Combining MALDI mass spectrometry imaging and droplet-base surface sampling analysis for tissue distribution, metabolite profiling, and relative quantification of cyclic peptide melanotan II

Peptides have become a fast-growing segment of the pharmaceutical industry over the past few decades. It is essential to develop cutting edge analytical techniques to support the discovery and development of peptide therapeutics, especially to examine their absorption, distribution, metabolism and excretion (ADME) properties. Herein, we utilized two label-free mass spectrometry (MS) based techniques to investigate representative challenges in developing therapeutic peptides, such as tissue distribution, metabolic stability and clearance. A tool proof-of-concept cyclic peptide, melanotan II, was used in this study. Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI), which is a well-developed label-free imaging technique, was used to map the detailed molecular distribution of melanotan II and its metabolites. Droplet-based liquid microjunction surface sampling liquid chromatography-high resolution mass spectrometry (LMJ-SSP-LC-HRMS) was used in combination with MALDI-MSI to rapidly profile molecular information and provide structural insights on drug and metabolites. Using both techniques in parallel allowed a more comprehensive and complementary data set than using either technique independently. We envision MALDI-MSI and droplet-based LMJ-SSP-LC-HRMS, which can be used in combination or as standalone techniques, to become valuable tools for assessing the in vivo fate of peptide therapeutics in support of drug discovery and development.

Non-Destructive Chemical Analysis of a Garcinia mangostana L. (Mangosteen) Herbarium Voucher Specimen

Herbarium voucher specimens are used primarily for taxonomic confirmation. However, they also afford a record of the metabolic profile of a plant, potentially at the time it was collected, or at the very least, at the time of analysis. Even with the enhanced sensitivity of modern analytical techniques, analysis of the metabolites of a herbarium voucher requires removal and consumption of at least part of an entire specimen. We present herein a non-destructive method to analyze the metabolites of herbarium voucher specimens with the droplet-liquid microjunction-surface sampling probe (droplet probe) coupled to ultra-performance liquid chromatography and highresolution mass spectrometry. As proof of concept, a herbarium voucher specimen of Garcinia mangostana (mangosteen) was utilized due to the well-characterized xanthones biosynthesized by this plant, which are of interest as potential anticancer agents. Also, the juice of the fruits of this plant is used widely in the United States and in other countries as a botanical dietary supplement. Metabolite profiles of the sampled surfaces were compared to a subset of xanthone standards. Using this innovative method on the herbarium voucher specimen, we were able to readily identify cytotoxic prenylated xanthones while maintaining the integrity of the entire specimen.

Quantitative biodistribution of biotherapeutics at whole body, organ and cellular levels by autoradiography

Aim: Tools for mapping and quantifying monoclonal antibody (mAb) and peptide biotherapeutics distribumtion were evaluated by comparing data from three independent methods conducted at the whole body, organ or tissue, and cellular levels. Materials & methods: [3H]-mAb1 and [3H]-peptide A were administered intravenously to rats followed by quantitative whole-body autoradiography, kidney macro-autoradiography and micro-autoradiography. Results: [3H]-mAb1 and [3H]-peptide A concentrations were measured in anatomical regions ranging from whole body to whole organ to sub-organ level, such as the kidney glomerulus, with increasing resolution. The tissue/blood [3H]-mAb1 concentrations in selected kidney microenvironments were comparable among the three quantitative methods. Conclusion: Quantitative whole-body autoradiography, tissue macro-autoradiography and micro-autoradiography all provide useful tools for quantifying the concentrations of biotherapeutics at different anatomical levels in tissues, facilitating better predictions of efficacy and toxicity.

Recent development and trends in sample extraction and preparation for mass spectrometric analysis of nucleotides, nucleosides, and proteins

This review describes the recent developments in sample extraction and preparation techniques for mass spectrometric analysis of nucleotides, nucleosides, and proteins. Unique materials and techniques have been developed for highly selective extraction of nucleotides and nucleosides by solid-phase extraction strategies using various affinities. However, for proteins, the analysis of small-scale sections of diseased tissues (formalin-fixed, paraffin-embedded tissues) and the direct analysis of an exact lesion on the surface of diseased tissues (liquid extraction surface analysis) have become important advances in this field. In this review, we focus on the latest developments of these techniques and strategies.

