Matrix-assisted laser desorption/ionization imaging mass spectrometry for direct measurement of clozapine in rat brain tissue

Mass spectrometry imaging as a tool for surgical decision-making

Despite significant advances in image-guided therapy, surgeons are still too often left with uncertainty when deciding to remove tissue. This binary decision between removing and leaving tissue during surgery implies that the surgeon should be able to distinguish tumor from healthy tissue. In neurosurgery, current image-guidance approaches such as magnetic resonance imaging (MRI) combined with neuronavigation offer a map as to where the tumor should be, but the only definitive method to characterize the tissue at stake is histopathology. Although extremely valuable information is derived from this gold standard approach, it is limited to very few samples during surgery and is not practically used for the delineation of tumor margins. The development and implementation of faster, comprehensive, and complementary approaches for tissue characterization are required to support surgical decision-making--an incremental and iterative process with tumor removed in multiple and often minute biopsies. The development of atmospheric pressure ionization sources makes it possible to analyze tissue specimens with little to no sample preparation. Here, we highlight the value of desorption electrospray ionization as one of many available approaches for the analysis of surgical tissue. Twelve surgical samples resected from a patient during surgery were analyzed and diagnosed as glioblastoma tumor or necrotic tissue by standard histopathology, and mass spectrometry results were further correlated to histopathology for critical validation of the approach. The use of a robust statistical approach reiterated results from the qualitative detection of potential biomarkers of these tissue types. The correlation of the mass spectrometry and histopathology results to MRI brings significant insight into tumor presentation that could not only serve to guide tumor resection, but that is also worthy of more detailed studies on our understanding of tumor presentation on MRI.

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.

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.

Imaging mass spectrometry for assessing temporal proteomics: analysis of calprotectin in Acinetobacter baumannii pulmonary infection

Imaging MS is routinely used to show spatial localization of proteins within a tissue sample and can also be employed to study temporal protein dynamics. The antimicrobial S100 protein calprotectin, a heterodimer of subunits S100A8 and S100A9, is an abundant cytosolic component of neutrophils. Using imaging MS, calprotectin can be detected as a marker of the inflammatory response to bacterial challenge. In a murine model of Acinetobacter baumannii pneumonia, protein images of S100A8 and S100A9 collected at different time points throughout infection aid in visualization of the innate immune response to this pathogen. Calprotectin is detectable within 6 h of infection as immune cells respond to the invading pathogen. As the bacterial burden decreases, signals from the inflammatory proteins decrease. Calprotectin is no longer detectable 96-144 h post infection, correlating to a lack of detectable bacterial burden in lungs. These experiments provide a label-free, multiplexed approach to study host response to a bacterial threat and eventual clearance of the pathogen over time.

Imaging nicotine in rat brain tissue by use of nanospray desorption electrospray ionization mass spectrometry

Imaging mass spectrometry offers simultaneous spatially resolved detection of drugs, drug metabolites, and endogenous substances in a single experiment. This is important when evaluating effects of a drug on a complex organ system such as the brain, where there is a need to understand how regional drug distribution impacts function. Nanospray desorption electrospray ionization, nano-DESI, is a new ambient technique that enables spatially resolved analysis of a variety of samples without special sample pretreatment. This study introduces an experimental approach for accurate spatial mapping of drugs and metabolites in tissue sections by nano-DESI imaging. In this approach, an isotopically labeled standard is added to the nano-DESI solvent to compensate for matrix effects and ion suppression. The analyte image is obtained by normalizing the analyte signal to the signal of the standard in each pixel. We demonstrate that the presence of internal standard enables online quantification of analyte molecules extracted from tissue sections. Ion images are subsequently mapped to the anatomical brain regions in the analyzed section by use of an atlas mesh deformed to match the optical image of the section. Atlas-based registration accounts for the physical variability between animals, which is important for data interpretation. The new approach was used for mapping the distribution of nicotine in rat brain tissue sections following in vivo drug administration. We demonstrate the utility of nano-DESI imaging for sensitive detection of the drug in tissue sections with subfemtomole sensitivity in each pixel of a 27 μm × 150 μm area. Such sensitivity is necessary for spatially resolved detection of low-abundance molecules in complex matrices.

Evaluation of an accurate mass approach for the simultaneous detection of drug and metabolite distributions via whole-body mass spectrometric imaging

In drug discovery and development, in vitro absorption and metabolism assays along with in vivo pharmacokinetic (PK), pharmacodynamic (PD), and toxicokinetic (TK) studies are used to evaluate a potential drug candidate. More recently, imaging mass spectrometry approaches have been successfully reported to aid in the preclinical assessment of drug candidates, resulting in the rapid and noteworthy acceptance of the technique in pharmaceutical research. Traditionally, drug distribution studies via mass spectrometric imaging (MSI) are performed as targeted MS/MS analyses, where the analytes of interest, drug and/or metabolite, are known before the imaging experiment is performed. The study presented here describes a whole-body mass spectrometric imaging (WB-MSI) approach using a hybrid MALDI-LTQ-Orbitrap-MS to detect the distribution of reserpine at 2 h post a 20 mg/kg oral dose. This study effectively demonstrates the utility of obtaining accurate mass measurements across a wide mass range combined with postprocessing tools to efficiently identify drug and metabolite distributions without the need for any a priori knowledge.

