Quantitative high-throughput analysis of drugs in biological matrices by mass spectrometry

Metabolic stability and determination of cytochrome P450 isoenzymes' contribution to the metabolism of medetomidine in dog liver microsomes

Medetomidine is a potent and selective alpha2-adrenergic agonist. The activation of alpha2-adrenergic receptor mediates a variety of effects including sedation, analgesia, relief of anxiety, vasoconstriction and bradycardia. However, our main interest is the sedative effects of medetomidine when used as a premedicant prior surgery in companion animals, especially in dogs. Recently, data suggested that following intravenous infusion at six dosing regiments non-linear pharmacokinetics was observed. Major causes of non-linear pharmacokinetics are the elimination of the drug not following a simple first-order kinetics and/or the elimination half-life changing due to saturation of an enzyme system. The goal of this study was to establish the metabolic stability and determine the metabolic pathway of medetomidine in dog liver microsomes. Consequently, Michaelis-Menten parameters (V(max), K(m)), T(1/2) and CL(i) were determined. The incubations were performed in a microcentrifuge tube and containing various concentrations of medetomidine (10-5000 nM), 1 mg/mL of microsomal proteins suspended in 0.1 M phosphate buffer, pH 7.4. Microsomal suspensions were preincubated with NADPH (1 mM) for 5 min at 37 degrees C prior to fortification with medetomidine. Samples were taken at various time points for kinetic information and the initial velocity (v(i)) was determined after 10 min incubation. The reaction was stopped by the addition of an internal standard solution (100 ng/mL of dextrometorphan in acetone). Medetomidine concentrations were determined using a selective and sensitive HPLC-ESI/MS/MS method. Using non-linear regression, we determined a K(m) value of 577 nM, indicating relatively low threshold enzyme saturation consistent with previous in vivo observation. The metabolic stability was determined at a concentration of 100 nm (<<K(m)) and the observed T(1/2) was 90 min with a CL(i) of 0.008 mL/min indicating moderately low clearance in dog liver microsomes, also consistent with previous in vivo data. Moreover, results suggest that principally medetomidine is metabolized by the CYP3A with a small contribution from CYP2D and CYP2E. The participation of CYP3A is an important discovery since medetomidine is used as a premedicant in combination with fentanyl, ketamine and/or midazolam. These findings combined with a low K(m) value may indicate that medetomidine can competitively inhibit the metabolism of these drugs and consequently significantly impair metabolic clearance.

Liquid chromatography/tandem mass spectrometry sensitivity enhancement via online sample dilution and trapping: applications in microdosing and dried blood spot (DBS) bioanalysis

A simple online sample dilution, enrichment, and cleanup technique was developed for sensitive microdosing and dried blood spot (DBS) liquid chromatography/tandem mass spectrometric (LC/MS/MS) bioanalysis. Samples are diluted online with water and enriched in a trap column which is subsequently switched inline with the analytical column. Excellent lansoprazole (in acetonitrile) peak shape is maintained even with an 80-microL injection. In comparison, similar chromatographic peaks were observed only when a small volume of the same solution, i.e., 1 microL, was injected on a regular high-performance liquid chromatography (HPLC) system, where an injection of 5 microL resulted in severe peak fronting. A substantial enhancement in sensitivity is realized in the trapping mode using large injection volumes. The trap column is washed at the beginning and at the end of each injection with aqueous and organic solvent respectively to remove matrix components. This ultimately leads to reduction of matrix effects and mass spectrometer noise, thus facilitating the utilization of protein precipitation as the sample preparation for plasma samples. A lower limit of quantitation (LLOQ) of 0.5 pg/mL was demonstrated for lansoprazole in human plasma with a signal-to-noise (S/N) ratio of 13 using a 100 microL injection. Excellent intra-day precision and accuracy were established for lansoprazole in human plasma with good linearity (R(2) > 0.999) from 0.5 to 500 pg/mL. This level of LLOQ makes LC/MS/MS a practical alternative for microdosing bioanalysis, where the dose is typically 100 times lower than the therapeutic dose. The same technique was applied to quantitate lansoprazole in human whole blood employing DBS technology. With a single 3-mm punch, i.e. approximately 2 microL of whole blood or approximately 1 microL plasma, a LLOQ of 0.1 ng/mL showed sufficient S/N ratio (40) for lansoprazole when 75 microL of extract was injected. In all, the online sample dilution, cleanup, and enrichment technique demonstrated the practical utility of LC/MS/MS in microdosing and DBS bioanalysis.

