Utility of the parent-neutral loss scan screening technique: Partial characterization of urinary metabolites of U-78875 in monkey urine

In vivo metabolite detection and identification in drug discovery via LC-MS/MS with data-dependent scanning and postacquisition data mining

An important aspect in drug discovery is the early structural identification of the metabolites of potential new drugs. This gives information on the metabolically labile points in the molecules under investigation, suggesting structural modifications to improve their metabolic stability, and allowing an early safety assessment via the identification of metabolic activation products. From an analytical point of view, metabolite identification still remains a challenging task, especially for in vivo samples, in which they occur at trace levels together with high amounts of endogenous compounds. Here we describe a method, based on LC-ion trap tandem MS, for the rapid in vivo metabolite identification. It is based on the automatic, data-dependent acquisition of multiple product ion MS/MS scans, followed by a postacquisition search, within the entire MS/MS data set obtained, for specific neutral losses or marker ions in the tandem mass spectra of parent molecule and putative metabolites. One advantage of the method is speed, since it requires minimum sample preparation and all the necessary data can be obtained in one chromatographic run. In addition, it is highly sensitive and selective, allowing detection of trace metabolites even in the presence of a complex matrix. As an example of application, we present the studies of the in vivo metabolism of the compound MEN 15916 (1). The method allowed identification of monohydroxy ([M + H](+) = m/z 655), dihydroxy ([M + H](+) = m/z 671), and trihydroxy ([M + H](+) = m/z 687) metabolites, as well as some unexpected biotransformation products such as a carboxylic acid ([M + H](+) = m/z 669), a N-dealkylated metabolite ([M + H](+) = m/z 541), and its hydroxy-analog ([M + H](+) = m/z 557).

Characterization of metabolites of Celecoxib in rabbits by liquid chromatography/tandem mass spectrometry

The metabolism of the anti-inflammatory drug Celecoxib in rabbits was characterized using liquid chromatography (LC)/tandem mass spectrometry (MS/MS) with precursor ion and constant neutral loss scans followed by product ion scans. After separation by on-line liquid chromatography, the crude urine samples and plasma and fecal extracts were analyzed with turbo-ionspray ionization in negative ion mode using a precursor ion scan of m/z 69 (CF(3)) and a neutral loss scan of 176 (dehydroglucuronic acid). The subsequent product ion scans of the [M - H] ions of these metabolites yielded the identification of three phase I and four phase II metabolites. The phase I metabolites had hydroxylations at the methyl group or on the phenyl ring of Celecoxib, and the subsequent oxidation product of the hydroxymethyl metabolite formed the carboxylic acid metabolite. The phase II metabolites included four positional isomers of acyl glucuronide conjugates of the carboxylic acid metabolite. These positional isomers were caused by the alkaline pH of the rabbit urine and were not found in rabbit plasma. The chemical structures of the metabolites were characterized by interpretation of their product ion spectra and comparison of their LC retention times and the product ion spectra with those of the authentic synthesized standards.

Identification of novel metabolites of prostaglandin E2 formed by isolated rat hepatocytes

The metabolism of prostaglandin E2 (PGE2) in isolated rat hepatocytes led to the formation of four major as well as several minor products which were structurally characterized using electrospray tandem mass spectrometry. The major metabolites identified included dinor-PGE1, dinor-PGE2, and tetranor-PGE1 and the taurine conjugates of dinor-PGE1 and dinor-PGE2. Several minor metabolites including the taurine conjugates of PGE2 and tetranor PGE1 along with a glucuronide conjugate of PGE2 were also identified. These taurine conjugates had not been previously identified in studies of PGE2 metabolism, yet comprised nearly 50% of the mixture of metabolites after 40-min incubations. Experiments carried out with deuterium-labeled PGE2 ([3,3,4,4-D4]PGE2) resulted in the complete loss of all deuterium atoms in dinor-PGE1, dinor-PGE2, and tetranor metabolites during incubation with hepatocytes. Metabolism via classic beta-oxidation pathways would predict one deuterium atom retained by dinor-PGE1 and two deuterium atoms retained by dinor-PGE2. When PGE2 was incubated with isolated rat hepatocytes in buffer containing 30% D2O, substantial incorporation (30%) of one deuterium atom could be observed in the dinor metabolites along with 10% incorporation into the tetranor and residual PGE2. Deuterium-labeled PGE1 ([3,3,4,4-D4]PGE1) was metabolized to D2-dinor-PGE1, tetranor-PGE1, and the taurine conjugate of D2-dinor-PGE1 by isolated rat hepatocytes. The loss of deuterium during metabolism of the deuterated substrates of PGE2, but not PGE1, as well as the incorporation of deuterium atoms from the aqueous solvent into PGE2 metabolites suggested that the delta 5 double bond and sequential isomerization reactions lead to eventual exchange of the protons from carbon atom 4 of PGE2 with water.