Analytical approach to characterize the intratumoral pharmacokinetics and pharmacodynamics of gefitinib in a glioblastoma model

Heterogeneity in brain tumors can result in variable drug distribution and possibly drug response; however, there are no readily accessible means to obtain regional pharmacokinetic (PK)/pharmacodynamic (PD) information in preclinical tumor models that typically rely on average drug concentration measurements. On the basis of a novel serial brain tumor sectioning protocol, sensitive and robust methods were developed to characterize the intratumoral PK [liquid chromatography with tandem mass spectrometry detection (LC/MS/MS)] and PD (phosphorylated extracellular-signal-regulated kinase, antibody-based detection) of gefitinib in small amounts of glioblastoma tumor samples obtained from mice bearing intracerebral tumors administered 150 mg/kg of gefitinib. LC/MS/MS method was accurate (±15%) and precise (coefficient of variation ≤15%). For PD analysis, two antibody-based assay systems [enzyme-linked immunosorbent assay and meso scale discovery (MSD)] were compared and the more sensitive method (MSD) was selected. Gefitinib concentrations showed up to 2.4 ± 0.7-fold intratumoral variability in PK and 1.5 ± 0.20-fold variability in PD. The methods are sufficiently accessible and could be applied to other anticancer drugs and tumor models to obtain greater resolution of intratumoral PKs and PDs.

Quantitative analysis of pharmaceutical drug distribution in multiple organs by imaging mass spectrometry

RATIONALE

Recently, the requirement for a quantitative research method using imaging mass spectrometry (IMS) to be developed has been discussed. Specifically, the simultaneous quantification of a drug in multiple organs by using whole-body sections could be insightful for the pharmaceutical industry in the study of drug distribution.

METHODS

Frozen whole-body sections were obtained from mice injected with raclopride, a dopamine D2 receptor selective antagonist, and coated with a matrix-assisted laser desorption/ionization (MALDI) matrix compound. The whole-body sections were then analyzed using a linear ion trap mass spectrometer equipped with a MALDI source. The concentration of raclopride in each tissue was determined using liquid chromatography/tandem mass spectrometry (LC/MS/MS).

RESULTS

The IMS-based signal intensity of raclopride strongly correlated with the concentration of the drug in the tissue samples (R=0.94; p <0.001) of six different organs. Furthermore, the spatial information obtained by IMS was very similar to that obtained by autoradiography, which is a traditional technique used for the study of drug distribution.

CONCLUSIONS

This study suggests that IMS enables the quantitative analysis of drug distribution in multiple organs simultaneously. In addition, it enhances ideal drug candidate selection in terms of efficient evaluations.

Use of mass spectrometry for imaging metabolites in plants

We discuss and illustrate recent advances that have been made to image the distribution of metabolites among cells and tissues of plants using different mass spectrometry technologies. These technologies include matrix-assisted laser desorption ionization, desorption electrospray ionization, and secondary ion mass spectrometry. These are relatively new technological applications of mass spectrometry and they are providing highly spatially resolved data concerning the cellular distribution of metabolites. We discuss the advantages and limitations of each of these mass spectrometric methods, and provide a description of the technical barriers that are currently limiting the technology to the level of single-cell resolution. However, we anticipate that advances in the next few years will increase the resolving power of the technology to provide unprecedented data on the distribution of metabolites at the subcellular level, which will increase our ability to decipher new knowledge concerning the spatial organization of metabolic processes in plants.

Combined elemental and biomolecular mass spectrometry imaging for probing the inventory of tissue at a micrometer scale

Several complementary mass spectrometric imaging techniques allow mapping of various analytes within biological tissue sections. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) quantitatively detects elements and isotopes with very high sensitivity and a particularly high dynamical range. Matrix-assisted laser desorption/ionization ion mobility mass spectrometry (MALDI-IM-MS) allows a pixel-by-pixel classification and identification of biomolecules. In order to dispose of the healthy hemisphere as an internal calibrant in addition to routinely used external standards, adjacent brain sections of mice with a unilateral 6-OHDA lesion of the medial forebrain bundle were chosen as exemplary samples. We demonstrate a comprehensive way of data acquisition and analysis by coregistering mass spectrometric data on photomicrographs as common reference space and thus providing trimodal spatial information. Registering subsequent planar element maps yielded continuous 3-dimensional data sets. Furthermore, we introduce a correction of MSI data for variable slice thickness applicable to all MSI techniques. In the present case, we observed increased concentrations of iron, manganese, and copper in the lesioned substantia nigra while monounsaturated lipid levels were decreased in the identical region of interest. Our techniques provide new insights into the intricate spatial relationship of morphology and chemistry within tissue.

