Fragmentation of N-oxides (deoxygenation) in atmospheric pressure ionization: investigation of the activation process

Stable isotope labeling for detection of ozonation byproducts in effluent organic matter with FT-ICR-MS

Despite effluent organic matter (EfOM) being a major consumer of ozone during wastewater treatment, little is known about ozonation byproducts (OBPs) produced from EfOM. To unambiguously identify OBPs, heavy ozone was used to ozonate EfOM, resulting in ¹⁸O labeled and unlabeled OBPs. Labeled OBPs mostly represent a single ¹⁸O transfer and were classified as either direct or indirect OBPs based on the ¹⁸O/¹⁶O intensity ratios of the isotopologues. Of the 929 labeled OBPs, 84 were unequivocally classified as direct OBPs. The remainder suggest a major contribution by indirect, hydroxyl radical induced formation of OBPs in EfOM. Overall, labelled OBPs possess a low degree of unsaturation and contributed most to OBP peak intensity - marking them as potential end products. A few direct and indirect OBPs with high peak intensity containing ¹⁸O and heteroatoms (N, S) were fragmented with CID FT-ICR-MS/MS and screened for indicative neutral losses carrying heavy oxygen. The neutral loss screening was used to detect the ¹⁸O location on the OBP and indicate the original functional group in EfOM based on known reaction mechanisms. We identified sulfoxide and sulfonic acid functional groups in selected OBPs - implying the presence of reduced sulfur in EfOM molecules - while no evidence for nitrogen containing functional groups reacting with ozone was found.

Flavin-Dependent Monooxygenase-Mediated 1,2-Oxazine Construction via Meisenheimer Rearrangement in the Biosynthesis of Paeciloxazine

The [1,2]-Meisenheimer rearrangement is well known as the [1,2]-migration of an O-substituted hydroxylamine from a tertiary amine N-oxide, and it is frequently employed in organic synthesis to enforce adjacent carbon oxidation or install a 1,2-oxazine core, which is a prevalent structural feature and pharmacophore of many bioactive natural products. Although the [1,2]-Meisenheimer rearrangement was proposed to occur in the biosynthesis of a number of 1,2-oxazine-containing natural products, it has never been proved biosynthetically. Here, we identified the biosynthetic gene cluster of an insecticidal natural product, paeciloxazine (1), from Penicillium janthinellum and characterized a flavin-dependent monooxygenase, PaxA, as the first example that mediates the formation of a 1,2-oxazine moiety via Meisenheimer rearrangement. In vitro biochemical assays, site-directed mutations, docking and molecular dynamics simulations, and density functional theory calculations support the mechanism that PaxA first catalyzes N-oxidation to form an N-oxide intermediate, which undergoes [1,2]-Meisenheimer rearrangement with the assistance of an amino acid with proton transfer property. This study expands the repertoire of rearrangement reactions during the biosynthesis of natural products and provides a new strategy for discovering natural products with N-O tethers by genome mining.

Optical and mass spectral characterization of the electrospray ionization/corona discharge ionization interface

The interchange between electrospray ionization (ESI) and corona discharge ionization (CDI) with respect to applied bias on the needle is customarily placed at the point where light production begins at the tip of the needle. If a liquid sample is flowing through a needle that is observed to produce light, the ionization process is assumed to be harsher and the term coronaspray ionization has been coined to describe this hybrid ionization mechanism. In this work, the transition between ESI and CDI is investigated with respect to applied bias through optical and mass spectrometric measurements. As a function of applied bias potential, the optical signal at the tip of the needle was recorded simultaneously with the resultant ionization products. In this effort, the production of ions from an electrospray ionization needle has been demonstrated to produce light regardless of bias if ions are also formed. With this understanding, an ESI/CDI needle was designed to allow the bias to be temporarily pulsed over the 'onset' voltage necessary for ionization and the rise and decay of the optical signal was measured. Positive mode CDI onset to a stable discharge state within 0.05 ms, while positive ESI required 1.9 ms to reach a stable condition. In the negative mode, the stability of the ionization process was highly variable in both ESI and CDI modes, though CDI was generally faster to reach the stable mode of operation. When the resultant ions were investigated, the effect of increased bias on an ESI needle was found to be species-dependent. Recognizing that the range of compounds probed was limited, for those examined, it appears that stable, non-labile species may be investigated via ESI under extremely high biases while labile species demonstrate a narrow range of stable biases before significant fragmentation occurs.

LC-ESI-QTOF-MS analysis utilizing gas-phase fragmentation reactions subjected to ESI-IS-CID and ESI-CID-MS/MS conditions to study the degradation behaviour of sorafenib tosylate: NMR and in vitro cytotoxicity and apoptosis detection studies of hydrolytic degradation products

The present study was to investigate the degradation profile of sorafenib tosylate (SORA), a potent oral multi-kinase inhibitor under various stress conditions as per ICH (Q1A (R2)) guidelines. Separation of SORA and its degradation products (DP-1-DP-5) was achieved on Acquity UPLC BEH C18 (100 mm × 2.1 mm × 1.7 μm) column using a gradient elution of 0.1% formic acid and acetonitrile at a flow rate of 0.3 mL/min within 12 min. High resolution quadruple time-of-flight mass spectrometer (Q-TOF/MS) was utilized for characterization of all DPs. In ESI/CID-MS/MS experiments, the protonated DP-1 and DP-2 exhibited few interesting product ions which provide a compelling evidence for the compounds to undergo gas phase rearrangement reaction justified by its mechanistic explanation in support with density functional theory (DFT). In-source collision-induced dissociation (IS-CID) fragmentation using ESI/APCI-MS analysis exhibited the formation of N-deoxygenated product ion peak corresponds to pyridine N-oxide moiety as in DP-5. Further, major hydrolytic DPs (DP-2 and DP-3) were isolated on preparative HPLC and structural elucidation was done using ID NMR (¹H, ¹³C and DEPT-135) experiments. In vitro cytotoxicity study for SORA and its isolated DPs were assessed by observing morphological changes in HepG2 cell lines under phase-contrast microscopy and MTT assay. Taken together, it was known that DP-2 and DP-3 were less potent with a cell viability of more than 90% and IC₅₀ >50 μM in comparison with SORA (IC₅₀ = 2.99 ± 0.35 μM). The developed method was validated in terms of specificity, limit of detection, limit of quantification, linearity, accuracy, precision and robustness.

