Volumetric absorptive microsampling as an effective microsampling technique for LC-MS/MS bioanalysis of biomarkers in drug discovery

Aim: Develop and validate a volumetric absorptive microsampling (VAMS)-based LC-MS/MS method to support the bioanalysis of amino acid and carboxylic acid biomarkers in mouse whole blood. Method: Mouse whole blood was collected using a 10 μl VAMS device. The analytes in VAMS were extracted and analyzed using an LC-MS/MS method. Results: The VAMS-based LC-MS/MS assay exhibited a linearity range of 10.0-10,000 ng/ml with acceptable precision and accuracy and consistent recovery. The analyte stability in mouse whole blood VAMS was shown for 7 days at ambient conditions and at -80°C, as well as with three freeze/thaw cycles. Conclusion: A simple and robust VAMS-based LC-MS/MS method was developed and further validated for simultaneous bioanalysis of nine biomarkers in mouse whole blood.

Reconstruction of Glutathione Metabolism in the Neuronal Model of Rotenone-Induced Neurodegeneration Using Mass Isotopologue Analysis with Hydrophilic Interaction Liquid Chromatography-Zeno High-Resolution Multiple Reaction Monitoring

Accurate reconstruction of metabolic pathways is an important prerequisite for interpreting metabolomics changes and understanding the diverse biological processes in disease models. A tracer-based metabolomics strategy utilizes stable isotope-labeled precursors to resolve complex pathways by tracing the labeled atom(s) to downstream metabolites through enzymatic reactions. Isotope enrichment analysis is informative and achieved by counting total labeled atoms and acquiring the mass isotopologue distribution (MID) of the intact metabolite. However, quantitative analysis of labeled metabolite substructures/moieties (MS² fragments) can offer more valuable insights into the reaction connections through measuring metabolite transformation. In order to acquire the isotopic labeling information at the intact metabolite and moiety level simultaneously, we developed a method that couples hydrophilic interaction liquid chromatography (HILIC) with Zeno trap-enabled high-resolution multiple reaction monitoring (MRM^HR). The method enabled accurate and reproducible MID quantification for intact metabolites as well as their fragmented moieties, with notably high sensitivity in the MS² fragmentation mode based on the measurement of ¹³C- or ¹⁵N-labeled cellular samples. The method was applied to human-induced pluripotent stem cell-derived neurons to trace the fate of ¹³C/¹⁵N atoms from D-¹³C₆-glucose/L-¹⁵N₂-glutamine added to the media. With the MID analysis of both intact metabolites and fragmented moieties, we validated the pathway reconstruction of de novo glutathione synthesis in mid-brain neurons. We discovered increased glutathione oxidization from both basal and newly synthesized glutathione pools under neuronal oxidative stress. Furthermore, the significantly decreased de novo glutathione synthesis was investigated and associated with altered activities of several key enzymes, as evidenced by suppressed glutamate supply via glucose metabolism and a diminished flux of glutathione synthetic reaction in the neuronal model of rotenone-induced neurodegeneration.

High-throughput proteomics of nanogram-scale samples with Zeno SWATH MS

The possibility to record proteomes in high throughput and at high quality has opened new avenues for biomedical research, drug discovery, systems biology, and clinical translation. However, high-throughput proteomic experiments often require high sample amounts and can be less sensitive compared to conventional proteomic experiments. Here, we introduce and benchmark Zeno SWATH MS, a data-independent acquisition technique that employs a linear ion trap pulsing (Zeno trap pulsing) to increase the sensitivity in high-throughput proteomic experiments. We demonstrate that when combined with fast micro- or analytical flow-rate chromatography, Zeno SWATH MS increases protein identification with low sample amounts. For instance, using 20 min micro-flow-rate chromatography, Zeno SWATH MS identified more than 5000 proteins consistently, and with a coefficient of variation of 6%, from a 62.5 ng load of human cell line tryptic digest. Using 5 min analytical flow-rate chromatography (800 µl/min), Zeno SWATH MS identified 4907 proteins from a triplicate injection of 2 µg of a human cell lysate, or more than 3000 proteins from a 250 ng tryptic digest. Zeno SWATH MS hence facilitates sensitive high-throughput proteomic experiments with low sample amounts, mitigating the current bottlenecks of high-throughput proteomics.

Relation of plasma ceramides to visceral adiposity, insulin resistance and the development of type 2 diabetes mellitus: the Dallas Heart Study

AIMS/HYPOTHESIS

Ceramides are sphingolipids that contribute to insulin resistance in preclinical studies. We hypothesised that plasma ceramides would be associated with body fat distribution, insulin resistance and incident type 2 diabetes in a multi-ethnic cohort.

METHODS

A total of 1557 participants in the Dallas Heart Study without type 2 diabetes underwent measurements of metabolic biomarkers, fat depots by MRI and plasma ceramides by liquid chromatography-mass spectrometry. Diabetes outcomes were assessed after 7 years. Associations of body fat and insulin resistance with ceramides at baseline and of ceramides with incident diabetes outcomes were analysed.

RESULTS

The cohort had a mean age of 43 years, with 58% women, 45% black participants and a mean BMI of 28 kg/m². Total cholesterol levels were associated with all ceramides, but higher triacylglycerols and lower HDL-cholesterol and adiponectin were associated only with saturated fatty acid chain ceramides (p < 0.0003). After adjusting for clinical characteristics and total body fat, visceral adipose tissue was positively associated with saturated fatty acid ceramides (per SD, β = 0.16 to 0.18) and inversely associated with polyunsaturated fatty acid ceramides (β = -0.14 to -0.16, p < 0.001 for all). Lower-body subcutaneous fat showed an opposite pattern to that for visceral fat. HOMA-IR was positively associated with saturated (β = 0.08 to 0.09, p < 0.001) and inversely with polyunsaturated ceramides (β = -0.06 to -0.07, p < 0.05). Ceramides were not associated with incident type 2 diabetes after adjustment for clinical factors.

CONCLUSIONS/INTERPRETATION

Plasma ceramides demonstrate a biologically complex relationship with metabolic and imaging indicators of dysfunctional adiposity. The role of ceramides in a shared pathway of metabolic dysfunction linking visceral adiposity and insulin resistance requires further investigation.

Cyp8b1 ablation prevents Western diet-induced weight gain and hepatic steatosis because of impaired fat absorption

Bile acids (BAs) are cholesterol derivatives that regulate lipid metabolism, through their dual abilities to promote lipid absorption and activate BA receptors. However, different BA species have varying abilities to perform these functions. Eliminating 12α-hydroxy BAs in mice via Cyp8b1 knockout causes low body weight and improved glucose tolerance. The goal of this study was to determine mechanisms of low body weight in Cyp8b1^-/- mice. We challenged Cyp8b1^-/- mice with a Western-type diet and assessed body weight and composition. We measured energy expenditure, fecal calories, and lipid absorption and performed lipidomic studies on feces and intestine. We investigated the requirement for dietary fat in the phenotype using a fat-free diet. Cyp8b1^-/- mice were resistant to Western diet-induced body weight gain, hepatic steatosis, and insulin resistance. These changes were associated with increased fecal calories, due to malabsorption of hydrolyzed dietary triglycerides. This was reversed by treating the mice with taurocholic acid, the major 12α-hydroxylated BA species. The improvements in body weight and steatosis were normalized by feeding mice a fat-free diet. The effects of BA composition on intestinal lipid handling are important for whole body energy homeostasis. Thus modulating BA composition is a potential tool for obesity or diabetes therapy.