Stable isotope-resolved metabolomics and applications for drug development

Advances in analytical methodologies, principally nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS), during the last decade have made large-scale analysis of the human metabolome a reality. This is leading to the reawakening of the importance of metabolism in human diseases, particularly cancer. The metabolome is the functional readout of the genome, functional genome, and proteome; it is also an integral partner in molecular regulations for homeostasis. The interrogation of the metabolome, or metabolomics, is now being applied to numerous diseases, largely by metabolite profiling for biomarker discovery, but also in pharmacology and therapeutics. Recent advances in stable isotope tracer-based metabolomic approaches enable unambiguous tracking of individual atoms through compartmentalized metabolic networks directly in human subjects, which promises to decipher the complexity of the human metabolome at an unprecedented pace. This knowledge will revolutionize our understanding of complex human diseases, clinical diagnostics, as well as individualized therapeutics and drug response. In this review, we focus on the use of stable isotope tracers with metabolomics technologies for understanding metabolic network dynamics in both model systems and in clinical applications. Atom-resolved isotope tracing via the two major analytical platforms, NMR and MS, has the power to determine novel metabolic reprogramming in diseases, discover new drug targets, and facilitates ADME studies. We also illustrate new metabolic tracer-based imaging technologies, which enable direct visualization of metabolic processes in vivo. We further outline current practices and future requirements for biochemoinformatics development, which is an integral part of translating stable isotope-resolved metabolomics into clinical reality.

Localization and imaging of gangliosides in mouse brain tissue sections by laserspray ionization inlet

A new ionization method for the analysis of fragile gangliosides without undesired fragmentation or salt adduction is presented. In laserspray ionization inlet (LSII), the matrix/analyte sample is ablated at atmospheric pressure, and ionization takes place in the ion transfer capillary of the mass spectrometer inlet by a process that is independent of a laser wavelength or voltage. The softness of LSII allows the identification of gangliosides up to GQ1 with negligible sialic acid loss. This is of importance to the field of MS imaging, as undesired fragmentation has made it difficult to accurately map the spatial distribution of fragile ganglioside lipids in tissue. Proof-of-principle structural characterization of endogenous gangliosides using MS(n) fragmentation of multiply charged negative ions on a LTQ Velos and subsequent imaging of the GD1 ganglioside is demonstrated. This is the first report of multiply charged negative ions using inlet ionization. We find that GD1 is detected at higher levels in the mouse cortex and hippocampus compared with the thalamus. In LSII with the laser aligned in transmission geometry relative to the inlet, images were obtained in approximately 60 min using an inexpensive nitrogen laser.

MALDI mass spectrometric imaging of cardiac tissue following myocardial infarction in a rat coronary artery ligation model

Although acute myocardial infarction (MI) is consistently among the top causes of death in the United States, the spatial distribution of lipids and metabolites following MI remains to be elucidated. This work presents the investigation of an in vivo rat model of MI using mass spectrometric imaging (MSI) and multivariate data analysis. MSI was conducted on cardiac tissue following a 24-h left anterior descending coronary artery ligation to analyze multiple compound classes. First, the spatial distribution of a small metabolite, creatine, was used to identify areas of infarcted myocardium. Second, multivariate data analysis and tandem mass spectrometry were used to identify phospholipid (PL) markers of MI. A number of lysophospholipids demonstrated increased ion signal in areas of infarction. In contrast, select intact PLs demonstrated decreased ion signal in the area of infarction. The complementary nature of these two lipid classes suggests increased activity of phospholipase A(2), an enzyme that has been implicated in coronary heart disease and inflammation.

MALDI in-source decay, from sequencing to imaging

Matrix-assisted laser desorption/ionization (MALDI) is now a mature method allowing the identification and, more challenging, the quantification of biopolymers (proteins, nucleic acids, glycans, etc). MALDI spectra show mostly intact singly charged ions. To obtain fragments, the activation of singly charged precursors is necessary, but not efficient above 3.5 kDa, thus making MALDI MS/MS difficult for large species. In-source decay (ISD) is a prompt fragmentation reaction that can be induced thermally or by radicals. As fragments are formed in the source, precursor ions cannot be selected; however, the technique is not limited by the mass of the analyzed compounds and pseudo MS3 can be performed on intense fragments. The discovery of new matrices that enhance the ISD yield, combined with the high sensitivity of MALDI mass spectrometers, and software development, opens new perspectives. We first review the mechanisms involved in the ISD processes, then discuss ISD applications like top-down sequencing and post-translational modifications (PTMs) studies, and finally review MALDI-ISD tissue imaging applications.

Tissue analysis by imaging MS

Imaging MS (IMS) is generating tremendous interest in scientific communities because of its unparalleled capabilities to provide chemical analysis of intact tissue. Advances in analytical chemistry and MS are providing new insights into chemical and biological processes. This review will discuss various IMS platforms and their applications in biomedical and pharmaceutical research.

