A Capillary Electrophoresis Electrospray Ionization-Mass Spectrometry Method using a Borate Background Electrolyte for the Fingerprinting Analysis of Flavonoids in Ginkgo biloba Herbal Supplements

A laboratory-built sheath liquid capillary electrophoresis-mass spectrometry interface was used to develop a qualitative method for fingerprinting analysis of 14 structurally similar flavones, flavonols, flavonones, and several representative glycosides in plant samples. The migration order of the flavonoids was dependent on a the number of hydroxyl groups present on the flavonoid B-ring, extent of conjugation, number of glycosidic functionalities, and ability of the flavonoid to form stable borate complexes with the background electrolyte. Parent ion scans of the flavonoids yielded [M-H]-, except for catechol containing flavonoids, which were detected as borate adducts. These adducts can be used diagnostically to determine the presence or absence of catechol groups on unknown polyphenolic compounds. Product ion scans of the flavonoid glycosides and borate adducts typically yielded the deprotonated aglycone fragment as the base peak, which could be used to confirm the base structure of the flavonoid. This method's utility was demonstrated by analyzing flavonoids present in ethanolic extracts of Ginkgo biloba herbal supplements.

MRP4 Modulation of the Guanylate Cyclase-C/cGMP Pathway: Effects on Linaclotide-Induced Electrolyte Secretion and cGMP Efflux

MRP4 mediates the efflux of cGMP and cAMP and acts as an important regulator of these secondary messengers, thereby affecting signaling events mediated by cGMP and cAMP. Immunofluorescence staining showed high MRP4 expression localized predominantly in the apical membrane of rat colonic epithelium. In vitro studies were performed using a rat colonic mucosal layer mounted in an Ussing chamber. Linaclotide activation of the guanylate cyclase-C (GC-C)/cGMP pathway induced a concentration-dependent increase in transepithelial ion current [short-circuit current (Isc)] across rat colonic mucosa (EC50: 9.2 nM). Pretreatment of colonic mucosa with the specific MRP4 inhibitor MK571 potentiated linaclotide-induced electrolyte secretion and augmented linaclotide-stimulated intracellular cGMP accumulation. Notably, pretreatment with the phosphodiesterase 5 inhibitor sildenafil increased basal Isc, but had no amplifying effect on linaclotide-induced Isc. MRP4 inhibition selectively affected the activation phase, but not the deactivation phase, of linaclotide. In contrast, incubation with a GC-C/Fc chimera binding to linaclotide abrogated linaclotide-induced Isc, returning to baseline. Furthermore, linaclotide activation of GC-C induced cGMP secretion from the apical and basolateral membranes of colonic epithelium. MRP4 inhibition blocked cGMP efflux from the apical membrane, but not the basolateral membrane. These data reveal a novel, previously unrecognized mechanism that functionally couples GC-C-induced luminal electrolyte transport and cGMP secretion to spatially restricted, compartmentalized regulation by MRP4 at the apical membrane of intestinal epithelium. These findings have important implications for gastrointestinal disorders with symptoms associated with dysregulated fluid homeostasis, such as irritable bowel syndrome with constipation, chronic idiopathic constipation, and secretory diarrhea.

Development of an on-animal separation-based sensor for monitoring drug metabolism in freely roaming sheep

The development of an on-animal separation-based sensor that can be employed for monitoring drug metabolism in a freely roaming sheep is described. The system consists of microdialysis sampling coupled to microchip electrophoresis with electrochemical detection (MD-ME-EC). Separations were accomplished using an all-glass chip with integrated platinum working and reference electrodes. Discrete samples from the microdialysis flow were introduced into the electrophoresis chip using a flow-gated injection approach. Electrochemical detection was accomplished in-channel using a two-electrode isolated potentiostat. Nitrite was separated by microchip electrophoresis using reverse polarity and a run buffer consisting of 50 mM phosphate at pH 7.4. The entire system was under telemetry control. The system was first tested with rats to monitor the production of nitrite following perfusion of nitroglycerin into the subdermal tissue using a linear probe. The data acquired using the on-line MD-ME-EC system were compared to those obtained by off-line analysis using liquid chromatography with electrochemical detection (LC-EC), using a second microdialysis probe implanted parallel to the first probe in the same animal. The MD-ME-EC device was then used on-animal to monitor the subdermal metabolism of nitroglycerin in sheep. The ultimate goal is to use this device to simultaneously monitor drug metabolism and behavior in a freely roaming animal.

N-acetyl-S-(N,N-diethylcarbamoyl) cysteine in rat nucleus accumbens, medial prefrontal cortex, and in rat and human plasma after disulfiram administration

Disulfiram (DSF), a treatment for alcohol use disorders, has shown some clinical effectiveness in treating addiction to cocaine, nicotine, and pathological gambling. The mechanism of action of DSF for treating these addictions is unclear but it is unlikely to involve the inhibition of liver aldehyde dehydrogenase (ALDH2). DSF is a pro-drug and forms a number of metabolites, one of which is N-acetyl-S-(N,N-diethylcarbamoyl) cysteine (DETC-NAC). Here we describe a LCMS/MS method on a QQQ type instrument to quantify DETC-NAC in plasma and intracellular fluid from mammalian brain. An internal standard, the N,N-di-isopropylcarbamoyl homolog (MIM: 291>128) is easily separable from DETC-NAC (MIM: 263>100) on C18 RP media with a methanol gradient. The method's linear range is 0.5-500 nM from plasma and dialysate salt solution with all precisions better than 10% RSD. DETC-NAC and internal standards were recovered at better than 95% from all matrices, perchloric acid precipitation (plasma) or formic acid addition (salt) and is stable in plasma or salt at low pH for up to 24 h. Stability is observed through three freeze-thaw cycles per day for 7 days. No HPLC peak area matrix effect was greater than 10%. A human plasma sample from a prior analysis for S-(N,N-diethylcarbamoyl) glutathione (CARB) was found to have DETC NAC as well. In other human plasma samples from 62.5 mg/d and 250 mg/d dosing, CARB concentration peaks at 0.3 and 4 nM at 3 h followed by DETC-NAC peaks of 11 and 70 nM 2 h later. Employing microdialysis sampling, DETC-NAC levels in the nucleus accumbens (NAc), medial prefrontal cortex (mPFC), and plasma of rats treated with DSF reached 1.1, 2.5 and 80 nM at 6h. The correlation between the appearance and long duration of DETC-NAC concentration in rat brain and the persistence of DSF-induced changes in neurotransmitters observed by Faiman et al. (Neuropharmacology, 2013, 75C, 95-105) is discussed.