Hydride transfer reactions via ion-neutral complex: fragmentation of protonated N-benzylpiperidines and protonated N-benzylpiperazines in mass spectrometry

An ion-neutral complex (INC)-mediated hydride transfer reaction was observed in the fragmentation of protonated N-benzylpiperidines and protonated N-benzylpiperazines in electrospray ionization mass spectrometry. Upon protonation at the nitrogen atom, these compounds initially dissociated to an INC consisting of [RC(6)H(4)CH(2)](+) (R = substituent) and piperidine or piperazine. Although this INC was unstable, it did exist and was supported by both experiments and density functional theory (DFT) calculations. In the subsequent fragmentation, hydride transfer from the neutral partner to the cation species competed with the direct separation. The distribution of the two corresponding product ions was found to depend on the stabilization energy of this INC, and it was also approved by the study of substituent effects. For monosubstituted N-benzylpiperidines, strong electron-donating substituents favored the formation of [RC(6)H(4)CH(2)](+), whereas strong electron-withdrawing substituents favored the competing hydride transfer reaction leading to a loss of toluene. The logarithmic values of the abundance ratios of the two ions were well correlated with the nature of the substituents, or rather, the stabilization energy of this INC.

Identification, characterization and quantification of specific neuropeptides in rat spinal cord by liquid chromatography electrospray quadrupole ion trap mass spectrometry

Substance P and CGRP play a central role in neuropathic pain development and maintenance. Additionally, dynorphin A is an endogenous ligand of opioid receptors implicated in the modulation of neurotransmitters including neuropeptides, such as substance P and CGRP. This manuscript proposes a method to characterize, identify and quantify substance P, CGRP and dynorphin A in rat spinal cord by HPLC-ESI/MS/MS. Rat spinal cords were collected and homogenized into a TFA solution. Samples were chromatographed using a microbore C(8) 100 x 1 mm column and a 19 min linear gradient (0:100 --> 40:60; ACN:0.2% formic acid in water) at a flow rate of 75 microL/min for a total run time of 32 min. The peptides were identified in rat spinal cord based on full-scan MS/MS spectra. Substance P, CGRP and dynorphin A were predominantly identified by the presence of specific b CID fragments. Extracted ion chromatogram (XIC) suggested selected mass transitions of 674 --> [600 + 254], 952 --> [1215 + 963] and 717 --> [944 + 630] for substance P, CGRP and dynorphin A can be used for isolation and quantitative analysis. A linear regression (weighted 1/x) was used and coefficients of correlations (r) ranging from 0.990 to 0.999 were observed. The precision (%CV) and accuracy (%NOM) observed were 10.9-14.4% and 8.9-14.2%, 8.8-13.0% and 91.0-110.2% and 97.2-107.3% and 91.8-97.3% for substance P, CGRP and dynorphin A respectively. Following the analysis of rat spinal cords, the mean endogenous concentrations were 110.7, 2541 and 779.4 pmol/g for substance P, CGRP and dynorphin A respectively. The results obtained show that the method provides adequate figures of merit to support targeted peptidomic studies aimed to determine neuropeptide regulation in animal neuropathic and chronic pain models.

Accelerated tryptic digestion for the analysis of biopharmaceutical monoclonal antibodies in plasma by liquid chromatography with tandem mass spectrometric detection

Accelerated tryptic digestion of a therapeutic protein including microwave irradiation and thermal transfer by convection at 60 degrees C and 37 degrees C was investigated. An analytical setup was devised to follow the protein digestion rate using 1D gel electrophoresis and liquid chromatography coupled a triple quadrupole linear ion trap mass spectrometer. The formation kinetic of its tryptic peptides was monitored in the selected monitoring mode (LC-SRM/MS). Different digestion end points (e.g. 2, 5, 10, 15, 30 and 60min) as well as an overnight digestion were tested using a therapeutic human monoclonal antibody (mAb) with the goal of its LC-SRM/MS quantification in human plasma. The peptides from the human mAb were generated at different rates and were classified into three categories: (1) the fast forming peptides, (2) the slow forming peptides and (3) the peptides degrading over time. For many monitored peptides, a heating temperature of 37 degrees C with a 750rpm mixing applied for at least 30min provided equivalent results to microwave-assisted digestion and generally allowed the achievement of an equivalent peptide concentration as an overnight digestion carried out at 37 degrees C. The disappearance of the protein of the heavy and light chains can be monitored by 1D gel electrophoresis but was found not to be representative of the final tryptic peptide concentrations. For quantitative purposes a stable isotope labeled version ((13)C(4), (15)N(1)) of the therapeutic protein was used. The labeled protein as internal standard was found to be very efficient to compensate for incomplete digestion or losses during sample preparation.