Qualitative and quantitative MALDI imaging of the positron emission tomography ligands raclopride (a D2 dopamine antagonist) and SCH 23390 (a D1 dopamine antagonist) in rat brain tissue sections using a solvent-free dry matrix application method

The distributions of positron emission tomography (PET) ligands in rat brain tissue sections were analyzed by matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI). The detection of the PET ligands was possible following the use of a solvent-free dry MALDI matrix application method employing finely ground dry α-cyano-4-hydroxycinnamic acid (CHCA). The D2 dopamine receptor antagonist 3,5-dichloro-N-{[(2S)-1-ethylpyrrolidin-2-yl]methyl}-2-hydroxy-6-methoxybenzamide (raclopride) and the D1 dopamine receptor antagonist 7-chloro-3-methyl-1-phenyl-1,2,4,5-tetrahydro-3-benzazepin-8-ol (SCH 23390) were both detected at decreasing abundance at increasing period postdosing. Confirmation of the compound identifications and distributions was achieved by a combination of mass-to-charge ratio accurate mass, isotope distribution, and MS/MS fragmentation imaging directly from tissue sections (performed using MALDI TOF/TOF, MALDI q-TOF, and 12T MALDI-FT-ICR mass spectrometers). Quantitative data was obtained by comparing signal abundances from tissues to those obtained from quantitation control spots of the target compound applied to adjacent vehicle control tissue sections (analyzed during the same experiment). Following a single intravenous dose of raclopride (7.5 mg/kg), an average tissue concentration of approximately 60 nM was detected compared to 15 nM when the drug was dosed at 2 mg/kg, indicating a linear response between dose and detected abundance. SCH 23390 was established to have an average tissue concentration of approximately 15 μM following a single intravenous dose at 5 mg/kg. Both target compounds were also detected in kidney tissue sections when employing the same MSI methodology. This study illustrates that a MSI may well be readily applied to PET ligand research development when using a solvent-free dry matrix coating.

Imaging of phospholipids in formalin fixed rat brain sections by matrix assisted laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is a valuable tool for the analysis of molecules directly from tissue. Imaging of phospholipids is gaining widespread interest, particularly as these lipids have been implicated in a variety of pathologic processes. Formalin fixation (FF) is the standard protocol used in histology laboratories worldwide to preserve tissue for analysis, in order to aid in the diagnosis and prognosis of diseases. This study assesses MALDI imaging of phospholipids directly in formalin fixed tissue, with a view to future analysis of archival tissue. This investigation proves the viability of MALDI-MSI for studying the distribution of lipids directly in formalin fixed tissue, without any pretreatment protocols. High quality molecular images for several phosphatidylcholine (PC) and sphingomyelin (SM) species are presented. Images correspond well with previously published data for the analysis of lipids directly from freshly prepared tissue. Different ionization pathways are observed when analyzing fixed tissue compared with fresh, and this change was found to be associated with formalin buffers employed in fixation protocols. The ability to analyze lipids directly from formalin fixed tissue opens up new doors in the investigation of disease profiles. Pathologic specimens taken for histologic investigation can be analyzed by MALDI-MS to provide greater information on the involvement of lipids in diseased tissue.

Determination of loperamide in mdr1a/1b knock-out mouse brain tissue using matrix-assisted laser desorption/ionization mass spectrometry and comparison with quantitative electrospray-triple quadrupole mass spectrometry analysis

Recently matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) imaging has been used to analyze small molecule pharmaceutical compounds directly on tissue sections to determine spatial distribution within target tissue and organs. The data presented to date usually indicate relative amounts of drug within the tissue. The determination of absolute amounts is still done using tissue homogenization followed by traditional liquid chromatography-tandem mass spectrometry (LC-MS/MS). In this study, the quantitative determination of loperamide, an antidiarrheal agent and a P-glycoprotein substrate, in mdr1a/1b (-/-) mouse brain tissue sections using MALDI MS on a quadrupole time-of-flight mass spectrometry is described. 5 mg/mL α-cyano-4-hydroxycinnamic acid in 50% acetonitrile with 0.1% trifluoroacetic acid and 0.5 μM reserpine was used as the MALDI matrix. The calibration curve constructed by the peak intensities of standard samples from MALDI MS was linear from 0.025 to 0.5 μM with r² = 0.9989. The accuracy of calibration curve standards was 78.3-105.9% and the percent deviation was less than 25%. Comparison between direct MALDI tissue analysis and conventional tissue analysis using homogenization followed by electrospray LC-MS/MS was also explored.