Correlation of 3-mercaptopropionic acid induced seizures and changes in striatal neurotransmitters monitored by microdialysis

OBJECTIVES

The goal of this study was to use a status epilepticus steady-state chemical model in rats using the convulsant, 3-mercaptopropionic acid (3-MPA), and to compare the changes in striatal neurotransmission on a slow (5min) and fast (60s) timescale. In vivo microdialysis was combined with electrophysiological methods in order to provide a complete evaluation of the dynamics of the results obtained.

OBJECTIVE

To compare the effects of a steady-state chemical model pof status epilepticus on striatal amino-acid and amine neurotransmitters contents, as measured via in vivo microdialysis combined with electrophysiological methods. Measurements were performed on samples collected every 60s and every 5min. "Fast" (60s) and "slow" (5min) sampling timescales were selected, to gain more insight into the dynamics of GABA synthesis inhibition and of its effects on other neurotransmitters and on cortical electrical activity.

METHODS

3-MPA was administered in the form of an intra-venous load (60mg/kg) followed by a constant infusion (50mg/kg/min) for min. Microdialysis samples were collected from the striatum at intervals of 5min and 60s and analyzed for biogenic amine and amino acid neurotransmitters. ECoG activity was monitored via screws placed over the cortex.

RESULTS

In the 5min samples, glutamate (Glu) increased and γ-aminobutyric acid (GABA) decreased monotonically while changes in dopamine (DA) concentration were bimodal. In the sixty second samples, Glu changes were bimodal, a feature that was not apparent with the 5min samples. ECoG activity was indicative of status epilepticus.

CONCLUSIONS

This study describes the combination of in vivo microdialysis with electrophysiology to monitor the effect of 3-MPA on neurotransmission in the brain. This led to a better understanding of the chemical changes in the striatum due to the applied 3-MPA chemical model of status epilepticus.

S-(N, N-diethylcarbamoyl)glutathione (carbamathione), a disulfiram metabolite and its effect on nucleus accumbens and prefrontal cortex dopamine, GABA, and glutamate: a microdialysis study

Disulfiram (DSF), used for the treatment of alcohol use disorders (AUDs) for over six decades, most recently has shown promise for treating cocaine dependence. Although DSF's mechanism of action in alcohol abuse is due to the inhibition of liver mitochondrial aldehyde dehydrogenase (ALDH2), its mechanism of action in the treatment of cocaine dependence is unknown. DSF is a pro-drug, forming a number of metabolites each with discrete pharmacological actions. One metabolite formed during DSF bioactivation is S-(N, N-diethylcarbamoyl) glutathione (carbamathione) (carb). We previously showed that carb affects glutamate binding. In the present studies, we employed microdialysis techniques to investigate the effect of carb administration on dopamine (DA), GABA, and glutamate (Glu) in the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC), two brain regions implicated in substance abuse dependence. The effect of DSF on DA, GABA, and Glu in the NAc also was determined. Both studies were carried out in male rats. Carb (20, 50, 200 mg/kg i v) in a dose-dependent manner increased DA, decreased GABA, and had a biphasic effect on Glu, first increasing and then decreasing Glu in both the NAc and mPFC. These changes all occurred concurrently. After carb administration, NAc and mPFC carb, as well as carb in plasma, were rapidly eliminated with a half-life for each approximately 4 min, while the changes in DA, GABA, and GLu in the NAc and mPFC persisted for approximately two hours. The maximal increase in carb (Cmax) in the NAc and mPFC after carb administration was dose-dependent, as was the area under the curve (AUC). DSF (200 mg/kg i p) also increased DA, decreased GABA, and had a biphasic effect on Glu in the NAc similar to that observed in the NAc after carb administration. When the cytochrome P450 inhibitor N-benzylimidazole (NBI) (20 mg/kg i p) was administered before DSF dosing, no carb could be detected in the NAc and plasma and also no changes in NAc DA, GABA, and GLu occurred. Changes in these neurotransmitters occurred only if carb was formed from DSF. When NBI was administered prior to dosing with carb, the increase in DA, decrease in GABA, and biphasic effect on GLu was similar to that seen after dosing with carb only. The i p or i v administration of carb showed similar changes in DA, GABA, and GLu, except the time to reach Cmax for DA as well as the changes in GABA, and GLu after i p administration occurred later. The elimination half-life of carb and the area under the curve (AUC) were similar after both routes of administration. It is concluded that carb must be formed from DSF before any changes in DA, GABA, and GLu in the NAc and mPFC are observed. DSF and carb, when administered to rats, co-release DA, GABA, and GLu. Carb, once formed can cross the blood brain barrier and enter the brain. Although inhibition of liver ALDH2 is the accepted mechanism for DSF's action in treating AUDs, the concurrent changes in DA, GABA, and GLu in the NAc and mPFC after DSF administration suggest that changes in these neurotransmitters as a potential mechanism of action not only for AUDs, but also for cocaine dependence cannot be excluded.

A parallel dual-electrode detector for capillary electrophoresis

An approach to on-capillary dual-electrode detection for CE using a parallel electrode configuration has been developed. The parallel configuration provides two operating modes. In the first mode, one working electrode is held at an oxidizing potential and the second working electrode is held at a reducing potential. This results in redox cycling of analytes between the oxidized and reduced forms, enhancing sensitivity compared to single-electrode detection. In the second mode, both working electrodes are held at different oxidizing potentials. This mode provides electrochemical characterization of electrophoretic peaks. In the redox cyclying mode, signal enhancement of up to twofold was observed for the dual-electrode detection of phenolic acid standards compared to single-electrode detection. Variation in response of less than 10% from electrode to electrode was determined (at a concentration of 60 nM) indicating reproducible fabrication. LODs were determined to be as low as 5.0 nM for dual-electrode configuration. Using the dual-potential mode peak identification of targeted phenolic acids in whiskey samples were confirmed based on both migration time and current ratios.