Abstract 186: Cyp8b1 Ablation Prevents Western Diet-Induced Weight Gain and Hepatic Steatosis due to Impaired Fat Absorption

Bile acids (BAs) are cholesterol derivatives that are well-known for their role in facilitating intestinal lipid absorption. Furthermore, they can regulate glucose and lipid metabolism by activating nuclear and cell surface receptors. Insulin resistance is correlated with alterations in bile acid composition, in particular higher levels of 12&#945;-hydroxylated BAs. These BA species are generated by the enzyme Cyp8b1. We hypothesized that elevated levels of 12&#945;-hydroxylated BAs could be a link between insulin resistance and defects of lipid metabolism. To study the role of Cyp8b1 in the regulation of lipid metabolism, we used Cyp8b1 deficient mice-which are unable to produce 12&#945;-hydroxy BAs-and challenged them with a western type diet. We found that Cyp8b1-/- mice gained less weight compared to controls, which was entirely accounted for by fat mass. Triglyceride and cholesterol accumulation in the liver of Cyp8b1-/- mice were also strongly reduced. We found that these improvements were due to reduced lipid absorption in the intestine of Cyp8b1-/- mice, which could be rescued by replenishing the pool with taurocholic acid. The lipid malabsorption resulted in higher caloric excretion in the feces, due to excess excretion of hydrolyzed dietary lipids. When we fed the mice with a fat-free diet, these differences between genotypes were normalized, confirming the central role of BA composition-not just overall levels-in intestinal lipid absorption and whole-body lipid homeostasis. Based on these findings, it is possible that reducing 12&#945;-hydroxy BAs could be a therapeutic option for the control of obesity and lipid homeostasis.

The MBOAT7-TMC4 Variant rs641738 Increases Risk of Nonalcoholic Fatty Liver Disease in Individuals of European Descent

BACKGROUND & AIMS

Nonalcoholic fatty liver disease (NAFLD) is a leading cause of liver damage and is characterized by steatosis. Genetic factors increase risk for progressive NAFLD. A genome-wide association study showed that the rs641738 C>T variant in the locus that contains the membrane bound O-acyltransferase domain-containing 7 gene (MBOAT7, also called LPIAT1) and transmembrane channel-like 4 gene (TMC4) increased the risk for cirrhosis in alcohol abusers. We investigated whether the MBOAT7-TMC4 is a susceptibility locus for the development and progression of NAFLD.

METHODS

We genotyped rs641738 in DNA collected from 3854 participants from the Dallas Heart Study (a multi-ethnic population-based probability sample of Dallas County residents) and 1149 European individuals from the Liver Biopsy Cross-Sectional Cohort. Clinical and anthropometric data were collected, and biochemical and lipidomics were measured in plasma samples from participants. A total of 2736 participants from the Dallas Heart Study also underwent proton magnetic resonance spectroscopy to measure hepatic triglyceride content. In the Liver Biopsy Cross-Sectional Cohort, a total of 1149 individuals underwent liver biopsy to diagnose liver disease and disease severity.

RESULTS

The genotype rs641738 at the MBOAT7-TMC4 locus associated with increased hepatic fat content in the 2 cohorts, and with more severe liver damage and increased risk of fibrosis compared with subjects without the variant. MBOAT7, but not TMC4, was found to be highly expressed in the liver. The MBOAT7 rs641738 T allele was associated with lower protein expression in the liver and changes in plasma phosphatidylinositol species consistent with decreased MBOAT7 function.

CONCLUSIONS

We provide evidence for an association between the MBOAT7 rs641738 variant and the development and severity of NAFLD in individuals of European descent. This association seems to be mediated by changes in the hepatic phosphatidylinositol acyl-chain remodeling.

Increased Bile Acid Synthesis and Impaired Bile Acid Transport in Human Obesity

CONTEXT

Alterations in bile acid (BA) synthesis and transport have the potential to affect multiple metabolic pathways in the pathophysiology of obesity.

OBJECTIVE

The objective of the study was to investigate the effects of obesity on serum fluctuations of BAs and markers of BA synthesis.

DESIGN

We measured BA fluctuations in 11 nonobese and 32 obese subjects and BA transporter expression in liver specimens from 42 individuals and specimens of duodenum, jejunum, ileum, colon, and pancreas from nine individuals.

MAIN OUTCOME MEASURES

We analyzed serum BAs and markers of BA synthesis after overnight fasting, during a hyperinsulinemic-euglycemic clamp, or a mixed-meal tolerance test and the association of BA transporter expression with body mass index.

RESULTS

BA synthesis markers were 2-fold higher (P < .01) and preferentially 12α-hydroxylated (P < .05) in obese subjects, and both measures were correlated with clamp-derived insulin sensitivity (r = -0.62, P < .0001, and r = -0.39, P = .01, respectively). Insulin infusion acutely reduced serum BAs in nonobese subjects, but this effect was blunted in obese subjects (δBAs -44.2% vs -4.2%, P < .05). The rise in serum BAs postprandially was also relatively blunted in obese subjects (δBAs +402% vs +133%, P < .01). Liver expression of the Na+-taurocholate cotransporting polypeptide and the bile salt export pump were negatively correlated with body mass index (r = -0.37, P = .02, and r = -0.48, P = .001, respectively).

CONCLUSIONS

Obesity is associated with increased BA synthesis, preferential 12α-hydroxylation, and impaired serum BA fluctuations. The findings reveal new pathophysiological aspects of BA action in obesity that may lend themselves to therapeutic targeting in metabolic disease.

Abstract 46: Bile Acid Synthesis and 12-Hydroxylation are Increased, and Bile Acid Transport is Impaired in Human Obesity

Bile acids (BAs) are now recognized as important signaling molecules. They have the ability to regulate a variety of physiologic processes, including those governing metabolism of glucose, triglycerides, and cholesterol. We hypothesize that obesity is associated with alterations in BA synthesis or turnover that may contribute to metabolic dysregulation. To test this, we measured serum BAs and markers of BA synthesis after overnight fasting, during a hyperinsulinemic-euglycemic clamp, or a mixed meal tolerance test in 11 nonobese and 32 obese subjects. We also analyzed BA transporter expression in specimens of human liver, duodenum, jejunum, ileum, colon, and pancreas. We found that BA synthesis markers were twofold higher (P<0.01) and preferentially 12-hydroxylated (P<0.05) in obese subjects, and both measures were correlated with clamp-derived insulin sensitivity (r=-0.62, P<0.0001 and r=-0.39, P=0.01, respectively). Insulin infusion acutely reduced serum BAs in nonobese subjects, but this effect was blunted in obese subjects (-44.2% versus -4.2%, P<0.05). The rise in serum BAs postprandially was also relatively blunted in obese subjects (+402% versus +133%, P<0.01). Liver expression of the Na+-taurocholate cotransporting polypeptide (NTCP) and the bile salt export pump (BSEP) were negatively correlated with BMI (r=-0.37, P=0.02 and r=-0.48, P=0.001, respectively). We conclude that obesity is associated with increased BA synthesis, preferential 12-hydroxylation, and decreased hepatic BA transport. These findings reveal new pathophysiological aspects of BA action in obesity that may lend themselves to therapeutic targeting in metabolic disease.

Increased Bile Acid Synthesis and Deconjugation After Biliopancreatic Diversion

Biliopancreatic diversion (BPD) improves insulin sensitivity and decreases serum cholesterol out of proportion with weight loss. Mechanisms of these effects are unknown. One set of proposed contributors to metabolic improvements after bariatric surgeries is bile acids (BAs). We investigated the early and late effects of BPD on plasma BA levels, composition, and markers of BA synthesis in 15 patients with type 2 diabetes (T2D). We compared these to the early and late effects of Roux-en-Y gastric bypass (RYGB) in 22 patients with T2D and 16 with normal glucose tolerance. Seven weeks after BPD, insulin sensitivity had doubled and serum cholesterol had halved. At this time, BA synthesis markers and total plasma BAs, particularly unconjugated BAs, had markedly risen; this effect could not be entirely explained by low FGF19. In contrast, after RYGB, insulin sensitivity improved gradually with weight loss and cholesterol levels declined marginally; BA synthesis markers were decreased at an early time point (2 weeks) after surgery and returned to the normal range 1 year later. These findings indicate that BA synthesis contributes to the decreased serum cholesterol after BPD. Moreover, they suggest a potential role for altered enterohepatic circulation of BAs in improving insulin sensitivity and cholesterol metabolism after BPD.