On-tissue chemical derivatization of 3-methoxysalicylamine for MALDI-imaging mass spectrometry

MALDI-imaging mass spectrometry (IMS) has been shown to be a powerful tool to study drug distributions in organ tissue as well as whole animal bodies. Nevertheless, not all drugs are amenable to MALDI while others may be limited by poor sensitivity poor sensitivity. The use of chemical derivatization to improve detection of small molecules by mass spectrometry techniques is well documented. To our knowledge, however, this approach has not been applied to direct tissue analysis of small organic molecules. In this manuscript, we demonstrate the use of on-tissue chemical derivatization of a small organic molecule, 3-methoxysalicylamine (3-MoSA) a scavenger of γ-ketoaldehydes. Derivatization of 3-MoSA with 1,1'-thiocarbonyldiimidazole (TCDI) results in an oxothiazolidine derivative which is detected with much greater sensitivity by MALDI than 3-MoSA itself. TCDI treatment of tissue from mice dosed with 3-MoSA allowed images to be obtained showing its spatial distribution as well as its pharmacokinetic profile in different organs. These images correlated well with results obtained from HPLC-MS/MS analyses of the same tissues. These results provide proof-of-concept that on-tissue chemical derivatization can be used to improve detection of a small organic molecule by MALDI-IMS.

MALDI imaging mass spectrometry for direct tissue analysis: technological advancements and recent applications

Matrix assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) is a method that allows the investigation of the molecular content of tissues within its morphological context. Since it is able to measure the distribution of hundreds of analytes at once, while being label free, this method has great potential which has been increasingly recognized in the field of tissue-based research. In the last few years, MALDI-IMS has been successfully used for the molecular assessment of tissue samples mainly in biomedical research and also in other scientific fields. The present article will give an update on the application of MALDI-IMS in clinical and preclinical research. It will also give an overview of the multitude of technical advancements of this method in recent years. This includes developments in instrumentation, sample preparation, computational data analysis and protein identification. It will also highlight a number of emerging fields for application of MALDI-IMS like drug imaging where MALDI-IMS is used for studying the spatial distribution of drugs in tissues.

MALDI tissue imaging: from biomarker discovery to clinical applications

Matrix-assisted laser desorption ionization (MALDI) imaging mass spectrometry (IMS) is a powerful tool for the generation of multidimensional spatial expression maps of biomolecules directly from a tissue section. From a clinical proteomics perspective, this method correlates molecular detail to histopathological changes found in patient-derived tissues, enhancing the ability to identify candidates for disease biomarkers. The unbiased analysis and spatial mapping of a variety of molecules directly from clinical tissue sections can be achieved through this method. Conversely, targeted IMS, by the incorporation of laser-reactive molecular tags onto antibodies, aptamers, and other affinity molecules, enables analysis of specific molecules or a class of molecules. In addition to exploring tissue during biomarker discovery, the integration of MALDI-IMS methods into existing clinical pathology laboratory practices could prove beneficial to diagnostics. Querying tissue for the expression of specific biomarkers in a biopsy is a critical component in clinical decision-making and such markers are a major goal of translational research. An important challenge in cancer diagnostics will be to assay multiple parameters in a single slide when tissue quantities are limited. The development of multiplexed assays that maximize the yield of information from a small biopsy will help meet a critical challenge to current biomarker research. This review focuses on the use of MALDI-IMS in biomarker discovery and its potential as a clinical diagnostic tool with specific reference to our application of this technology to prostate cancer.

Distribution of chloroquine in ocular tissue of pigmented rat using matrix-assisted laser desorption/ionization imaging quadrupole time-of-flight tandem mass spectrometry

In pharmacology and toxicology, localization of the distribution of a drug molecule in its target tissue provides very important in vivo biological information. Traditionally, this has been examined using autoradiography (ARG). However, there are significant limitations in this application. One is the synthesis and use of radiolabeled compounds, the other is that the image generated expresses an undifferentiated mixture of the parent drug and/or its metabolites. The objective of the study was to define the specific distribution of the parent drug in rat ocular tissue containing melanin (e.g. the retina) using non-labeled chloroquine by MALDI Imaging tandem mass spectrometry (MS/MS). After single oral administration (at 20 mg/kg) of chloroquine, sections (10 µm) of rat eye tissue were prepared at 24 h. The MS system used was a quadrupole time-of flight (Q-TOF) tandem mass spectrometer (MALDI Synapt™, Waters, Milford, MA, USA). Tissue sections were sprayed with CHCA (α-cyano-4-hydroxycinnamic acid, 5 mg/mL) in 80% acetonitrile (ACN) containing 5% formic acid (FA) using either a manual sprayer (airbrush) or an automated sprayer (TM-Sprayer™, HTX Technologies, Carrboro, NC, USA). Chloroquine was readily detected in the MS/MS mode by monitoring one of its major fragment ions (m/z 247.10) and imaged through the rat eye tissue. The image of the specific distribution within the retina in the rat eye tissue was confirmed, and found to be similar to autoradiograms after oral administration of (14)C-chloroquine reported previously.

