Ultra-low flow nanospray for the normalization of conventional liquid chromatography/mass spectrometry through equimolar response: standard-free quantitative estimation of metabolite levels in drug discovery

Characterization of Electrospray Drop Size Distributions by Mobility-Classified Mass Spectrometry: Implications for Ion Clustering in Solution and Ion Formation Pathways

One of the outcomes of electrospray ionization is the size distribution of the droplets, which determines, together with the solvent composition and the source gas temperature, the minimum distance from the sprayer tip to the mass spectrometer inlet and therefore the ion transfer efficiency. Even more importantly, the average number of analyte molecules and, if present, contaminant species per droplet depend on the drop size. Consequently, the drop size distribution is a key parameter in nonspecific ion clustering in solution and ion suppression. The finding that small droplet sizes improve the mass spectral quality led to the development of nanoelectrospray sources, which dispense liquid flow rates below 0.1 μL/min and can generate drops with diameters smaller than 100 nm. However, current discussions on the effect of drop size on ion formation pathways and efficiencies remain qualitative because the exact drop size distributions are unknown. Here, we show that ion mobility-classified mass spectrometry of raffinose cluster ions allows us to determine very precisely the drop size distribution generated by the electrospray source in positive- and negative-ion modes. Based on the derived drop size distributions, we can quantitatively predict nonspecific ion clustering and can extract accurate probabilities for emission of species from parent drops upon Coulomb fission.

Direct Liquid Extraction and Ionization Techniques for Understanding Multimolecular Environments in Biological Systems (Secondary Publication)

A combination of direct liquid extraction using a small volume of solvent and electrospray ionization allows the rapid measurement of complex chemical components in biological samples and visualization of their distribution in tissue sections. This review describes the development of such techniques and their application to biological research since the first reports in the early 2000s. An overview of electrospray ionization, ion suppression in samples, and the acceleration of specific chemical reactions in charged droplets is also presented. Potential future applications for visualizing multimolecular environments in biological systems are discussed.

Nested-channel for on-demand alternation between electrospray ionization regimes

On-demand electrospray ionization from different liquid channels in the same emitter was realized using filamented capillary and gas phase charge supply. The solution sub-channel was formed when back-filling solution to the emitter tip by capillary action along the filament. Gas phase charge carriers were used to trigger electrospray ionization from the solution meniscus at the tip. The meniscus at the tip opening may be fully filled or partially empty to generate electrospray ionization in main-channel regime and sub-channel regime, respectively. For emitters with 4 μm tip opening, the two nested electrospray (nested-ESI) channels accommodated ESI flow rates ranging from 50 pL min⁻¹ to 150 nL min⁻¹. The platform enabled on-demand regime alternations within one sample run, in which the sub-channel regime generated smaller charged droplets. Ionization efficiencies for saccharides, glycopeptide, and proteins were enhanced in the sub-channel regime. Non-specific salt adducts were reduced and identified by regime alternation. Surprisingly, the sub-channel regime produced more uniform responses for a peptide mixture whose relative ionization efficiencies were insensitive to ESI conditions in previous picoelectrospray study. The nested channels also allowed effective washing of emitter tip for multiple sampling and analysis operations.

Nested electrospray ionization alternates on-demand between microscale main-channel and nanscale sub-channels.

Advances in mass spectrometry-based metabolomics for investigation of metabolites

Metabolomics is the systematic study of all the metabolites present within a biological system, which consists of a mass of molecules, having a variety of physical and chemical properties and existing over an extensive dynamic range in biological samples. Diverse analytical techniques are needed to achieve higher coverage of metabolites. The application of mass spectrometry (MS) in metabolomics has increased exponentially since the discovery and development of electrospray ionization and matrix-assisted laser desorption ionization techniques. Significant advances have also occurred in separation-based MS techniques (gas chromatography-mass spectrometry, liquid chromatography-mass spectrometry, capillary electrophoresis-mass spectrometry, and ion mobility-mass spectrometry), as well as separation-free MS techniques (direct infusion-mass spectrometry, matrix-assisted laser desorption ionization-mass spectrometry, mass spectrometry imaging, and direct analysis in real time mass spectrometry) in the past decades. This review presents a brief overview of the recent advanced MS techniques and their latest applications in metabolomics. The software/websites for MS result analyses are also reviewed.