Multiplexed Quantification of Proglucagon-Derived Peptides by Immunoaffinity Enrichment and Tandem Mass Spectrometry after a Meal Tolerance Test

BACKGROUND

Proglucagon-derived peptides (PGDPs), which include glucagon-like peptide (GLP)-1, glucagon, and oxyntomodulin, are key regulators of glucose homeostasis and satiety. These peptide hormones are typically measured with immuno-based assays (e.g., ELISA, RIA), which often suffer from issues of selectivity.

METHODS

We developed a multiplexed assay for measuring PGDPs including GLP-1 (7-36) amide, GLP-1 (9-36) amide, glucagon, and oxyntomodulin by mass spectrometry and used this assay to examine the effect of a meal tolerance test on circulating concentrations of these hormones. Participants fasted overnight and were either given a meal (n = 8) or continued to fast (n = 4), with multiple blood collections over the course of 3 h. Plasma samples were analyzed by microflow immunoaffinity (IA)-LC-MS/MS with an isotope dilution strategy.

RESULTS

Assay performance characteristics were examined and established during analytical validation for all peptides. Intra- and interassay imprecision were found to be 2.2%-10.7% and 6.8%-22.5%, respectively. Spike recovery was >76%, and dilution linearity was established up to a 16-fold dilution. Immediately after the meal tolerance test, GLP-1 and oxyntomodulin concentrations increased and had an almost identical temporal relationship, and glucagon concentrations increased with a slight delay.

CONCLUSIONS

IA-LC-MS/MS was used for the simultaneous and selective measurement of PGDPs. This work includes the first indication of the physiological concentrations and modulation of oxyntomodulin after a meal.

Effect of Error Propagation in Stable Isotope Tracer Studies: An Approach for Estimating Impact on Apparent Biochemical Flux

Stable isotope tracers are widely used to quantify metabolic rates, and yet a limited number of studies have considered the impact of analytical error on estimates of flux. For example, when estimating the contribution of de novo lipogenesis, one typically measures a minimum of four isotope ratios, i.e., the precursor and product labeling pre- and posttracer administration. This seemingly simple problem has 1 correct solution and 80 erroneous outcomes. In this report, we outline a methodology for evaluating the effect of error propagation on apparent physiological endpoints. We demonstrate examples of how to evaluate the influence of analytical error in case studies concerning lipid and protein synthesis; we have focused on (2)H2O as a tracer and contrast different mass spectrometry platforms including GC-quadrupole-MS, GC-pyrolysis-IRMS, LC-quadrupole-MS, and high-resolution FT-ICR-MS. The method outlined herein can be used to determine how to minimize variations in the apparent biology by altering the dose and/or the type of tracer. Likewise, one can facilitate biological studies by estimating the reduction in the noise of an outcome that is expected for a given increase in the number of replicate injections.

Discovery of Triazole CYP11B2 Inhibitors with in Vivo Activity in Rhesus Monkeys

Hit-to-lead efforts resulted in the discovery of compound 19, a potent CYP11B2 inhibitor that displays high selectivity vs related CYPs, good pharmacokinetic properties in rat and rhesus, and lead-like physical properties. In a rhesus pharmacodynamic model, compound 19 displays robust, dose-dependent aldosterone lowering efficacy, with no apparent effect on cortisol levels.

Inhibition of cholesteryl ester transfer protein increases cholesteryl ester content of large HDL independently of HDL-to-HDL homotypic transfer: in vitro vs in vivo comparison using anacetrapib and dalcetrapib

The increase in high density lipoprotein (HDL)-cholesterol observed with cholesteryl ester transfer protein (CETP) inhibition is commonly attributed to blockade of cholesteryl ester (CE) transfer from HDL to low density lipoprotein particles. In vitro, it has been observed that CETP can mediate transfer of CE between HDL particles ("homotypic transfer"), and it is postulated that this contributes to HDL remodeling and generation of anti-atherogenic pre-beta HDL. Inhibition of CETP could limit this beneficial remodeling and reduce pre-beta HDL levels. We observed that anacetrapib does not reduce pre-beta HDL in vivo, but the role of HDL homotypic transfer was not examined. This study evaluated the effects of anacetrapib on homotypic transfer from HDL3 to HDL2 in vivo using deuterium-labeled HDL3, and compared this to in vitro settings, where homotypic transfer was previously described. In vitro, both anacetrapib and dalcetrapib inhibited transfer of CE from HDL3 to HDL2 particles. In CETP transgenic mice, anacetrapib did not inhibit the appearance of labeled CE derived from HDL3 in HDL2 particles, but rather promoted the appearance of labeled CE in HDL2. We concluded that inhibition of CETP by anacetrapib promoted HDL particle remodeling, and does not impair the flux of cholesterol ester into larger HDL particles when studied in vivo, which is not consistent with in vitro observations. We further conclude, therefore, that the in vitro conditions used to examine HDL-to-HDL homotypic transfer may not recapitulate the in vivo condition, where multiple mechanisms contribute to cholesteryl ester flux into and out of the HDL pool.

Discovery of Benzimidazole CYP11B2 Inhibitors with in Vivo Activity in Rhesus Monkeys

We report the discovery of a benzimidazole series of CYP11B2 inhibitors. Hit-to-lead and lead optimization studies identified compounds such as 32, which displays potent CYP11B2 inhibition, high selectivity versus related CYP targets, and good pharmacokinetic properties in rat and rhesus. In a rhesus pharmacodynamic model, 32 produces dose-dependent aldosterone lowering efficacy, with no apparent effect on cortisol levels.

Evaluation of cynomolgus monkeys for the identification of endogenous biomarkers for hepatic transporter inhibition and as a translatable model to predict pharmacokinetic interactions with statins in humans

Inhibition of hepatic transporters such as organic anion transporting polypeptides (OATPs) 1B can cause drug-drug interactions (DDIs). Determining the impact of perpetrator drugs on the plasma exposure of endogenous substrates for OATP1B could be valuable to assess the risk for DDIs early in drug development. As OATP1B orthologs are well conserved between human and monkey, we assessed in cynomolgus monkeys the endogenous OATP1B substrates that are potentially suitable to assess DDI risk in humans. The effect of rifampin (RIF), a potent inhibitor for OATP1B, on plasma exposure of endogenous substrates of hepatic transporters was measured. From the 18 biomarkers tested, RIF (18 mg/kg, oral) caused significant elevation of plasma unconjugated and conjugated bilirubin, which may be attributed to inhibition of cOATP1B1 and cOATP1B3 based on in vitro to in vivo extrapolation analysis. To further evaluate whether cynomolgus monkeys are a suitable translational model to study OATP1B-mediated DDIs, we determined the inhibitory effect of RIF on in vitro transport and pharmacokinetics of rosuvastatin (RSV) and atorvastatin (ATV). RIF strongly inhibited the uptake of RSV and ATV by cOATP1B1 and cOATP1B3 in vitro. In agreement with clinical observations, RIF (18 mg/kg, oral) significantly decreased plasma clearance and increased the area under the plasma concentration curve (AUC) of intravenously administered RSV by 2.8- and 2.7-fold, and increased the AUC and maximum plasma concentration of orally administered RSV by 6- and 10.3-fold, respectively. In contrast to clinical findings, RIF did not significantly increase plasma exposure of either intravenous or orally administered ATV, indicating species differences in the rate-limiting elimination pathways.

In vivo isotopically labeled atherosclerotic aorta plaques in ApoE KO mice and molecular profiling by matrix-assisted laser desorption/ionization mass spectrometric imaging

RATIONALE

The ability to quantify rates of formation, regression and/or remodeling of atherosclerotic plaque should facilitate a better understanding of the pathogenesis and management of cardiovascular disease. In the current study, we coupled a stable isotope labeled tracer protocol with matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) to examine spatial and temporal lipid dynamics in atherosclerotic plaque.