The degradation of levofloxacin in infusions exposed to daylight with an identification of a degradation product with HPLC-MS

In this paper the decomposition product of levofloxacin was identified. Levofloxacin was dissolved in 0.9% NaCl, 5% glucose, and Ringer's solution. The solutions were divided into two batches: the first one was exposed to daylight and the second one was protected from it. The solutions were stored at the room temperature. The qualitative analysis of the degradation product was performed using MS and TOF detectors. The quantitative assay was done by a validated HPLC method. Visual inspection and pH assessment were done. Levofloxacin protected from daylight remained stable in 0.9% NaCl, 5% dextrose, and Ringer's solution. A slight decomposition of the analyte was observed in the solutions exposed to daylight with the fastest decomposition rate in Ringer's solution as compared with 0.9% NaCl and 5% dextrose solutions. The degradation product of levofloxacin detected with MS was levofloxacin N-oxide. Levofloxacin solutions should be protected from direct daylight to maintain drug stability. Levofloxacin N-oxide is formed regardless of the solvent used.

Identification and characterization of in vitro and in vivo generated metabolites of the adiponectin receptor agonists AdipoRon and 112254

Peroxisome proliferator-activated receptors (PPARs), peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), sirtuin 1 (SIRT1) and adenosine monophosphate-activated protein kinase (AMPK) are regulators of transcriptional processes and effects of exercise and pseudo-exercise situations. Compounds occasionally referred to as endurance exercise mimetics such as AdipoRon and 112254, both adiponectin receptor agonists, can be used to simulate the physiology of endurance exercise via pathways including these transcriptional regulators. Adiponectin supports fatty acid utilization and triglyceride-content reduction in cells and increases both the mitochondrial biogenesis and the oxidative metabolism in muscle cells. In routine doping control analysis, knowledge about phase-I and -II metabolic products of target analytes is essential. Hence, in vitro- and in vivo-metabolism experiments are frequently employed tools in preventive doping research to determine potential urinary metabolites for sports drug testing purposes, especially concerning new, (yet) unapproved compounds. In the present study, in vitro assays were conducted using human liver microsomal and S9 fractions, and rat in vivo experiments were performed using both AdipoRon and 112254. For AdipoRon, obtained samples were analyzed using liquid chromatography-high resolution/high accuracy (tandem) mass spectrometry with both electrospray ionization or atmospheric-pressure chemical ionization techniques. Overall, more than five phase I-metabolites were found in vitro and in vivo, including particularly monohydroxylated and hydrogenated species. No phase II-metabolites were found in vitro; conversely, signals suggesting the presence of glucuronic acid or other conjugates in samples collected from in vivo experiment were observed, the structures of which were however not conclusively identified. Also for 112254, several phase-I metabolites were found in vitro, e.g. monohydroxylated and demethylated species. Here, no phase II-metabolites were observed neither using in vitro nor in vivo samples. Based on the generated data, the implementation of metabolites and unmodified drug candidates into routine doping control protocols is deemed warranted for comprehensive sports drug testing programs until human elimination study data are available.

Multiported pulsed valve interface for a linear quadrupole ion trap mass spectrometer to enable rapid screening of multiple functional-group selective ion-molecule reactions

Ion-molecule reactions provide a powerful tool for structural elucidation of ionized pharmaceutical analytes in tandem mass spectrometry. However, all previous interfaces for the introduction of reagents for ion-molecule reactions have utilized a single reagent approach. In this study, a multiported pulsed valve system was designed and characterized for rapid introduction of three neutral reagents into a linear quadrupole ion trap. Additionally, automatic triggering was used to allow for the introduction of the reagents on a chromatographic time scale. This system enables automatic, high throughput screening of complex mixtures by using at least three different ion-molecule reactions. Further, rapid testing of new neutral reagents is also possible.

Persistence and dissipation pathways of the antidepressant sertraline in agricultural soils

Sertraline is a widely-used antidepressant that is one of the selective serotonin reuptake inhibitors. It has been detected in biosolids and effluents from sewage treatment plants. Since sertraline can reach agriculture land through the application of municipal biosolids or reclaimed water, the persistence and dissipation pathways of (3)H-sertraline were determined in laboratory incubations using three agriculture soils varying in textures and properties. The total solvent extractable radioactivity decreased in all three soils with times to dissipate 50% of material (DT50) ranging from 48.1±3.5 (loam soil) to 84.5±13.8 (clay soil) days. Two hydroxylated sertraline transformation products were identified in all three soils by high performance liquid chromatography with time-of-flight mass spectrometry (HPLC-TOF-MS), but the accumulation did not exceed 10% of the initial parent concentration. The addition of liquid municipal biosolids to the loam soil had no effect on the rate of sertraline dissipation, or production of transformation products. In summary, sertraline was persistent in agricultural soils with major dissipation pathways including the production of non-extractable soil-bound residues, and accumulation of hydroxylated transformation products. The biologically active sertraline transformation product norsertraline was not detected in soil.

