Use of Ion Chromatography/Mass Spectrometry for Targeted Metabolite Profiling of Polar Organic Acids.

Organic acids (OAs) serve as metabolites that play pivotal roles in a host of different metabolic and regulatory pathways. The polar nature of many OAs poses a challenge to their measurement using widely practiced analytical methods. In this study, a targeted metabolomics method was developed using ion chromatography/triple quadrupole mass spectrometry (IC/MS) to quantitate 28 polar OAs with limits of quantitation ranging from 0.25 to 50 µM. The interday assay precisions ranged from 1% to 19%, with accuracies ranging from 82% to 115%. The IC/MS assay was used to quantitate OAs in quadriceps muscle from sedentary mice compared to fatigued mice subjected to either a low intensity, long duration (LILD) or high intensity, short duration (HISD) forced treadmill regimen. Among the OAs examined, significant differences were detected for hippuric acid, malic acid, fumaric acid, and 2-ketoglutaric acid between the sedentary and fatigued mice. In conclusion, the IC/MS method enabled the separation and quantitative survey of a broad range of polar OAs that are difficult to analyze by chromatographic techniques.

Measurement of Pyridine Nucleotides in Biological Samples Using LC-MS/MS.

Pyridine nucleotides which include NAD+, NADH, NADP, and NADPH play vital roles in many different biological processes. These metabolites can be accurately quantified in a wide variety of biological samples using LC-MS/MS. The quality and precision of these measurements was enhanced using heavy isotope-labeled internal standards and carefully crafted protocols for sample processing.

Boosting NAD+ with a small molecule that activates NAMPT.

Pharmacological strategies that boost intracellular NAD+ are highly coveted for their therapeutic potential. One approach is activation of nicotinamide phosphoribosyltransferase (NAMPT) to increase production of nicotinamide mononucleotide (NMN), the predominant NAD+ precursor in mammalian cells. A high-throughput screen for NAMPT activators and hit-to-lead campaign yielded SBI-797812, a compound that is structurally similar to active-site directed NAMPT inhibitors and blocks binding of these inhibitors to NAMPT. SBI-797812 shifts the NAMPT reaction equilibrium towards NMN formation, increases NAMPT affinity for ATP, stabilizes phosphorylated NAMPT at His247, promotes consumption of the pyrophosphate by-product, and blunts feedback inhibition by NAD+. These effects of SBI-797812 turn NAMPT into a "super catalyst" that more efficiently generates NMN. Treatment of cultured cells with SBI-797812 increases intracellular NMN and NAD+. Dosing of mice with SBI-797812 elevates liver NAD+. Small molecule NAMPT activators such as SBI-797812 are a pioneering approach to raise intracellular NAD+ and realize its associated salutary effects.

Skeletal muscle overexpression of nicotinamide phosphoribosyl transferase in mice coupled with voluntary exercise augments exercise endurance.

OBJECTIVE: Nicotinamide phosphoribosyl transferase (NAMPT) is the rate-limiting enzyme in the salvage pathway that produces nicotinamide adenine dinucleotide (NAD+), an essential co-substrate regulating a myriad of signaling pathways. We produced a mouse that overexpressed NAMPT in skeletal muscle (NamptTg) and hypothesized that NamptTg mice would have increased oxidative capacity, endurance performance, and mitochondrial gene expression, and would be rescued from metabolic abnormalities that developed with high fat diet (HFD) feeding. METHODS: Insulin sensitivity (hyperinsulinemic-euglycemic clamp) was assessed in NamptTg and WT mice fed very high fat diet (VHFD, 60% by kcal) or chow diet (CD). The aerobic capacity (VO2max) and endurance performance of NamptTg and WT mice before and after 7 weeks of voluntary exercise training (running wheel in home cage) or sedentary conditions (no running wheel) were measured. Skeletal muscle mitochondrial gene expression was also measured in exercised and sedentary mice and in mice fed HFD (45% by kcal) or low fat diet (LFD, 10% by kcal). RESULTS: NAMPT enzyme activity in skeletal muscle was 7-fold higher in NamptTg mice versus WT mice. There was a concomitant 1.6-fold elevation of skeletal muscle NAD+. NamptTg mice fed VHFD were partially protected against body weight gain, but not against insulin resistance. Notably, voluntary exercise training elicited a 3-fold higher exercise endurance in NamptTg versus WT mice. Mitochondrial gene expression was higher in NamptTg mice compared to WT mice, especially when fed HFD. Mitochondrial gene expression was higher in exercised NamptTg mice than in sedentary WT mice. CONCLUSIONS: Our studies have unveiled a fascinating interaction between elevated NAMPT activity in skeletal muscle and voluntary exercise that was manifest as a striking improvement in exercise endurance.