METHODS

To promote plaque formation in the aorta region, ApoE KO mice were fed a high cholesterol diet (0.15% cholesterol) and orally dosed with (2,2,3,4,4,6-d(6))-cholesterol over several weeks. Tissue sections of ~10 µm thickness were analyzed by MALDI-MSI using matrix deposition by either chemical sublimation or acoustic droplet ejection.

RESULTS

MALDI-MSI yielded distinct spatial distribution information for a variety of lipid classes including specific lysophosphatidylcholines typically associated with atherosclerosis-related tissue damage such as phospholipase 2 (Lp-PLA(2)) that mediate chemotactic responses to inflammation (e.g. LPC 16:0, LPC 18:0 and LPC 18:1) as well as free cholesterol and cholesteryl esters that contribute to atheroma formation. MALDI mass spectra acquired from aorta tissue sections clearly distinguished non-esterified and esterified versions of (2,2,3,4,4,6-d(6))-cholesterol within aortic plaque regions and showed distinct spatial accumulation of the cholesterol tracer.

CONCLUSIONS

The ability to couple stable isotope based protocols with MALDI-MSI enables a novel strategy to characterize the effects of therapeutic treatments on atherosclerotic plaque formation, regression and potential remodeling of the complex lipid components with high chemical specificity and spatiotemporal information.

Static and turnover kinetic measurement of protein biomarkers involved in triglyceride metabolism including apoB48 and apoA5 by LC/MS/MS

LC/MS quantification of multiple plasma proteins that differ by several orders of magnitude in concentration from a single sample is challenging. We present a strategy that allows the simultaneous determination of the concentration and turnover kinetics of higher and lower abundant proteins from a single digestion mixture. Our attention was directed at a cluster of proteins that interact to affect the absorption and interorgan lipid trafficking. We demonstrate that apos involved in TG metabolism such as apoC2, C3, E, and A4 (micromolar concentration), and apoB48 and apoA5 (single-digit nanomolar concentration) can be quantified from a single digestion mixture. A high degree of correlation between LC/MS and immunobased measurements for apoC2, C3, E, and B48 was observed. Moreover, apoA5 fractional synthesis rate was measured in humans for the first time. Finally, the method can be directly applied to studies involving nonhuman primates because peptide sequences used in the method are conserved between humans and nonhuman primates.

Determination of low levels of 2H-labeling using high-resolution mass spectrometry: application in studies of lipid flux and beyond

RATIONALE

The ability to measure low levels of (2)H-labeling is important in studies of metabolic flux, e.g. one can estimate lipid synthesis by administering (2)H2O and then measuring the incorporation of (2)H into fatty acids. Unfortunately, the analyses are complicated by the presence of more abundant naturally occurring stable isotopes, e.g. (13)C. Conventional approaches rely on coupling gas chromatographic separation of lipids with either quadrupole-mass spectrometry (q-MS) and/or pyrolysis-isotope ratio mass spectrometry (IRMS). The former is limited by high background labeling (primarily from (13)C) whereas the latter is not suitable for routine high-throughput analyses.

METHODS

We have contrasted the use of continuous flow-pyrolysis-IRMS against high-resolution mass spectrometry (i.e. Qq-FT-ICR MS) for measuring the (2)H-enrichment of fatty acids and peptides.

RESULTS

In contrast to IRMS, which requires ~30 min per analysis, it is possible to measure the (2)H-enrichment of palmitate via direct infusion high-resolution mass spectrometry (HRMS) in ~3 min per sample. In addition, Qq-FT-ICR MS enabled measurements of the (2)H-enrichment of peptides (which is not possible using IRMS).

CONCLUSIONS

High-resolution mass spectrometry can be used to measure low levels of (2)H-labeling so we expect that this approach will enhance studies of metabolic flux that rely on (2)H-labeled tracers, e.g. (2)H2O. However, since the high-resolution analyses require greater amounts of a given analyte one potential limitation centers on the overall sensitivity. Presumably, future advances can overcome this barrier.

In vivo effects of anacetrapib on preβ HDL: improvement in HDL remodeling without effects on cholesterol absorption

Cholesteryl ester transfer protein (CETP) transfers cholesteryl ester and triglyceride between HDL and apoB-containing lipoproteins. Anacetrapib (ANA), a reversible inhibitor of CETP, raises HDL cholesterol and lowers LDL cholesterol in dyslipidemic patients. We previously demonstrated that ANA increases macrophage-to-feces reverse cholesterol transport and fecal cholesterol excretion in hamsters, and increased preβ HDL-dependent cholesterol efflux via ABCA1 in vitro. However, the effects of ANA on in vivo preβ HDL have not been characterized. In vitro, ANA inhibited the formation of preβ, however in ANA-treated dyslipidemic hamsters, preβ HDL levels (measured by two-dimensional gel electrophoresis) were increased, in contrast to in vitro findings. Because changes in plasma preβ HDL have been proposed to potentially affect markers of cholesterol absorption with other CETP inhibitors, a dual stable isotope method was used to directly measure cholesterol absorption in hamsters. ANA treatment of hamsters (on either dyslipidemic or normal diet) had no effect on cholesterol absorption, while dalcetrapib-treated hamsters displayed an increase in cholesterol absorption. Taken together, these data support the notion that ANA promotes preβ HDL functionality in vivo, with no effects on cholesterol absorption.

Measurement of apo(a) kinetics in human subjects using a microfluidic device with tandem mass spectrometry

RATIONALE

Apolipoprotein(a) [apo(a)] is the defining protein component of lipoprotein(a) [Lp(a)], an independent risk factor for cardiovascular disease. The regulation of Lp(a) levels in blood is poorly understood in part due to technical challenges in measuring Lp(a) kinetics. Improvements in the ability to readily and reliably measure the kinetics of apo(a) using a stable isotope labeled tracer is expected to facilitate studies of the role of Lp(a) in cardiovascular disease. Since investigators typically determine the isotopic labeling of protein-bound amino acids following acid-catalyzed hydrolysis of a protein of interest [e.g., apo(a)], studies of protein synthesis require extensive protein purification which limits throughput and often requires large sample volumes. We aimed to develop a rapid and efficient method for studying apo(a) kinetics that is suitable for use in studies involving human subjects.

METHODS

Microfluidic device and tandem mass spectrometry were used to quantify the incorporation of [(2)H3]-leucine tracer into protein-derived peptides.

RESULTS

We demonstrated that it is feasible to quantify the incorporation of [(2)H3]-leucine tracer into a proteolytic peptide from the non-kringle repeat region of apo(a) in human subjects. Specific attention was directed toward optimizing the multiple reaction monitoring (MRM) transitions, mass spectrometer settings, and chromatography (i.e., critical parameters that affect the sensitivity and reproducibility of isotopic enrichment measurements). The results demonstrated significant advantages with the use of a microfluidic device technology for studying apo(a) kinetics, including enhanced sensitivity relative to conventional micro-flow chromatography, a virtually drift-free elution profile, and a stable and robust electrospray.

CONCLUSIONS

The technological advances described herein enabled the implementation of a novel method for studying the kinetics of apo(a) in human subjects infused with [(2)H3]-leucine.