Determination of GABA, glutamate and carbamathione in brain microdialysis samples by capillary electrophoresis with fluorescence detection

Disulfiram has been used as a deterrent in the treatment of alcohol abuse for almost 60 years. Our laboratory has shown that a disulfiram metabolite, S-(N,N-diethylcarbamoyl) glutathione (carbamathione), is formed from disulfiram and appears in the brain after the administration of disulfiram. Carbamathione does not inhibit aldehyde dehydrogenase but has been shown to be a partial non-competitive inhibitor of the N-methyl-D-aspartic acid glutamate (Glu) receptor. In light of disulfiram's apparent clinical effectiveness in cocaine dependence, and carbamathione's effect on the N-methyl-D-aspartic acid receptor, the effect of carbamathione on brain Glu and γ-aminobutyric acid (GABA) needs to be further examined. A CE-LIF method based on derivatization with napthalene-2,3-dicarboxyaldehyde to simultaneously detect both neurotransmitter amino acids and carbamathione in brain microdialysis samples is described. The separation of Glu, GABA and carbamathione was carried out using a 50 mmol/L boric acid buffer (pH 9.6) on a 75 cm×50 μm id fused-silica capillary (60 cm effective) at +27.5 kV voltage with a run time of 11 min. The detection limits for Glu, GABA and carbamathione were 6, 10 and 15 nmol/L, respectively. This method was used to monitor carbamathione and the amino acid neurotransmitters in brain microdialysis samples from the nucleus accumbens after the administration of an intravenous dose of the drug (200 mg/kg) and revealed a carbamathione-induced change in GABA and Glu levels. This method demonstrates a simple, rapid and accurate measurement of two amino acid neurotransmitters and carbamathione for in vivo monitoring in the brain using microdialysis sampling.

Determination of in plasma samples by dual-electrode amperometric detection and liquid chromatography

Co-Q10 is a lipid-soluble benzoquinone that is an important factor in free radical scavenging, mitochondrial membrane stability and ATP synthesis. Dietary Co-Q10 is a powerful antioxidant that has been useful in lessening the damage associated with ischemia-reperfusion injuries and aiding in the recovery of myocardial function after myocardial infarction. However, the role of dietary Co-Q10 in oxidative damage and repair is not well understood. Previous LC-EC methods have used packed carbon bed electrodes with high overpotentials that were sufficient to oxidize and reduce several biological compounds, thereby decreasing the selectivity that can be achieved with EC detection. Thin-layer cell dual electrode detection enables monitoring of reduced and oxidized forms of Co-Q10 simultaneously and selectively. The oxidation (+0.45 V vs. Ag/AgCl) and reduction (-0.4 V vs. Ag/AgCl) electrode potentials were optimized to oxidize and reduce the electroactive quinone moiety. The reduced form of Co-Q10 was prepared from the commercially available oxidized form using a Jones reductor. Confirmation of its formation was determined using the current ratios of the peak and half wave potentials from previously generated hydrodynamic voltammograms, using the oxidized form with electrodes in a series configuration. This analytical system was successfully applied to determine basal concentrations of oxidized (510 nM) and reduced (500 nM) Co-Q10 in human plasma. Peak identity of oxidized and reduced Co-Q10 was confirmed by two orthogonal methods: by the current ratios at +0.45 V and +0.25 V and -0.4 V and -0.2 V (vs. Ag/AgCl) as well as by retention time. Detection limits were determined to be 5 nM, with a linear range of three orders of magnitude.

LC-MS/MS determination of carbamathione in microdialysis samples from rat brain and plasma

A selective liquid chromatography-tandem mass spectrometric (LC-MS/MS) method was developed for the determination of S-(N, N-diethylcarbamoyl) glutathione (carbamathione) in microdialysis samples from rat brain and plasma. S-(N, N-Diethylcarbamoyl) glutathione (carbamathione) is a metabolite of disulfiram. This metabolite may be responsible for disulfiram's effectiveness in the treatment of cocaine dependence. Chromatographic separations were carried out on an Alltech Altima C-18 (50 mm long x 2.1 mm i.d., 3 microm particles) analytical column at a flow rate of 0.3 ml/min. Solvent A consisted of 10 mM ammonium formate, methanol, and formic acid (99:1:0.06, v/v/v). Solvent B consisted of methanol, 10 mM ammonium formate and formic acid (99:1:0.06, v/v/v). A 20 min linear gradient from 95% aqueous to 95% organic was used. Tandem mass spectra were acquired on a Micromass Quattro Ultima "triple" quadrupole mass spectrometer equipped with an ESI interface. Quantitative mass spectrometric analysis was conducted in positive ion mode selected reaction monitoring (SRM) mode looking at the transition of m/z 407-100 and 175 for carbamathione and m/z 392-263 for the internal standard S-hexyl glutathione. The simultaneous collection of microdialysate from blood and brain was used to monitor carbamathione concentrations centrally and peripherally. Good linearity was obtained over a concentration range of 0.25-10,000 nM. The lowest limit of quantification (LLOQ) was determined to be 1 nM and the lowest limit of detection (LLOD) was calculated to be 0.25 nM. Intra- and inter-day accuracy and precision were determined and for all the samples evaluated, the variability was less that 10% (R.S.D.).

The development of multiple probe microdialysis sampling in the stomach

A multiple probe approach of implanting microdialysis probes into each separate tissue layer would better represent sampling from the stomach. Presently, microdialysis sampling experiments are performed with only a single probe in the submucosa to represent sampling from the stomach tissue. The focus of this research was to develop a four-probe microdialysis sampling design to simultaneously monitor the stomach lumen, mucosa, submucosa and in the blood of the rat. Due to the small outer diameter of the microdialysis probe (350 microm), implantation into each separate layer was achieved with confirmation of probe location from histological examination. To assess the significance of sampling by this approach, multiple probe microdialysis sampling was used to monitor drug absorption in the stomach. Salicylic acid, caffeine and metoprolol were individually dosed to the ligated stomach. Analysis of the dialysate samples was performed by HPLC-UV and concentration-time curves and pharmacokinetics analysis were used to determine differences between the different probe locations.