Rapid simultaneous quantitative determination of different small pharmaceutical drugs using a conventional matrix-assisted laser desorption/ionization time-of-flight mass spectrometry system

The present study establishes a simple, rapid and sensitive method for the simultaneous quantification of different small pharmaceutical drugs using a matrix-assisted laser desorption/ionization source (MALDI) coupled with a time-of-flight (TOF) mass analyzer. Neither time-consuming sample preparation, nor special target plates, isotopically labelled internal standards or other extra equipment are necessary. A simple standard dried-droplet preparation with the common matrix alpha-cyano-4-hydroxycinnamic acid (CHCA) was used. The background signals of CHCA in the low-mass region did not pose the presumed problem, because the sensitivity, resolution and mass accuracy of a modern MALDI-TOF MS system is sufficient to overcome this difficulty. Four experiments were performed in order to verify the quantification method. First, ten different phenothiazines were quantified in the range of 5-2000 nM (1-880 ng/mL). A good precision (relative standard deviation (RSD) 4.4-9.3%), linearity (R2 >0.99) and accuracy (error 4.7-11%) was obtained in all cases. Additionally, simultaneous quantification of these ten phenothiazines was carried out in human plasma without prior chromatographic separation in the range of 2-1750 ng/mL yielding good linearity, precision and accuracy (mean RSD 7.6%; R2 >0.99, mean error 8.0%). Accordingly, a quantitative analysis of ten chemically and pharmaceutically unrelated drugs was performed in the same way. A comparable linearity (R2 >0.99), precision (mean RSD 7.6%) and accuracy (mean error 8.3%) was obtained in the range of 5-2000 nM. Finally, the prazosin content of a commercial tablet was directly determined without further purification steps.

Atmospheric pressure ion sources

This review of atmospheric pressure ion sources discusses major developments that have occurred since 1991. Advances in the instrumentation and understanding of the key physical principles are the primary focus. Developments with electrospray and atmospheric pressure chemical ionization and variations encompassing adaptations for surface analysis, ambient air analysis, high throughput, and modification of the ionization mechanism are covered. An important and limiting consequence of atmospheric pressure chemical ionization, chemical noise, is discussed as is techniques being employed to ameliorate the problem. Ion transfer and transport from atmospheric pressure into deep vacuum is an area undergoing constant improvement and refinement so is given considerable consideration in this review.

Overcoming matrix effects in liquid chromatography-mass spectrometry

A major limitation in quantitative analysis with electrospray ionization mass spectrometry (ESI-MS) is represented by the so-called matrix effects in which the matrix coextracted with the analytes can alter the signal response, causing either suppression or enhancement, resulting in poor analytical accuracy, linearity, and reproducibility. In the direct electron ionization liquid chromatography-mass spectrometry (direct-EI LC-MS) interface the ionization process is based on electron impact ionization, and it occurs in the gas phase and is not influenced by coeluted matrix compounds. In this work we quantitatively evaluated matrix effects on enriched environmental and biological samples, with different extraction procedures, using ESI and direct-EI LC-MS. As expected, the samples analyzed with direct-EI were not affected by matrix composition, whereas with ESI we observed either signal suppression or enhancement, depending on the sample nature.

Comparison of fused-core and conventional particle size columns by LC-MS/MS and UV: application to pharmacokinetic study

The chromatographic performance of fused-core (superficially porous) HPLC packing materials was compared with conventional fully porous particle materials for LC-MS/MS analysis of two pharmaceuticals in rat plasma. Two commercially available antidepressants, imipramine and desipramine, were assayed using a conventional analytical C(18) column (5 microm, 2.0 mm x 30 mm) and a fused-core C(18) column (2.7 microm, 2.1 mm x 30 mm). Retention time, column efficiency, pressure drop, resolution, and loading capacity were compared under the same operating conditions. The fused-core column demonstrated reduced assay time by 34% and 2-3-fold increased efficiency (N). Loading capacity up to 25 microl of extract injected on column showed no peak distortion. The registered back-pressure from a flow rate of 1.0 ml/min did not exceed 3400 psi making it compatible with standard HPLC equipment (typically rated to 6000 psi). Two mobile phases were examined, and morpholine as an organic base modifier yielded a 2-5-fold increase in S/N near the limit of detection over triethylamine. The 2.7 microm fused-core column was applied to the analysis of imipramine and desipramine in extracted, protein precipitated rat plasma by LC-MS/MS. The calibration curves were linear in the concentration range of 0.5-1000 ng/ml for both imipramine and desipramine. Intra-run precisions (%CV) and accuracies (%bias) were within +/-7.8% and +/-7.3% at three QC levels and within 14.7% and 14.4% at the LOQ level for both analytes. Following a single method qualification run, the method was applied to the quantitation of pharmacokinetic study samples after oral administration of imipramine to male rats.