Behavior of N-oxide derivatives in atmospheric pressure ionization mass spectrometry

RATIONALE

Indolone-N-oxide derivatives possess interesting biological properties. The analysis of these compounds using mass spectrometry (MS) may lead to interference or under-estimation due to the tendency of the N-oxides to lose oxygen. All the previous works focused only on the temperature of the heated parts (vaporizer and ion-transfer tube) of the mass spectrometer without investigating other parameters. This work is extended to the investigation of other parameters.

METHODS

The behavior of N-oxides during atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) has been investigated using MS(n) ion trap mass spectrometry. Different parameters were investigated to clarify the factors implicated in the deoxygenation process. The investigated parameters were vaporizer temperature (APCI), ion-transfer tube temperature, solvent type, and the flow rates of the sheath gas, auxiliary gas, sweep gas and mobile phase.

RESULTS

The deoxygenation increased when the vaporizer temperature increased. The extent of the 'thermally' induced deoxygenation was inversely proportional to the ion-transfer tube temperature and auxiliary gas flow rate and in direct proportion to the mobile phase flow rate. Deoxygenation was not detected under MS/MS fragmentation and hence it is a non-collision-induced dissociation. N-Oxides have the tendency to form abundant 'non-classical' dimers under ESI, which fragment via dehydration rather than giving their corresponding monomer.

CONCLUSIONS

Deoxygenation is not solely a 'classical' thermal process but it is a thermal process that is solvent-mediated in the source. Deoxygenation was maximal with an APCI source while dimerization was predominant with an ESI source. Therefore, attention should be paid to these molecular changes in the mass spectrometer as well as to the choice of the ionization mode for N-oxides.

Persistence of the tricyclic antidepressant drugs amitriptyline and nortriptyline in agriculture soils

Amitriptyline and nortriptyline are widely used tricyclic antidepressant drugs. They have been detected in wastewater, surface runoff, and effluents from sewage treatment plants. As such, they could potentially reach agriculture land through the application of municipal biosolids or reclaimed water. In the absence of data on their fate in the environment, the persistence and dissipation pathways of radiolabeled amitriptyline were determined in three agriculture soils varying widely in texture and chemical properties (loam soil, clay loam soil, and sandy loam soil). Tritiated amitriptyline was added to laboratory microcosms containing soils, and the metabolism of the extractable (3) H was monitored during incubation at 30°C. The total solvent extractable radioactivity decreased in all three soils with times to dissipate 50% of material (DT50) ranging from 34.1 ± 3.2 (loam soil) to 85.3 ± 3.2 d (sandy soil). Nortriptyline (N-desmethyl amitriptyline) and amitriptyline-N-oxide were identified as major transformation products in all three soils by high performance liquid chromatography with photodiode array detector and time-of-flight mass spectrometry (HPLC-TOF-MS/UV). The addition of liquid municipal biosolids to the loam soil had no effect on the dissipation of amitriptyline. The persistence of nortriptyline was evaluated in the loam soil. The DT50 of nortriptyline was 40.5 ± 3.2 d estimated with HPLC-TOF-MS/UV. Approximately 10% of added nortriptyline was converted to hydroxylated products after 50 d of incubation. In summary, amitriptyline persisted in agricultural soils with major dissipation mechanisms, including forming nonextractable residues and producing various transformation products including the psychoactive drug nortriptyline.

The stability of amitriptyline N-oxide and clozapine N-oxide on treated and untreated dry blood spot cards

Procedures for drug monitoring based on Dried Blood Spot (DBS) sampling are gaining acceptance for an increasing number of clinical and preclinical applications, where ease of use, small sample requirement, and improved sample stability have been shown to offer advantages over blood tube sampling. However, to-date, the vast majority of this work has described the analysis of well characterized drugs. Using amitriptyline, clozapine, and their potentially labile N-oxide metabolites as model compounds, we consider the merits of using DBS for discovery pharmacokinetic (PK) studies where the metabolic fate of test compounds are often unknown. Both N-oxide metabolites reverted to parent compound under standard drying (2hr) and extraction conditions. Card type significantly affected the outcome, with 14% and 22% degradation occurring for clozapine-N-oxide and amitriptyline-N-oxide on a brand of untreated DBS cards, compared to 59 and 88% on a brand of treated DBS cards. Enrichment of the parent compound ex vivo leads to overestimation of circulating blood concentration and inaccurate determination of the PK profile.

Bioanalytical aspects of clinical mass balance studies in oncology

Clinical mass balance studies aim to investigate the absorption, distribution, metabolism and excretion (ADME) of a(n) (often radiolabeled) drug, following a single administration to humans. They are perfectly suited to determine the disposition and major metabolic pathways of a drug, the exposure to the parent drug and its metabolites, and the rate and route of elimination. A mass balance study, however, poses interesting challenges to the analysis of parent drug and metabolites in different biological matrices. Using recent clinical mass balance studies in oncology as an example, this review focuses on the aspects of mass balance studies, from bioanalytical assay development, analysis of clinical samples to reporting of study results. Along the way, it discusses bioanalytical problems and practical solutions.

Thermally induced intramolecular oxygen migration of N-oxides in atmospheric pressure chemical ionization mass spectrometry

N-Oxides are known to undergo three main thermal degradation reactions, namely deoxygenation, Cope elimination (for N-oxides containing a β-hydrogen) and Meisenheimer rearrangement, in atmospheric pressure chemical ionization mass spectrometry (APCI-MS). The ions corresponding to these thermal degradants observed in the ensuing APCI mass spectra have been used to identify N-oxides as well as to determine the N-oxidation site when the analyte contains multiple tertiary amine groups. In this paper, we report a thermally induced oxygen migration from one N-oxide amine to another tert-amine group present in the same molecule through a six-membered ring transition state during APCI-MS analysis. The observed intramolecular oxygen migration resulted in the formation of a new isomeric N-oxide, rendering the results of the APCI-MS analysis more difficult to interpret and potentially misleading. In addition, we observed novel degradation behavior that happened after the Meisenheimer rearrangement of the newly formed N-oxide: a homolytic cleavage of the N-O bond instead of elimination of an aldehyde or a ketone that usually follows the rearrangement. Understanding of these unusual degradation pathways, which have not been reported previously, should facilitate structural elucidation of N-oxides using APCI-MS analysis.

