An unusual rearrangement of Zofenopril, a new ACE inhibitor drug: mass spectrometric and conformational studies

High-throughput approaches towards the definitive identification of pharmaceutical drug metabolites. 2. An example of how unexpected dissociation behaviour could preclude correct assignment of sites of metabolism

S-oxidation is a common metabolic route for sulfur-containing compounds. Whilst investigating the dissociation of a series of chemically synthesised model S-oxide metabolites, two unexpected losses of 62 m/z units were observed in the collision-induced dissociation (CID) product ion spectrum of protonated 3-dimethylaminomethyl-4-(4-methanesulfinyl-3-methylphenoxy)benzenesulfonamide. A single loss was initially assigned using the low-resolution product ion spectrum, acquired by electrospray ionisation quadrupole ion trap mass spectrometry (ESI-QIT-MS), as methanethial, S-oxide via a charge-remote, four-centred rearrangement. This assignment was consistent with well-documented hydrogen rearrangements in the literature. Further, the loss was not observed for the parent compound. Thus, it was inferred that the site of metabolism was involved in the dissociation and the attractive nature of the four-centred rearrangement meant that the loss of methanethial, S-oxide was a logical assignment. However, deuterium-labelling experiments and accurate mass measurements, performed using electrospray ionisation Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS), showed that the nominal loss of 62 m/z units occurs via two distinct dissociation pathways. Neither of these losses was of methanethial, S-oxide as initially hypothesised from the low-resolution product ion spectrum of the protonated molecule. Mechanisms consistent with the experimental findings are postulated. An MS(3) spectrum of the fully exchanged, deuterated species supported the proposed mechanisms by suggesting that 3-dimethylaminomethyl-4-(4-methanesulfinyl-3-methylphenoxy)benzenesulfonamide has multiple sites of protonation in the gas phase. The planar structures of the posited product ions are likely to provide the driving force for the rearrangements. The relevance of the observations with regards to pharmaceutical drug metabolite identification is discussed.

Development and validation of a liquid chromatographic/electrospray ionization mass spectrometric method for the determination of benazepril, benazeprilat and hydrochlorothiazide in human plasma

A new method was developed and fully validated for the quantitation of benazepril, benazeprilat and hydrochlorothiazide in human plasma. Sample pretreatment was achieved by solid-phase extraction (SPE) using Oasis HLB cartridges. The extracts were analysed by high-performance liquid chromatography (HPLC) coupled to a single-quadrupole mass spectrometer (MS) with an electrospray ionization interface. The MS system was operated in selected ion monitoring (SIM) modes. HPLC was performed isocratically on a reversed-phase porous graphitized carbon (PGC) analytical column (2.1 x 125.0 mm i.d., particle size 5 microm). The mobile phase consisted of 55% acetonitrile in water containing 0.3% v/v formic acid and pumped at a flow rate of 0.15 ml min(-1). Chlorthalidone was used as the internal standard (IS) for quantitation. The assay was linear over a concentration range of 5.0-500 ng ml(-1) for all the compounds analysed, with a limit of quantitation of 5 ng ml(-1) for all the compounds. Quality control (QC) samples (5, 10, 100 and 500 ng ml(-1)) in five replicates from three different runs of analyses demonstrated intra-assay precision (coefficient of variation (CV) < or =14.6%), inter-assay precision (CV < or = 5.6%) and overall accuracy (relative error less than -8.0%). The method can be used to quantify benazepril, benazeprilat and hydrochlorothiazide in human plasma, covering a variety of pharmacokinetic or bioequivalence studies.

Novel acetylation-aided migrating rearrangement of uridine-diphosphate-N-acetylglucosamine in electrospray ionization multistage tandem mass spectrometry

Uridine 5'-diphospho-N-acetylglucosamine (UDP-GlcNAc) is the final product of hexosamine biosynthetic pathway (HSP) and the donor substrate for the modification of nucleocytoplasmic proteins at serine and threonine residues with N-acetylglucosamine (GlcNAc) catalyzed by O-GlcNAc transferase (OGT). Many analogs of UDP-GlcNAc were designed to interfere with the process of protein O-glycosylation by blocking OGT. A novel rearrangement reaction was observed in which phosphate-N-acetylglucosamine moiety migrated to 3' terminus of ribose in ESI-MS(n) of UDP-GlcNAc. Results from tandem mass spectrometry, control experiments and calculation showed that the phosphate-N-acetylglucosamine migration might undergo a pentacoordinate phosphoric intermediate. Furthermore, the acetylation of glucosamine in UDP-GlcNAc was essential in the migration process.

Mass spectral fragmentation reactions of angiotensin-converting enzyme (ACE) inhibitors

A variety of mass spectrometric techniques have been employed in the study of a series of structurally similar compounds used in the treatment of hypertension. The compounds, known collectively as angiotensin-converting enzyme (ACE) inhibitors, all share the amino acid residue proline or some variant thereof, as a common structural element. The gas phase fragmentation behavior of these compounds has been explored systematically using various instruments and techniques. An interesting dissociation process (rearrangement) unique to one of the compounds, lisinopril, has been investigated using isotopic labeling experiments and exact mass measurements. The general nature of the process has been probed through both the positive and negative ion analyses of fourteen related compounds exhibiting structural homology.