Lipid analysis of flat-mounted eye tissue by imaging mass spectrometry with identification of contaminants in preservation

Matrix-assisted laser desorption/ionization imaging mass spectrometry was used to analyze donor eye tissue specimens for phospholipid content to evaluate lipid distribution. Phosphatidylcholines and sphingomyelins were detected in the positive ion mode using 2,5-dihydroxybenzoic acid as the matrix. During this study, unknown ion signals in the lower m/z region (less than m/z 400) were detected, mainly in the far periphery of human flat-mounted tissue but not in age-matched rhesus monkey tissue prepared in a similar manner. The unknown ion signals occurred at m/z 304, 332, 360, and 388. These ions were subjected to tandem mass spectrometry directly from the tissue sample, and exact mass measurements of extracts were prepared for further identification. These ions were identified as alkyl dimethylbenzylammonium surfactants (benzalkonium chlorides (BACs)). The classification of these species was verified by comparing an eye tissue extract to an over-the-counter eye-care product containing BACs.

Protein signatures for survival and recurrence in metastatic melanoma

Patients with melanoma metastatic to regional lymph nodes exhibit a range in tumor progression, survival, and treatment. Current approaches to stratify patients with this stage of disease predominantly involve clinical and histological methods. Molecular classification thus far has focused almost exclusively on genetic mutations. In this study, proteomic data from 69 melanoma lymph node metastases and 17 disease free lymph nodes acquired by histology-directed MALDI imaging mass spectrometry were used to classify tumor from control lymph node and to molecularly sub-classify patients with stage III disease. From these data, 12 survival associated protein signals and 3 recurrence associated signals in the acquired mass spectra were combined to generate a multiplex molecular signature to group patients into either poor or favorable groups for recurrence and survival. Proteins represented in the signature include cytochrome c, s100 A6, histone H4, and cleaved forms of thymosin β-4, thymosin β-10, and ubiquitin. In total over 40 protein signals from the tissue were identified.

Mass spectrometry imaging for drugs and metabolites

Mass spectrometric imaging (MSI) is a powerful analytical technique that provides two- and three-dimensional spatial maps of multiple compounds in a single experiment. This technique has been routinely applied to protein, peptide, and lipid molecules with much less research reporting small molecule distributions, especially pharmaceutical drugs. This review's main focus is to provide readers with an up-to-date description of the substrates and compounds that have been analyzed for drug and metabolite composition using MSI technology. Additionally, ionization techniques, sample preparation, and instrumentation developments are discussed.

A quantitation method for mass spectrometry imaging

A new quantitation method for mass spectrometry imaging (MSI) with matrix-assisted laser desorption/ionization (MALDI) has been developed. In this method, drug concentrations were determined by tissue homogenization of five 10 µm tissue sections adjacent to those analyzed by MSI. Drug levels in tissue extracts were measured by liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS). The integrated MSI response was correlated to the LC/MS/MS drug concentrations to determine the amount of drug detected per MSI ion count. The study reported here evaluates olanzapine in liver tissue. Tissue samples containing a range of concentrations were created from liver harvested from rats administered a single dose of olanzapine at 0, 1, 4, 8, 16, 30, or 100 mg/kg. The liver samples were then analyzed by MALDI-MSI and LC/MS/MS. The MALDI-MSI and LC/MS/MS correlation was determined for tissue concentrations of ~300 to 60,000 ng/g and yielded a linear relationship over two orders of magnitude (R(2) = 0.9792). From this correlation, a conversion factor of 6.3 ± 0.23 fg/ion count was used to quantitate MSI responses at the pixel level (100 µm). The details of the method, its importance in pharmaceutical analysis, and the considerations necessary when implementing it are presented.

Nanostructure-initiator mass spectrometry metabolite analysis and imaging

Nanostructure-Initiator Mass Spectrometry (NIMS) is a matrix-free desorption/ionization approach that is particularly well-suited for unbiased (untargeted) metabolomics. An overview of the NIMS technology and its application in the detection of biofluid and tissue metabolites are presented. (To listen to a podcast about this feature, please go to the Analytical Chemistry multimedia page at pubs.acs.org/page/ancham/audio/index.html .).

Desorption electrospray ionization imaging mass spectrometry of lipids in rat spinal cord

Imaging mass spectrometry allows for the direct investigation of tissue samples to identify specific biological compounds and determine their spatial distributions. Desorption electrospray ionization (DESI) mass spectrometry has been used for the imaging and analysis of rat spinal cord cross sections. Glycerophospholipids and sphingolipids, as well as fatty acids, were detected in both the negative and positive ion modes and identified through tandem mass spectrometry (MS/MS) product ion scans using collision-induced dissociation and accurate mass measurements. Differences in the relative abundances of lipids and free fatty acids were present between white and gray matter areas in both the negative and positive ion modes. DESI-MS images of the corresponding ions allow the determination of their spatial distributions within a cross section of the rat spinal cord, by scanning the DESI probe across the entire sample surface. Glycerophospholipids and sphingolipids were mostly detected in the white matter, while the free fatty acids were present in the gray matter. These results show parallels with reported distributions of lipids in studies of rat brain. This suggests that the spatial intensity distribution reflects relative concentration differences of the lipid and fatty acid compounds in the spinal cord tissue. The "butterfly" shape of the gray matter in the spinal cord cross section was resolved in the corresponding ion images, indicating that a lateral resolution of better than 200 mum was achieved. The selected ion images of lipids are directly correlated with anatomic features on the spinal cord corresponding to the white and the gray matter.