Integrity profiling of high throughput screening hits using LC-MS and related techniques.

Integrity profiling of HTS hits is valuable for verification of the hit identity and purity. This provides early discovery researchers with more confident SAR theories. Methodology for integrity profiling of HTS hits must be high throughput, consume little material, and selectively provide structure-based data. Analytical techniques that can be utilized for integrity profiling methods are reviewed for their appropriateness in sample preparation, component separation, detection, purity quantitation, identity confirmation, and follow-up.

Differentiation of stereoisomeric steroids by reactions with phosphenium ions.

A chemical ionization method is reported for distinction of diastereomeric hydroxysteroids by using Fourier-transform ion cyclotron mass spectrometry (FT-ICR). Certain phosphenium ions are demonstrated to react with stereoisomeric steroids to yield qualitatively different product ions. For example, 1,3,5(10)-estratriene-3,16beta,17beta-triol (cis-estriol) reacts with the dimethoxy phosphenium ion to form a diagnostic product ion (not formed for the trans-estriol) through addition followed by the loss of two molecules of methanol. In an analogous manner, the 1,3-dioxolan-2-phosphenium ion produces a diagnostic product ion through the loss of ethylene glycol from the adduct of cis-estriol only. The 1,3,5(10)-estratriene-3,16alpha,17beta-triol (trans-estriol), on the other hand, reacts with each phosphenium ion only via hydroxide abstraction-initiated pathways that indicate the presence of at least two hydroxyl groups in the molecule. These specific reactions take place for all hydroxysteroids examined, independent of their stereochemistry. Another isomer pair, cholestan-3alpha,5alpha-diol (cis-cholestandiol) and cholestan-3beta,5alpha-diol (trans-cholestandiol), is differentiated based on selective elimination of water only from the adduct of the cis-isomer. However, the method does not allow distinction between the stereoisomeric 5beta-pregnane-3alpha,17alpha,20alpha-triol and 5beta-pregnane-3alpha,17alpha,20beta-triol. The different reactivities of the three pairs of steroid isomers and of each diastereomeric compound pair are rationalized by reaction enthalpies and steric effects based on straightforward and predictable reaction mechanisms.

Trace LC/MS/MS quantitation of 17beta-estradiol as a biomarker for selective estrogen receptor modulator activity in the rat brain.

A sensitive LC/MS/MS method has been developed by derivatization of 17beta-estradiol (E2) with dansyl chloride to quantitate 17beta-E2 in female rat serum. The use of E2-d(5) minimized interferences from endogenous 17beta-E2 in order to achieve a limit of quantitation (LOQ) of 2.5 pg/ml using 150 microl of female rat serum. The recovery of the dansyl derivative was 95% or greater in quality control samples. The intra and interday assay precision was better than 8.2 and 6.2%, respectively, with accuracies ranging from 97 to 101% in the quality control samples. The assay was used for the quantitation of serum E2 as a biomarker for the estrogen receptor (ER) antagonist activity of small molecule SERMs (selective estrogen receptor modulators) in the female rat brain. The study revealed that a statistically significant upregulation of serum 17beta-E2 occurred for rats dosed with SERMs that are known to penetrate the brain and disrupt the hypothalamic-pituitary-ovarian (HPO) axis. Variations in 17beta-E2 in ascending dose studies also correlated with the corresponding trends in CYP17a1 levels, an mRNA biomarker for ovarian hyperstimulation. This biomarker assay has provided a useful screen for medicinal chemistry optimization to produce SERMs that do not interfere with negative feedback of estrogens on the brain and for biological hypothesis testing.

Activation of farnesoid X receptor prevents atherosclerotic lesion formation in LDLR-/- and apoE-/- mice.

The role of farnesoid X receptor (FXR) in the development of atherosclerosis has been unclear. Here, LDL receptor (LDLR(-/-)) or apolipoprotein E (apoE(-/-)) female or male mice were fed a Western diet and treated with a potent synthetic FXR agonist, WAY-362450. Activation of FXR blocked diet-induced hypertriglyceridemia and elevations of non-HDL cholesterol and produced a near complete inhibition of aortic lesion formation. WAY-362450 also induced small heterodimer partner (SHP) expression and repressed cholesterol 7alpha-hydroxylase (CYP7A1) and sterol 12 alpha-hydroxylase (CYP8B1) expression. To determine if SHP was essential for these protective activities, LDLR(-/-)SHP(-/-) and apoE(-/-)SHP(-/-) mice were similarly treated with WAY-362450. Surprisingly, a notable sex difference was observed in these mice. In male LDLR(-/-)SHP(-/-) or apoE(-/-)SHP(-/-) mice, WAY-362450 still repressed CYP7A1 and CYP8B1 expression by 10-fold and still strongly reduced non-HDL cholesterol levels and aortic lesion area. In contrast, in the female LDLR(-/-)SHP(-/-) or apoE(-/-)SHP(-/-) mice, WAY-362450 only slightly repressed CYP7A1 and CYP8B1 expression and did not reduce non-HDL cholesterol or aortic lesion size. WAY-362450 inhibition of hypertriglyceridemia remained intact in LDLR(-/-) or apoE(-/-) mice lacking SHP of both sexes. These results suggest that activation of FXR protects against atherosclerosis in the mouse, and this protective effect correlates with repression of bile acid synthetic genes, with mechanistic differences between male and female mice.