Metabolomics is the systematic study of all the metabolites present within a biological system, supply functional information and has received extensive attention in the field of life sciences.

Evaluation of relative MS response factors of drug metabolites for semi-quantitative assessment of chemical liabilities in drug discovery

Drug metabolism studies are performed in drug discovery to identify metabolic soft spots, detect potentially toxic or reactive metabolites and provide an early insight into potential species differences. The relative peak area approach is often used to semi-quantitatively estimate the abundance of metabolites. Differences in the liquid chromatography-mass spectrometry responses result in an underestimation or overestimation of the metabolite and misinterpretation of results. The relative MS response factors (RF) of 132 structurally diverse drug candidates and their 233 corresponding metabolites were evaluated using a capillary-liquid chromatography/high-resolution mass spectrometry system. All of the synthesized metabolites discussed here were previously identified as key biotransformation products in discovery investigations or predicted to be formed. The most commonly occurring biotransformation mechanisms such as oxygenation, dealkylation and amide cleavage are represented within this dataset. However, relatively few phase II metabolites were evaluated because of the limited availability of authentic standards. Approximately 85% of these metabolites had a relative RF in the range between 0.2 (fivefold under-prediction) and 2.0 (twofold over-prediction), and the median MS RF was 0.6. Exceptions to this included very small metabolites that were hardly detectable. Additional experiments performed to understand the impact of the MS platform, flow rate and concentration suggested that these parameters do not have a significant impact on the RF of the compounds tested. This indicates that the use of relative peak areas to semi-quantitatively estimate the abundance of metabolites is justified in the drug discovery setting in order to guide medicinal chemistry efforts. Copyright © 2017 John Wiley & Sons, Ltd.

Picoelectrospray ionization mass spectrometry using narrow-bore chemically etched emitters

Electrospray ionization mass spectrometry (ESI-MS) at flow rates below ~10 nL/min has been only sporadically explored because of difficulty in reproducibly fabricating emitters that can operate at lower flow rates. Here we demonstrate narrow orifice chemically etched emitters for stable electrospray at flow rates as low as 400 pL/min. Depending on the analyte concentration, we observe two types of MS signal response as a function of flow rate. At low concentrations, an optimum flow rate is observed slightly above 1 nL/min, whereas the signal decreases monotonically with decreasing flow rates at higher concentrations. For example, consumption of 500 zmol of sample yielded signal-to-noise ratios ~10 for some peptides. In spite of lower MS signal, the ion utilization efficiency increases exponentially with decreasing flow rate in all cases. Significant variations in ionization efficiency were observed within this flow rate range for an equimolar mixture of peptide, indicating that ionization efficiency is an analyte-dependent characteristic for the present experimental conditions. Mass-limited samples benefit strongly from the use of low flow rates and avoiding unnecessary sample dilution. These findings have important implications for the analysis of trace biological samples.

Mass spectrometry strategies for clinical metabolomics and lipidomics in psychiatry, neurology, and neuro-oncology

Metabolomics research has the potential to provide biomarkers for the detection of disease, for subtyping complex disease populations, for monitoring disease progression and therapy, and for defining new molecular targets for therapeutic intervention. These potentials are far from being realized because of a number of technical, conceptual, financial, and bioinformatics issues. Mass spectrometry provides analytical platforms that address the technical barriers to success in metabolomics research; however, the limited commercial availability of analytical and stable isotope standards has created a bottleneck for the absolute quantitation of a number of metabolites. Conceptual and financial factors contribute to the generation of statistically under-powered clinical studies, whereas bioinformatics issues result in the publication of a large number of unidentified metabolites. The path forward in this field involves targeted metabolomics analyses of large control and patient populations to define both the normal range of a defined metabolite and the potential heterogeneity (eg, bimodal) in complex patient populations. This approach requires that metabolomics research groups, in addition to developing a number of analytical platforms, build sufficient chemistry resources to supply the analytical standards required for absolute metabolite quantitation. Examples of metabolomics evaluations of sulfur amino-acid metabolism in psychiatry, neurology, and neuro-oncology and of lipidomics in neurology will be reviewed.