Integration of Ion Mobility MSE after Fully Automated, Online, High-Resolution Liquid Extraction Surface Analysis Micro-Liquid Chromatography

Direct analysis by mass spectrometry (imaging) has become increasingly deployed in preclinical and clinical research due to its rapid and accurate readouts. However, when it comes to biomarker discovery or histopathological diagnostics, more sensitive and in-depth profiling from localized areas is required. We developed a comprehensive, fully automated online platform for high-resolution liquid extraction surface analysis (HR-LESA) followed by micro–liquid chromatography (LC) separation and a data-independent acquisition strategy for untargeted and low abundant analyte identification directly from tissue sections. Applied to tissue sections of rat pituitary, the platform demonstrated improved spatial resolution, allowing sample areas as small as 400 μm to be studied, a major advantage over conventional LESA. The platform integrates an online buffer exchange and washing step for removal of salts and other endogenous contamination that originates from local tissue extraction. Our carry over–free platform showed high reproducibility, with an interextraction variability below 30%. Another strength of the platform is the additional selectivity provided by a postsampling gas-phase ion mobility separation. This allowed distinguishing coeluted isobaric compounds without requiring additional separation time. Furthermore, we identified untargeted and low-abundance analytes, including neuropeptides deriving from the pro-opiomelanocortin precursor protein and localized a specific area of the pituitary gland (i.e., adenohypophysis) known to secrete neuropeptides and other small metabolites related to development, growth, and metabolism. This platform can thus be applied for the in-depth study of small samples of complex tissues with histologic features of ∼400 μm or more, including potential neuropeptide markers involved in many diseases such as neurodegenerative diseases, obesity, bulimia, and anorexia nervosa.

Extraction efficiency and implications for absolute quantitation of propranolol in mouse brain, liver and kidney tissue sections using droplet-based liquid microjunction surface sampling high-performance liquid chromatography/electrospray ionization tandem mass spectrometry

RATIONALE

Currently, the absolute quantitation aspects of droplet-based surface sampling for tissue analysis using a fully automated autosampler/high-performance liquid chromatography/electrospray ionization tandem mass spectrometry (HPLC/ESI-MS/MS) system have not been fully evaluated. Knowledge of extraction efficiency and its reproducibility is required to judge the potential of the method for absolute quantitation of analytes from tissue sections.

METHODS

Adjacent tissue sections of propranolol-dosed mouse brain (10-μm-thick), kidney (10-μm-thick) and liver (8-, 10-, 16- and 24-μm-thick) were obtained. The absolute concentration of propranolol was determined in tissue punches from serial sections using standard bulk tissue extraction protocols and subsequent HPLC separations and MS/MS analysis. These values were used to determine propranolol extraction efficiency from the tissues with the droplet-based surface sampling approach.

RESULTS

Extraction efficiency of propranolol using 10-μm-thick brain, kidney and liver tissues using droplet-based surface sampling varied between ~45 and 63%. The extraction efficiency decreased from ~65% to ~36% with liver thickness increasing from 8 μm to 24 μm. Selecting half of the samples as standards, the precision and accuracy of propranolol concentrations were determined for the other half of the samples that were employed as a quality control data set. The resulting precision (±15%) and accuracy (±3%) were within acceptable limits.

CONCLUSIONS

Quantitation of adjacent mouse tissue sections of different organs and of various thicknesses by droplet-based surface sampling in comparison with bulk extraction of tissue punches showed that extraction efficiency was incomplete using the former method, and that it depended on the organ and tissue thickness. However, once extraction efficiency was determined and applied, the droplet-based approach provided satisfactory quantitation accuracy and precision for assay validations. Thus, once the extraction efficiency was calibrated for a given tissue type, tissue thickness and drug, the droplet-based approach provides a non-labour-intensive and high-throughput means to acquire spatially resolved quantitative analysis of multiple samples of the same type. Published in 2016. This article is a U.S. Government work and is in the public domain in the USA.