High-throughput quantitative analysis by desorption electrospray ionization mass spectrometry

A newly developed high-throughput desorption electrospray ionization (DESI) source was characterized in terms of its performance in quantitative analysis. A 96-sample array, containing pharmaceuticals in various matrices, was analyzed in a single run with a total analysis time of 3 min. These solution-phase samples were examined from a hydrophobic PTFE ink printed on glass. The quantitative accuracy, precision, and limit of detection (LOD) were characterized. Chemical background-free samples of propranolol (PRN) with PRN-d(7) as internal standard (IS) and carbamazepine (CBZ) with CBZ-d(10) as IS were examined. So were two other sample sets consisting of PRN/PRN-d(7) at varying concentration in a biological milieu of 10% urine or porcine brain total lipid extract, total lipid concentration 250 ng/microL. The background-free samples, examined in a total analysis time of 1.5 s/sample, showed good quantitative accuracy and precision, with a relative error (RE) and relative standard deviation (RSD) generally less than 3% and 5%, respectively. The samples in urine and the lipid extract required a longer analysis time (2.5 s/sample) and showed RSD values of around 10% for the samples in urine and 4% for the lipid extract samples and RE values of less than 3% for both sets. The LOD for PRN and CBZ when analyzed without chemical background was 10 and 30 fmol, respectively. The LOD of PRN increased to 400 fmol analyzed in 10% urine, and 200 fmol when analyzed in the brain lipid extract.

Reducing glycerophosphocholine lipid matrix interference effects in biological fluid assays by using high-turbulence liquid chromatography

Matrix interferences can severely affect quantitative assays of biological samples when electrospray ionization (ESI) is employed with liquid chromatography/tandem mass spectrometry (LC/MS/MS). A major source of matrix interferences for plasma sample analyses is the presence of glycerophosphocholine (GPCho) lipids. The efficiency of online high-turbulence liquid chromatography (HTLC) extraction for eliminating these lipids is evaluated and the interfering effects of endogenous lipids on human plasma assays are measured for pharmaceutical compounds having a wide variety of chemical properties. It is found that GPCho lipids, represented by 16:0, 18:1 and 18:0 LPC (lysophosphatidylcholine) and 16:0-18:2 PC, cause variations for hydrophobic compound analyses even when optimal online HTLC extraction conditions are employed. The efficiency for lipid removal depends on the organic content of the transfer solvent, but turbulent flow loading has no significant effect.

Enzyme inhibitor screening by electrospray mass spectrometry with immobilized enzyme on magnetic silica microspheres

In this study, a novel technique for screening inhibitors by electrospray mass spectrometry (ESI-MS) with immobilized enzyme on magnetic microspheres has been demonstrated. First, the model enzyme acetylcholinesterase (AChE) is immobilized onto the 3-glycidoxypropyltrimethoxysilane (GLYMO)-modified magnetic silica microspheres. AChE activity was monitored by biochemical assay that is based on mixing of AChE immobilized microspheres and model substrate acetylcholine, separating and detecting the product through ESI-MS. Stability of the enzyme-immobilized microspheres was investigated. No apparent loss of enzyme activity was observed after fivefold reuse of AChE-immobilized microspheres. The enzyme-immobilized bioassay was used to effectively identify AChE inhibitors among two standard samples, huperzine A and huperzine B, and their source herbal Huperzia serrata, all of which were spiked into the substrate. The inhibition was determined by measuring a decrease of product formation using ESI-MS.

Metabolism and bioavailability of flavonoids in chemoprevention: current analytical strategies and future prospectus

Flavonoids are structurally diverse and among the most ubiquitous groups of dietary polyphenols distributed in various fruits and vegetables. Many have been proposed to be bioactive compounds in the diet that are responsible for lowering the risk of cancer and have been used in chemoprevention studies using animal models of this disease. As for any xenobiotic, to evaluate the potential risks and benefits of bioflavonoids to human health, an understanding of the physiological behavior of these compounds following oral ingestion is needed as well as their absorption, distribution, metabolism, and excretion (ADME). The study on metabolism and bioavailability is very important in defining the pharmacological and toxicological profile of these compounds. Due to great structural diversity among flavonoids, these profiles differ greatly from one compound to another, so that the most abundant polyphenols in our diet are not necessarily the ones that reach target tissues. Therefore, careful analysis of flavonoids and their metabolites in biological systems is critical. Mass spectrometry in various combinations with chromatographic methods has been a mainstay in applications that involve profiling and quantification of metabolites in complex biological samples. Because of its speed, sensitivity and specificity, liquid chromatography-tandem mass spectrometry (LC-MS/MS) has become the technology of choice for sample analysis. This review describes the chemistry of polyphenols and flavonoids, their ADME, and the various mass spectrometry-based strategies used in the analysis of flavonoids, including future trends in this field.

The application of cassette dosing for pharmacokinetic screening in small-molecule cancer drug discovery

Pharmacokinetic evaluation is an essential component of drug discovery and should be conducted early in the process so that those compounds with the best chance of success are prioritized and progressed. However, pharmacokinetic analysis has become a serious bottleneck during the 'hit-to-lead' and lead optimization phases due to the availability of new targets and the large numbers of compounds resulting from advances in synthesis and screening technologies. Cassette dosing, which involves the simultaneous administration of several compounds to a single animal followed by rapid sample analysis by liquid chromatography/tandem mass spectrometry, was developed to increase the throughput of in vivo pharmacokinetic screening. Although cassette dosing is advantageous in terms of resources and throughput, there are possible complications associated with this approach, such as the potential for compound interactions. Following an overview of the cassette dosing literature, this article focuses on the application of the technique in anticancer drug discovery. Specific examples are discussed, including the evaluation of cassette dosing to assess pharmacokinetic properties in the development of cyclin-dependent kinase and heat shock protein 90 inhibitors. Subject to critical analysis and validation in each case, the use of cassette dosing is recommended in appropriate chemical series to enhance the efficiency of drug discovery and reduce animal usage.