CE-LIF method for the separation of anthracyclines: application to protein binding analysis in plasma using ultrafiltration

Anthracyclines are chemotherapeutic drugs that are widely used in the treatment of cancers such as lung and ovarian cancers. The simultaneous determination of the anthracyclines, daunorubicin, doxorubicin and epirubicin, was achieved using CE coupled to LIF, with an excitation and emission wavelength of 488 and 560 nm, respectively. Using a borate buffer (105 mM, pH 9.0) and 30% MeOH, a stable and reproducible separation of the three anthracyclines was obtained. The method developed was shown to be capable of monitoring the therapeutic concentrations (50-50 000 ng/mL) of anthracyclines. LODs of 10 ng/mL, calculated at an S/N = 3, were achieved. Using the CE method developed, the in vitro protein binding to plasma was measured by ultrafiltration, and from this investigation the estimated protein binding was determined to be in the range of 77-94%.

Investigation of drug delivery by iontophoresis in a surgical wound utilizing microdialysis

PURPOSE

This study investigated the penetration of lidocaine around and through a sutured incision following the application of iontophoretic and passive patches in the CD Hairless rat.

MATERIALS AND METHODS

Concentrations in localized areas (suture, dermis, subcutaneous, and vascular) were determined using microdialysis sampling followed by analysis using liquid chromatography with UV detection.

RESULTS

Iontophoresis significantly enhanced the dermal penetration of lidocaine. In an intact skin model, dermal concentrations were 40 times greater following iontophoretic delivery compared to passive delivery. In a sutured incision model, iontophoresis enhanced localized concentrations in the dermis, suture, and subcutaneous regions by 6-, 15-, and 20-fold, respectively. Iontophoretic delivery to a region containing a sutured incision was focused to the incision resulting in a greater increase in the suture concentration and in the subcutaneous region directly below the incision.

CONCLUSIONS

The four microdialysis probe design was successful in the determination of localized drug penetration in a sutured incision model. Iontophoresis enhanced skin penetration and allowed for site specific delivery when applied to a sutured incision.

The direct comparison of health and ulcerated stomach tissue: a multiple probe microdialysis sampling approach

The ability to directly compare gastric ulcerated and healthy tissue would aid in the understanding of the physiological differences between these tissue types. Presently, these comparisons can only be drawn by the use of separate animal groups, which results in increased study variability. The focus of this research was to develop a four-probe microdialysis sampling approach to directly compare ulcerated and healthy tissue in the same animal. After controlled chemical ulcer induction, probes were implanted in the ulcerated and healthy stomach submucosa, stomach lumen and in the blood. To assess the significance of this multiple probe approach, drug concentrations in each probe location were monitored after selected compounds were dosed to the ligated stomach by oral gavage. Analysis of the dialysate samples was performed by HPLC-UV and concentration-time curves and pharmacokinetics analyses were used to determine differences between these tissue types.

Enhanced pH-mediated stacking of anions for CE incorporating a dynamic pH junction

A technique has been developed to enhance analyte focusing for CE for the analysis of physiological samples. High-ionic-strength samples are titrated to low-ionic-strength on-line using pH-mediated sample stacking in conjunction with a dynamic pH junction. This method concentrates analytes by reducing their electrophoretic mobility during field-amplification. Parameters responsible for enhanced focusing were investigated, and an enhanced pH-mediated stacking method was optimized for anionic nucleosides. The process results in ultra-narrow peak widths, for example, 0.28 s for thymidine with a 10 min analysis time. Peak width and resolution with the enhanced stacking method were also compared to normal base stacking and electrokinetic injection. With this technique, mass-loading capacity can be increased without degradation in peak shape and resolution is dramatically improved.

Development of tissue-targeted metabonomics. Part 1. Analytical considerations

Tissue-targeted metabonomics provides tissue specific metabolic information while still retaining the profiling approach of traditional metabonomics. Microdialysis sampling is used to generate site-specific samples of endogenous metabolites. The dialysate samples are subjected to proton NMR analysis with data analysis by principal components analysis and partial least squares regression. In this study, sample and data pretreatment methods were examined for their impact on the quality of the data analysis. Specifically, the effects of speed vacuuming, sample solubility, sample pH stability, and sample storage stability were examined. Data pretreatment methods examined included the effects of standardization and normalization to internal standards. In addition, the ability of tissue-targeted metabonomics to generate time trend data was explored and more fully characterized using principal components analysis and partial least squares regression.

[Norepinephrine of the rat myocardium during repeated ischemia]

BACKGROUND

Ischemic preconditioning improves myocardium survival during test ischemia and removes the start of massive norepinephrine (NE) release from sympathetic terminals into the myocardial interstitium. We studied the influence of long and relatively short occlusions on NE release during test occlusion. The possibility has been studied to provoke NE release by local tyramine application on the background of ischemia.

METHODS AND RESULTS

In rats, anesthetized with urethane, interstitial catecholamine concentrations were measured by implanted microdialysis probe in the perfusion areas of the left anterior descending coronary artery (LAD). After stabilization, the LAD was occluded for 10-60 minutes, reperfused for 20-120 minutes and occluded again for 30-80 minutes. Mean dialysate concentration of NE increased to 50 minute of first LAD occlusion from 0.208±0.038 ng/ml to 6,73±1,42 (maximal increase was by 232). Effect of occlusion was highly significant (p <0,00005). 30 min LAD occlusion suppressed massive NE release during subsequent 30 min test occlusion, but 10 min preocclusion did not. Local tyramine application through microdialysis probe provoked big NE overflow to the myocardial interstitium even on the background of occlusion.

CONCLUSION

30 min LAD occlusion damages the mechanism of massive NE release under the influence of ischemia but not of tyramine application.