Structural elucidation of N-oxidized clemastine metabolites by liquid chromatography/tandem mass spectrometry and the use of Cunninghamella elegans to facilitate drug metabolite identification

Cunninghamella elegans is a filamentous fungus that has been shown to biotransform drugs into the same metabolites as mammals. In this paper we describe the use of C. elegans to aid the identification of clemastine metabolites since high concentrations of the metabolites were produced and MS(n) experiments were facilitated. The combination of liquid chromatography and tandem mass spectrometry with two different ionization techniques and hydrogen/deuterium exchange were used for structural elucidation of the clemastine metabolites. Norclemastine, four isomers of hydroxylated clemastine, and two N-oxide metabolites were described for the first time in C. elegans incubations. The N-oxidations were confirmed by hydrogen/deuterium exchange and deoxygenation (-16 Da) upon atmospheric pressure chemical ionization mass spectrometry. By MS(n) fragmentation it was concluded that two of the hydroxylated metabolites were oxidized on the methylpyrridyl moiety, one on the aromatic ring with the chloro substituent, and one on the aromatic ring without the chlorine.

An experimental and computational study on the dissociation behavior of hydroxypyridine N-oxides in atmospheric pressure ionization mass spectrometry

A tandem mass spectrometric study of protonated isomeric hydroxypyridine N-oxides was carried out with a hybrid quadrupole/time-of-flight mass spectrometer coupled with different atmospheric pressure ionization sources. The behavior observed in the collision-induced dissociation (CID) mass spectra of the parent cations, was similar irrespective of the source employed. However, there were intrinsic differences in the intensities of the two fragments observed for each isomer. The major fragment because of elimination of a hydroxyl radical, dominated the CID spectra (in contrast with weaker water loss) at different energy thresholds. Therefore, it was possible to differentiate both isomers at collision energies above 13 eV by comparing the ratio of intensities of the major fragment relative to the precursor cation. In addition, quantum chemical calculations at the B3LYP/6-31 + + G(d,p) level of theory were performed for the protonated isomers of hydroxypyridine N-oxide and their radical cation products in order to gain insight into the major routes of dissociation. The results suggest that dissociation from the lowest triplet excited state of the protonated species would provide a reasonable rationalization for the difference in behavior of both isomers.

Decomposition of N-hydroxylated compounds during atmospheric pressure chemical ionization

N-Hydroxylated polyamine derivatives were found to decompose during the ionization process of liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry (LC-APCI-MS) experiments. The phenomenon was studied with a model compound, a synthetic N-hydroxylated tetraamine derivative. It was found that reduction, oxidation and water elimination occurred during APCI to generate the corresponding amine, N-oxide, and imine. The investigation further revealed that decomposition of hydroxylamines during APCI depends upon the concentration of the analyte and on the acidity of the solution introduced into the ionization source. The pH-dependence of decomposition was utilized for the development of an MS method that allows for the unambiguous identification of N-OH functionalities. This method was applied for the study of natural products including polyamine toxins from the venom of the spider Agelenopsis aperta and mayfoline, a cyclic polyamine derivative of the shrub Maytenus buxifolia.

Liquid chromatography/tandem mass spectrometry utilizing ion-molecule reactions and collision-activated dissociation for the identification of N-oxide drug metabolites

A liquid chromatography/tandem mass spectrometry (LC/MS(3)) method based on ion-molecule reactions and collision-activated dissociation (CAD) is presented for the identification of analytes with the N-oxide functional group directly in mixtures. Tri(dimethylamino)borane (TDMAB) rapidly and selectively derivatizes protonated N-oxides in a modified commercial linear quadrupole ion trap (LQIT) mass spectrometer to yield a distinct product ion (adduct-(CH(3))(2)NH). The LQIT was outfitted with an external reagent-mixing manifold that allows TDMAB to be mixed with the helium buffer gas used in the trap. The derivatized analytes are readily identified on the basis of a shift of 98 Th (Thomson) relative to the m/z value of the protonated analyte. Further probing of the derivatized analytes via isolation followed by CAD can be used to confirm the presence of an N-oxide, and distinguish between aliphatic and aromatic tertiary N-oxides. Since the ion-molecule reaction is fast, these experiments can be accomplished on the same time scale as typical CAD-based MS(n) experiments, thus maintaining the duty cycle of the instrument for this type of experiment. To demonstrate real world applicability, the method was tested on real active pharmaceutical ingredients and their derivatives.

Characterization of imatinib metabolites in rat and human liver microsomes: differentiation of hydroxylation from N-oxidation by liquid chromatography/atmospheric pressure chemical ionization mass spectrometry

In vitro metabolism of imatinib was investigated in rat and human liver microsomes. Atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) was applied in differentiating hydroxyl metabolites from N-oxides of imatinib because N-oxides are known to undergo deoxygenation during APCI. In addition, the major oxidative metabolite (M9, N-oxidation on the piperazine ring) was observed to undergo in-source fragmentation by elimination of formaldehyde. This fragment ion resulted from Meisenheimer rearrangement with migration of the N-methyl group to the corresponding N-methoxyl piperazine, followed by elimination of formaldehyde due to thermal energy activation at the vaporizer of APCI source. The presence of this fragment ion distinguished not only N-oxide from isomeric hydroxylated metabolite, but also unambiguously indicated that oxidation occurred on the N-4 of the piperazine ring where the methyl group was attached.

