Parallel high-throughput accurate mass measurement using a nine-channel multiplexed electrospray liquid chromatography ultraviolet time-of-flight mass spectrometry system

Screening for pharmaco-toxicologically relevant compounds in biosamples using high-resolution mass spectrometry: a 'metabolomic' approach to the discrimination between isomers

High-resolution mass spectrometry (HRMS) enables the identification of a chemical formula of small molecules through the accurate measurement of mass and isotopic pattern. However, the identification of an unknown compound starting from the chemical formula requires additional tools: (1) a database associating chemical formulas to compound names and (2) a way to discriminate between isomers. The aim of this present study is to evaluate the ability of a novel 'metabolomic' approach to reduce the list of candidates with identical chemical formula. Urine/blood/hair samples collected from real positive cases were submitted to a screening procedure using ESI-MS-TOF (positive-ion mode) combined with either capillary electrophoresis or reversed phase liquid chromatography (LC). Detected peaks were searched against a Pharmaco/Toxicologically Relevant Compounds database (ca 50,500 compounds and phase I and phase II metabolites) consisting of a subset of PubChem compounds and a list of candidates was retrieved. Then, starting from the mass of unknown, mass shifts corresponding to pre-defined biotransformations (e.g. demethylation, glucuronidation, etc.) were calculated and corresponding mass chromatograms were extracted from the total ion current (TIC) in order to search for metabolite peaks. For each candidate, the number of different functional groups in the molecule was automatically calculated using E-Dragon software (Talete srl, Milan, Italy). Then, the presence of metabolites in the TIC was matched with functional groups data in order to exclude candidates with structures not compatible with observed biotransformations (e.g. loss of methyl from a structure not bearing methyls). The procedure was tested on 108 pharmaco-toxicologically relevant compounds (PTRC) and their phase I metabolites were detected in real positive samples. The mean list length (MLL) of candidates retrieved from the database was 7.01 +/- 4.77 (median, 7; range, 1-28) before the application of the 'metabolomic' approach, and after the application it was reduced to 4.08 +/- 3.11 (median 3, range 1-17). HRMS allows a much broader screening for PTRC than other screening approaches (e.g. library search on mass spectra databases). The 'metabolomic' approach enables the reduction of the list of candidate isomers.

Mass spectrometry analysis of new chemical entities for pharmaceutical discovery

In this Section, we review the applications of mass spectrometry for the analysis and purification of new chemical entities (NCEs) for pharmaceutical discovery. Since the speed of synthesis of NCEs has dramatically increased over the last few years, new high throughput analytical techniques have been developed to keep pace with the synthetic developments. In this Section, we review both novel, as well as modifications of commonly used mass spectrometry techniques that have helped increase the speed of the analytical process. Part of the review is devoted to the purification of NCEs, which has undergone significant development in recent years, and the close integral association between characterization and purification to drive high throughput operations. At the end of the Section, we review potential future directions based on promising and exciting new developments.

Quantification of Drugs in Plasma Without Primary Reference Standards by Liquid Chromatography-Chemiluminescence Nitrogen Detection: Application to Tramadol Metabolite Ratios

Lack of availability of reference standards for drug metabolites, newly released drugs, and illicit drugs hinders the analysis of these substances in biologic samples. To counter this problem, an approach is presented here for quantitative drug analysis in plasma without primary reference standards by liquid chromatography-chemiluminescence nitrogen detection (LC-CLND). To demonstrate the feasibility of the method, metabolic ratios of the opioid drug tramadol were determined in the setting of a pharmacogenetic study. Four volunteers were given a single 100-mg oral dose of tramadol, and a blood sample was collected from each subject 1 hour later. Tramadol, O-desmethyltramadol, and nortramadol were determined in plasma by LC-CLND without reference standards and by a gas chromatography-mass spectrometry reference method. In contrast to previous CLND studies lacking an extraction step, a liquid-liquid extraction system was created for 5-mL plasma samples using n-butyl chloride-isopropyl alcohol (98 + 2) at pH 10. Extraction recovery estimation was based on model compounds chosen according to their similar physicochemical characteristics (retention time, pKa, logD). Instrument calibration was performed with a single secondary standard (caffeine) using the equimolar response of the detector to nitrogen. The mean differences between the results of the LC-CLND and gas chromatography-mass spectrometry methods for tramadol, O-desmethyltramadol, and nortramadol were 8%, 32%, and 19%, respectively. The sensitivity of LC-CLND was sufficient for therapeutic concentrations of tramadol and metabolites. A good correlation was obtained between genotype, expressed by the number of functional genes, and the plasma metabolite ratios. This experiment suggests that a recovery-corrected LC-CLND analysis produces sufficiently accurate results to be useful in a clinical context, particularly in instances in which reference standards are not readily accessible.