AAPS-FDA workshop white paper: microdialysis principles, application and regulatory perspectives

Many decisions in drug development and medical practice are based on measuring blood concentrations of endogenous and exogenous molecules. Yet most biochemical and pharmacological events take place in the tissues. Also, most drugs with few notable exceptions exert their effects not within the bloodstream, but in defined target tissues into which drugs have to distribute from the central compartment. Assessing tissue drug chemistry has, thus, for long been viewed as a more rational way to provide clinically meaningful data rather than gaining information from blood samples. More specifically, it is often the extracellular (interstitial) tissue space that is most closely related to the site of action (biophase) of the drug. Currently microdialysis (microD) is the only tool available that explicitly provides data on the extracellular space. Although microD as a preclinical and clinical tool has been available for two decades, there is still uncertainty about the use of microD in drug research and development, both from a methodological and a regulatory point of view. In an attempt to reduce this uncertainty and to provide an overview of the principles and applications of microD in preclinical and clinical settings, an AAPS-FDA workshop took place in November 2005 in Nashville, TN, USA. Stakeholders from academia, industry and regulatory agencies presented their views on microD as a tool in drug research and development.

An investigation into the pharmacokinetics of 3-mercaptopropionic acid and development of a steady-state chemical seizure model using in vivo microdialysis and electrophysiological monitoring

OBJECTIVES

The goal of the present study was to develop a chemical seizure model using the convulsant, 3-mercaptopropionic acid (3-MPA). A pharmacodynamics approach was taken, combining in vivo microdialysis sampling with electrophysiological methods to simultaneously monitor, in real-time, the 3-MPA concentration in the brain and the corresponding electrocorticographic (ECoG) activity.

METHODS

The 3-MPA was administered in two doses (50 and 100 mg/kg) in order to study its pharmacokinetics. Microdialysis samples were collected from the striatum, hippocampus, and jugular vein every 5 min. The microdialysates were analyzed using high-performance liquid chromatography with electrochemical detection (HPLC-EC). The ECoG activity was monitored via screws placed onto the cortex. Noncompartmental pharmacokinetics analysis was performed to obtain the elimination constants (K(e)), the maximum concentration (C(max)), the time to achieve maximum concentration (T(max)), and the area under the concentration-time curves (AUC(inf)).

RESULTS

The average brain K(e) for the 50 and the 100mg/kg doses were 0.060 and 0.018 min(-1), respectively. The brain AUC(inf) for the 50 and 100mg/kg doses were 353 and 2168 mg min(-1)mL(-1), respectively. This led to a 67-fold increase in the observed number of seizures in the higher dose with the average seizure intensity double that of the smaller dose. These data led to the dosing scheme for the chemical seizure model of administering a 3-MPA loading dose of 60 mg/kg followed by a constant infusion of 50 mg/(kg min(-1)).

CONCLUSIONS

This study describes, to our knowledge, the first successful attempt to combine in vivo microdialysis with electrophysiology to monitor in real-time, the concentration and effects of 3-MPA in the brain. This led to the development of a steady-state chemical seizure model.

Concentration profiling in rat tissue by high-resolution magic-angle spinning NMR spectroscopy: investigation of a model drug

The utility of high-resolution magic-angle spinning (HR-MAS) NMR for studying drug delivery in whole tissues was explored by dosing female Sprague-Dawley rats with topical or injectable benzoic acid (BA). In principle, HR-MAS NMR permits the detection of both intra- and extracellular compounds. This is an advantage over the previous detection of topically applied BA using microdialysis coupled to HPLC/UV as microdialysis samples only the extracellular space. Skin and muscle samples were analyzed by (1)H HR-MAS NMR, and BA levels were determined using an external standard solution added to the sample rotor. One to two percent of the BA topical dose was detected in the muscle, showing that BA penetrated through the dermal and subcutaneous layers. Since BA was not detected in the muscle in the microdialysis studies, the NMR spectra revealed the intracellular localization of BA. The amount of BA detected in muscle after subcutaneous injection correlated with the distance from the dosing site. Overall, the results suggest that HR-MAS NMR can distinguish differences in the local concentration of BA varying with tissue type, dosage method, and tissue proximity to the dosing site. The results illustrate the potential of this technique for quantitative analysis of drug delivery and distribution and the challenges to be addressed as the method is refined.

Application of a column selection system and DryLab software for high-performance liquid chromatography method development

This paper describes a strategy for the development of chromatographic methods for drug candidates based upon the use of simple MS compatible mobile phases and optimization of the chromatographic selectivity through variations of the stationary phase and mobile phase pH. The strategy employs an automated column selection system and a series of HPLC columns, varying in hydrophobicity and silanol activity, in combination with DryLab software to develop chromatographic methods for the separation of mixtures of bupivacaine and its metabolites; acidic, basic, and neutral compounds; and atenolol, nitrendipine, and their degradation products.

On-column preconcentration of glutathione and glutathione disulfide using pH-mediated base stacking for the analysis of microdialysis samples by capillary electrophoresis

Capillary electrophoresis (CE) has become a useful analytical tool for the analysis of microdialysis samples. However, CE with UV detection (CE-UV) does not provide detection limits sufficient to quantify glutathione (GSH) and glutathione disulfide (GSSG) in biological samples such as liver microdialysates, because of the small optical path length in the capillary. To overcome this limitation, an on-column preconcentration technique, pH-mediated base stacking, was used in this study to improve the sensitivity of CE-UV. This stacking technique allowed large volumes of high ionic strength sample injection without deterioration of the separation efficiency and resolution. A 26-fold increase in sensitivity was achieved for both GSH and GSSG using the pH-mediated base stacking, relative to normal injection without stacking. The limit of detection for GSH and GSSG was found to be 0.75 microM (S/N=6) and 0.25 microM (S/N=6), respectively. The developed method was used to analyze GSH and GSSG in liver microdialysates of anesthetized Sprague Dawley male rats. The basal concentrations of GSH and GSSG in the liver microdialysates of male rats were found to be 4.73+/-2.08 microM (n=7) and 5.52+/-3.66 microM (n=7), respectively.