Application of monolithic supports to online extraction and LC-MS analysis of benzodiazepines in whole blood samples

A column-switching (CS) LC-MS method allowing high-speed determination of benzodiazepines (BZDs) in whole blood is presented. After protein precipitation with ACN followed by evaporation and reconstitution with the loading mobile phase, the online sample clean-up was carried out using a CS device. Two extractive precolumns were evaluated: a conventional restricted access material (RAM) sorbent and a monolithic silica support. Separation was achieved using a Chromolith Performance RP-18e (100 mmx4.6 mm id) monolithic silica column, and detection was performed by atmospheric pressure chemical ionization (APCI) MS. The method with both supports has been fully validated according to an accuracy profile approach. Finally, the monolithic silica column, demonstrating better validation data and a higher robustness than the RAM sorbent, was used for the analysis of several real forensic cases.

Liquid chromatography combined with mass spectrometry for 13C isotopic analysis in life science research

Among the different disciplines covered by mass spectrometry, measurement of (13)C/(12)C isotopic ratio crosses a large section of disciplines from a tool revealing the origin of compounds to more recent approaches such as metabolomics and proteomics. Isotope ratio mass spectrometry (IRMS) and molecular mass spectrometry (MS) are the two most mature techniques for (13)C isotopic analysis of compounds, respectively, for high and low-isotopic precision. For the sample introduction, the coupling of gas chromatography (GC) to either IRMS or MS is state of the art technique for targeted isotopic analysis of volatile analytes. However, liquid chromatography (LC) also needs to be considered as a tool for the sample introduction into IRMS or MS for (13)C isotopic analyses of non-volatile analytes at natural abundance as well as for (13)C-labeled compounds. This review presents the past and the current processes used to perform (13)C isotopic analysis in combination with LC. It gives particular attention to the combination of LC with IRMS which started in the 1990's with the moving wire transport, then subsequently moved to the chemical reaction interface (CRI) and was made commercially available in 2004 with the wet chemical oxidation interface (LC-IRMS). The LC-IRMS method development is also discussed in this review, including the possible approaches for increasing selectivity and efficiency, for example, using a 100% aqueous mobile phase for the LC separation. In addition, applications for measuring (13)C isotopic enrichments using atmospheric pressure LC-MS instruments with a quadrupole, a time-of-flight, and an ion trap analyzer are also discussed as well as a LC-ICPMS using a prototype instrument with two quadrupoles.

Two-dimensional reverse phase-reverse phase chromatography: A simple and robust platform for sensitive quantitative analysis of peptides by LC/MS. Hardware design

We have revised current two-dimensional RP-RP approaches and developed a new robust 2-D RP-RP platform. This platform was implemented on an Agilent 1100 2-D liquid chromatography system and is based on high pressure switching between two high-resolution RP columns. An independent binary gradient was implemented for each dimension. The powerful combination of dual analytical columns with independent gradient elution achieves high analyte purity, effectively eliminates matrix effects, and maximizes MS sensitivity in Q1 SIM comparable to the sensitivity enhancements of MS/MS-based methods. Implementation of dual simultaneous gradient profiles (overlapped gradients) reduces 2-D method run-time to the scale of 1-D method run-times. This robust and sensitive approach is particularly suitable for hydrophobic peptides and small proteins and can be used as a routine standard technique for enhanced on-line peptide purification coupled with mass spectrometric detection.

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

A qualitative and quantitative analysis of erlotinib (RO0508231) and its metabolites was carried out on rat tissue sections from liver, spleen and muscle. Following oral administration at a dose of 5 mg/kg, samples were analyzed by matrix-assisted laser desorption ionization (MALDI) with mass spectrometry (MS) using an orthogonal quadrupole time-of-flight instrument. The parent compound was detected in all tissues analyzed. The metabolites following drug O-dealkylation could also be detected in liver sections. Sinapinic acid (SA) matrix combined with the dried-droplet method resulted in better conditions for our analysis on tissues. Drug quantitation was investigated by the standard addition method and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis on the tissue extracts. The presence of the parent compound and of its O-demethylated metabolites was confirmed in all tissue types and their absolute amounts calculated. In liver the intact drug was found to be 3.76 ng/mg tissue, while in spleen and muscle 6- and 30-fold lower values, respectively, were estimated. These results were compared with drug quantitation obtained by whole-body autoradiography, which was found to be similar. The potential for direct quantitation on tissue sections in the presence of an internal standard was also investigated using MALDI-MS. The use of alpha-cyano-4-hydroxycinnamic acid (CHCA) as the matrix resulted in better linearity for the calibration curves obtained with reference solutions of the drug when compared to SA, but on tissue samples no reliable quantitative analysis was possible owing to the large variability in the signal response. MS imaging experiments using MALDI in MS/MS mode allowed visualizing the distribution of the parent compound in liver and spleen tissues. By calculating the ratio between the total ion intensities of MS images for liver and spleen sections, a value of 6 : 1 was found, which is in good agreement with the quantitative data obtained by LC-MS/MS analysis.

