, DP5, DP6, DP7, DP8, DP10, and DP11. This demonstrates that complex fructans such as agavins can be located and their isoforms resolved using a combination of MALDI, IMS, and MSI, without the need for extraction or derivatization. Use of this methodology uncovered patterns of fructan localization consistent with functional differences where higher DP fructans are found toward the central section of the stem supporting a role in long term carbohydrate storage whereas lower DP fructans are concentrated in the highly vascularized central core of rhizomes supporting a role in mobilization of carbohydrates from the mother plant to developing offsets. Tissue specific patterns of expression of genes encoding enzymes involved in fructan metabolism are consistent with fructan structures and localization.

Role of MALDI-MSI in combination with 3D tissue models for early stage efficacy and safety testing of drugs and toxicants.

Introduction: Three-dimensional (3D) cell cultures have become increasingly important materials to investigate biological processes and drug efficacy and toxicity. The ability of 3D cultures to mimic the physiology of primary tissues and organs in the human body enables further insight into cellular behavior and is hence highly desirable in early-stage drug development. Analyzing the spatial distribution of drug compounds and endogenous molecules provides an insight into the efficacy of a drug whilst simultaneously giving information on biological responses. Areas Covered: In this review we will examine the main 3D cell culture systems employed and applications, which describe their integration with mass spectrometry imaging (MSI). Expert Opinion: MSI is a powerful technique that can map a vast range of molecules simultaneously in tissues without the addition of labels that can provide insights into the efficacy and safety of a new drug. The combination of MSI and 3D cell cultures has emerged as a promising tool in early-stage drug analysis. However, the most common administration route for pharmaceutical drugs is via oral delivery. The use of MSI in combination with models of the GI tract is an area that has been little explored to date, the reasons for this are discussed.

Mass spectrometry imaging of endogenous metabolites in response to doxorubicin in a novel 3D osteosarcoma cell culture model.

Three-dimensional (3D) cell culture is a rapidly emerging field, which mimics some of the physiological conditions of human tissues. In cancer biology, it is considered a useful tool in predicting in vivo chemotherapy responses, compared with conventional two-dimensional (2D) cell culture. We have developed a novel 3D cell culture model of osteosarcoma composed of aggregated proliferative tumour spheroids, which shows regions of tumour heterogeneity formed by aggregated spheroids of polyclonal tumour cells. Aggregated spheroids show local necrotic and apoptotic regions and have sizes suitable for the study of spatial distribution of metabolites by mass spectrometry imaging (MSI). We have used this model to perform a proof-of-principle study showing a heterogeneous distribution of endogenous metabolites that colocalise with the necrotic core and apoptotic regions in this model. Cytotoxic chemotherapy (doxorubicin) responses were significantly attenuated in our 3D cell culture model compared with those of standard cell culture, as determined by resazurin assay, despite sufficient doxorubicin diffusion demonstrated by localisation throughout the 3D constructs. Finally, changes to the distribution of endogenous metabolites in response to doxorubicin were readily detected by MSI. Principal component analysis identified 50 metabolites which differed most in their abundance between treatment groups, and of these, 10 were identified by both in-software t test and mixed-effects analysis of variance (ANOVA). Subsequent independent MSIs of identified species were consistent with principle component analysis findings. This proof-of-principle study shows for the first time that chemotherapy-induced changes in metabolite abundance and distribution may be determined in 3D cell culture by MSI, highlighting this method as a potentially useful tool in the elucidation of chemotherapy responses as an alternative to in vivo testing.

Quantitative Investigation of Terbinafine Hydrochloride Absorption into a Living Skin Equivalent Model by MALDI-MSI.

The combination of microspotting of analytical and internal standards, matrix sublimation, and recently developed software for quantitative mass spectrometry imaging has been used to develop a high-resolution method for the determination of terbinafine hydrochloride in the epidermal region of a full thickness living skin equivalent model. A quantitative assessment of the effect of the addition of the penetration enhancer (dimethyl isosorbide (DMI)) to the delivery vehicle has also been performed, and data have been compared to those obtained from LC-MS/MS measurements of homogenates of isolated epidermal tissue. At 10% DMI, the levels of signal detected for the drug in the epidermis were 0.20 ± 0.072 mg/g tissue for QMSI and 0.28 ± 0.040 mg/g tissue for LC-MS/MS at 50% DMI 0.69 ± 0.23 mg/g tissue for QMSI and 0.66 ± 0.057 mg/g tissue for LC-MS/MS. Comparison of means and standard deviations indicates no significant difference between the values obtained by the two methods.

Quantitation of Endogenous Metabolites in Mouse Tumors Using Mass-Spectrometry Imaging.

Described is a quantitative-mass-spectrometry-imaging (qMSI) methodology for the analysis of lactate and glutamate distributions in order to delineate heterogeneity among mouse tumor models used to support drug-discovery efficacy testing. We evaluate and report on preanalysis-stabilization methods aimed at improving the reproducibility and efficiency of quantitative assessments of endogenous molecules in tissues. Stability experiments demonstrate that optimum stabilization protocols consist of frozen-tissue embedding, post-tissue-sectioning desiccation, and storage at -80 °C of tissue sections sealed in vacuum-tight containers. Optimized stabilization protocols are used in combination with qMSI methodology for the absolute quantitation of lactate and glutamate in tumors, incorporating the use of two different stable-isotope-labeled versions of each analyte and spectral-clustering performed on each tissue section using k-means clustering to allow region-specific, pixel-by-pixel quantitation. Region-specific qMSI was used to screen different tumor models and identify a phenotype that has low lactate heterogeneity, which will enable accurate measurements of lactate modulation in future drug-discovery studies. We conclude that using optimized qMSI protocols, it is possible to quantify endogenous metabolites within tumors, and region-specific quantitation can provide valuable insight into tissue heterogeneity and the tumor microenvironment.