Determination of 8-oxoguanine and 8-hydroxy-2'-deoxyguanosine in the rat cerebral cortex using microdialysis sampling and capillary electrophoresis with electrochemical detection

A rapid and sensitive method to determine 8-oxoguanine (8oxoG) and 8-hydroxydeoxyguanosine (8OHdG), biomarkers for oxidative DNA damage, in cerebral cortex microdialysate samples using capillary electrophoresis (CE) with electrochemical detection (CEEC) was developed. Samples were concentrated on-column using pH-mediated stacking for anions. On-column anodic detection was performed with a carbon fiber working electrode and laser-etched decoupler. The method is linear over the expected extracellular concentration range for 8oxoG and 8-OHdG during induced ischemia-reperfusion, with R.S.D. values Collapse

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MESH Headings

• Animals
• Biomarkers/analysis
• Cerebral Cortex/chemistry
• Chromatography, Liquid/methods
• Deoxyadenosines/analysis
• Electrochemistry
• Electrophoresis, Capillary/methods
• Female
• Guanine/analogs & derivatives
• Guanine/analysis
• Mass Spectrometry/methods
• Microdialysis/methods
• Rats
• Rats, Sprague-Dawley

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Grants

• R01 EB000247 NIBIB NIH HHS
• R01 HL069014-02 NHLBI NIH HHS
• R01EB00247 NIBIB NIH HHS
• T32 CA009242 NCI NIH HHS
• R01 HL069014 NHLBI NIH HHS
• T32CA09242 NCI NIH HHS

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Tissue targeted metabonomics: metabolic profiling by microdialysis sampling and microcoil NMR

The concentration of low molecular weight compounds in tissues can yield valuable information about the metabolic state of an organism. Studies of changes in the metabolic state or metabonomics can reflect disease pathways, drug action, or toxicity. This research aims to develop a new approach, tissue targeted metabonomics. Microdialysis sampling and microcoil NMR analysis are employed to compare basal and ischemic metabolic states of various tissues (blood, brain, and heart) of Sprague-Dawley rats. Microdialysis sampling is localized, making the metabolic profile tissue specific. Coupling to NMR analysis is highly advantageous, because a complete metabolic profile is obtained in a single spectrum. However, small sample volumes and low analyte concentrations make analysis of microdialysis samples challenging. Microcoil NMR uses low sample volumes and has improved mass sensitivity, relative to standard 5 mm probes. The coupling of these techniques is a potentially powerful tool for metabonomics analysis.

pH-mediated acid stacking with reverse pressure for the analysis of cationic pharmaceuticals in capillary electrophoresis

When using capillary electrophoresis (CE) for the analysis of biological samples, it is often necessary to employ techniques to overcome peak-broadening that results from having a high-conductivity sample matrix. To improve the concentration detection limits and separation efficiency of cationic pharmaceuticals in CE, pH-mediated acid stacking was performed to electrofocus the sample, improving separation sensitivity for the analyzed cations by 60-fold. However, this method introduces a large titrated acid plug into the capillary. To overcome the limitations this low-conductivity plug poses to stacking, the plug was removed prior to the separation step by applying reverse pressure to force it out of the anode of the capillary. Employing this technique allows for roughly twice the volume of sample to be injected. A maximum sample injection time of 240 s was attainable with baseline peak resolution compared to a maximum sample injection time of 120 s without reverse pressure, leading to a twofold decrease in the limits of detection of the analytes used. Separation efficiency overall is also improved when utilizing the reverse pressure step. For example, a 60 s sample injection time results in 94,000 theoretical plates as compared to 60,500 theoretical plates without reverse pressure. This reverse-pressure method was used for detection and quantitation of several cationic pharmaceuticals that were prepared in Ringer's solution to simulate microdialysis sampling conditions.

Application of capillary electrophoresis with pH-mediated sample stacking to analysis of coumarin metabolites in microsomal incubations

A sensitive method for the analysis of metabolites of coumarin by capillary electrophoresis (CE), incorporating pH-mediated sample stacking, was developed. The analytes were detected in phosphate buffer (pH 7.5; 25 mM), the matrix of the microsomal incubations. Detection was by direct UV absorbance. The three metabolites studied were 7-hydroxycoumarin (7-OHC), 4-hydroxycoumarin (4-OHC) and 2-hydroxyphenylacetic acid (HPAA), and the limits of detection of the analytes were 0.1, 0.5 and 0.3 microM, respectively. The developed method was then applied to microsomal incubations of coumarin. Male Cynomologus monkey microsomes were used in the study and 7-OHC was detected in the incubation mixture.

Determination of bupivacaine and metabolites in rat urine using capillary electrophoresis with mass spectrometric detection

A method using capillary electrophoresis-mass spectrometry (CE-MS) was developed for the structural elucidation of bupivacaine and metabolites in rat urine. Prior to CE-MS analysis, solid-phase extraction (SPE) was used for sample cleanup and preconcentration purposes. Exact mass and tandem mass spectrometric (MS/MS) experiments were performed to obtain structural information about the unknown metabolites. Two instruments with different mass analyzers were used for mass spectrometric detection. A quadrupole time-of-flight (Q-TOF) and a magnetic sector hybrid instrument were coupled to CE and used for the analysis of urine extracts. Hydroxybupivacaine as well as five other isomerically different metabolites were detected including methoxylated bupivacaine.

Cyclodextrin-modified micellar electrokinetic chromatography for the analysis of Esterom, a topical product consisting of hydrolyzed benzoylecgonine in propylene glycol

Esterom, a new drug currently in human clinical trials, is a mixture of compounds in a propylene glycol vehicle. It is being evaluated as a topical treatment to aid in the relief of muscle pain and to increase range of motion. Benzoylecgonine is the major component of Esterom and there are at least nine other minor constituents, including four hydroxypropyl esters that have multiple diasteriomers. The aim of the study was to develop a capillary electrophoresis method for the simultaneous separation of the main components in Esterom, including the multiple proposed diastereomers of the esters. Due to the complex sample composition, the use of micelles and cyclodextrins as buffer modifiers was evaluated. A cyclodextrin-modified micellar electrokinetic chromatography method was able to determine 7 of the 8 UV-active Esterom components, with baseline separation of 7 of the 10 diastereomers of the hydroxypropyl esters.