Mass spectrometry for enzyme assays and inhibitor screening: an emerging application in pharmaceutical research

Robust methods that monitor enzyme activity and inhibitor potency are crucial to drug discovery and development. Over the past 20 years, mass spectrometric methods have increasingly been used to measure enzyme activity and kinetics. However, for rapid screening of inhibitory compounds, various forms of fluorescence and chemiluminscence readout have continued to dominate the market. As the sensitivity, speed, and miniaturization of mass spectrometry methods continue to advance, opportunities to couple mass spectrometry with screening will continue to come to the forefront. To appreciate the tremendous potential for MS-based screening assays, it becomes necessary to understand the current state of capabilities in this arena. Thus, this review is intended to capture how mass spectrometry for studying enzymes activity has progressed from simple qualitative questions (i.e., is the product detected?) to quantitative measures of enzyme activity and kinetics and then as a tool for rapidly screening inhibitory compounds as an alternative to current methods of high throughput drug screening.

A strategy for high-throughput analysis of levosimendan and its metabolites in human plasma samples using sequential negative and positive ionization liquid chromatography/tandem mass spectrometric detection

Levosimendan (Simdax) is an approved drug in approximately 40 countries and currently in phase III clinical studies in the USA and Europe. An accurate, high-throughput and rugged assay is critical to support these clinical trials. Due to the mechanism of drug metabolism, the drug and its active metabolites often have significant differences in their chemical properties. In order to achieve high assay throughput and low sample volumes, a single bioanalytical assay for the drug and its metabolites is preferred. However, this need may prevent the optimization of both high-performance liquid chromatography (HPLC) and mass spectrometric ionization conditions. The chemical properties of levosimendan are significantly different from those of its two active metabolites, OR-1855 and OR-1896. Here, we present a novel strategy for high-throughput analysis of levosimendan and its metabolites. A 96-well liquid/liquid extraction procedure was developed for sample preparation. A single liquid chromatography/tandem mass spectrometry (LC/MS/MS) system with two separate mobile phases, shared backwash solvent and conditioning solvent, was developed to perform sequential LC separation for levosimendan and the metabolites. Levosimendan was eluted by 5 mM ammonium acetate in 33.3% acetonitrile and detected using negative ionization mode MS/MS monitoring. The metabolites were eluted by 5 mM ammonium acetate and 0.2% acetic acid in 20% acetonitrile and detected with positive ionization mode MS/MS monitoring. The method has been demonstrated to have excellent precision and accuracy, with high assay ruggedness during method validation and clinical sample analysis. The linear dynamic ranges were approximately 200-50,000 pg/mL for levosimendan and approximately 500-130,000 pg/mL for both metabolites. The coefficient of determination (r2) for all analytes was greater than 0.9985. The intra-assay %CVs for QC samples were from 0.9% to 2.0% for levosimendan, 0.9% to 3.2% for OR-1855, and 0.4% to 4.9% for OR-1896. The inter-assay %CVs for QC samples were from 1.2% to 1.8% for levosimendan, 1.3% to 2.7% for OR-1855, and 1.4% to 3.4% for OR-1896. The mean % biases for QC samples were from 1.5% to 5.5% for levosimendan, -1.4% to 2.6% for OR-1855, and -0.3% to 4.5% for OR-1896. By using a single extraction approach coupled with sequential LC/MS/MS analysis for levosimendan and its metabolites, the assay maintained high throughput and low sample volume usage.

Method development aspects for the quantitation of pharmaceutical compounds in human plasma with a matrix-assisted laser desorption/ionization source in the multiple reaction monitoring mode

The present work investigates various method development aspects for the quantitative analysis of pharmaceutical compounds in human plasma using matrix-assisted laser desorption/ionization and multiple reaction monitoring (MALDI-MRM). Talinolol was selected as a model analyte. Liquid-liquid extraction (LLE) and protein precipitation were evaluated regarding sensitivity and throughput for the MALDI-MRM technique and its applicability without and with chromatographic separation. Compared to classical electrospray liquid chromatography/mass spectrometry (LC/ESI-MS) method development, with MALDI-MRM the tuning of the analyte in single MS mode is more challenging due to interfering matrix background ions. An approach is proposed using background subtraction. With LLE and using a 200 microL human plasma aliquot acceptable precision and accuracy could be obtained in the range of 1 to 1000 ng/mL without any LC separation. Approximately 3 s were required for one analysis. A full calibration curve and its quality control samples (20 samples) can be analyzed within 1 min. Combining LC with the MALDI analysis allowed improving the linearity down to 50 pg/mL, while reducing the throughput potential only by two-fold. Matrix effects are still a significant issue with MALDI but can be monitored in a similar way to that used for LC/ESI-MS analysis.