Characterization of metabolites of a novel histamine H(2)-receptor antagonist, lafutidine, in human liver microsomes by liquid chromatography coupled with ion trap mass spectrometry

The metabolism of lafutidine in human liver microsomes was studied using liquid chromatography/ion trap mass spectrometry with electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) sources. A total of 14 metabolites were identified including hydroxylated lafutidine and sulfonyl lafutidine as the major metabolites. The chemical properties and the MS(n) behaviors of lafutidine and all of its identified metabolites were studied in detail. Lafutidine had a fragmentation pattern as a result of homolytic bond cleavage in the MS/MS spectrum. This cleavage can form an odd-electron ion with the loss of furan-2-ylmethyl radical (-81 Da with a proton shift), which then sequentially loses neutral groups in the MS(3) spectrum. This fragmentation sequence was also observed from the metabolites with the unchanged sulfinyl moiety. When the sulfinyl moiety was oxidized to the sulfonyl moiety, this fragmentation sequence did not exist, which could be used to identify S-oxidation metabolites of lafutidine. In general, N-oxides could produce distinct [M+H-O](+) ions under LC/APCI-MS due to the thermal activation in the desolvation region of the API source, which could be used to identify N-oxidation metabolites of lafutidine. In order to avoid the possibility of false positives, the MS/MS spectrum of the [M+H-O](+) ion was compared with that of the non-N-oxidation metabolites or parent drug in the APCI source. If they were consistent, the structure could be finally confirmed. The exact masses for lafutidine and lafutidine N-oxide fragment ions were determined using an LTQ/Orbitrap mass spectrometer.

Identification of human liver cytochrome P450 enzymes involved in biotransformation of vicriviroc, a CCR5 receptor antagonist

Vicriviroc (SCH 417690), a CCR5 receptor antagonist, is currently under investigation for the treatment of human immunodeficiency virus infection. The objective of this study was to identify human liver cytochrome P450 enzyme(s) responsible for the metabolism of vicriviroc. Human liver microsomes metabolized vicriviroc via N-oxidation (M2/M3), O-demethylation (M15), N,N-dealkylation (M16), N-dealkylation (M41), and oxidation to a carboxylic acid metabolite (M35b/M37a). Recombinant human CYP3A4 catalyzed the formation of all these metabolites, whereas CYP3A5 catalyzed the formation of M2/M3 and M41. CYP2C9 only catalyzed the formation of M15. There was a high correlation between the rates of formation of M2/M3, M15, and M41, which was determined using 10 human liver microsomal samples and testosterone 6beta-hydroxylation catalyzed by CYP3A4/5 (r > or = 0.91). Ketoconazole and azamulin (inhibitors of CYP3A4) were potent inhibitors of the formation of M2/M3, M15, M41, and M35b/M37a from human liver microsomes. A CYP3A4/5-specific monoclonal antibody (1 microg/microg of protein) inhibited the formation of all metabolites from human liver microsomes by 86 to 100%. The results of this study suggest that formation of the major vicriviroc metabolites in human liver microsomes is primarily mediated via CYP3A4. CYP2C9 and CYP3A5 most likely play a minor role in the biotransformation of this compound. These enzymology data will provide guidance to design clinical studies to address any potential drug-drug interactions.

Tracking problems and possible solutions in the quantitative determination of small molecule drugs and metabolites in biological fluids using liquid chromatography–mass spectrometry

During the last decade, quantification of low molecular weight molecules using liquid chromatography-tandem mass spectrometry in biological fluids has become a common procedure in many preclinical and clinical laboratories. This overview highlights a number of issues involving "small molecule drugs", bioanalytical liquid chromatography-tandem mass spectrometry, which are frequently encountered during assay development. In addition, possible solutions to these issues are proposed with examples in some of the case studies. Topics such as chromatographic peak shape, carry-over, cross-talk, standard curve non-linearity, internal standard selection, matrix effect, and metabolite interference are presented. Since plasma is one of the most widely adopted biological fluid in drug discovery and development, the focus of this discussion will be limited to plasma analysis. This article is not intended to be a comprehensive overview and readers are encouraged to refer to the citations herein.

The tyrosine kinase inhibitor PD153035: implication of labeling position on radiometabolites formed in vitro

INTRODUCTION

The epidermal growth factor receptor is highly expressed in several types of cancers. Molecules with high affinity to its intracellular tyrosine kinase domain are being developed as in vivo imaging probes. The 4-anilinoquinazoline PD153035 has promising in vitro and in vivo properties for development as a reversible radioligand. Labeling it with carbon-11 in either of its two methoxy positions can potentially give rise to different radiometabolites and, consequently, different imaging capabilities. An evaluation of the radiotracers' metabolism was needed to determine the potential significance of the labeling position.

METHODS

PD153035 was labeled in the 6- and 7-O-methoxy positions by reacting the corresponding O-desmethyl precursors with [(11)C]methyl iodide. The two radiolabeled compounds were each incubated for 1 h with human and rat liver microsomes. At five time points, the radiolabeled metabolites were examined using radio-liquid chromatography. One metabolite was isolated and subjected to mass spectroscopic analysis.

RESULTS

A major polar metabolite was obtained in all incubations. Its molecular weight was consistent with an addition of oxygen, and its fragmentation was consistent with an N-oxidation rather than an aromatic hydroxylation. Regioselective 7-O-dealkylation was also observed, albeit in substantial amounts only in the assay using human microsomes.

