Direct analysis of free amino acids by mixed-mode chromatography with tandem mass spectrometry

Automated Liquid Handling Extraction and Rapid Quantification of Underivatized Amino Acids and Tryptophan Metabolites from Human Serum and Plasma Using Dual-Column U(H)PLC-MRM-MS and Its Application to Prostate Cancer Study

Amino acids (AAs) and their metabolites are important building blocks, energy sources, and signaling molecules associated with various pathological phenotypes. The quantification of AA and tryptophan (TRP) metabolites in human serum and plasma is therefore of great diagnostic interest. Therefore, robust, reproducible sample extraction and processing workflows as well as rapid, sensitive absolute quantification are required to identify candidate biomarkers and to improve screening methods. We developed a validated semi-automated robotic liquid extraction and processing workflow and a rapid method for absolute quantification of 20 free, underivatized AAs and six TRP metabolites using dual-column U(H)PLC-MRM-MS. The extraction and sample preparation workflow in a 96-well plate was optimized for robust, reproducible high sample throughput allowing for transfer of samples to the U(H)PLC autosampler directly without additional cleanup steps. The U(H)PLC-MRM-MS method, using a mixed-mode reversed-phase anion exchange column with formic acid and a high-strength silica reversed-phase column with difluoro-acetic acid as mobile phase additive, provided absolute quantification with nanomolar lower limits of quantification within 7.9 min. The semi-automated extraction workflow and dual-column U(H)PLC-MRM-MS method was applied to a human prostate cancer study and was shown to discriminate between treatment regimens and to identify metabolites responsible for discriminating between healthy controls and patients on active surveillance.

What is the role of current mass spectrometry in pharmaceutical analysis?

The role of mass spectrometry (MS) has become more important in most application domains in recent years. Pharmaceutical analysis is specific due to its stringent regulation procedures, the need for good laboratory/manufacturing practices, and a large number of routine quality control analyses to be carried out. The role of MS is, therefore, very different throughout the whole drug development cycle. While it dominates within the drug discovery and development phase, in routine quality control, the role of MS is minor and indispensable only for selected applications. Moreover, its role is very different in the case of analysis of small molecule pharmaceuticals and biopharmaceuticals. Our review explains the role of current MS in the analysis of both small-molecule chemical drugs and biopharmaceuticals. Important features of MS-based technologies being implemented, method requirements, and related challenges are discussed. The differences in analytical procedures for small molecule pharmaceuticals and biopharmaceuticals are pointed out. While a single method or a small set of methods is usually sufficient for quality control in the case of small molecule pharmaceuticals and MS is often not indispensable, a large panel of methods including extensive use of MS must be used for quality control of biopharmaceuticals. Finally, expected development and future trends are outlined.

Comprehensive characterization of amino acids and water-soluble vitamins in a pentylenetetrazole-induced seizures rat model

Epilepsy is a complex neurological disease characterized by spontaneous recurrent seizures that affect around 1% of the global population. Despite the significant progress in the mechanisms of epileptogenesis, there is still about 60% of cases in which the cause is unknown. Thus, revealing the molecular mechanisms of epileptogenesis will greatly improve the development of epilepsy treatment. Since the comprehensive characterization of amino acids and water-soluble vitamins is important in understanding the underlying mechanisms of epilepsy or seizures, we developed two liquid chromatography-tandem mass spectrometry methods to quantify 17 water-soluble vitamins and 46 amino acids and applied them to our pentylenetetrazole-induced kindling rat model. All water-soluble vitamins were detected with a linearity of r > 0.992 and limits of quantitation between 0.1 and 5 ng/ml except for nicotinic acid. For amino acids, the linearities obtained were good with correlation coefficients higher than 0.99, and matrix effects were between 85.3% and 110%. To handle the multidimensional data more effectively, multivariate statistical analysis approaches used in non-targeted metabolomics were creatively exploited in the visualization, interpretation, and exploration of the results.

Quantitation of non-derivatized free amino acids for detecting inborn errors of metabolism by incorporating mixed-mode chromatography with tandem mass spectrometry

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Analysis of 39 free amino acids in 15 min LC-MS/MS run.

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Validation of method in both plasma and urine.

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Chromatographically resolves leucine, isoleucine, and allo-isoleucine.

Introduction

Amino acids are critical biomarkers for many inborn errors of metabolism, but amino acid analysis is challenging due to the range of chemical properties inherent in these small molecules. Techniques are available for amino acid analysis, but they can suffer from long run times, laborious derivatization, and/or poor resolution of isobaric compounds.

Objective

To develop and validate a method for the quantitation of a non-derivatized free amino acid profile in both plasma and urine samples using mixed-mode chromatography and tandem mass spectrometry.