New methodologies for studying lipid synthesis and turnover: looking backwards to enable moving forwards

Our ability to understand the pathogenesis of problems surrounding lipid accretion requires attention towards quantifying lipid kinetics. In addition, studies of metabolic flux should also help unravel mechanisms that lead to imbalances in inter-organ lipid trafficking which contribute to dyslipidemia and/or peripheral lipid accumulation (e.g. hepatic fat deposits). This review aims to outline the development and use of novel methods for studying lipid kinetics in vivo. Although our focus is directed towards some of the approaches that are currently reported in the literature, we include a discussion of the older literature in order to put "new" methods in better perspective and inform readers of valuable historical research. Presumably, future advances in understanding lipid dynamics will benefit from a careful consideration of the past efforts, where possible we have tried to identify seminal papers or those that provide clear data to emphasize essential points. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Abstract 119: LCAT Activation in Combination with the CETP Inhibitor Anacetrapib Results in Enhanced Cholesterol Efflux and a Favorable Lipid Profile in Dyslipidemic Hamsters

Lecithin cholesterol acyltransferase (LCAT) is a key enzyme involved in the metabolism of HDL. LCAT synthesizes cholesterol esters (CE) in plasma by catalyzing the transfer of the sn-2 fatty acid of phosphatidylcholine to cholesterol on HDL. The hydrophobic CE moves to the core of the HDL particle which results in HDL maturation and a progressive enlargement of the particle. Cholesteryl ester transfer protein (CETP) is a plasma protein that facilitates the transport of CE from HDL to ApoB-containing lipoproteins. CETP inhibition has been identified as a potential strategy for lowering LDL-c and raising HDL cholesterol levels for the treatment of CVD.

The purpose of the current study was to test the hypothesis that CETP inhibition in the context of LCAT activation might yield an unfavorable lipoprotein profile (e.g. dysfunctional HDL). We measured plasma lipoprotein (via FPLC), particle size and ex vivo cholesterol efflux from dyslipidemic hamsters chronically treated with a small molecule LCAT activator (20 and 60 mpk/day) in the presence or absence of the CETP inhibitor anacetrapib (20mpk/day). As expected, anacetrapib significantly reduced LDL-c (-45%) and increased HDL-c (+63%) after chronic treatment. LCAT activation reduced TG (-35%) and increased HDL-c (+40-68%) but had no effect on LDL-c. Both treatments led to an apparent increase in HDL particle size as measured by FPLC. Additionally, LCAT activation showed a significant reduction in ABCA1-mediated efflux, presumably due to the resulting increase in HDL particle size. Animals that received both anacetrapib and the LCAT activator maintained the favorable lipid profile observed with single treatment. Moreover, HDL from hamsters treated with both LCATa and CETPi demonstrated a significant increase in ABCG1- and SRB1-mediated efflux whereas single treatment did not exhibit a measurable effect. We provide evidence that LCAT activation in the context of a CETP inhibitor, maintains a favorable lipid profile (lower LDL-c) and appear s to have a synergistic effect on cholesterol efflux.

Chronic overexpression of PNPLA3I148M in mouse liver causes hepatic steatosis

A genetic variant in PNPLA3 (PNPLA3(I148M)), a triacylglycerol (TAG) hydrolase, is a major risk factor for nonalcoholic fatty liver disease (NAFLD); however, the mechanism underlying this association is not known. To develop an animal model of PNPLA3-induced fatty liver disease, we generated transgenic mice that overexpress similar amounts of wild-type PNPLA3 (PNPLA3(WT)) or mutant PNPLA3 (PNPLA3(I148M)) either in liver or adipose tissue. Overexpression of the transgenes in adipose tissue did not affect liver fat content. Expression of PNPLA3(I148M), but not PNPLA3(WT), in liver recapitulated the fatty liver phenotype as well as other metabolic features associated with this allele in humans. Metabolic studies provided evidence for 3 distinct alterations in hepatic TAG metabolism in PNPLA3(I148M) transgenic mice: increased formation of fatty acids and TAG, impaired hydrolysis of TAG, and relative depletion of TAG long-chain polyunsaturated fatty acids. These findings suggest that PNPLA3 plays a role in remodeling TAG in lipid droplets, as they accumulate in response to food intake, and that the increase in hepatic TAG levels associated with the I148M substitution results from multiple changes in hepatic TAG metabolism. The development of an animal model that recapitulates the metabolic phenotype of the allele in humans provides a new platform in which to elucidate the role of PNLPA3(I148M) in NAFLD.

Tracking fatty acid kinetics in distinct lipoprotein fractions in vivo: a novel high-throughput approach for studying dyslipidemia in rodent models

Isotopic tracers have been used to examine lipid trafficking for many years, and data from those studies have typically yielded novel insight regarding the pathophysiology of dyslipidemia. Previous experimental designs were suitable for studies in humans because relatively large volumes of plasma could be regularly sampled. We have expanded on the earlier logic by applying high-throughput analytical methods that require reduced sample volumes. Specifically, we have examined the possibility of coupling gel-based separations of lipoproteins (e.g., lipoprint) with LC-MS/MS analyses of complex lipid mixtures as a way to routinely measure the labeling profiles of distinct lipids in discrete lipoprotein subfractions. We demonstrate the ability to measure the incorporation of [U-(13)C]oleate into triglycerides (TG), PLs (PL), and cholesterol esters (CE) in VLDL, LDL, and HDL particles in mice. Although rodent models of dyslipidemia are inherently different from humans because of alterations in enzyme activities and underlying metabolism, rodent models can be used to screen novel compounds for efficacy in altering a given biochemical pathway and therein enable studies of target engagement in vivo. We expect that it is possible to translate our approach for application in other systems, including studies in humans.

Abstract 168: Effects of the CETP Inhibitor Anacetrapib on HDL3-to-HDL2 Transfer: Comparison of in Vitro and in Vivo Methodologies

Cholesteryl ester transfer protein (CETP) is a target for the treatment of dyslipidemia and coronary artery disease. In addition to the well-known effect of CETP to transfer CE from HDL to LDL and to VLDL, in vitro , CETP has been reported to transfer CE between small and large HDL particles (HDL2 and HDL3, respectively). We sought to understand how the CETP inhibitor anacetrapib (ANA) affects HDL3-to-HDL2 transfer under both in vitro and in vivo conditions. In vitro , ANA dose-dependently inhibited transfer of 3 H-CE from total HDL to LDL (IC50 30nM), and from isolated HDL3 to HDL2 particles (IC50 200nM). In human CETP transgenic mice, animals treated with a single dose of ANA (100mg/kg) displayed 80% maximal reduction in plasma CETP activity and a 22% increase in total HDL cholesterol. In animals treated with either vehicle or ANA, 3 H-CE-labeled HDL3 was injected intravenously and 3 H-tracer was monitored in lipoprotein fractions following injection. Animals treated with ANA showed an increase in the amount 3 H-tracer present in HDL2 compared to vehicle over time (20-70% increase across 6 hrs post 3 H-CE-HDL3 injection, P<0.05 vs vehicle). The HDL2 CE pool was also increased with ANA treatment, and 3 H-cholesterol flux into HDL2 was increased with ANA treatment when adjusted to the change in pool size (at 2 and 4 hrs post 3H-CE-HDL3 injection). No change in HDL2 3 H-tracer was seen in C57BL6 mice (lacking CETP) treated with ANA. These results indicate that in contrast to in vitro findings, ANA increases flux of CE into HDL2 in vivo , a process which likely involves multiple pathways. Therefore, the in vitro phenomena of 1) HDL3-to-HDL2 transfer by CETP and 2) inhibition of this transfer by CETP inhibitors are not recapitulated in vivo . It is clear that in vivo approaches are necessary to understand the relevance of HDL3-to-HDL2 transfer in vivo , and to accurately study the effects of CETP inhibition on lipoprotein metabolism.