Performance assessment of microflow LC combined with high-resolution MS in bioanalysis

BACKGROUND

There continues to be consistent pressure for bioanalytical scientists to achieve lower limits of quantitation. The reasons range from smaller sample volumes available for analysis, to more potent analytes and the growth of biologics in drug development. This has led scientists to investigate alternative LC techniques, including microflow and nanoflow. These techniques have been shown to increase sensitivity of electrospray methods and reduce ionization matrix effects. Because high-resolution MS has significant benefits for the analysis of biologics, this type of mass spectrometer is becoming increasingly important in bioanalysis.

RESULTS

For microflow analysis, a new ion source and significant extra sample preparation or chromatographic separation are not required. However, increased sensitivity and reduced matrix effects were consistently demonstrated when compared with UHPLC flow rates. The extent of matrix effects observed were compound dependent.

DISCUSSION

This paper presents the utility of combining high-resolution/accurate mass with microflow LC from a quantitative standpoint. This includes evaluating the typical quantitative parameters of sensitivity, linearity/dynamic range, precision and accuracy. It also includes the evaluation of changes in signal suppression using microflow LC and microspray ionization. The benefits and disadvantages of using the combination of these two technologies for quantitative bioanalysis are also discussed.

Electrospray ionization from nanopipette emitters with tip diameters of less than 100 nm

Work presented here demonstrates application of nanopipettes pulled to orifice diameters of less than 100 nm as electrospray ionization emitters for mass spectrometry. Mass spectrometric analysis of a series of peptides and proteins electrosprayed from pulled-quartz capillary nanopipette emitters with internal diameters ranging from 37 to 70 nm is detailed. Overall, the use of nanopipette emitters causes a shift toward the production of ions of higher charge states and leads to a reduction in width of charge-state distribution as compared to typical nanospray conditions. Further, nanopipettes show improved S/N and the same signal precision as typical nanospray, despite the much smaller dimensions. As characterized by SEM images acquired before and after spray, nanopipettes are shown to be robust under conditions employed. Analytical calculations and numerical simulations are used to calculate the electric field at the emitter tip, which can be significant for the small diameter tips used.

Determination of rosiglitazone and 5-hydroxy rosiglitazone in rat plasma using LC–HRMS by direct and indirect quantitative analysis: a new approach for metabolite quantification

Background: With recent advances in mass spectrometry instrumentation, HRMS is of increasing interest for quantitative bioanalysis due to its high sensitivity, rapid acquisition of full scan data, and advanced software for metabolite identification. In particular, there is strong interest in use of HRMS for simultaneous quantification of parent drug and metabolites without authentic metabolite standard materials. Materials & methods: Rosiglitazone and 5-hydroxy rosiglitazone in rat plasma were analyzed using LC–Q-TOF by both direct and indirect quantitative analysis. Direct quantitative analysis used an authentic metabolite standard (5-hydroxy rosiglitazone). Indirect quantitative analysis firstly used the parent drug (rosiglitazone) calibration curve to provide a semiquantitative measure of metabolite concentration. A correction factor was then applied to the original data to re-calculate the 5-hydroxy rosiglitazone metabolite concentration. Results: The ratio of the calibration curve slope of rosiglitazone to that of 5-hydroxy rosiglitazone was determined to be 2.09 ± 0.28 using different batches of mobile phases and columns. The correction factor 2.09 was then used to correct for the 5-hydroxy rosiglitazone concentrations found from the semiquantitative results using the rosiglitazone calibration standard curve. The concentrations of 5-hydroxy rosiglitazone found by direct and indirect quantitative analysis were highly comparable (within ±8%). Conclusion: Indirect quantitative analysis provides an alternative approach for metabolite quantification for discovery PK studies.