CONCLUSIONS

Radiolabeling in the 7-O-methoxy position is advocated, since the labeled metabolites produced in the 7-O-demethylation are polar and probably rapidly cleared. The differences observed in the incubations with rat and human microsomes suggest that in vivo positron emission tomography studies with (11)C-labeled PD153035 in rodents may not be directly predictive for studies in humans.

Applications of LC/MS in structure identifications of small molecules and proteins in drug discovery

With advancements in ionization methods and instrumentation, liquid chromatography/mass spectrometry (LC/MS) has become a powerful technology for the characterization of small molecules and proteins. This article will illustrate the role of LC/MS analysis in drug discovery process. Examples will be given on high-throughput analysis, structural analysis of trace level impurities in drug substances, identification of metabolites, and characterization of therapeutic protein products for process improvement. Some unique MS techniques will also be discussed to demonstrate their effectiveness in facilitating structural identifications.

Investigating the in vitro metabolism of fipexide: characterization of reactive metabolites using liquid chromatography/mass spectrometry

The in vitro metabolism of the nootropic drug fipexide was studied using different liquid chromatography/mass spectrometry (LC/MS) techniques. This drug has been withdrawn from the market due to toxic effects. No previous reports have investigated the possible involvement of reactive metabolites in the toxicity of fipexide. The hydrolysis of this drug leads to the formation of two potentially toxic species, 3,4-methylenedioxybenzylpiperazine (MDBP) and 4-chlorophenoxyacetic acid (4-CPA). Here, we investigate the in vitro metabolism of fipexide in human, rat, mouse and dog, as well as of MDBP and 4-CPA in human and rat, while focusing on the formation of reactive metabolites. A combination of LC/MS analyses on a hybrid quadrupole-linear ion trap instrument and accurate mass data from QqTOF measurements was employed for the characterization of these metabolites. Microsomal metabolites of fipexide were MDBP, 4-CPA, fipexide N-oxide or hydroxyl, demethylenated fipexide and other minor ones, all of which were investigated by tandem mass spectrometry. Reactive metabolites were detected using several trapping procedures with small molecules such as glutathione, its ethyl ester derivative and N-acetylcysteine. The demethylenated metabolite, a catechol, formed its corresponding ortho-quinone, which readily reacts with these nucleophiles. MDBP was studied in a similar manner, due to its ability to form an analogous catechol. Because of its acidic nature, 4-CPA was assessed for possible acylglucuronide and acyl-CoA thioester metabolites, which could also be involved in bioactivation pathways. Several important metabolites were identified as potential mediators of toxicity via protein binding.

Structural characterization of sulfadiazine metabolites using H/D exchange combined with various MS/MS experiments

Two major metabolites and one minor metabolite of sulfadiazine were found in pig manure, using a special combination of different MS techniques like parent and product ion scans, H/D exchange, accurate mass measurement, and MS/MS experiments with substructures. N4-acetylsulfadiazine and 4-hydroxysulfadiazine were identified as major metabolites. N4-acetylsulfadiazine could be verified by H/D exchange and comparison with product ion spectra of a synthetic reference compound. In the case of 4-hydroxysulfadiazine, the majority of possible isomers could be discounted after H/D exchange. Substructure-specific MS/MS experiments with fragment ions and comparison with product ion spectra of two references revealed the presence of 4-hydroxysulfadiazine. The minor metabolite was characterized to some degree using H/D exchange and tandem mass spectrometry in combination with a high-resolution time of flight mass spectrometer. The aminopyrimidine moiety contained an additional modification with a likely elemental composition of C2H4O and no further acidic hydrogen.

Compound Screening for the Presence of the Primary N-Oxide Functionality via Ion−Molecule Reactions in a Mass Spectrometer

A mass spectrometry method is presented for the identification of compounds that contain the primary N-oxide functional group. This method utilizes a gas-phase ion-molecule reaction with dimethyl disulfide that rapidly and selectively derivatizes the protonated primary N-oxide functional group in a mass spectrometer to yield an ionic reaction product (with 31 Da higher mass than that of the protonated molecule) that is diagnostic for the presence of a primary N-oxide functionality. A variety of protonated analytes containing different functional groups were tested in Fourier transform ion-cyclotron resonance and triple quadrupole mass spectrometers to probe the selectivity of the reaction. Only molecules containing the protonated primary N-oxide functional group yielded the diagnostic reaction product; all other protonated molecules gave protonated dimethyl disulfide or no reaction products. The feasibility of this method for compound screening was tested by examining six analytes with the same molecular formula but different atom connectivity. The one analyte that contained the primary N-oxide functional group was readily differentiated from the other analytes.

Thermally Induced N-to-O Rearrangement of tert-N-Oxides in Atmospheric Pressure Chemical Ionization and Atmospheric Pressure Photoionization Mass Spectrometry: Differentiation of N-Oxidation from Hydroxylation and Potential Determination of N-Oxidation Site

N-Oxides are known to undergo deoxygenation during atmospheric pressure chemical ionization (Ramanathan, R.; Su, A.-D.; Alvarez, N.; Blumenkrantz, N.; Chowdhury, S. K.; Alton, K.; Patrick, J. Anal. Chem. 2000, 72, 1352-1359) resulting from thermal energy activation at the vaporizer of the APCI source. In addition to deoxygenation, tert-N-oxides containing an alkyl or benzyl group on the N-oxide nitrogen also undergo an N-R to O-R rearrangement (Meisenheimer arrangement, where R = alkyl or benzyl), followed by elimination of an aldehyde (or a ketone) through an internal hydrogen transfer. This has been observed under both atmospheric pressure chemical ionization and atmospheric pressure photoionization conditions. These fragment ions were not observed in the product ion spectra from the protonated molecules of the corresponding N-oxides. The elimination of an aldehyde or a ketone, thus, results from thermal energy activation at the vaporizer and is not induced by collisional activation. These fragmentations not only distinguish N-oxides from isomeric hydroxylated metabolites but also provide a potential way to determine the position of N-oxidation when a metabolite (or molecule) contains multiple N-oxidation sites that are in different chemical environments.

