Metabolite identification in rat brain microdialysates by direct infusion nanoelectrospray ionization after desalting on a ZipTip and LTQ/Orbitrap mass spectrometry.

Analyzing brain microdialysate samples by mass spectrometry is challenging due to the high salt content of the artificial cerebral spinal fluid (aCSF), low analyte concentrations and small sample volumes collected. A drug and its major metabolites can be examined in brain microdialysates by targeted approaches such as selected reaction monitoring (SRM) which provides selectivity and high sensitivity. However, this approach is not well suited for metabolite profiling in the brain which aims to determine biotransformation pathways. Identifying minor metabolites, or metabolites that arise from brain metabolism, remains a challenge and, for a drug in early discovery, identification of metabolites present in the brain can provide useful information for understanding the pharmacological activity and potential toxicological liabilities of the drug. A method is described here for rapid metabolite profiling in brain microdialysates that involves sample clean-up using C18 ZipTips to remove salts followed by direct infusion nanoelectrospray with an LTQ/Orbitrap mass spectrometer using real-time internal recalibration. Full scan mass spectra acquired at high resolving power (100 K at m/z 400) were examined manually and with mass defect filtering. Metabolite identification was aided by sub-parts-per-million mass accuracy and structural characterization was accomplished by tandem mass spectrometry (MS/MS) experiments in the Orbitrap or LTQ depending on the abundance of the metabolite. Using this approach, brain microdialysate samples from rats dosed with one of four CNS drugs (imipramine, reboxetine, citalopram or trazodone) were examined for metabolites. For each drug investigated, metabolites, some of which not previously reported in rat brain, were identified and characterized.

Spectral accuracy of molecular ions in an LTQ/Orbitrap mass spectrometer and implications for elemental composition determination.

In addition to mass accuracy, the ability of a mass spectrometer to faithfully measure the isotopic distribution of an ion, defined as spectral accuracy, is also important. Although time-of-flight mass spectrometers are reported to possess high spectral accuracy capability compared with other mass spectrometers, the Orbitrap has not yet been investigated. Ten natural products (moxidectin, erythromycin, digoxin, rifampicin, amphotericin B, rapamycin, gramicidin S, cyclosporin A, vancomycin, and thiostrepton) ranging in molecular weight from 639 to 1663 Da were measured on an LTQ/Orbitrap mass spectrometer with resolving power settings of 7.5, 15, 30, 60, and 100 K. The difference in the observed profile isotope pattern compared with the theoretical calculation after peak shape calibration, denoted spectral error, was calculated using the program MassWorks (Cerno Bioscience, Danbury, CT, USA). Spectral errors were least at 7.5 K resolving power (< or = 3%) but exceeded 10% for some compounds at 100 K. The increasing spectral error observed at higher resolving power for compounds with complex fine structure might be explained by the phenomena of isotopic beat patterns as observed in FTICR. Several compounds with prominent doubly charged ions allowed comparison of spectral accuracies of singly- versus doubly-charged ions. When using spectral error to rank elemental compositions with formula constraints (C(0-100)H(0-200)N(0-50)O(0-50)Cl(0-5)S(0-5)) and a mass tolerance < or = 2 parts-per-million, the correct formula was ranked first 35% of the time. However, spectral error considerations eliminated >99% of possible elemental formulas for compounds with molecular weight >900 Da.

Liquid chromatography/mass spectrometry determination of endogenous plasma acetyl and palmitoyl carnitines as potential biomarkers of beta-oxidation in mice.

A robust bioanalytical method capable of measuring acetyl and palmitoyl carnitines was developed and validated. Application of hydrophilic interaction chromatography (HILIC) enabled retention of these highly polar and difficult to analyze compounds on a silica HPLC column. The chromatography was conducted with a high percentage of an organic component in the mobile phase, allowing high sensitivity for the pre-existing positively charged quaternary ammonium ions by electrospray ionization mass spectrometry. Successful application of the method to reliably quantify naturally occurring acyl carnitines in mouse plasma depended on the use of corresponding deuterated analogues. The specificity of the method, achieved through the use of stable isotope labeled compounds in combination with a mass spectral multiple reaction monitoring technique, permitted a non-invasive assessment of the overall change in the levels of these acyl carnitines in the plasma of intact animals administered peroxisome proliferator activated receptor (PPAR) agents. These acyl carnitines, as carriers of the corresponding long-chain fatty acids for transport into mitochondria, can be employed as potential biomarkers for significant alteration in the beta-oxidation process in an intact animal.