Methods

Chromatographic conditions were optimized to separate leucine, isoleucine, and allo-isoleucine and maintain analytical runtime at less than 15 min. Sample preparation included a quick protein precipitation followed by LC-MS/MS analysis. Matrix effects, interferences, linearity, carryover, acceptable dilution limits, precision, accuracy, and stability were evaluated in both plasma and urine specimen types.

Results

A total of 38 amino acids and related compounds were successfully quantitated with this method. In addition, argininosuccinic acid was qualitatively analyzed. A full clinical validation was performed that included method comparison to a reference laboratory for plasma and urine with Deming regression slopes ranging from 0.38 to 1.26.

Conclusion

This method represents an alternative to derivatization-based methods, especially in urine samples where interference from metabolites and medications is prevalent.

Characterization and comparison of mixed-mode and reversed-phase columns; interaction abilities and applicability for peptide separation

In this work, two mixed-mode columns from a different manufacturers and one marketed as a reversed-phase column were characterized and compared in the terms of their interaction abilities, retentivity, peak symmetry, and applicability for peptide separation. All the tested columns contain octadecyl ligand and positively charged modifier, i.e. pyridyl group for the reversed-phase column XSelect CSH C18, quaternary alkylamine for mixed-mode column Atlantis PREMIER BEH C18 AX, and permanently charged moiety (details not available from the manufacturer) for mixed-mode column Luna Omega PS C18. For detailed characterization and comparison of their interaction potential, several approaches were used. First, a simple Walters test was performed to estimate hydrophobic and silanophilic interactions of the tested columns. The highest values of both parameters were observed for column Atlantis PREMIER BEH C18 AX. To investigate the effect of pH and buffer concentration on retention, mobile phases composed of acetonitrile and buffer (ammonium formate, pH 3.0; ammonium acetate pH 4.7 and pH 6.9) in various concentrations (5mM; 10mM; 15mM and 20mM) were used. The analysis of permanently charged compounds was used to describe the electrostatic interaction abilities of the stationary phases. The most significant contribution of electrostatic interactions to the retention was observed for Atlantis PREMIER BEH C18 AX column in the mobile phase with buffer of pH 3.0. A set of ten dipeptides, three pentapeptides and one octapeptide was used to investigate the effects of pH and buffer concentration on retention and peak symmetry. Each of the tested columns provides the optimal peak shape under different buffer pH and concentration. The gradient separation of the 14 tested peptides was used to verify the application potential of the tested columns for peptide separation. The best separation was achieved within 4 minutes on column Atlantis PREMIER BEH C18 AX.

Synthesis and evaluation of a diethylammonio-propylsulfate amphoteric ionic column for the high-performance liquid chromatography-mass spectrometric separation and detection of amino acids

A diethylammonio-propylsulfate amphoteric ionic resin was synthesized and employed as the stationary phase for high-performance liquid chromatography-mass spectrometry (LC-MS) separation and detection of amino acids. The influence of experimental conditions such as mobile phase composition, column length and temperature upon the amino acid separations was evaluated. However, temperature, addition of water-miscible organic solvent to the mobile phase and mobile phase gradients were not effective at improving the separations. In contrast, the use of an unbuffered pure water mobile phase proved successful for the separation and detection of amino acids. The observed order of elution seems to parallel the isoelectric points of the respective amino acids and suggests that this diethylammonio-propylsulfate stationary phase column functions as an amphoteric ion-exchanger. Under optimized chromatographic conditions, the detection limits for the amino acids were in range of 0.07-0.44 pmol (pico-mole). The method was also successfully applied for the analysis of an actual commercial sample.

Metabolic profiling by reversed-phase/ion-exchange mass spectrometry

Metabolic profiling is commonly achieved by mass spectrometry (MS) following reversed-phase (RP) and hydrophilic interaction chromatography (HILIC) either performed independently, leading to overlapping datasets, or in a coupled configuration, requiring multiple liquid chromatography (LC) systems. To overcome these limitations, we developed a single, 20-minute chromatographic method using an in-line RP-ion-exchange (IEX) column arrangement and a single LC system. This configuration separates clinically significant polar and non-polar compounds without derivatization or ion-pairing reagents, allowing ionization in both polarities. An in-house library was created with 397 authentic standards, including acylcarnitines, amino acids, bile acids, nucleosides, organic acids, steroid hormones, and vitamins. Analysis of pooled plasma and urine samples revealed 5445 and 4111 ion features, leading to 88 and 82 confirmed metabolite identifications, respectively. Metabolites were detected at clinically relevant concentrations with good precision, and good chromatographic separation was demonstrated for clinically significant isomers including methylmalonic acid and succinic acid, as well as alloisoleucine and isoleucine/leucine. Evaluation of the samples by unsupervised principal component analysis showed excellent analytical quality.