Small molecule activation of lecithin cholesterol acyltransferase modulates lipoprotein metabolism in mice and hamsters

The objective was to assess whether pharmacological activation of lecithin cholesterol acyltransferase (LCAT) could exert beneficial effects on lipoprotein metabolism. A putative small molecule activator (compound A) was used as a tool compound in in vitro and in vivo studies. Compound A increased LCAT activity in vitro in plasma from mouse, hamster, rhesus monkey, and human. To assess the acute pharmacodynamic effects of compound A, C57Bl/6 mice and hamsters received a single dose (20 mg/kg) of compound A. Both species displayed a significant increase in high-density lipoprotein cholesterol (HDLc) and a significant decrease in non-HDLc and triglycerides acutely after dosing; these changes tracked with ex vivo plasma LCAT activity. To examine compound A's chronic effect on lipoprotein metabolism, hamsters received a daily dosing of vehicle or of 20 or 60 mg/kg of compound A for 2 weeks. At study termination, compound treatment resulted in a significant increase in HDLc, HDL particle size, plasma apolipoprotein A-I level, and plasma cholesteryl ester (CE) to free cholesterol ratio, and a significant reduction in very low-density lipoprotein cholesterol. The increase in plasma CE mirrored the increase in HDL CE. Triglycerides trended toward a dose-dependent decrease in very low-density lipoprotein and HDL, with multiple triglyceride species reaching statistical significance. Gallbladder bile acids content displayed a significant and more than 2-fold increase with the 60 mg/kg treatment. We characterized pharmacological activation of LCAT by a small molecule extensively for the first time, and our findings support the potential of this approach in treating dyslipidemia and atherosclerosis; our analyses also provide mechanistic insight on LCAT's role in lipoprotein metabolism.

Anacetrapib promotes reverse cholesterol transport and bulk cholesterol excretion in Syrian golden hamsters

Cholesteryl ester transfer protein (CETP) transfers cholesteryl ester (CE) and triglyceride between HDL and apoB-containing lipoproteins. Anacetrapib (ANA), a reversible inhibitor of CETP, raises HDL cholesterol (HDL-C) and lowers LDL cholesterol in dyslipidemic patients; however, the effects of ANA on cholesterol/lipoprotein metabolism in a dyslipidemic hamster model have not been demonstrated. To test whether ANA (60 mg/kg/day, 2 weeks) promoted reverse cholesterol transport (RCT), ³H-cholesterol-loaded macrophages were injected and (3)H-tracer levels were measured in HDL, liver, and feces. Compared to controls, ANA inhibited CETP (94%) and increased HDL-C (47%). ³H-tracer in HDL increased by 69% in hamsters treated with ANA, suggesting increased cholesterol efflux from macrophages to HDL. ³H-tracer in fecal cholesterol and bile acids increased by 90% and 57%, respectively, indicating increased macrophage-to-feces RCT. Mass spectrometry analysis of HDL from ANA-treated hamsters revealed an increase in free unlabeled cholesterol and CE. Furthermore, bulk cholesterol and cholic acid were increased in feces from ANA-treated hamsters. Using two independent approaches to assess cholesterol metabolism, the current study demonstrates that CETP inhibition with ANA promotes macrophage-to-feces RCT and results in increased fecal cholesterol/bile acid excretion, further supporting its development as a novel lipid therapy for the treatment of dyslipidemia and atherosclerotic vascular disease.

Localization of fatty acyl and double bond positions in phosphatidylcholines using a dual stage CID fragmentation coupled with ion mobility mass spectrometry

A high content molecular fragmentation for the analysis of phosphatidylcholines (PC) was achieved utilizing a two-stage [trap (first generation fragmentation) and transfer (second generation fragmentation)] collision-induced dissociation (CID) in combination with travelling-wave ion mobility spectrometry (TWIMS). The novel aspects of this work reside in the fact that a TWIMS arrangement was used to obtain a high level structural information including location of fatty acyl substituents and double bonds for PCs in plasma, and the presence of alkali metal adduct ions such as [M + Li](+) was not required to obtain double bond positions. Elemental compositions for fragment ions were confirmed by accurate mass measurements. A very specific first generation fragment ion m/z 577 (M-phosphoryl choline) from the PC [16:0/18:1 (9Z)] was produced, which by further CID generated acylium ions containing either the fatty acyl 16:0 (C(15)H(31)CO(+), m/z 239) or 18:1 (9Z) (C(17)H(33)CO(+), m/z 265) substituent. Subsequent water loss from these acylium ions was key in producing hydrocarbon fragment ions mainly from the α-proximal position of the carbonyl group such as the hydrocarbon ion m/z 67 (+H(2)C-HC = CH-CH = CH(2)). Formation of these ions was of important significance for determining double bonds in the fatty acyl chains. In addition to this, and with the aid of (13)C labeled lyso-phosphatidylcholine (LPC) 18:1 (9Z) in the ω-position (methyl) TAP fragmentation produced the ion at m/z 57. And was proven to be derived from the α-proximal (carboxylate) or distant ω-position (methyl) in the LPC.

siRNA-induced liver ApoB knockdown lowers serum LDL-cholesterol in a mouse model with human-like serum lipids

Increased serum apolipoprotein (apo)B and associated LDL levels are well-correlated with an increased risk of coronary disease. ApoE⁻/⁻ and low density lipoprotein receptor (LDLr)⁻/⁻ mice have been extensively used for studies of coronary atherosclerosis. These animals show atherosclerotic lesions similar to those in humans, but their serum lipids are low in apoB-containing LDL particles. We describe the development of a new mouse model with a human-like lipid profile. Ldlr CETP⁺/⁻ hemizygous mice carry a single copy of the human CETP transgene and a single copy of a LDL receptor mutation. To evaluate the apoB pathways in this mouse model, we used novel short-interfering RNAs (siRNA) formulated in lipid nanoparticles (LNP). ApoB siRNAs induced up to 95% reduction of liver ApoB mRNA and serum apoB protein, and a significant lowering of serum LDL in Ldlr CETP⁺/⁻ mice. ApoB targeting is specific and dose-dependent, and it shows lipid-lowering effects for over three weeks. Although specific triglycerides (TG) were affected by ApoB mRNA knockdown (KD) and the total plasma lipid levels were decreased by 70%, the overall lipid distribution did not change. Results presented here demonstrate a new mouse model for investigating additional targets within the ApoB pathways using the siRNA modality.

Collision cross-section determination and tandem mass spectrometric analysis of isomeric carotenoids using electrospray ion mobility time-of-flight mass spectrometry

Carotenoids are natural pigments with provitamin A and antioxidant activities. Biosynthesized in plants as their all-trans isomers, carotenoids isomerize in solution and in humans to multiple cis isomers which can have different bioavailabilities and functions. Since separation and characterization of isomeric carotenoids using high-pressure liquid chromatography (HPLC) or liquid chromatography-tandem mass spectrometry (LC-MS/MS) is time-consuming, the potential for ion mobility mass spectrometry (IM-MS) to resolve and characterize carotenoid isomers rapidly without chromatography was investigated using traveling-wave ion mobility spectrometry on a quadrupole time-of-flight mass spectrometer. The all-trans isomers of lycopene and β-carotene were separated by several milliseconds from the cis-isomers which were detected as partially overlapping peaks. The collision cross-section values of these carotenoid isomers were determined using IM-MS to be 180 and 236 Å(2) for cis-lycopene and all-trans-lycopene, and 181 and 225 Å(2) for cis-β-carotene and all-trans-β-carotene, respectively. Collision-induced dissociation MS/MS of ion mobility-resolved isomers indicated that cis and all-trans carotenoid isomers can be distinguished by their fragmentation patterns. Previous MS/MS studies of cis- and all trans-carotenoids had suggested that they produced identical tandem mass spectra, but this appears to have been the result of isomerization during ionization. Introduction of specific cis or trans isomers by infusion or HPLC resulted in cis/trans isomerization in the ion source during electrospray, and the relative levels of cis carotenoids forming in the ion source compared to the all-trans isomers were temperature dependent.