Strategies for characterization of drug metabolites using liquid chromatography–tandem mass spectrometry in conjunction with chemical derivatization and on-line H/D exchange approaches

Strategies using high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) in conjunction with techniques such as chemical derivatization and on-line hydrogen/deuterium (H/D) exchange for structural elucidation of drug metabolites in crude samples are reviewed. Useful mass spectrometric scan techniques discussed include product ion scan, constant neutral-loss scan, precursor ion scan, multistage MS(n), and accurate mass measurements. In biological systems, xenobiotics are transformed into metabolites, which usually involves introduction of one or more polar functional groups or removal or blockage of such structural moieties. Therefore, chemical derivatization strategies for determination of functional groups and on-line H/D exchange approaches for probing number of exchangeable hydrogens are powerful tools for structural elucidation of drug metabolites in drug metabolism studies. More importantly, these experiments can be carried out on crude samples in microscale, providing sufficient material for LC-MS/MS analysis. Therefore, labor intensive and technically challenging purification of low levels of drug metabolites from complex biological matrices can be avoided. It is the authors' conclusion that strategies such as chemical derivatization and on-line H/D exchange should be used more routinely in drug metabolism studies in order to facilitate metabolite identification.

N-Deethylation and N-oxidation of etamiphylline: identification of etamiphylline-N-oxide in greyhound urine by high performance liquid chromatography–mass spectrometry

Millophyline-V, (etamiphylline camsylate) was administered intramuscularly to two racing greyhounds at a dose of 10 mg kg(-1). Unhydrolysed pre- and post-administration urine samples were extracted using mixed mode solid phase extraction (SPE) cartridges, the basic isolates derivatised as trimethylsilyl ethers and analysed by positive ion electron ionisation gas chromatography-mass spectrometry (GC/EI+/MS). The parent drug and one metabolite, N-desethyletamiphylline, were detected in urine for up to 72 h. For semi-quantification, urine samples were extracted on-line using a Prospekt sample handler. The analytes retained on the C2 SPE cartridge were eluted by the mobile phase directly on to the analytical high performance liquid chromatography column and analysed by positive ion atmospheric pressure chemical ionisation (LC/APCI+) MS in the multiple selective-ion recording mode. A major peak containing both ions (m/z) 280 and (m/z) 252 was observed. Full scan LC/APCI+/MS of the unknown indicated that the ion at (m/z) 280 was formed by the loss of an oxygen atom [MH+ -->(MH+-O)]. Samples were analysed by positive ion electrospray ionisation LC/MS on two different instruments and the unknown compound was identified as an N-oxide of the tert. nitrogen atom of the 2-(diethylamino)ethyl substituent on N7 of the theophylline nucleus. This compound has not been reported previously either as an in vivo or in vitro metabolite of etamiphylline in any species. Thermal decomposition of the N-oxide could lead to an increase the detection period of the parent drug during routine GC/MS screening of post-competition greyhound urine samples.

Characterisation of sulphoxides by atmospheric pressure ionisation mass spectrometry

An observation that a series of proprietary compounds containing a methyl thiophenyl group all underwent metabolic S-oxidation, and that the product ion spectra of the resulting S-oxides showed methyl radical loss under low-energy atmospheric pressure ionisation tandem mass spectrometry (API-MS/MS) conditions, has led to an investigation of the fragmentation of commercially available sulphoxides. The phenyl methyl sulphoxides studied do lose methyl radicals under MS/MS conditions on triple quadrupole mass spectrometers. In addition, the phenyl sulphoxides, with simple substituents other than a methyl group, also showed a tendency to lose the substituent as a radical. It was concluded that radical loss from these simple sulphoxides was characteristic of S-oxidation of these molecules. Radical losses, such as those reported here, are used in-house to distinguish S-oxidation from N- and C-oxidation in metabolism studies.

Pyridine N-oxide and pyridine-d5 N-oxide: an electrospray/tandem mass spectrometric study carried out at high mass resolution

A mass spectrometric study of pyridine N-oxide and pyridine-d5 N-oxide was carried out with a hybrid quadrupole/time-of-flight (TOF) mass spectrometer coupled with an electrospray (ES) source. In addition to the observation of protonated, sodiated, and proton-bound dimers of pyridine N-oxide and pyridine-d5 N-oxide, mass scans revealed the presence of several doubly-charged ion species. Doubly-charged ions of m/z 191 were identified as diprotonated tetramers of pyridine N-oxide; a structure has been proposed for the diprotonated tetramer and its energy relative to that of protonated pyridine N-oxide has been obtained from geometry optimizations. The principal ion species observed were subjected to collision-induced dissociation; accurate mass measurements were made of each fragment ion so as to determine its elemental composition. On the basis of mass spectrometric evidence, it is suggested that dissociation of pyridine N-oxide may occur during the ES process and the resulting fragments become embedded in doubly-charged ions. The proton affinity for both pyridine N-oxide and pyridine-d5 N-oxide was calculated; the difference between these proton affinities was compared with an experimentally determined difference between the proton affinities of pyridine N-oxide and pyridine-d5 N-oxide.