Histo-proteomic profiling of formalin-fixed, paraffin-embedded tissue

In the functional proteome era, the proteomic profiling of clinicopathologic-annotated tissues is an essential step for mining and evaluating candidate biomarkers for disease. For many diseases, but especially cancer, the development of predictive biomarkers requires performing assays directly on the diseased tissue. The last decade has seen the explosion of both prognostic and predictive biomarkers in the research setting but few of these biomarkers have entered widespread clinical use. Previously, application of routine proteomic methodologies to clinical formalin-fixed and paraffin-embedded tissue specimens has provided unsatisfactory results. In this paper, we will discuss recent advancements in proteomic profiling technology for clinical applications. These approaches focus on the retention of histomorphologic information as an element of the proteomic analysis.

Detection of carbohydrates and steroids by cation-enhanced nanostructure-initiator mass spectrometry (NIMS) for biofluid analysis and tissue imaging

Nanostructure-initiator mass spectrometry (NIMS) is a highly sensitive, matrix-free technique that is well suited for biofluid analysis and imaging of biological tissues. Here we provide a new technical variation of NIMS to analyze carbohydrates and steroids, molecules that are challenging to detect with traditional mass spectrometric approaches. Analysis of carbohydrates and steroids was accomplished by spray depositing NaCl or AgNO(3) on the NIMS porous silicon surface to provide a uniform environment rich with cationization agents prior to desorption of the fluorinated polymer initiator. Laser desorption/ionization of the ion-coated NIMS surface allowed for Na(+) cationization of carbohydrates and Ag(+) cationization of steroids. The reliability of the approach is quantitatively demonstrated with a calibration curve over the physiological range of glucose and cholesterol concentrations in human serum (1-200 microM). Additionally, we illustrate the sensitivity of the method by showing its ability to detect carbohydrates and steroids down to the 800-amol and 100-fmol levels, respectively. The technique developed is well suited for tissue imaging of biologically significant metabolites such as sucrose and cholesterol. To highlight its applicability, we used cation-enhanced NIMS to image the distribution of sucrose in a Gerbera jamesonii flower stem and the distribution of cholesterol in a mouse brain. The flower stem and brain sections were placed directly on the ion-coated NIMS surface without further preparation and analyzed directly. The overall results reported underscore the potential of NIMS to analyze and image chemically diverse compounds that have been traditionally challenging to observe with mass spectrometry-based techniques.

Essential tactics of tissue preparation and matrix nano-spotting for successful compound imaging mass spectrometry

The ultimate goal of MALDI-Imaging Mass Spectrometry (MALDI-IMS) is to achieve spatial localization of analytes in tissue sections down to individual tissue compartments or even at the level of a few cells. With compound tissue imaging, it is possible to track the transportation of an unlabelled, inhaled reference compound within lung tissue, through the application of MALDI-IMS. The procedure for isolation and preparation of lung tissues is found to be crucial in order to preserve the anatomy and structure of the pulmonary compartments. To avoid delocalization of analytes within lung tissue compartments we have applied an in-house designed nano-spotter, based on a microdispenser mounted on an XY table, of which movement and spotting functionality were fully computer controlled. We demonstrate the usefulness of this platform in lung tissue sections isolated from rodent in vivo model, applied to compound tissue imaging as exemplified with the determination of the spatial distribution of (1alpha,2beta,4beta,7beta)-7-[(hydroxidi-2-thienylacetyl)oxy]-9,9-dimethyl-3-oxa-9-azoniatricyclo[3.3.1.0(2,4)]nonane, also known as tiotropium. We provide details on tissue preparation protocols and sample spotting technology for successful identification of drug in mouse lung tissue by using MALDI-Orbitrap instrumentation.

Mass spectrometry imaging of pharmacological compounds in tissue sections

The use of MS imaging (MSI) to resolve the spatial and pharmacodynamic distributions of compounds in tissues is emerging as a powerful tool for pharmacological research. Unlike established imaging techniques, only limited a priori knowledge is required and no extensive manipulation (e.g., radiolabeling) of drugs is necessary prior to dosing. MS provides highly multiplexed detection, making it possible to identify compounds, their metabolites and other changes in biomolecular abundances directly off tissue sections in a single pass. This can be employed to obtain near cellular, or potentially subcellular, resolution images. Consideration of technical limitations that affect the process is required, from sample preparation through to analyte ionization and detection. The techniques have only recently been adapted for imaging and novel variations to the established MSI methodologies will further enhance the application of MSI for pharmacological research.