Rapid metabolite identification with sub parts-per-million mass accuracy from biological matrices by direct infusion nanoelectrospray ionization after clean-up on a ZipTip and LTQ/Orbitrap mass spectrometry.

Metabolite identification studies remain an integral part of pre-clinical and clinical drug development programs. Analysis of biological matrices, such as plasma, urine, feces and bile, pose challenges due to the large amounts of endogenous components that can mask a drug and its metabolites. Although direct infusion nanoelectrospray using capillaries has been used routinely for proteomic studies, metabolite identification has traditionally employed liquid chromatographic (LC) separation prior to analysis. A method is described here for rapid metabolite profiling in biological fluids that involves initial sample clean-up using pipette tips packed with reversed-phase material (i.e. ZipTips) to remove matrix components followed by direct infusion nanoelectrospray on an LTQ/Orbitrap mass spectrometer using a protonated polydimethylcyclosiloxane cluster ion for internal calibration. We re-examined samples collected from a prazosin metabolism study in the rat. Results are presented that demonstrate that sub parts-per-million accuracies can be achieved on molecular ions, facilitating identification of metabolites, and on product ions, facilitating structural assignments. The data also show that the high-resolution measurements (R = 100,000 at m/z 400) enable metabolites of interest to be resolved from endogenous components. The extended analysis times available with nanospray enables signal averaging for 1 min or more that is valuable when metabolites are present in low concentrations as encountered here in plasma and brain. Using this approach, the metabolic fate of a drug can be quickly obtained. A limitation of this approach is that metabolites that are structural isomers cannot be distinguished, although such information can be collected by LC/MS during follow-on experiments.

Metabolism of prazosin in rat, dog, and human liver microsomes and cryopreserved rat and human hepatocytes and characterization of metabolites by liquid chromatography/tandem mass spectrometry.

Prazosin (2-[4-(2-furanoyl)-piperazin-1-yl]-4-amino-6,7-dimethoxyquinazoline) is an antihypertensive agent that was introduced to the market in 1976. It has since established an excellent safety record. However, in vitro metabolism of prazosin has not been investigated. This study describes the in vitro biotransformation of prazosin in liver microsomes from rats, dogs, and humans, as well as rat and human cryopreserved hepatocytes and characterization of metabolites using liquid chromatography/tandem mass spectrometry. The major in vivo biotransformation pathways reported previously in rats and dogs include demethylation, amide hydrolysis, and O-glucuronidation. These metabolic pathways were also confirmed in our study. In addition, several new metabolites were characterized, including a stable carbinolamine, an iminium species, and an enamine-all formed via oxidation of the piperazine ring. Two ring-opened metabolites generated following oxidative cleavage of the furan ring were also identified. Using semicarbazide hydrochloride as a trapping agent, an intermediate arising from opening of the furan ring was captured as a pyridazine product. In the presence of glutathione, three glutathione conjugates were detected in microsomal incubations, although they were not detected in cryopreserved hepatocytes. These data support ring opening of the furan via a reactive gamma-keto-alpha,beta-unsaturated aldehyde intermediate. In the presence of UDP-glucuronic acid, prazosin underwent conjugation to form an N-glucuronide not reported previously. Our in vitro investigations have revealed additional metabolic transformations of prazosin and have shown the potential of prazosin to undergo bioactivation through metabolism of the furan ring to a reactive intermediate.

Drug interactions: concerns and current approaches.