The advantages of microflow LC–MS/MS compared with conventional HPLC–MS/MS for the analysis of methotrexate from human plasma

Background: In support of bioanalysis, there has always been a desire to improve detection limits and reduce scale. Microflow LC (MFLC) coupled with MS accomplishes both of these goals. Results: As such, MFLC coupled with an MS system was used to generate bioanalytical validation data that met US FDA criteria. The MFLC–MS/MS data was compared with the same method with the use of conventional HPLC–MS/MS and a more than 14× S/N improvement was found with the MFLC–MS/MS method. Methotrexate was used as a model molecule to demonstrate the validation of the method from human plasma. The MFLC–MS/MS method was demonstrated to be accurate (±7%) and precise (12.9% at the LLOQ and a maximum of 11.6% at all other concentrations) across the dynamic range of the assay (1–1000 ng/ml) and compared well with the HPLC–MS/MS method. The MFLC bioanalytical validation was performed at a flow rate of 35 µl/min on a 0.5-mm inner diameter (I.D.) column, whereas, for the same linear velocities on the 2.0-mm I.D. column, the conventional HPLC bioanalytical validation was performed at 700 µl/min. Since the flow rate of the MFLC system is 20-times less than the HPLC system, the consumable solvent and disposal cost to perform the MFLC validation was significantly less. Conclusion: MFLC–MS/MS can be used to perform bioanalytical method validations with increased MS signal, reduced source contamination and reduced solvent consumption.

Conventional liquid chromatography/triple quadrupole mass spectrometry based metabolite identification and semi-quantitative estimation approach in the investigation of in vitro dabigatran etexilate metabolism

Dabigatran etexilate (DABE) is an oral prodrug that is rapidly converted by esterases to dabigatran (DAB), a direct inhibitor of thrombin. To elucidate the esterase-mediated metabolic pathway of DABE, a high-performance liquid chromatography/mass spectrometry based metabolite identification and semi-quantitative estimation approach was developed. To overcome the poor full-scan sensitivity of conventional triple quadrupole mass spectrometry, precursor-product ion pairs were predicted to search for the potential in vitro metabolites. The detected metabolites were confirmed by the product ion scan. A dilution method was introduced to evaluate the matrix effects on tentatively identified metabolites without chemical standards. Quantitative information on detected metabolites was obtained using "metabolite standards" generated from incubation samples that contain a high concentration of metabolite in combination with a correction factor for mass spectrometry response. Two in vitro metabolites of DABE (M1 and M2) were identified, and quantified by the semi-quantitative estimation approach. It is noteworthy that CES1 converts DABE to M1 while CES2 mediates the conversion of DABE to M2. M1 and M2 were further metabolized to DAB by CES2 and CES1, respectively. The approach presented here provides a solution to a bioanalytical need for fast identification and semi-quantitative estimation of CES metabolites in preclinical samples.

CE-MS for proteomics: Advances in interface development and application

Capillary electrophoresis-mass spectrometry (CE-MS) has emerged as a powerful technique for the analysis of proteins and peptides. Over the past few years, significant progress has been made in the development of novel and more effective interfaces for hyphenating CE to MS. This review provides an overview of these new interfacing techniques for coupling CE to MS, covering the scientific literature from January 2007 to December 2011. The potential of these new CE-MS interfacing techniques is demonstrated within the field of (clinical) proteomics, more specifically "bottom-up" proteomics, by showing examples of the analysis of various biological samples. The relevant papers on CE-MS for proteomics are comprehensively summarized in tables, including, e.g. information on sample type and pretreatment, interfacing and MS detection mode. Finally, general conclusions and future perspectives are provided.