METABOLISM AND EXCRETION OF CAPRAVIRINE, A NEW NON-NUCLEOSIDE REVERSE TRANSCRIPTASE INHIBITOR, ALONE AND IN COMBINATION WITH RITONAVIR IN HEALTHY VOLUNTEERS

Metabolism and disposition of capravirine, a new non-nucleoside reverse transcriptase inhibitor, were studied in healthy male volunteers who were randomly divided into two groups (A and B) with five subjects in each group. Group A received a single oral dose of [(14)C]capravirine (1400 mg) and group B received multiple oral doses of ritonavir (100 mg), followed by a single oral dose of [(14)C]capravirine (1400 mg). Mean total recoveries of radioactivity for groups A and B were 86.3% and 79.0%, respectively, with a mean cumulative recovery in urine comparable with that in feces for both groups. Excretion of unchanged capravirine was negligible in urine and low in feces for both groups. The results suggest that capravirine was well absorbed, with metabolism as the principal mechanism of clearance. Capravirine underwent extensive metabolism to a variety of metabolites via oxygenations (mono-, di-, tri-, and tetra-) representing the predominant pathway, glucuronidation, and sulfation in humans. No useful plasma profiles of group A were obtained due to extremely low levels of plasma radioactivity. Analysis of group B plasma indicated that unchanged capravirine was the major radiochemical component, with three monooxygenated products and a glucuronide of capravirine as the major circulating metabolites. Nineteen metabolites were identified using liquid chromatography-multistage ion-trap mass spectrometry methodologies. In summary, coadministration of low-dose ritonavir (a potent CYP3A4 inhibitor) drastically decreased the levels of sequential oxygenated metabolites and markedly increased the levels of the parent drug and primary oxygenated metabolites overall in plasma, urine, and feces.

Bioanalytical method validation: a risk-based approach?

Bioanalysis frequently involves the measurement of very low analyte concentrations in complex and potentially variable matrices. It is not possible to test in validation every possible circumstance that may be encountered when analyzing study samples; logically, therefore, some risk of obtaining erroneous results exists when validated methods are applied to study samples. An initial attempt has been made to apply a risk management tool to the bioanalytical situation, with the hope that this will stimulate further discussion on the idea of more formally addressing "risk" with regards to bioanalytical method validation.

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

To support pharmacokinetic and drug metabolism studies, LC-MS/MS plays more and more an essential role for the quantitation of drugs and their metabolites in biological matrices. With the new challenges encountered in drug discovery and drug development, new strategies are put in place to achieve high-throughput analysis, using serial and parallel approaches. To speed-up method development and validation, generic approaches with the direct injection of biological fluids is highly desirable. Column-switching, using various packing materials for the extraction columns, is widely applied. Improvement of mass spectrometers performance, and in particular triple quadrupoles, also strongly influences sample preparation strategies, which remain a key element in the bioanalytical process.

In vivo and in vitro disposition profile of tezosentan, an intravenous dual endothelin receptor antagonist, in humans

1. The plasma concentration-time profile of the (14)C-labelled endothelin receptor antagonist tezosentan in healthy male volunteers after a 1-h infusion at 100 mg h(-1) followed a biphasic decline with half-lives of 3-5 min for the initial disposition phase and approximately 4 h for the terminal phase. 2. Tezosentan was predominantly excreted unchanged into faeces, whereas less than 5% of the dose was excreted as unchanged drug in urine. Two isomeric, hydroxylated metabolites (M1, M2) were detected in faeces representing 2-5% of the total radioactivity. 3. In vitro, with human liver microsomes and primary hepatocytes, tezosentan was metabolized at very low rates. Upon prolonged incubation with human hepatocytes for 24 h, formation of the hydroxylated metabolite M1 and a glucuronic acid conjugate, M3, was observed. 4. No relevant inhibition of the human cytochrome P450 (CYP) forms, CYP1A2, 2C9, 2C19, 2D6 and 3A4, was observed in vitro at tezosentan concentrations largely exceeding those observed in clinical trials. 5. In human blood, tezosentan was highly bound to plasma proteins, mainly albumin, and hardly penetrated into red blood cells.

Liquid chromatography/atmospheric pressure ionization-mass spectrometry in drug metabolism studies

The study of the metabolic fate of drugs is an essential and important part of the drug development process. The analysis of metabolites is a challenging task and several different analytical methods have been used in these studies. However, after the introduction of the atmospheric pressure ionization (API) technique, electrospray and atmospheric pressure chemical ionization, liquid chromatography/mass spectrometry (LC/MS) has become an important and widely used method in the analysis of metabolites owing to its superior specificity, sensitivity and efficiency. In this paper the feasibility of LC/API-MS techniques in the identification, structure characterization and quantitation of drug metabolites is reviewed. Sample preparation, LC techniques, isotope labeling, suitability of different MS techniques, such as tandem mass spectrometry, and high-resolution MS in drug metabolite analysis, are summarized and discussed. Automation of data acquisition and interpretation, special techniques and possible future trends are also the topics of the review.

Collection of selected reaction monitoring and full scan data on a time scale suitable for target compound quantitative analysis by liquid chromatography/tandem mass spectrometry

A hybrid linear ion trap/triple quadrupole mass spectrometer was used to demonstrate the value of collecting full scan qualitative data during quantitative analysis of target compounds. We present examples of the additional information that can be obtained from plasma samples analyzed primarily for target compound concentrations. This information includes detection of circulating metabolites, dosing vehicle, interfering matrix components, and potential interfering drug conjugates. Additionally, the quantitative results from selected reaction monitoring (SRM) analysis and from combined full scan and SRM analysis (SRM/EMS) were compared. The quantitative data in both scan modes are acceptable in terms of sensitivity, accuracy and precision. One can conclude from this work that the hybrid linear ion trap/triple quadrupole mass analyzer can provide in a single analysis both useful qualitative data, and accurate and precise quantitative data from the samples routinely prepared and analyzed for target drug concentrations.