MALDI imaging of neutral cuticular lipids in insects and plants

The spatial distribution of neutral lipids and hydrocarbons has been imaged using MALDI-TOF mass spectrometry on intact plant and insect surfaces, namely wings and legs of the gray flesh fly (Neobellieria bullata), wings of common fruit fly (Drosophila melanogaster), leaves of thale cress (Arabidopsis thaliana), and leaves of date palm tree (Phoenix sp.). The distribution of wax esters (WEs) and saturated and unsaturated hydrocarbons (HCs) was visualized. The samples were attached on a target and multiply sprayed with lithium or sodium 2,5-dihydroxybenzoate. The deposits were homogenous, consisting of small islands (50-150 microm) of matrix crystals separated by small areas (10 microm) of uncovered cuticle. Samples of N. bullata wings were found to contain HCs and WEs distributed close to their basal parts. The distribution of sodium and potassium ions was visualized on samples prepared by sublimation of 2,5-dihydroxybenzoic acid. Pheromonal dienes were detected on D. melanogaster female wings. A homogenous distribution of saturated WEs was observed on A. thaliana and Phoenix sp. leaf samples. The optimum number of laser shots per pixel was found to be higher than for polar compounds imaging.

Imaging of lipids in spinal cord using intermediate pressure matrix-assisted laser desorption-linear ion trap/Orbitrap MS

A hybrid linear ion trap/Orbitrap mass spectrometer was used to perform tandem mass spectrometry (MS/MS) experiments and high-resolution mass analysis of lipids desorbed from nerve tissue. A dramatic improvement in mass spectral resolution and a decrease in background are observed in the spectra collected from the Orbitrap mass analyzer, which allows generation of more accurate mass spectrometric images of the distribution of lipids within nerve tissue. Employment of both mass analyzers provides a rapid and reliable means of compound identification based on MS/MS fragmentation and high-resolution mass spectrometry (HRMS) accurate mass.

Tandem mass spectrometric methods for phospholipid analysis from brain tissue

We describe the utility of intermediate-pressure MALDI and tandem mass spectrometry (MS/MS and MS( n )) for the characterization and imaging of phospholipids in brain tissue sections. The use of both MS/MS spectra and MS/MS images allows for identification of isobaric compounds. The structural characterization of phosphatidylcholines, phosphatidylserines, phosphatidylethanolamines, and sphingomyelins directly from tissue sections is described.

Mapping pharmaceuticals in rat brain sections using MALDI imaging mass spectrometry

Matrix-assisted laser desorption/ionization-tandem mass spectrometric method (MALDI-MS/MS) has proven to be a reliable tool for direct measurement of the disposition of small molecules in animal tissue sections. As example, MALDI-MS/MS imaging system was employed for visualizing the spatial distribution of astemizole and its primary metabolite in rat brain tissues. Astemizole is a second-generation antihistamine, a block peripheral H1 receptor, which was introduced to provide comparable therapeutic benefit but was withdrawn in most countries due to toxicity risks. Astemizole was observed to be heterogeneously distributed to most parts of brain tissue slices including cortex, hippocampus, hypothalamic, thalamus, and ventricle regions while its major metabolite, desmethylastemizole, was only found around ventricle sites. We have shown that astemizole alone is likely to be responsible for the central nervous system (CNS) side effects when its exposures became elevated.

Metabolite identification in rat brain microdialysates by direct infusion nanoelectrospray ionization after desalting on a ZipTip and LTQ/Orbitrap mass spectrometry

Analyzing brain microdialysate samples by mass spectrometry is challenging due to the high salt content of the artificial cerebral spinal fluid (aCSF), low analyte concentrations and small sample volumes collected. A drug and its major metabolites can be examined in brain microdialysates by targeted approaches such as selected reaction monitoring (SRM) which provides selectivity and high sensitivity. However, this approach is not well suited for metabolite profiling in the brain which aims to determine biotransformation pathways. Identifying minor metabolites, or metabolites that arise from brain metabolism, remains a challenge and, for a drug in early discovery, identification of metabolites present in the brain can provide useful information for understanding the pharmacological activity and potential toxicological liabilities of the drug. A method is described here for rapid metabolite profiling in brain microdialysates that involves sample clean-up using C18 ZipTips to remove salts followed by direct infusion nanoelectrospray with an LTQ/Orbitrap mass spectrometer using real-time internal recalibration. Full scan mass spectra acquired at high resolving power (100 K at m/z 400) were examined manually and with mass defect filtering. Metabolite identification was aided by sub-parts-per-million mass accuracy and structural characterization was accomplished by tandem mass spectrometry (MS/MS) experiments in the Orbitrap or LTQ depending on the abundance of the metabolite. Using this approach, brain microdialysate samples from rats dosed with one of four CNS drugs (imipramine, reboxetine, citalopram or trazodone) were examined for metabolites. For each drug investigated, metabolites, some of which not previously reported in rat brain, were identified and characterized.