Integrated quantitative and qualitative workflow for in vivo bioanalytical support in drug discovery using hybrid Q-TOF-MS

Background: UHPLC coupled with orthogonal acceleration hybrid quadrupole-TOF (Q-TOF)-MS is an emerging technique offering new strategies for the efficient screening of new chemical entities and related molecules at the early discovery stage within the pharmaceutical industry. In the first part of this article, we examine the main instrumental parameters that are critical for the integration of UHPLC–Q-TOF technology to existing bioanalytical workflows, in order to provide simultaneous quantitative and qualitative bioanalysis of samples generated following in vivo studies. Material & Methods: Three modern Q-TOF mass spectrometers, including Bruker maXis™, Agilent 6540 and Sciex TripleTOF™ 5600, all interfaced with UHPLC systems, are evaluated in the second part of the article. The scope of this work is to demonstrate the potential of Q-TOF for the analysis of typical small molecules, therapeutic peptides (molecular weight <6000 Da), and enzymatically (i.e., trypsin, chymotrypsin and pepsin) cleaved peptides from larger proteins. Results & Discussion: This work focuses mainly on full-scan TOF data obtained under ESI conditions, the major mode of TOF operation in discovery bioanalytical research, where the compounds are selected based on their pharmacokinetic/pharmacodynamic behaviors using animal models prior to selecting a few desirable candidates for further development. Finally, important emerging TOF technologies that could potentially benefit bioanalytical research in the semi-quantification of metabolites without synthesized standards are discussed. Particularly, the utility of captive spray ionization coupled with TripleTOF 5600 was evaluated for improving sensitivity and providing normalized MS response for drugs and their metabolites. The workflow proposed compromises neither the efficiency, nor the quality of pharmacokinetic data in support of early drug discovery programs.

Application of on-line nano-liquid chromatography/mass spectrometry in metabolite identification studies

Metabolite identification is an important part of the drug discovery and development process. High sensitivity is necessary to identify metabolic products in vitro and in vivo. The most common method utilizes standard high-performance liquid chromatography (4.6  mm i.d. column and 1  mL/min flow rate) coupled to tandem mass spectrometry (HPLC/MS/MS). We have developed a method that utilizes a nano-LC system coupled to a high-resolution tandem mass spectrometer to identify metabolites from in vitro and in vivo samples. Using this approach, we were able to increase the sensitivity of analysis by approximately 1000-fold over HPLC/MS. In vitro samples were analyzed after simple acetonitrile precipitation, centrifugation, and dilution. The significant improvement in sensitivity enabled us to conduct experiments at very low substrate concentrations (0.01  μM), and very low incubation volumes (20  μL). In vivo samples were injected after simple dilution without any pre-purification. All the metabolites identified by conventional HPLC/MS/MS were also identified using the nano-LC method. This study demonstrates a very sensitive approach to identifying phase I and II metabolites with throughput and separation equivalent to the standard HPLC/MS/MS method.

Evaluation of relative LC/MS response of metabolites to parent drug in LC/nanospray ionization mass spectrometry: potential implications in MIST assessment

There is an increasing demand for quantitative data on metabolite exposure triggered by regulatory guidances. This contribution describes the accuracy of nanoelectrospray ionization mass spectrometry response of drug compounds and their metabolites from biological matrices compared with radiometric quantification. This is a comprehensive investigation of a set of real-life pharmaceutical compounds in relevant matrices such as urine, bile, feces and plasma. The data suggest that nanoelectrospray mass spectrometry can be used for semi-quantitation of metabolites in the absence of reference standards. Therefore, this approach is suitable to screen out non-relevant metabolites early in development as long as an adequate analytical error margin is applied thus balancing risks and resources.

Small molecule quantification by liquid chromatography-mass spectrometry for metabolites of drugs and drug candidates

Identification and quantification of the metabolites of drugs and drug candidates are routinely performed using liquid chromatography-mass spectrometry (LC-MS). The best practice is to generate a standard curve with the metabolite versus the internal standard. However, to avoid the difficulties in metabolite synthesis, standard curves are sometimes prepared using the substrate, assuming that the signal for substrate and the metabolite will be equivalent. We have tested the errors associated with this assumption using a series of very similar compounds that undergo common metabolic reactions using both conventional flow electrospray ionization LC-MS and low-flow captive spray ionization (CSI) LC-MS. The differences in standard curves for four different types of transformations (O-demethylation, N-demethylation, aromatic hydroxylation, and benzylic hydroxylation) are presented. The results demonstrate that the signals of the substrates compared with those of the metabolites are statistically different in 18 of the 20 substrate-metabolite combinations for both methods. The ratio of the slopes of the standard curves varied up to 4-fold but was slightly less for the CSI method.