In vivo D2O labeling to quantify static and dynamic changes in cholesterol and cholesterol esters by high resolution LC/MS

High resolution LC/MS-MS and LC/APPI-MS methods have been established for the quantitation of flux in the turnover of cholesterol and cholesterol ester. Attention was directed toward quantifying the monoisotopic mass (M0) and that of the singly deuterated labeled (M+1) isotope. A good degree of isotopic dynamic range has been achieved by LC/MS-MS ranging from 3-4 orders of magnitude. Correlation between the linearity of GC/MS and LC atmospheric pressure photoionization (APPI)-MS are complimentary (r² = 0.9409). To prove the viability of this particular approach, male C57Bl/6 mice on either a high carbohydrate (HC) or a high fat (HF) diet were treated with ²H₂O for 96 h. Gene expression analysis showed an increase in the activity of stearoyl-CoA desaturase (Scd1) in the HC diet up to 69-fold (P < 0.0008) compared with the HF diet. This result was supported by the quantitative flux measurement of the isotopic incorporation of ²H into the respective cholesterol and cholesterol ester (CE) pools. We concluded that it is possible to readily obtain static and dynamic measurement of cholesterol and CEs in vivo by coupling novel LC/MS methods with stable isotope-based protocols.

Fractional mass filtering as a means to assess circulating metabolites in early human clinical studies

Recent changes in the regulatory environment have led to a need for new methods to assess circulating human drug metabolites in early clinical studies with respect to their potential toxicological impact. The specific goals of such studies are to determine if the metabolites present in human plasma following administration of a drug candidate also are observed in plasma from the animal studies employed for preclinical toxicological evaluation, and to estimate corresponding exposure margins (animal:human) for the major metabolites. Until recently, the accepted best practice for the characterization of circulating drug metabolites utilized liquid chromatography/tandem mass spectrometry (LC/MS/MS)-based methodologies, in conjunction with authentic chemical standards, for the detection and quantitative analyses of metabolites predicted from both animal studies and experiments with human liver preparations in vitro. While this approach is satisfactory for anticipated biotransformation products, metabolites that were not expected to circulate in human plasma frequently escape detection. Current accurate mass instruments enable the use of the technique of fractional mass filtering to detect both expected and unexpected metabolites in a rapid, less resource-intensive and more robust manner. Application of this technology to several clinical development programs at Merck Research Laboratories has demonstrated the value of fractional mass filtering in the assessment of circulating drug metabolites in early clinical trials.

Metabolism of [14C]-5-chloro-1,3-benzodioxol-4-amine in male Wistar-derived rats following intraperitoneal administration

[14C]-5-chloro-1,3-benzodioxol-4-amine was administered intraperitoneally (i.p.) to bile duct-cannulated rats (Alpk:ApfSD, Wistar derived) at 25 mg kg-1 to determine the rates and routes of excretion of the compound and to investigate its metabolic fate. A total of 89.1% of the dose was excreted in the 48 h following administration, the majority being recovered in the urine during the first 12 h. The main metabolite in both urine and bile, detected by high-performance liquid chromatography (HPLC) with radioprofiling and mass spectrometry, was identified as a demethylenated monosulfate conjugate. Unchanged parent compound formed a major component of the radiolabel excreted in urine and, in addition to unchanged parent and demethylenated sulphate conjugate, a large number of minor metabolites were detected in urine and bile. The overall metabolic fate of 5-chloro-1,3-benzodioxol-4-amine in the rat was complex, with some similarities to previously studied methylenedioxyphenyl compounds.

High-throughput, accurate mass liquid chromatography/tandem mass spectrometry on a quadrupole time-of-flight system as a 'first-line' approach for metabolite identification studies

Throughput for drug metabolite identification studies has been increased significantly by the combined use of accurate mass liquid chromatography/tandem mass spectrometry (LC/MS/MS) data on a quadrupole time-of-flight (QTOF) system and targeted data analysis procedures. Employed in concert, these tools have led to the implementation of a semi-automated high-throughput metabolite identification strategy that has been incorporated successfully into lead optimization efforts in drug discovery. The availability of elemental composition data on precursor and all fragment ions in each spectrum has greatly enhanced confidence in ion structure assignments, while computer-based algorithms for defining sites of biotransformation based upon mass shifts of diagnostic fragment ions have facilitated identification of positions of metabolic transformation in drug candidates. Adoption of this technology as the 'first-line' approach for in vitro metabolite profiling has resulted in the analysis of as many as 21 new chemical entities on one day from diverse structural classes and therapeutic programs.

Generation of ultrahigh peak capacity LC separations via elevated temperatures and high linear mobile-phase velocities

The use of a combination of ultraperformance liquid chromatography at approximately 11,000 psi on sub 2-microm particles combined with reversed-phase gradient chromatography at a temperature of 90 degrees C is described as applied to the analysis of endogenous and drug metabolites in human and animal urine. By using elevated temperatures, back pressures can be reduced while maintaining high flow rates and chromatographic efficiency, with peaks 1-3 s wide at the base. Application to urine samples provided a peak capacity of approximately 700 for a 10-min analysis and greater than approximately 1000 in 1 h. Despite the narrow nature of the peaks, good quality mass spectra were also obtained, allowing the identification of typical drug and endogenous metabolites. These ultra-high-resolution chromatograms should be ideal for the analysis of complex samples in, for example, metabolite identification, impurity identification, and metabonomic/metabolomic studies. Applications in natural product analysis and proteomics can also be envisaged.

MSE with mass defect filtering for in vitro and in vivo metabolite identification

Metabolite identification studies involve the detection and structural characterization of the biotransformation products of drug candidates. These experiments are necessary throughout the drug discovery and development process. The use of high-resolution chromatography and high-resolution mass spectrometry together with data processing using mass defect filtering is described for in vitro and in vivo metabolite identification studies. Data collection was done using UPLC coupled with an orthogonal hybrid quadrupole time-of-flight mass spectrometer. This experimental approach enabled the use of MS(E) data collection (where E represents collision energy) which has previously been shown to be a powerful approach for metabolite identification studies. Post-acquisition processing with a prototype mass defect filtering program was used to eliminate endogenous interferences in the study samples, greatly enhancing the discovery of metabolites. The ease of this approach is illustrated by results showing the detection and structural characterization of metabolites in plasma from a preclinical rat pharmacokinetic study.

Narrow-bore sample trapping and chromatography combined with quadrupole/time-of-flight mass spectrometry for ultra-sensitive identification of in vivo and in vitro metabolites

The identification of in vitro and in vivo metabolites is vital to the discovery and development of new pharmaceutical therapies. Analytical strategies to identify metabolites at different stages of this process vary, but all involve the use of liquid chromatography separations combined with detection via mass spectrometry (HPLC/MS). Reported here is the use of narrow-bore column (0.5-1.0 mm i.d.) trapping of metabolites, followed by back-flushing onto a matching analytical column. Separated metabolites were then identified using quadrupole time-of-flight mass spectrometry (MS) and tandem MS. Metabolites in human plasma and from low-level in vitro incubations, that were not identified using standard HPLC/MS approaches, were characterized using the instrumental configuration described here.

High resolution "ultra performance" liquid chromatography coupled to oa-TOF mass spectrometry as a tool for differential metabolic pathway profiling in functional genomic studies

The combination of a new 1.7 mum reversed-phase packing material, and a chromatographic system, operating at ca. 12,000 psi, (so-called ultra performance liquid chromatography, UPLC) has enabled dramatic increases in chromatographic performance to be obtained for complex mixture separation. This increase in performance is manifested in improved peak resolution, together with increased speed and sensitivity. Here, we show that UPLC offers significant advantages over conventional reversed-phase HPLC amounting to a more than doubling of peak capacity, an almost 10-fold increase in speed and a 3- to 5-fold increase in sensitivity compared to that generated with a conventional 3.5 microm stationary phase. The first functional genomic application of UPLC-MS technology is illustrated here with respect to multivariate metabolic profiling of urines from males and females of two groups of phenotypically normal mouse strains (C57BL19J and Alpk:ApfCD) and a "nude mouse" strain. We have also compared this technology to conventional HPLC-MS under similar analytical conditions and show improved phenotypic classification capability of UPLC-MS analysis together with increased ability to probe differential pathway activities between strains as a result of improved analytical sensitivity and resolution.