Investigation of the mechanism of pH-mediated stacking of anions for the analysis of physiological samples by capillary electrophoresis

Capillary electrophoresis has been widely used for the analysis of physiological samples such as plasma and microdialysate. However, sample destacking can occur during the analysis of these high-ionic strength samples, resulting in poor separation efficiency and reduced sensitivity. A technique termed pH-mediated stacking of anions (base stacking) has previously been developed to analyze microdialysate samples and achieve on-line preconcentration of analytes by following sample injection with an injection of sodium hydroxide. In this work, the mechanism of base stacking was investigated. Peak efficiency was shown to be a function of background electrolyte and sample ionic strength. Analytes representing several classes of compounds with a wide range of mobilities were used to study the effects of multiple parameters on sample stacking. The length of hydroxide injection required for stacking was shown to be dependent on analyte mobility and the type of amine background electrolyte used. Combinations of electrokinetic and hydrodynamic injections of sample and hydroxide were examined and it was concluded that although stacking could be achieved with several injection modes, electrokinetic injection of both sample and hydroxide was most effective for sample stacking. The mechanism of pH-mediated stacking for each of these modes is presented.

On-column electrochemical detection for microchip capillary electrophoresis

The development of a cellulose acetate decoupler for on-column electrochemical detection in microchip capillary electrophoresis is presented. The capillary based laser-etched decoupler is translated to the planar format to isolate the detector circuit from the separation circuit. The decoupler is constructed by aligning a series of 20 30-microm holes through the coverplate of the microchip with the separation channel and casting a thin film of cellulose acetate within the holes. The decoupler shows excellent isolation of the detection circuit for separation currents up to 60 microA, with noise levels at or below 1 pA at a carbon fiber electrode. Detection limits of 25 nM were achieved for dopamine. This decoupler design combines excellent mechanical stability, effective shunting of high separation currents, and ease of manufacture.

A chip-based electrophoresis system with electrochemical detection and hydrodynamic injection

A simple capillary electrophoresis system in the planar format that uses a new, hydrodynamic injection principle is described. The system was realized with poly(dimethylsiloxane)-glass chips and microdisk electrodes for amperometric detection. Using a double-tee injector, no precise voltage control of the electrolyte reservoirs was needed, thus making the microchip CE system more user-friendly. The analytical characteristics of chip-based CE-EC were evaluated using ascorbic acid as the model analyte. The reproducibility of migration time and signal height was expressed by relative standard deviations of 1.2% and 5.1%, respectively (n = 5). The limit of detection for ascorbic acid was approximately 5 microM at a signal-to-noise ratio of 3. Practical application concerning the determination of physiologically relevant compounds such as noradrenaline and L-dopa are discussed.

Determination of cannabinoids in hair using high-pH* non-aqueous electrolytes and electrochemical detection. Some aspects of sensitivity and selectivity

Non-aqueous capillary electrophoresis with electrochemical detection (NACE-ED) was applied to the determination of cannabinoids in hair. The effect of different electrolyte compositions on the selectivity of the separation of tetrahydrocannabinol (THC), cannabinol (CBN), cannabidiol (CBD) and tetrahydrocannabinol carboxylic acid (THCA) was studied. Complete electrophoretic resolution was obtained using a strongly basic background electrolyte consisting of 5 mM sodium hydroxide dissolved in acetonitrile-methanol (1:1). Electrochemical detection yielded well defined signals in the oxidation mode. In order to obtain low limits of detection experimental parameters, which determine the sensitivity and the noise level, were optimized. A crucial parameter for sensitive measurements using a wall-tube flow cell as end-column detector is the distance between the capillary outlet and the working electrode. The highest signal-to-noise ratio using a 50 microm I.D. capillary was obtained at a distance of 25 microm. When the capillary outlet was moved away from the working electrode, thus reducing the strength of the separation field present at the working electrode, a large low frequency noise developed. This rise was attributed to disturbances of the hydrodynamic pattern in the flow cell. Analytical aspects such as sensitivity, reproducibility and selectivity were addressed in this work. The precision of NACE-ED regarding migration time and peak height for a sample containing 1 microg/ml THC was 0.4% and 1.1% (RSD), respectively (n=5). The calibration curve was linear for concentrations ranging between 0.1 and 10 microg/ml (r=0.998). The limit of detection for THC was 37 ng/ml, which is almost two orders of magnitude lower when compared with on-column UV detection. The method was evaluated using hair samples containing cannabinoids as sample material.

Cellulose acetate decoupler for on-column electrochemical detection in capillary electrophoresis

A new decoupler for on-column electrochemical detection in capillary electrophoresis is presented. The decoupler is constructed by etching a series of holes through the side of the separation capillary with a CO2 laser and then coating the holes with cellulose acetate. The decoupler shows isolation of the detection circuit for separation currents up to 30 microA. Detection limits below 1 nM were achieved for four model compounds, including anions, neutrals, and cations, using the laser-etched decoupler. This decoupler design combines excellent mechanical stability, effective shunting of high separation currents, and ease of manufacture.

pH-mediated field-amplified sample stacking of pharmaceutical cations in high-ionic strength samples

Capillary electrophoretic separation of samples of physiological origin typically have both poor resolution and efficiency due to destacking. We have previously reported a stacking method for concentration of catecholamines in artificial dialysate, or Ringer's solution. However, pH-mediated sample stacking of other cations has not been investigated. In this report, pH-mediated stacking has been extended to eletripan, dofetilide, doxazosin, sildenafil, UK-103,320, UK-202,581, and CP-122,288. These compounds were chosen without prior structural screening except that they were cationic at the pH of our background electrolyte (BGE). Capillary electrophoretic behavior of samples in BGE is compared with those of samples in Ringer's solution with and without pH-mediated acid stacking. Results indicate that the peak heights and efficiencies for acid-stacked samples are increased compared to the unstacked samples in Ringer's solution or BGE. For example, the peak efficiencies for 5 s injections of eletriptan in BGE and Ringer's solution are 138,000 and 72,000 plates, respectively. In contrast, a 10 s injection of eletriptan followed by acid injection for 16 s produces a peak with 246,000 plates. Evaluation of the stacking effect was performed by comparison of the peak height at similar peak efficiencies for samples in Ringer's solution with and without stacking. Using this method, pH-mediated acid stacking provides a 10- to 27-fold sensitivity enhancement for the seven cations.