Electrospray ionization mass spectrometry from discrete nanoliter-sized sample volumes

We describe a method for nanoelectrospray ionization mass spectrometry (nESI-MS) of very small sample volumes. Nanoliter-sized sample droplets were taken up by suction into a nanoelectrospray needle from a silicon microchip prior to ESI. To avoid a rapid evaporation of the small sample volumes, all manipulation steps were performed under a cover of fluorocarbon liquid. Sample volumes down to 1.5 nL were successfully analyzed, and an absolute limit of detection of 105 attomole of insulin (chain B, oxidized) was obtained. The open access to the sample droplets on the silicon chip provides the possibility to add reagents to the sample droplets and perform chemical reactions under an extended period of time. This was demonstrated in an example where we performed a tryptic digestion of cytochrome C in a nanoliter-sized sample volume for 2.5 h, followed by monitoring the outcome of the reaction with nESI-MS. The technology was also utilized for tandem mass spectrometry (MS/MS) sequencing analysis of a 2 nL solution of angiotensin I.

Analysis of samples stored as individual plugs in a capillary by electrospray ionization mass spectrometry

Droplets or plugs within multiphase microfluidic systems have rapidly gained interest as a way to manipulate samples and chemical reactions on the femtoliter to microliter scale. Chemical analysis of the plugs remains a challenge. We have discovered that nanoliter plugs of sample separated by air or oil can be analyzed by electrospray ionization mass spectrometry when pumped directly into a fused silica nanospray emitter tip. Using leucine-enkephalin in methanol and 1% acetic acid in water (50:50 v:v) as a model sample, we found carry-over between plugs was <0.1% and relative standard deviation of signal for a series of plugs was 3%. Detection limits were 1 nM. Sample analysis rates of 0.8 Hz were achieved by pumping 13 nL samples separated by 3 mm long air gaps in a 75 microm inner diameter tube. Analysis rates were limited by the scan time of the ion trap mass spectrometer. The system provides a robust, rapid, and information-rich method for chemical analysis of sample in segmented flow systems.

Single droplet separations and surface partition coefficient measurements using laser ablation mass spectrometry

Surface activity of analytes plays a significant role in many chemical and physical phenomena. We present here a mass spectrometric method to characterize surface activity and solute partitioning between bulk liquid and the gas-liquid interface in droplets. The approach employs ablation by an infrared (IR) laser from the surface of a microliter droplet deposited on a stainless steel post. The ablated material is ionized for mass spectrometric analysis by either droplet charging or by postionization in an electrospray plume. Three areas of application have been explored using this method (1) separations in a single droplet: continuous ablation by a series of many successive laser pulses results in faster depletion of more surface active analytes, effectively comprising a surface activity-based separation. (2) Partition coefficient measurements: droplet volume is held constant during ablation by continually replenishing lost solvent. This leads to analyte-specific ion signal decay curves that may be fitted to a model based on Langmuir adsorption isotherms and simple analytical expressions, yielding quantitative values for the analyte surface partition coefficients. (3) Studies of the relationship between surface partitioning and high-performance liquid chromatography (HPLC) phase partitioning: comparisons of surface activities measured by laser desorption with retention times in reversed-phase HPLC reveal that the relationship between the two partitioning processes is very sensitive to chemical structure. Poor correlation between the retention time and surface activity is also observed within a subcategory of analytes (peptides). This effect is attributed to multimodal solute-stationary phase interactions. The laser desorption approach presented here provides direct information on analyte surface activities free from the complications encountered in chromatographic methods due to chemical structure variations.