[Metabonomics and its applications]

The concept, characteristics and history of metabonomics are introduced. The techniques used in data acquisition and data analysis in metabonomics including their advantages and disadvantages are summarized. In data acquisition platform, NMR, GC/MS, LC/MS (/MS) are the prevalent techniques although at present, none of them is a perfect technique that could meet with the requirement of the metabonomics for measuring all metabolites. While in data analysis, the PCA, PLS and ANN are the major techniques. The researchers could select them according to the research destination. Recent advances and applications of metabonomics in disease diagnosis, drug toxicity evaluation, plant metabolomics and microbial metabolomics are reviewed. In addition, by giving the situation on the establishment of the related corporations, the conferences about metabonomics and proclamation of NIH roadmap the current boom of the metabonomics is reflected. It can be expected that with the development of the function genomics, metabonomics will play a major role in the discovery of the phynotype of the genome and searching for the disease diagnostic biomarkers, and it will also bring much benefit to the drug discovery, clinical diagnosis and nutrition science.

A high-throughput liquid chromatography/tandem mass spectrometry method for screening glutathione conjugates using exact mass neutral loss acquisition

Chemically reactive metabolites may cause hepatotoxicity and as a result liver failure or other adverse side reactions. Therefore, this is a vital topic of interest because early reactive metabolite screening may prevent compound failure at a later stage. In order to address this issue, a screening assay has been developed to detect the formation of reactive metabolites by using glutathione as a trapping reagent, which will allow us to search for phase I metabolites and also glutathiones during in vitro metabolite screening using liquid chromatography/tandem mass spectrometry (LC/MS/MS) with exact mass. Glutathione conjugations when fragmented by the mass spectrometer give a common loss corresponding to the pyroglutamic acid moiety, which can be monitored. Until recently, this work has been carried out with triple quadrupole technology using nominal mass. The advantage of the hybrid quadrupole time-of-flight mass spectrometer is the selectivity and sensitivity that can be achieved. Exact neutral loss detection is achieved via sequential low- and high-energy MS acquisitions. After detection of the loss of the pyroglutamic acid moiety, using a window of +/-20 mDa on the high-energy scan, MS/MS is carried out on the parent mass of interest to confirm the common neutral loss.

A rapid screening approach to metabonomics using UPLC and oa-TOF mass spectrometry: application to age, gender and diurnal variation in normal/Zucker obese rats and black, white and nude mice

The use of Ultra Performance Liquid Chromatography (UPLC), with a rapid 1.5 minute reversed-phase gradient separation on a 1.7 microm reversed-phase packing material to provide rapid "high throughput" support for metabonomic screening is demonstrated. The peak capacity and the number of marker ions detected using these fast UPLC separations and oa-TOF MS was found to be similar to that generated by conventional HPLC-MS methods with a 10 minute separation. The UPLC-MS methodology was applied to the analysis of urine samples from rodents, including normal and Zucker obese rats and three strains of mice (of both sexes), and was found to provide rapid discrimination between age, strain, gender and diurnal variation.

Increasing throughput and information content for in vitro drug metabolism experiments using ultra-performance liquid chromatography coupled to a quadrupole time-of-flight mass spectrometer

The field of drug metabolism has been revolutionized by liquid chromatography/mass spectrometry (LC/MS) applications with new technologies such as triple quadrupoles, ion traps and time-of-flight (ToF) instrumentation. Over the years, these developments have often relied on the improvements to the mass spectrometer hardware and software, which has allowed users to benefit from lower levels of detection and ease-of-use. One area in which the development pace has been slower is in high-performance liquid chromatography (HPLC). In the case of metabolite identification, where there are many challenges due to the complex nature of the biological matrices and the diversity of the metabolites produced, there is a need to obtain the most accurate data possible. Reactive or toxic metabolites need to be detected and identified as early as possible in the drug discovery process, in order to reduce the very costly attrition of compounds in late-phase development. High-resolution, exact mass measurement plays a very important role in metabolite identification because it allows the elimination of false positives and the determination of non-trivial metabolites in a much faster throughput environment than any other standard current methodology available to this field. By improving the chromatographic resolution, increased peak capacity can be achieved with a reduction in the number of co-eluting species leading to superior separations. The overall enhancement in the chromatographic resolution and peak capacity is transferred into a net reduction in ion suppression leading to an improvement in the MS sensitivity. To investigate this, a number of in vitro samples were analyzed using an ultra-performance liquid chromatography (UPLC) system, with columns packed with porous 1.7 mum particles, coupled to a hybrid quadrupole time-of-flight (ToF) mass spectrometer. This technique showed very clear examples for fundamental gains in sensitivity, chromatographic resolution and speed of analysis, which are all important factors for the demands of today's HTS in discovery.

Ultra-performance liquid chromatography coupled to quadrupole-orthogonal time-of-flight mass spectrometry

Ultra-performance liquid chromatography (UPLC) utilizes sub-2 microm particles with high linear solvent velocities to effect dramatic increases in resolution, sensitivity and speed of analysis. The reduction in particle size to below 2 microm requires instrumentation that can operate at pressures in the 6000-15,000 psi range. The typical peak widths generated by the UPLC system are in the order of 1-2 s for a 10-min separation. In the present work this technology has been applied to the study of in vivo drug metabolism, in particular the analysis of drug metabolites in bile. The reduction in peak width significantly increases analytical sensitivity by three- to five-fold, and the reduction in peak width, and concomitant increase in peak capacity, significantly reduces spectral overlap resulting in superior spectral quality in both MS and MS/MS modes. The application of UPLC/MS resulted in the detection of additional drug metabolites, superior separation and improved spectral quality.

Detection of mono- and di-hexoses as metabolites of 4-bromoaniline using HPLC-TOF-MS/MS

1. The metabolic fate of 4-bromoaniline (4-BrA) was investigated in rat following intraperitoneal administration at 50 mg kg(-1) using HPLC-TOF-MS/MS. 2. The sensitivity provided by the use of TOF-MS/MS, aided by the distinctive isotope pattern resulting from the presence of the bromine substituent in the molecule, enabled the detection of many previously uncharacterized metabolites in the samples. 3. Several groups of minor metabolites were detected in the urine that corresponded to a number of isomeric hexose and di-hexose-containing conjugates (possibly glucosides and diglucosides) of 4-BrA. 4. As well as hexose and di-hexose conjugates of 4-BrA, several further groups of metabolites that also contained either a sulphamate or sulphate group in addition to the sugar moieties were also detected.

Use of liquid chromatography/time-of-flight mass spectrometry and multivariate statistical analysis shows promise for the detection of drug metabolites in biological fluids

The process of metabolite identification is essential to the drug discovery and development process; this is usually achieved by liquid chromatography/tandem mass spectrometry (LC/MS/MS) or a combination of liquid chromatography/mass spectrometry (LC/MS) and nuclear magnetic resonance (NMR) spectroscopy. Metabolite identification is, however, a time-consuming process requiring an experienced skilled scientist. Multivariate statistical analysis has been used in the field of metabonomics to elucidate differences in endogenous biological profiling due to a toxic effect or a disease state. In this paper we show how a combination of liquid chromatography/time-of-flight mass spectrometry (LC/TOFMS) and multivariate statistical analysis can be used to detect drug metabolites in a biological fluid with no prior knowledge of the compound administered.

Characterisation of putative pentose-containing conjugates as minor metabolites of 4-bromoaniline present in the urine of rats following intraperitoneal administration

The metabolic fate of 4-bromoaniline (4-BrA) was investigated following intraperitoneal administration to the rat at 50 mg kg(-1), using high-performance liquid chromatography/time-of-flight tandem mass spectrometry (HPLC/TOF-MS/MS). Up to five metabolites were detected in urine that correspond to isomeric pentose conjugates (possibly ribosides) of a hydroxysulphate of 4-BrA. This identification is supported by further studies where the water used in the reversed-phase solvent system was replaced with deuterated water in order to confirm that the number of exchangeable protons present in the metabolites was consistent with the proposed structures.