Rapid screening of enzymes for the enzymatic hydrolysis of chiral esters in drug discovery.

Thirty-five enzymes were rapidly screened for their ability to selectively hydrolyze chiral esters to their corresponding carboxylic acids for the efficient generation of chiral intermediates in drug discovery. Optimization of the enzymatic reactions at various incubation times was performed using a robotic liquid handler. Enantiomeric pairs of chiral esters and carboxylic acids were then analyzed simultaneously by chiral GC/MS in a single analysis. This analytical approach is particularly useful for compounds that do not possess a conjugated chromophore or are volatile and difficult to analyze by chiral HPLC/UV or HPLC/MS. The resulting data was used to determine enantiomeric excesses and percent conversions to the desired enantiomer of the carboxylic acid for the selection of efficient enzymes for bioconversions in drug discovery in a pharmaceutical company.

Direct analysis in real time for reaction monitoring in drug discovery.

Direct analysis in real time (DART) is a novel ionization technique that provides for the rapid ionization of small molecules under ambient conditions. In this study, several commercially available drugs as well as actual compounds from drug discovery research were examined by LC/UV/ESI-MS and DART interfaced to a quadrupole mass spectrometer. For most compounds, the molecular ions observed by ESI-MS were observed by DART/MS. DART/MS was also studied as a means to quickly monitor synthetic organic reactions and to obtain nearly instantaneous molecular weight confirmations of final products in drug discovery. For simple, synthetic organic transformations, the trends in the intensities of the mass spectral signals for the reactant and product obtained by DART/MS scaled closely with those of the diode array or the total ion chromatogram obtained by LC/UV/ESI-MS. In summary, DART is a new tool that complements electrospray ionization for the rapid ionization and subsequent mass spectral analysis of compounds in drug discovery.

Automated accurate mass data processing using a gas chromatograph/time-of-flight mass spectrometer in drug discovery.

A gas chromatograph/time-of-flight (GCT) mass spectrometer, with high mass measurement accuracy to within 5 ppm, has been used for the automated accurate mass analysis of multicomponent mixtures and drug discovery compounds. A multicomponent mixture was analyzed several times over the course of a week to assess the reproducibility and ruggedness of the automated method while operating the GCT in electron ionization mode. For example, the data for 31 radical cations generated via electron ionization was processed using automated software (i.e. OpenLynx) to provide for mass accuracies less than 5 ppm for nearly 100% of the ions from multiple injection data. Mass accuracies of the radical anions of polyaromatic hydrocarbons generated via negative chemical ionization, and protonated pyridines and quinolines generated via methane chemical ionization, were mainly less than 5 ppm from multiple injection data. In addition, the automated method has been used for the accurate mass analysis of drug discovery compounds.

Hydrogen atom abstraction reactions of charged polyaromatic sigma-radicals related to the active intermediates of the enediyne antitumor drugs.

Polar effects are demonstrated to play an important role in controlling the reactivity of polyaromatic sigma-radicals that are structurally related to the active intermediates of the enediyne anticancer type antibiotics. This was accomplished by measuring the rate constants of hydrogen atom abstraction for novel, charged dehydroquinolines, dehydroisoquinolines, dehydrobenzenes, and dehydronaphthalenes in the gas phase by using Fourier-transform ion cyclotron resonance mass spectrometry. The reactivity trends observed for these radicals upon hydrogen atom abstraction from tetrahydrofuran and 2-methyltetrahydrofuran, simple models of deoxyribose, do not reflect differences in reaction exothermicities, radical sizes, exact location of the radical site in the ring system, or heteroatom-radical site distances. However, the reactivity trends match the trend in the calculated electron affinities of the radicals. The radicals' different electrophilicities result in variations in the reaction barrier due to different extents of polarization of the transition state. Generally, the reaction efficiencies are the greatest when the formally charged heteroatom is contained within the same ring system as the radical site. In this case, polar effects have the greatest influence on radical reactivity. Hence, insertion of a basic heteroatom (which gets protonated in biological systems) into specific locations in the polyaromatic ring system of the sigma-biradicals, which ultimately cause cleavage of DNA exposed to the enediyne antitumor drugs, should allow tuning of the reactivity of these radicals.