Narrow-bore liquid chromatography-tandem mass spectrometry with simultaneous radioactivity monitoring for partially characterizing the biliary metabolites of an arginine fluoroalkyl ketone analog of D-MePhe-Pro-Arg, a potent thrombin inhibitor

Characterizing components eluting from a HPLC column is enhanced when multiple detectors are incorporated in-line. The performance of a system consisting of a combination of two detectors-electrospray ionization mass spectrometry (and tandem mass spectrometry) and radioactivity monitoring, following gradient separation with a 250 x 2.1 mm I.D. (Vydac Protein and Peptide C18, 5 microns, 300 A) column-is evaluated with respect to chromatographic integrity and detection. The HPLC effluent was split (8:1) and a post-column make-up solvent was added to flow directed towards the radioactivity detector containing a solid glass cell. Trifluoracetic acid (0.1%) was added to the make-up flow solvent to prevent silanol interactions from degrading the profile displayed in the 14C trace. A 14C chromatographic peak representing 550 dpm was detected with signal-to-noise ratio of 3. This system was used for rapidly characterizing the biliary metabolites of an arginine fluoroalkyl ketone analog of D-MePhe-Pro-Arg, a potent thrombin inhibitor currently being evaluated as a drug candidate. These metabolites are shown to comprise of mono- and dihydroxylated drug as well as a reduced ketone form of the drug. Combining the radioactivity monitor in-line with the mass spectrometer ensured that all of the major metabolites (as evident from the 14C profile) were characterized by mass spectrometry.

Analysis of dog and rat plasma for metabolites of a new isoindoline anxiolytic, DN-2327, by liquid chromatography/thermospray mass spectrometry and tandem mass spectrometry

Combined liquid chromatography/thermospray mass spectrometry (full scan) and its tandem mass spectrometry (precursor ion, product ion and neutral loss scan) were used to characterize rat and dog plasma metabolites of an anxiolytic candidate (DN-2327; (+-)-2-(7-chloro-1,8-naphthyridin-2-yl)-3-[(1,4-dioxa-8- azaspiro[4.5]dec-8-yl)carbonylmethyl]isoindolin-1-one) . The results indicated that DN-2327 was metabolized to M-I by hydrolysis of the dioxolane ring which was subsequently reduced at the carbonyl moiety to form M-II. M-II was further metabolized to diol isomers, M-III and M-IV, by hydroxylation on the hydroxypiperidine moiety. M-V was an acyclic diol resulting from cleavage of the piperidine ring followed by reduction of the aldehyde. By the methodology used here, detailed structural information could be obtained without recourse to individual metabolite isolation and this provided a great saving in time and effort.

The use of liquid chromatography/thermospray mass spectrometry with on-line ultraviolet diode array and radiochemical detection: characterization of the putative metabolites of U-78875 in female rat faeces

The metabolites of an anxiolytic drug candidate U-78875 [3-(5-cyclopropyl-1,2,4-oxadiazol-3-yl)-5-(1-methylethyl)- imidazo[1,5-alpha]quinoxalin-4(5H)-one; I] were investigated in female rat faecal extracts following a single oral dose of (14C)I. Initial metabolite profiling was performed by high-performance liquid chromatography incorporating homogeneous (i.e. liquid scintillant added post-column) radiochemical detection (radio-HPLC). This indicated the presence of parent drug and one minor metabolite. Because liquid scintillant is incompatible with thermospray interfaces, subsequent analysis by thermospray/HPLC/MS (TSP/LC/MS) incorporated radiochemical detection in heterogeneous (i.e. solid scintillant) mode and ultraviolet (UV) diode array detection. This revealed that the peak thought previously to be parent drug contained two components: a metabolite (major) and parent drug (minor). The UV spectrum and TSP/LC/MS/MS daughter ion analysis led to the proposition of a bisamide structure for the major metabolite. Chemical synthesis was used to confirm this structure. TSP/LC/MS indicated that the molecular weight of the minor metabolite was 16 u higher than the bisamide, suggesting an oxidized analogue. Attempts to obtain a daughter ion spectrum on this minor metabolite proved unsuccessful. On-line radiochemical and UV diode array detection greatly facilitates the TSP/LC/MS characterization of metabolites from studies using radiolabelled drugs.

An evaluation of tandem mass spectrometry in drug metabolism studies

The use of precursor ion and constant neutral loss scanning as a means of rapidly detecting drug metabolites is evaluated. Four clinically useful drugs, namely (i) cyclophosphamide, (ii) mifentidine, (iii) cimetropium bromide and (iv) haloperidol, were subjected to microsomal incubations to afford phase I metabolites. Aside from a minor clean-up procedure involving zinc sulfate precipitation of microsomal proteins and solid-phase extraction of metabolites using a Sep-pak C-18 cartridge, the mixtures were analysed directly by fast atom bombardment tandem mass spectrometry. It is demonstrated that such screening strategies are important in detecting novel metabolites. However, there are some problems associated with only using such methods, including (i) the possibility of not detecting metabolites that undergo unusual collision-induced dissociation fragmentation pathways, (ii) the non-detection of metabolites that have undergone metabolic change at unusual sites of reactivity, and (iii) production of artifacts derived from the parent drug by the primary ionization process. Examples are discussed that highlight both the strengths and weaknesses of such an approach.