Optimizing production and evaluating biosynthesis in situ of a herbicidal compound, mevalocidin, from Coniolariella sp

Mevalocidin is a fungal secondary metabolite produced by Coniolariella sp. It is a unique phytotoxin that demonstrates broad spectrum post-emergent herbicidal properties. With limited options for weed control, the commercialization of a natural product pesticide would be beneficial to organic farming. In this study, two mevalocidin-producing fungal strains, coded MSX56446 and MSX92917, were explored under a variety of growth conditions, including time, temperature, and media. The concentration of mevalocidin was quantitatively measured via LC-MS to determine the optimal setting for each condition. Maximum production was achieved for each condition at 20 days, at 30 °C, with YESD + agar, and with a media containing 2.5 % dextrose. Furthermore, an advanced surface sampling technique was incorporated to gain a better understanding of the fungal culture's natural ability to biosynthesize and distribute this herbicide into its environment. It was shown that both fungi actively exude mevalocidin into their environment via liquid droplet formations known as guttates.

Quantitation of repaglinide and metabolites in mouse whole-body thin tissue sections using droplet-based liquid microjunction surface sampling-high-performance liquid chromatography-electrospray ionization tandem mass spectrometry

Herein, quantitation aspects of a fully automated autosampler/HPLC-MS/MS system applied for unattended droplet-based surface sampling of repaglinide dosed thin tissue sections with subsequent HPLC separation and mass spectrometric analysis of parent drug and various drug metabolites were studied. Major organs (brain, lung, liver, kidney and muscle) from whole-body thin tissue sections and corresponding organ homogenates prepared from repaglinide dosed mice were sampled by surface sampling and by bulk extraction, respectively, and analyzed by HPLC-MS/MS. A semi-quantitative agreement between data obtained by surface sampling and that by employing organ homogenate extraction was observed. Drug concentrations obtained by the two methods followed the same patterns for post-dose time points (0.25, 0.5, 1 and 2 h). Drug amounts determined in the specific tissues was typically higher when analyzing extracts from the organ homogenates. In addition, relative comparison of the levels of individual metabolites between the two analytical methods also revealed good semi-quantitative agreement.

An open port sampling interface for liquid introduction atmospheric pressure ionization mass spectrometry

RATIONALE

A simple method to introduce unprocessed samples into a solvent for rapid characterization by liquid introduction atmospheric pressure ionization mass spectrometry has been lacking. The continuous flow, self-cleaning open port sampling interface introduced here fills this void.

METHODS

The open port sampling interface used a vertically aligned, co-axial tube arrangement enabling solvent delivery to the sampling end of the device through the tubing annulus and solvent aspiration down the center tube and into the ionization source of the mass spectrometer via the commercial APCI emitter probe. The solvent delivery rate to the interface was set to exceed the aspiration rate, creating a continuous sampling interface along with a constant, self-cleaning spillover of solvent from the top of the probe.

RESULTS

Using the open port sampling interface with positive ion mode APCI and a hybrid quadrupole time-of-flight mass spectrometer, rapid, direct sampling and analysis possibilities are exemplified with plastics, ballpoint and felt tip ink pens, skin, and vegetable oils. These results demonstrated that the open port sampling interface could be used as a simple, versatile and self-cleaning system to rapidly introduce multiple types of unprocessed, sometimes highly concentrated and complex, samples into a solvent flow stream for subsequent ionization and analysis by mass spectrometry. The basic setup presented here could be incorporated with any self-aspirating liquid introduction ionization source (e.g., ESI, APCI, APPI, ICP, etc.) or any type of atmospheric pressure sampling-ready mass spectrometer system.

CONCLUSIONS

The open port sampling interface provides a means to introduce and quickly analyze unprocessed solid or liquid samples with the liquid introduction atmospheric pressure ionization source without fear of sampling interface or ionization source contamination.

Dereplicating and Spatial Mapping of Secondary Metabolites from Fungal Cultures in Situ

Ambient ionization mass spectrometry techniques have recently become prevalent in natural product research due to their ability to examine secondary metabolites in situ. These techniques retain invaluable spatial and temporal details that are lost through traditional extraction processes. However, most ambient ionization techniques do not collect mutually supportive data, such as chromatographic retention times and/or UV/vis spectra, and this can limit the ability to identify certain metabolites, such as differentiating isomers. To overcome this, the droplet-liquid microjunction-surface sampling probe (droplet-LMJ-SSP) was coupled with UPLC-PDA-HRMS-MS/MS, thus providing separation, retention times, MS data, and UV/vis data used in traditional dereplication protocols. By capturing these mutually supportive data, the identity of secondary metabolites can be confidently and rapidly assigned in situ. Using the droplet-LMJ-SSP, a protocol was constructed to analyze the secondary metabolite profile of fungal cultures without any sample preparation. The results demonstrate that fungal cultures can be dereplicated from the Petri dish, thus identifying secondary metabolites, including isomers, and confirming them against reference standards. Furthermore, heat maps, similar to mass spectrometry imaging, can be used to ascertain the location and relative concentration of secondary metabolites directly on the surface and/or surroundings of a fungal culture.