Two-dimensional liquid chromatography/mass spectrometry set-up for structural elucidation of metabolites in complex biological matrices

For absorption, distribution, metabolism and excretion (ADME) studies of drug candidates, mass spectrometry (MS) has become an indispensable tool for the characterization of biotransformation pathways. Samples from in vivo animal studies such as plasma, tissue extracts or excreta contain vast amounts of endogenous compounds. Therefore, the generation of metabolite patterns requires dedicated sample pre-treatment and sophisticated separation methods. Methodologies used for metabolite separation are often inappropriate for structure elucidation. Therefore, a two-dimensional liquid chromatography (LC) approach in combination with MS was developed. Study samples were analyzed using high-performance liquid chromatography (HPLC) for the generation of a qualitative and quantitative metabolite pattern (first dimension) with high reproducibility and recovery without extensive sample pre-treatment. Selected radioactive metabolite fractions were then applied to micro-HPLC with off-line radioactivity monitoring and subsequent MS detection (second dimension). Applying the two-dimensional HPLC/MS approach not only major metabolites could be identified, even minor and trace metabolites were characterized. The usage of sampled metabolite fractions allowed also the re-analysis of specific metabolites for additional investigations (e.g. H/D exchange experiments or product ion scanning experiments). It could be clearly shown that the two-dimensional HPLC/MS approach showed mass spectra with higher sensitivity and selectivity significantly improving the characterization of minor and trace metabolites in in vivo ADME studies.

A biosynthetic pathway for anandamide

The endocannabinoid arachidonoyl ethanolamine (anandamide) is a lipid transmitter synthesized and released "on demand" by neurons in the brain. Anandamide is also generated by macrophages where its endotoxin (LPS)-induced synthesis has been implicated in the hypotension of septic shock and advanced liver cirrhosis. Anandamide can be generated from its membrane precursor, N-arachidonoyl phosphatidylethanolamine (NAPE) through cleavage by a phospholipase D (NAPE-PLD). Here we document a biosynthetic pathway for anandamide in mouse brain and RAW264.7 macrophages that involves the phospholipase C (PLC)-catalyzed cleavage of NAPE to generate a lipid, phosphoanandamide, which is subsequently dephosphorylated by phosphatases, including PTPN22, previously described as a protein tyrosine phosphatase. Bacterial endotoxin (LPS)-induced synthesis of anandamide in macrophages is mediated exclusively by the PLC/phosphatase pathway, which is up-regulated by LPS, whereas NAPE-PLD is down-regulated by LPS and functions as a salvage pathway of anandamide synthesis when the PLC/phosphatase pathway is compromised. Both PTPN22 and endocannabinoids have been implicated in autoimmune diseases, suggesting that the PLC/phosphatase pathway of anandamide synthesis may be a pharmacotherapeutic target.

Asymmetric multicomponent reactions (AMCRs): the new frontier

Asymmetric multicomponent reactions involve the preparation of chiral compounds by the reaction of three or more reagents added simultaneously. This kind of addition and reaction has some advantages over classic divergent reaction strategies, such as lower costs, time, and energy, as well as environmentally friendlier aspects. All these advantages, together with the high level of stereoselectivity attained in some of these reactions, will force chemists in industry as in academia to adopt this new strategy of synthesis, or at least to consider it as a viable option. The positive aspects as well as the drawbacks of this strategy are discussed in this Review.

Development of a Multiplexed Interface for Capillary Electrophoresis−Electrospray Ion Trap Mass Spectrometry

A four-channel multiplexed electrospray capillary electrophoresis interface has been developed. This new interface permits up to four capillary electrophoresis columns to be sampled sequentially by means of a stepper motor and a notched rotating plate assembly, which at any instant occludes all but a single sprayer. In this design, four sheath liquid electrospray probes are oriented in a circular array situated 90 degrees relative to one another. The rotating metal disk, which contains a one-quarter notch, is mounted to the stepper motor assembly and is located between the sprayers and the entrance aperture of an ion trap mass spectrometer. By using the data acquisition signal from the ion trap mass spectrometer, the scan event is synchronized with the rotation of the metal disk. With this device, four discrete sample streams can be simultaneously analyzed, resulting in a 4-fold increase in analytical throughput.