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.

Multiplex multidimensional nanoLC-MS system for targeted proteomic analyses

The present work describes a dual-column and dual-sprayer LC-MS system for high-throughput proteomic analyses. This system consists of two precolumns for sample desalting and two analytical columns. Each column is terminated by a nanoelectrospray emitter mounted on a robotic arm enabling their sequential positioning in front of the sampling cone of the mass spectrometer. The effluent from each emitter is recorded in separate acquisition channels without detectable crosstalk. Gradient elution to both nanoLC columns is delivered by a single HPLC system via a flow splitter. The reproducibility of retention time and peak intensity of the present multiplex system were comparable to those obtainable using a single emitter configuration. Replicate injections of complex tryptic digests (n = 10) indicated that this system provided good reproducibility of retention time and peak intensity on both columns with RSD values of less than 0.9 and 18.6%, respectively. The application of this system is demonstrated for the monitoring of protein expression changes in U937 human monocyte cells with and without phorbol ester administration. Furthermore, we also demonstrated the use of this multiplex system in a 2-D LC configuration to increase sample loading and throughput for the analysis of biomarker samples of higher complexity. Variations in peptide abundance down to two-fold change were identified across salt fractions for spiked tryptic digests present at a level of 50 fmol in 1.5 microg of plasma samples.

Isotopic pattern and accurate mass determination in urine drug screening by liquid chromatography/time-of-flight mass spectrometry

An efficient method was developed for toxicological drug screening in urine by liquid chromatography coupled with electrospray ionization time-of-flight mass spectrometry. The method relies on a large target database of exact monoisotopic masses representing the elemental formulae of reference drugs and their metabolites. Mass spectral identification is based on matching measured accurate mass and isotopic pattern (SigmaFit) of a sample component with those in the database. Data post-processing software was developed for automated reporting of findings in an easily interpretable form. The mean and median of SigmaFit for true-positive findings were 0.0066 and 0.0051, respectively. The mean and median of mass error absolute values for true-positive findings were 2.51 and 2.17 ppm, respectively, corresponding to 0.65 and 0.60 mTh. For routine screening practice, a SigmaFit tolerance of 0.03 and a mass tolerance of 10 ppm were chosen. Ion abundance differences from urine extracts did not affect the accuracy of the automatically acquired SigmaFit or mass values. The results show that isotopic pattern matching by SigmaFit is a powerful means of identification in addition to accurate mass measurement.

Assigning product ions from complex MS/MS spectra: the importance of mass uncertainty and resolving power

This study offers a unique insight into the mass accuracy and resolving power requirements in MS/MS analyses of complex product ion spectra. In the examples presented here, accurate mass assignments were often difficult because of multiple isobaric interferences and centroid mass shifts. The question then arose whether the resolving power of a medium-resolution quadrupole time-of flight (QqTOF) is sufficient or high-resolution Fourier-transform ion cyclotron resonance (FT-ICR) is required for unambiguous assignments of elemental compositions. For the comparison, two paralytic shellfish poisons (PSP), saxitoxin (STX) and neosaxitoxin (NEO), with molecular weights of 299 and 315 g x mol(-1), respectively, were chosen because of the high peak density in their MS/MS spectra. The assessment of QqTOF collision-induced dissociation spectra and FT-ICR infrared multiphoton dissociation spectra revealed that several intrinsic dissociation pathways leading to isobaric fragment ions could not be resolved with the QqTOF instrument and required FT-ICR to distinguish very close mass differences. The second major source of interferences was M + 1 species originating from coactivated 13C12Cc-1 ion contributions of the protonated molecules of the PSPs. The problem in QqTOF MS results from internal mass calibration when the MH+ ions of analyte and mass calibrant are activated at the same time in the collision or trapping cell. Although FT-ICR MS readily resolved these interfering species, the QqTOF did not provide resolving power >20,000 (full width at half maximum) required to separate most isobaric species. We were able to develop a semi-internal QqTOF calibration technique that activated only the isolated 12C isotope species of the protonated molecules, thus reducing the M + 1 interferences significantly. In terms of overall automated elemental formulas assignment, FT-ICR MS achieved the first formula hit for 100% of the product ions, whereas the QqTOF MS hit rate was only 56 and 65% for STX and NEO product ions, respectively. External mass calibration from commercial FT-ICR and QqTOF instruments gave similar results.