Online nanoelectrospray/high-field asymmetric waveform ion mobility spectrometry as a potential tool for discovery pharmaceutical bioanalysis

Nanoelectrospray ionization (nESI) coupled online with high-field asymmetric waveform ion mobility spectrometry (FAIMS) for small molecule analysis in a discovery pharmaceutical setting was examined. A conventional capillary pump, autosampler and nESI source were used to introduce samples directly into the FAIMS device. The FAIMS device was used to separate gas-phase ions on a timescale that was compatible with the mass spectrometer. The capability of the nESI-FAIMS combination to efficiently remove metabolite interferences from the parent drug, and reduce ion suppression effects, was demonstrated. On average, 85% of the signal intensity obtained from a neat sample was preserved in the extracted plasma samples. Standard curves were prepared for several compounds. Linearity was obtained over approximately 3 to 4 orders of magnitude. Comparison of results from nESI-FAIMS with those from conventional LC/MS for a mouse pharmacokinetic study yielded concentration values differing by no more than 30%.

Quantitative-qualitative data acquisition using a benchtop Orbitrap mass spectrometer

Current approaches to discovery-stage drug metabolism studies (pharmacokinetics, microsomal stability, etc.) typically use triple-quadrupole-based approaches for quantitative analysis. This necessitates the optimization of parameters such as Q1 and Q3 m/z values, collision energy, and interface voltages. These studies detect only the specified compound and information about other components, such as metabolites, is lost. The ability to perform full-scan acquisition for quantitative analysis would eliminate the need for compound optimization while enabling the detection of metabolites and other non-drug-related endogenous components. Such an instrument would have to provide sensitivity, selectivity, dynamic range, and scan speed suitable for discovery-stage quantitative studies. In this study, a prototype benchtop Orbitrap-based mass analyzer was used to collect both quantitative and qualitative data from human microsomal incubation samples as well as rat plasma from pharmacokinetic studies. Instrumental parameters such as scan speed, resolution, and mass accuracy are discussed in relation to the requirements for a quantitative-qualitative workflow. The ability to perform highly selective quantitative analysis while simultaneously characterizing metabolites from both in vitro and in vivo studies is discussed.

Metabolite quantitation: detector technology and MIST implications

HPLC detector technology has advanced dramatically over the past 20 years, with a range of highly sensitive and specific detectors becoming available. What is still missing from the bioanalyst’s armoury, however, is a highly sensitive detector that gives an equimolar response independent of the compound. This would allow for quantification of compounds without the requirement for a synthetic standard or a radiolabeled analogue. In particular, such a detector applied to metabolism studies would establish the relative significance of the various metabolic routes. The recently issued US FDA guidelines on metabolites in safety testing (MIST) focus on the relative quantitation of human metabolites being obtained as soon as feasible in the drug-development process. In this article, current detector technology is reviewed with respect to its potential for quantitation without authentic standards or a radiolabel and put in the context of the MIST guidelines. The potential for future developments are explored.

Analytical strategies for the screening and evaluation of chemically reactive drug metabolites

BACKGROUND

Metabolic activation leading to formation of chemically reactive drug metabolites is a long-standing issue for drug development inasmuch as some, but not all, reactive intermediates play a role as mediators of drug-induced toxicities. The risk assessment profile/decision-making guide requires a comprehensive understanding of bioactivation mechanism(s), quantitative magnitude and cellular consequences of this principal and continued safety attrition.

OBJECTIVE

To evaluate analytical methodologies with improved sensitivity, selectivity and throughput for the analysis of reactive metabolites.

CONCLUSIONS

Identification and quantification of short-lived electrophilic intermediates through appropriate trapping experiments have become relatively straightforward. Minimizing the bioactivation potential of drug candidates during the discovery/lead optimization phase has been adopted as a default strategy. Together with advances of proteomics, metabolomics and toxicogenomics, an integrated multitier approach possibly provides a deeper insight into mechanistic aspects of drug-induced toxicities, and contributes to bridging the relationships between metabolic activation, drug-protein adduct formation and their toxicological consequences.

Liquid chromatography-mass spectrometry in in vitro drug metabolite screening

A combination of high performance liquid chromatography (HPLC) and mass spectrometry (LC/MS) has proven its status as the most powerful analytical tool for screening and identifying drug metabolites in modern drug discovery. These techniques have become irreplaceable for drug metabolism laboratories, providing high amounts of information from a wide variety of samples. This review focuses on the most common and useful applications of these techniques when working on in vitro metabolism, more specifically with screening and identification of chemically stable or reactive metabolites formed via biotransformation reactions. Matching specific tasks and suitable instruments is a recurring consideration; for many reasons, the time-of-flight or orbitrap mass spectrometry provides clearly increased efficiency in metabolite profiling compared to other types of mass spectrometry.