Profiling of adrenocorticotropic hormone and arginine vasopressin in human pituitary gland and tumor thin tissue sections using droplet-based liquid-microjunction surface-sampling-HPLC-ESI-MS-MS

Described here are the results from the profiling of the proteins arginine vasopressin (AVP) and adrenocorticotropic hormone (ACTH) from normal human pituitary gland and pituitary adenoma tissue sections, using a fully automated droplet-based liquid-microjunction surface-sampling-HPLC-ESI-MS-MS system for spatially resolved sampling, HPLC separation, and mass spectrometric detection. Excellent correlation was found between the protein distribution data obtained with this method and data obtained with matrix-assisted laser desorption/ionization (MALDI) chemical imaging analyses of serial sections of the same tissue. The protein distributions correlated with the visible anatomic pattern of the pituitary gland. AVP was most abundant in the posterior pituitary gland region (neurohypophysis), and ATCH was dominant in the anterior pituitary gland region (adenohypophysis). The relative amounts of AVP and ACTH sampled from a series of ACTH-secreting and non-secreting pituitary adenomas correlated with histopathological evaluation. ACTH was readily detected at significantly higher levels in regions of ACTH-secreting adenomas and in normal anterior adenohypophysis compared with non-secreting adenoma and neurohypophysis. AVP was mostly detected in normal neurohypophysis, as expected. This work reveals that a fully automated droplet-based liquid-microjunction surface-sampling system coupled to HPLC-ESI-MS-MS can be readily used for spatially resolved sampling, separation, detection, and semi-quantitation of physiologically-relevant peptide and protein hormones, including AVP and ACTH, directly from human tissue. In addition, the relative simplicity, rapidity, and specificity of this method support the potential of this basic technology, with further advancement, for assisting surgical decision-making. Graphical Abstract Mass spectrometry based profiling of hormones in human pituitary gland and tumor thin tissue sections.

Sampling reliability, spatial resolution, spatial precision, and extraction efficiency in droplet-based liquid microjunction surface sampling

RATIONALE

Droplet-based liquid extraction approaches for spatially resolved surface sampling coupled with high-performance liquid chromatography/mass spectrometry (HPLC/MS) provide the ability to deal with complex sample matrices and to identify isomeric compounds not distinguishable by MS methods alone. Improvements in sampling reliability, spatial resolution, spatial precision and extraction efficiency are required to further the analytical utility of such sampling systems.

METHODS

An autosampler capable of droplet-based liquid extraction was coupled with an HPLC/MS system. Visual inspection of the junction formation between the probe and a glass surface allowed evaluation of the liquid junction formation reliability, spatial location and size as a function of variable parameters such as solvent composition, probe-to-surface distance and droplet volume during solvent dispense and aspiration. Quantitative analysis of a component from a model surface using a weak extraction solvent was used to evaluate the effect of extraction time and number of extraction cycles on analyte extraction efficiency.

RESULTS

Reliable junction formation, independent of other variable parameters, was realized simply by maintaining a maximum distance of 0.4 mm between the probe and the sample surface. The smallest liquid junction diameter (1.6 mm) was observed when using a 1 μL dispensed volume and 90% aqueous extraction solvent with either methanol or acetonitrile. Good sampling precision was always achieved using an extraction solvent with at least 50% methanol or acetonitrile by volume. Quantitative sampling of rhodamine B from a magenta Sharpie ink surface using a weak extraction solvent showed that extraction efficiency could be improved by increasing the extraction time or the number of extraction cycles.

CONCLUSIONS

A platform employing a commercially available autosampler coupled to HPLC/MS was developed and successfully applied to investigate the effect of different sampling parameters on the reliability, spatial resolution, spatial precision and extraction efficiency of the liquid junction surface sampling process. This article is a U.S. Government work and is in the public domain in the USA.