High-throughput characterization and quality control of small-molecule combinatorial libraries

To fully realize the potential of combinatorial synthesis and high-throughput screening for increasing the efficiency of the drug discovery and development process, issues related to compound purity must be addressed. Impurities, often present after synthesis, can lead to ambiguous screening results and inhibit the development of quality structure-activity relationships. The demand for high-throughput analytical characterization of combinatorial libraries has prompted the development of more rapid methods to keep pace with compound production. Recent progress has focused upon the development of parallel separation methods, multiplexed detector interfaces, and synergistic combinations of different detectors possessing complementary selectivities.

Use of a five-channel multiplexed electrospray quadrupole time-of-flight hybrid mass spectrometer for metabolite identification

Metabolism data provided with reduced cycle time has become of increasing importance for the early evaluation of DMPK properties of drugs in discovery. In this regard, quadrupole time-of-flight hybrid mass spectrometers (Q-TOF) can provide very reliable metabolite identification via accurate mass measurement of ions and the consequent access to the elemental composition of the metabolite. However, due to their cost, they are often used for drug metabolism studies on later stage drug candidates or to address challenging metabolism questions. A new prototype, consisting of a five-channel multiplexed electrospray ionization (ESI) source on a Q-TOF with one channel used for lock-mass compound infusion, was evaluated for metabolite identification. The goal was to increase the sample throughput of a single ESI-MS system by a factor of 4, while maintaining efficient metabolite separation in high-performance liquid chromatography (HPLC) as well as adequate sensitivity and mass accuracy, and ultimately improve the speed and quality of metabolism studies supporting drug discovery. The analytical performance of the system was assessed by evaluating the sensitivity and mass accuracy (using real in vitro and in vivo samples), inter-channel differences in retention times, MS/UV response, and cross-talk among channels. The sensitivity using the multiplexed ESI source was on average 2-fold lower than with single ESI, correlating well with previous literature data. The mass accuracy was comparable to that obtained using single ESI in both MS and MS/MS modes, making the metabolite identification process using the multiplexed ESI source as reliable as with single ESI. Compound-dependent differences in ionization efficiencies were observed among channels, and were minimized by analyzing related samples on the same channel. Finally, the level of cross-talk among channels was acceptable (around 0.3%) and comparable to levels previously published for quantitative applications using multiplexed ESI. The paper also focuses on the advantages and disadvantages of this new approach compared to other approaches in the literature in the field of metabolite identification.

Analysis of Street Drugs in Seized Material without Primary Reference Standards

A novel approach was used to analyze street drugs in seized material without primary reference standards. Identification was performed by liquid chromatography/time-of-flight mass spectrometry (LC/TOFMS), essentially based on accurate mass determination using a target library of 735 exact monoisotopic masses. Quantification was carried out by liquid chromatography/chemiluminescence nitrogen detection (LC/CLND) with a single secondary standard (caffeine), utilizing the detector's equimolar response to nitrogen. Sample preparation comprised dilution, first with methanol and further with the LC mobile phase. Altogether 21 seized drug samples were analyzed blind by the present method, and results were compared to accredited reference methods utilizing identification by gas chromatography/mass spectrometry and quantification by gas chromatography or liquid chromatography. The 31 drug findings by LC/TOFMS comprised 19 different drugs-of-abuse, byproducts, and adulterants, including amphetamine and tryptamine designer drugs, with one unresolved pair of compounds having an identical mass. By the reference methods, 27 findings could be confirmed, and among the four unconfirmed findings, only 1 apparent false positive was found. In the quantitative analysis of 11 amphetamine, heroin, and cocaine findings, mean relative difference between the results of LC/CLND and the reference methods was 11% (range 4.2-21%), without any observable bias. Mean relative standard deviation for three parallel LC/CLND results was 6%. Results suggest that the present combination of LC/TOFMS and LC/CLND offers a simple solution for the analysis of scheduled and designer drugs in seized material, independent of the availability of primary reference standards.