Microdialysis sampling of the isothiazolone, PD-161374, and its thiol and disulfide metabolites

A method based on microdialysis sampling combined with high-performance liquid chromatography (HPLC) has been developed for monitoring the anti-HIV agent PD-161374 (isothiazolone) and its thiol and disulfide metabolites in blood. It was demonstrated that unlike blood withdraw and extraction, microdialysis sampling can preserve the distribution among the isothiazolone and its metabolites in blood. The use of a narrow-bore HPLC system, combined with the relatively high probe extraction efficiency (approximately 50%) from the flexible probe design in this work, allows the direct and quantitative determination of the drug and its major metabolites at submicromolar level.

Analytical considerations for microdialysis sampling

Adaptations in microdialysis probe designs have made it possible to obtain samples from the extracellular fluid of a variety of tissues with high temporal resolution. The resulting small volume samples, often with low concentration of the analyte(s) of interest, present a particular challenge to the analytical system. Rapid separations can be coupled on-line with microdialysis to provide near real-time data. By combining microdialysis sampling with a liquid chromatographic or capillary electrophoretic separation and a highly sensitive detection method, a separation-based sensor can be developed. Such sensors have been applied to the investigation of drug entities as well as to study endogenous analytes.

On-line preconcentration methods for capillary electrophoresis

The limits of detection (LOD) for capillary electrophoresis (CE) are constrained by the dimensions of the capillary. For example, the small volume of the capillary limits the total volume of sample that can be injected into the capillary. In addition, the reduced pathlength hinders common optical detection methods such as UV detection. Many different techniques have been developed to improve the LOD for CE. In general these techniques are designed to compress analyte bands within the capillary, thereby increasing the volume of sample that can be injected without loss of CE efficiency. This on-line sample preconcentration, generally referred to as stacking, is based on either the manipulation of differences in the electrophoretic mobility of analytes at the boundary of two buffers with differing resistivities or the partitioning of analytes into a stationary or pseudostationary phase. This article will discuss a number of different techniques, including field-amplified sample stacking, large-volume sample stacking, pH-mediated sample stacking, on-column isotachophoresis, chromatographic preconcentration, sample stacking for micellar electrokinetic chromatography, and sweeping.

Detection of a urinary biomaker for oxidative DNA damage 8-hydroxydeoxyguanosine by capillary electrophoresis with electrochemical detection

8-Hydroxydeoxyguanosine (8-OHdG) is present in urine as a result of oxidative DNA damage associated with age-related diseases such as cancer. In this report a method is presented for the detection of 8-OHdG in human morning urine utilizing capillary electrophoresis with electrochemical detection (CEEC). The limit of detection for a aqueous standard of 8-OHdG is 50 nM (signal to noise ratio S/N = 3). A single solid-phase extraction (SPE) step with a C18 column is used for sample cleanup and 20-fold preconcentration of the urine before analysis by CEEC. Optimized conditions for analysis of extracted urine are E(app) = 0.5 V vs. Ag/AgCl with 20 mM sodium borate/20% MeOH v/v, pH 9, as the background electrolyte, and a separation voltage of 22 kV. The concentration of 8-OHdG varied from 6 to 86 nM with an average value of 42 +/- 26.9 nM for four healthy female and four healthy male subjects between the ages of 23 and 43.

Analysis of illicit drugs by nonaqueous capillary electrophoresis and electrochemical detection

Nonaqueous capillary electrophoresis (NACE) was applied to the determination of illicit drugs. The complete separation of amphetamine, methamphetamine, 3,4-methylene dioxy amphetamine (MDA), 3,4-methylene dioxy methamphetamine (MDMA), mescaline, cocaine and benzoylecgonine was obtained using an acetonitrile based buffer solution containing 10 mM sodium acetate and 1 M acetic acid. Electrochemical detection using a Pt microdisk electrode set to a potential of +1.8 V was found to be selective for MDA, MDMA and mescaline. The detection limits for these compounds were in the low ng/mL range which is between 2 and 3 orders of magnitude lower compared to UV-detection.

Microdialysis sampling with on-line microbore HPLC for the determination of tirapazamine and its reduced metabolites in rats

An on-line microdialysis microbore HPLC method is described for the determination of the bioreductive anti-tumor agent, tirapazamine (3-amino-1,2,4-benzotriazine-1,4-di-N-oxide, SR4233, WIN59075, Tirazone, TPZ) and its two major reduced metabolites, 3-amino-1,2,4-benzotriazine-1-N-oxide (SR4317) and 3-amino-1,2,4-benzotriazine (SR4330). Detection limits of 0.003 microM, 0.005 microM and 0.007 microM were obtained for tirapazamine, SR4317 and SR4330, respectively. Linear ranges of 0.011-20 microM, 0.017-20 microM and 0.025-20 microM for tirapazamine, SR4317 and SR4330 permitted quantitative analysis of all three compounds in microdialysis samples. Typical intra-day reproducibilities (n = 7) of 4.1% (tirapazamine), 6.6% (SR4317), 9.9% (SR4317), and 1.8% (tirapazamine), 2.4% (SR4317) and 2.6% (SR4330) were obtained at the 0.12 microM and 1.2 microM levels, respectively. Inter-day reproducibilities (n = 5) of 3.4% (tirapazamine), 1.8% (SR4317), 4.5% (SR4330) and 2.5% (tirapazamine), 2.5% (SR4317) and 1.7% (SR4330) were obtained at the 0.12 microM and 1.2 microM levels, respectively. The use of an on-line microdialysis HPLC system, permitted the determination of tirapazamine, SR4317 and SR4330 in blood and muscle tissue of rats with a high temporal resolution of sampling. The pharmacokinetics of tirapazamine and its metabolites were studied in the muscle and blood of rats previously administered an intraperitoneal dose of tirapazamine.