MALDI-TOF MS as a label-free approach to rapid inhibitor screening

Mass Spectrometry (MS) has been widely reported for measuring the conversion of substrates to products for enzyme assays. These measurements are typically performed by time-consuming LC-MS to eliminate buffer salts that interfere with electrospray ionization MS. However, matrix-assisted laser desorption ionization, time-of-flight MS (MALDI-TOF MS) offers a label-free and direct readout of substrate and product, a fast sampling rate, and is tolerant of many buffer salts, reagents, and compounds that are typically found in enzyme reaction mixtures. In this report, a demonstration of how MALDI-TOF MS can be used to directly measure ratios of substrates and products to produce IC(50) curves for rapid enzyme assays and compound screening is provided. Typical reproducibility parameters were <7% RSD-a value comparable to ESI MS quantitative assays and well within the acceptable limits for screening assays. The speed of the MALDI readout is currently about 10 s per sample, thus allowing for over 7500 samples/day. From a simplicity standpoint, the enzymatic reaction mixtures are prepared by liquid handling robots, the reactions are stopped by addition of a 10 times volume of acidic matrix solution, and the samples are simultaneously transferred to MALDI target plate for analysis. Importantly, the ratios of substrate to product are of sufficient reproducibility to eliminate the need for internal standards and, thus, minimize the cost and increasing the speed of assay development.

Principles and applications of LC-MS in new drug discovery

The use of high-performance liquid chromatography combined with mass spectrometry (HPLC-MS) or tandem mass spectrometry (HPLC-MS-MS) has proven to be the analytical technique of choice for most assays used in various stages of new drug discovery. A summary of the key components of HPLC-MS systems, as well as an overview of major application areas that use this technique as part of the drug discovery process, will be described here. This review will also provide an introduction into the various types of mass spectrometers that can be selected for the multiple tasks that can be performed using LC-MS as the analytical tool. The strategies for optimizing the use of this technique and also the potential problems and how to avoid them will be highlighted.

High-throughput bioanalysis with simultaneous acquisition of metabolic route data using ultra performance liquid chromatography coupled with time-of-flight mass spectrometry

The capability of ultra performance liquid chromatography coupled with time-of-flight mass spectrometry (UPLC/TOFMS) in the high-throughput quantitative analysis of a drug candidate in plasma has been investigated. Data obtained were compared with results from conventional analysis by high-performance liquid chromatography with tandem mass spectrometric detection on a triple quadrupole instrument (HPLC/MS/MS). The accuracies and precisions of the two approaches were comparable. The UPLC/TOFMS system displayed excellent robustness over the course of 276 injections of protein-precipitated plasma samples. With the instrumentation used, the limits of detection and quantification were approximately five-fold higher with UPLC/TOFMS than for HPLC/MS/MS. Nevertheless, the UPLC/TOFMS system proved adequate to quantify plasma concentrations of a drug molecule administered orally to rats at a pharmacologically relevant dose of 4 mg/kg. As well as providing quantitative data on the test compound, it was also possible to extract data for eight different metabolites, including several isomeric species (three +O and three +2O) from the UPLC/TOFMS data sets, using an analytical method with a 2.5-minute run time. Selectivity for the test compound and its metabolites was derived from the accurate mass capabilities of the TOF instrument, and no MS method development was required.

Development of an on-line automated sample clean-up method and liquid chromatography-tandem mass spectrometry analysis: application in an in vitro proteolytic assay

Fluorescence detection has been a method of choice in industry for screening assays, including identification of enzyme inhibitors, owing to its high-throughput capabilities, excellent reproducibility, and sensitivity. Occasionally, inhibitors are identified that challenge the fluorescence assay limit, necessitating the development of more sensitive detection methods to assess these compounds. For data mining purposes, however, original assay conditions may be required. A direct method transfer to highly sensitive and specific LC-MS-based methods has not always been possible due to the presence of MS-incompatible neutral detergents and non-volatile salts in the assay matrix. Utilizing an in vitro proteolytic screening assay for the serine protease hepatitis C virus (HCV) nonstructural (NS) 3 protease as a test case, we report the development of an automated sample clean-up procedure implemented on-line with liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis to complement fluorescence detection. Ion exchange and peptide microtraps were employed to remove MS-incompatible assay matrix components. Three protease inhibitors were used to validate the MS/MS method. Comparable potencies were achieved for these compounds when assessed by fluorescence and MS/MS detection. Furthermore, four-fold less enzyme could be utilized when employing the MS/MS method compared to fluorescence detection. The longer analysis time, however, resulted in reduced sample capacity. The potency of our designed HCV NS3 protease inhibitors are thus routinely evaluated using a continuous fluorescence-based assay. Only pertinent inhibitors approaching the fluorescence assay sensitivity limit are subsequently analyzed further by LC-MS/MS. This methodology allows us to maintain a database and to compare results independent of the detection method. Despite the relatively slow sample turnaround time of this LC-MS approach, the versatility of the automated on-line clean-up procedure and sample analysis can be applied to assays containing reagents which were historically considered to be MS incompatible.