Following the recent withdrawal of several prominent drugs from US and European markets because of detrimental drug-drug interactions, metabolic drug interactions have received considerable attention in the pharmaceutical industry. In turn, the question of drug safety has received significant legal, regulatory and commercial emphasis, bringing this issue to the forefront of both industry and government drug agendas. The value of predicting the drug interactions of compounds as early as possible in the drug discovery process for all therapeutic areas cannot be underestimated. From 1964 to 1999, approximately 8% of the drugs approved by the FDA were later withdrawn from the US market. Pharmaceutical companies are facing increasing pressure to prove the long-term safety of their products, and this is complicated by the fact that animal models are not perfectly predictive of human responses, and may provide contradictory information. The failure to address safety concerns successfully during the drug optimization process may result in companies withdrawing any approved drugs from the market; drug safety issues not only present human health consequences, but also have a negative economic and public relations impact on the pharmaceutical industry. This paper discusses the significance of drug interactions, and addresses strategies to evaluate the potential of a drug candidate for drug interactions.

The impact of recent innovations in the use of liquid chromatography-mass spectrometry in support of drug metabolism studies: are we all the way there yet?

Absorption, distribution, metabolism, excretion and toxicology (ADMET) studies are widely used in drug discovery and development to help obtain the optimal balance of properties necessary to convert lead compounds into drugs that are safe and effective for human use. Drug discovery efforts have been aimed at identifying and addressing metabolism issues at the earliest possible stage, by developing and applying innovative liquid chromatography-mass spectrometry (LC-MS)-based techniques and instrumentation, which are both faster and more accurate than prior techniques. Such new approaches are demonstrating considerable potential to improve the overall safety profile of drug candidates throughout the drug discovery and development process. These emerging techniques streamline and accelerate the process by eliminating potentially harmful candidates earlier and improving the safety of new drugs. In the area of drug metabolism, for example, revolutionary changes have been achieved by the combination of LC-MS with innovative instrumentation such as triple quadrupoles, ion traps and time-of-flight mass spectrometry. In turn, most ADMET studies have come to rely on LC-MS for the analysis of an ever-increasing workload of potential candidates. This article provides a discussion on the importance of LC-MS in supporting drug metabolism studies, and highlights the relative merits of current applications for LC-MS in drug metabolism testing and analysis. These applications include in vitro and in vivo testing, pharmacokinetic profiling, chiral separations, stable isotope labeling, metabolic activation testing, metabolite characterization and radiolabeled-drug testing.

Metabolic activation of troglitazone: identification of a reactive metabolite and mechanisms involved.

Troglitazone (TGZ), the first glitazone used for the treatment of type II diabetes mellitus and removed from the market for liver toxicity, was shown to bind covalently to microsomal protein and glutathione (GSH) following activation by cytochrome P450 (P450). The covalent binding of (14)C-TGZ in dexamethasone-induced rat liver microsomes was NADPH-dependent and required the active form of P450; it was completely inhibited by ketoconazole (10 microM) and GSH (4 mM). The covalent binding in P450 3A4 Supersomes (9.2 nmol of TGZ Eq/nmol P450) was greater than that with P450 1A2 (0.7), 2C8 (3.7), 2C19 (1.4), 2E1 (0.6), and 2D6 (1.1) and 3A5 (3.0). The covalent binding in liver microsomes from rats pretreated with dexamethasone (5.3 nmol of TGZ Eq bound/nmol P450) was greater than that from rats pretreated with vehicle (3.5), beta-naphthoflavone (0.4), phenobarbital (1.1), or pyridine (2.5). A TGZ-GSH adduct was detected by liquid chromatography-tandem mass spectrometry and radioactivity detection with a deprotonated quasi-molecular ion [M-H](-) at m/z 745, with fragment ions at m/z 438 (deprotonated TGZ moiety), and at m/z 306 (deprotonated GSH moiety). The TGZ-GSH adduct was determined to be 5-glutathionyl-5-[4-(6-hydroxy-2,5,7,8-tetramethylchroman-2-ylmethoxy)benzyl]-thiazolidine-2,4-dione based on collision-induced dissociation fragmentation, and one- and two-dimensional NMR analysis of the isolated adduct. The synthetic 5-hydroxy TGZ and the benzylidene derivative of TGZ did not react with GSH or GSH ethyl ester. The mechanisms for metabolic activation of TGZ may involve an ultimate reactive sulfonium ion which could be formed from an initial sulfoxide followed by a formal Pummerer rearrangement, or a C5 thiazolidinedione radical or a sulfur cation radical.