Analytical strategies for the determination of amino acids: Past, present and future trends

This review describes the analytical methods that have been developed over the years to tackle the high polarity and non-chromophoric nature of amino acids (AAs). First, the historical methods are briefly presented, with a strong focus on the use of derivatization reagents to make AAs detectable with spectroscopic techniques (ultraviolet and fluorescence) and/or sufficiently retained in reversed phase liquid chromatography. Then, an overview of the current analytical strategies for achiral separation of AAs is provided, in which mass spectrometry (MS) becomes the most widely used detection mode in combination with innovative liquid chromatography or capillary electrophoresis conditions to detect AAs at very low concentration in complex matrixes. Finally, some future trends of AA analysis are provided in the last section of the review, including the use of supercritical fluid chromatography (SFC), multidimensional liquid chromatography and electrophoretic separations, hyphenation of ion exchange chromatography to mass spectrometry, and use of ion mobility spectrometry mass spectrometry (IM-MS). Various application examples will also be presented throughout the review to highlight the benefits and limitations of these different analytical approaches for AAs determination.

Rapid quantification of underivatized alloisoleucine and argininosuccinate using mixed-mode chromatography with tandem mass spectrometry

Plasma elevations of the amino acids alloisoleucine and argininosuccinic acid (ASA) are pathognomonic for maple syrup urine disease and argininosuccinate lyase deficiency, respectively. Reliable detection of these biomarkers is typically achieved using methods with tedious sample preparations or long chromatographic separations, and many published amino acid assays report poor specificity and/or sensitivity for one or both of these compounds. This report describes a novel liquid chromatography tandem mass spectrometry (LC-MS/MS) method that provides rapid quantification of alloisoleucine and ASA in human plasma. The basis of this method is a mixed-mode solid phase separation that achieves baseline resolution of alloisoleucine from isobaric interferents without the use of derivatization or ion pairing agents. The inject-to-inject time is 6 min including elution, column washing and re-equilibration. Validation studies demonstrate excellent limits of quantification (1 μmol/L), linearity (r = 0.999 from 1 to 250 μmol/L), accuracy (bias = -3.8% and -10.1%), and inter-assay imprecision (CV < 8.06%) for plasma analyses. Data from long-term clinical application confirms chromatographic consistency equivalent to more traditional reversed-phase or HILIC based columns. Additional matrix studies indicate low suppression (<10%) for a wide range of amino acids and compatibility with other matrixes such as blood spot analyses. Finally, analysis of our first 257 clinical specimens demonstrates high analytic specificity and sensitivity, allowing the detection of subtle but clinically relevant elevations of alloisoleucine and ASA that may be missed by other less sensitive methods. In conclusion, the novel LC-MS/MS method reported here overcomes a number of the challenges associated with alloisoleucine and ASA quantification. Combining this approach with published incomplete amino acid quantification methods allows, for the first time, a rapid and comprehensive LC-MS/MS analysis of underivatized amino acids without the use of ion pairing agents.

Determination of immunogenic proteins in biopharmaceuticals by UHPLC-MS amino acid analysis

Nowadays, there is a growing interest in innovative and more efficient therapeutics—biopharmaceuticals, based on peptides or proteins. There are increased demands on quality control of such therapeutics. One of the methods usually used for characterization and quantification of biopharmaceuticals is amino acid analysis. In this work, a modern advanced analytical method based on precolumn derivatization and reversed-phase ultra high-performance liquid chromatography in combination with single quadrupole mass spectrometer was developed for amino acid analysis in different protein samples—model sample of bovine serum albumin, sample of strong immunogenic protein keyhole limpet hemocyanin, and sample of drug etanercept present in commercially available biopharmaceutical Enbrel. The method used isotopically labeled internal standards and was validated according to the International Council for Harmonisation guideline. The developed method was characterized by favorable performance and validation parameters, such as time of analysis (6 min), specificity, linearity (r² ≥ 0.99), limit of detection (0.009–0.822 µM), limit of quantification (1–2.5 µM), accuracy (recovery in the range 90–102.8%), intra-day (RSD in the range 0.25–11.97%) and inter-day precision (RSD in the range 1.67–11.57%), or stability (RE ≤ 12%). According to these findings, the developed amino acid analysis approach is suitable for routine use in areas of peptide/protein quantification, such as quality control laboratories of biopharmaceutical companies.

A matching algorithm with isotope distribution pattern in LC-MS based on support vector machine (SVM) learning model

In proteomics, it is important to detect, analyze, and quantify complex peptide components and differences.