Surface roughening of a non-tapered open tubular emitter for improved electrospray ionization mass spectrometry performance at low flow rates

A non-tapered open tubular emitter with 75 microm internal diameter (i.d.) and 360 microm external diameter (o.d.) was developed by simply grinding the exit aperture of a fused-silica capillary. The roughened emitter, with a relatively large aperture, generates stable electrospray signals (generally <5% relative standard deviation (RSD) for most conditions studied) at less than 500 nL/min flow rates, and was characterized with atomic force microscopy. The surface treatment greatly extends the operational range of an open tubular emitter to lower flow rates, compared to that of a cleaved capillary with similar dimensions. The stabilized nanoelectrospray is attributed to the increased surface roughness and modified wetting characteristics of the emitter exit resulting from grinding. Electrospray performance was evaluated, and as a result of the enhanced sensitivity from a roughened emitter, five femtomoles of leucine enkephalin were detected at a 50 nL/min flow rate with a signal to noise (S/N) ratio of 48. Furthermore, trypsin-digested bovine serum albumin (BSA) was used to demonstrate the application of the emitter in protein identification, giving a sequence coverage of 60%. These emitters are robust, and may become a facile alternative to tapered emitters at moderate nano flow rates (e.g. 50 to 500 nL/min).

Silanization of inner surfaces of nanoelectrospray ionization emitters for reduced analyte adsorption

During the course of nanoelectrospray ionization (nanoESI) of substance P, an unusual type of signal reduction was observed with flow rates <10 nL/min. This reduction in signal appears to be induced by the adsorption of positively charged analytes onto negatively charged free silanol groups on the inner surface of emitters; analytes with higher pI values (such as substance P) exhibit greater tendency for adsorption. Support for this hypothesis is demonstrated by the decrease in signal reduction in the presence of concentrated salts or for emitters whose internal silanols have been covalently silanized. Emitters treated with hexamethyldisilazane or 3-aminopropyltriethoxysilane showed higher analyte signals for substance P than untreated emitters, suggesting a reduction of analyte adsorption onto the inner walls of silanized emitters. The efficacy of reduced peptide adsorption was demonstrated for emitters silanized with 3-aminopropyltriethoxysilane using a simple peptide mixture as well as a more complex peptide mixture (a tryptic digest of horse hemoglobin).

Capillary-based multi nanoelectrospray emitters: improvements in ion transmission efficiency and implementation with capillary reversed-phase LC-ESI-MS

We describe the coupling of liquid chromatography (LC) separations with mass spectrometry (MS) using nanoelectrospray ionization (nano-ESI) multiemitters. The array of 19 emitters reduced the flow rate delivered to each emitter, allowing the enhanced sensitivity that is characteristic of nano-ESI to be extended to higher flow rate separations. The signal for tryptic fragments from proteins spiked into a human plasma sample increased 11-fold on average when the multiemitters were employed, due to increased ionization efficiency and improved ion transfer efficiency through a newly designed heated multicapillary MS inlet. Additionally, the LC peak signal-to-noise ratio increased approximately 7-fold when the multiemitter configuration was used. The low dead volume of the emitter arrays preserved peak shape and resolution for robust capillary LC separations using total flow rates of 2 microL/min.

Microfluidics for drug discovery and development: from target selection to product lifecycle management

Microfluidic technologies' ability to miniaturize assays and increase experimental throughput have generated significant interest in the drug discovery and development domain. These characteristics make microfluidic systems a potentially valuable tool for many drug discovery and development applications. Here, we review the recent advances of microfluidic devices for drug discovery and development and highlight their applications in different stages of the process, including target selection, lead identification, preclinical tests, clinical trials, chemical synthesis, formulations studies and product management.