Incorporation of an oxygen from water into troglitazone quinone by cytochrome P450 and myeloperoxidase.

Troglitazone (TGZ) was the first glitazone used for the treatment of type II diabetes mellitus. TGZ undergoes an oxidative chroman ring-opening reaction to form a quinone product. Recently, cytochrome P450 (P450) was shown to be able to catalyze the formation of TGZ quinone. TGZ quinone was the major metabolite formed by dexamethasone-induced rat liver microsomes or myeloperoxidase (MPO) incubated with TGZ. The ultimate source for the quinone carbonyl oxygen atom of TGZ quinone was investigated using (18)O water in both enzyme reaction systems followed by liquid chromatography/tandem mass spectometry analysis of the TGZ quinone product. The resultant TGZ quinone formed by either liver microsomes or MPO contained a single atom of (18)O. The (18)O atom was determined to be the quinone carbonyl oxygen by collision-induced dissociation fragmentation of the (18)O-labeled TGZ quinone. The formation of TGZ quinone was inhibited approximately 90% by coincubation with ascorbic acid or cysteine in the MPO reaction system but only 10 to 20% in liver microsomes, which might reflect the difference in the mechanism by which TGZ quinone is formed by P450 and peroxidase. These results suggest that P450 catalyze an atypical reaction to form TGZ quinone, involving the incorporation of an oxygen from water into the quinone carbonyl position.

Strategies for dealing with metabolite elucidation in drug discovery and development.

Structural information on metabolites can be a considerable asset for enhancing and streamlining the process of developing new drug candidates. Modern approaches that generate and use metabolite structural information can accelerate the drug discovery and development process by eliminating potentially harmful candidates earlier in the process and improving the safety of new drugs. This review examines the relative merits of current and potential strategies for dealing with metabolite characterization.

Faciometrics: a new syntax for facial feature analysis.

A little neglect appears to exist in current cephalometric soft tissue analysis. Specifically, some areas of the facial integument are not accessible to cephalometric analysis. Further, no method of frontal cephalometric soft tissue analysis has been described. Faciometrics, a new cephalometric, nonradiographic system for facial feature analysis, may carry professional diagnostic capacity to another dimension by providing an evaluation for previously inaccessible facial areas and by supplying a beginning for frontal cephalometric soft tissue analysis. The reliability and the validity of the proposed system were tested on a random sample of 20 subjects. The system was found to be reliable and valid. A method for rapid checking of facial symmetry is also described, and some equations for facial proportions are presented as guidelines for good facial esthetics. The faciometric system is proposed as a complement to, rather than a replacement for, available soft tissue cephalometric analysis.

Diagnostic value of DNA content in keratosis and adult papilloma of the larynx.

Nuclear DNA content of laryngeal epithelial cells in smears was carried out by DNA cytometry with automatic microscope and T.V. image analysis system in 6 adult papilloma and 10 keratosis of the larynx patients. The method had high degree of precision confirmed by whole tissue biopsy and serial sections. The method is non invasive and allows fair diagnosis of suspicious and early malignant cases.

Osteoperiosteal flap in the treatment of ozena. New technique.

An osteoperiosteal flap taken from the anterior wall of the maxilla and pedicled on its periosteum was developed to narrow the nasal cavity of 15 patients with ozena. Good results were obtained in over 80% of cases. Crusting, fetor, and epistaxis disappeared. The nasal mucosa appeared healthy. The bulge in the lateral nasal wall persisted. The method is not liable to graft rejection or shrinkage. The advantages of this new technique are discussed.

Stapedius reflex after stapedectomy with preservation of the stapedius tendon.

Stapedectomy with preservation of the stapedius tendon was carried out on 25 patients. Impedance measurements confirm that the stapedius muscle does function thereafter. The results of stapedius reflex tests are shown by an x-y plotter on an Amplaid 702 for permanent records. The compliance of the drum is also increased by about 50 per cent. The characteristics of the reflex are reported and discussed as well as its clinical value. There is better speech discrimination and no pseudorecruitment.