Daniel G. Morgan

The Pneumotoxin 3-Methylindole Is a Substrate and a Mechanism-Based Inactivator of CYP2A13, a Human Cytochrome P450 Enzyme Preferentially Expressed in the Respiratory Tract

3-Methylindole (3MI), a respiratory tract toxicant, can be metabolized by a number of cytochromes P450 (P450), primarily through either dehydrogenation or epoxidation of the indole. In the present study, we assessed the bioactivation of 3MI by recombinant CYP2A13, a human P450 predominantly expressed in the respiratory tract. Four metabolites were detected, and the two principal ones were identified as indole-3-carbinol (I-3-C) and 3-methyloxindole (MOI). Bioactivation of 3MI by CYP2A13 was verified by the observation of three glutathione (GSH) adducts designated as GS-A1 (glutathione adduct 1), GS-A2 (glutathione adduct 2), and GS-A3 (glutathione adduct 3) in a NADPH- and GSH-fortified reaction system. GS-A1 and GS-A2 gave the same molecular ion at m/z 437, an increase of 305 Da over 3MI. Their structures are assigned to be 3-glutathionyl-S-methylindole and 3-methyl-2-glutathionyl-S-indole, respectively, on the basis of the mass fragmentation data obtained by high-resolution mass spectrometry. Kinetic parameters were determined for the formation of I-3-C (Vmax = 1.5 nmol/min/nmol of P450; Km = 14 μM), MOI (Vmax = 1.9 nmol/min/nmol of P450; Km = 15 μM) and 3-glutathionyl-S-methylindole (Vmax = 0.7 nmol/min/nmol of P450; Km = 13 μM). The structure of GS-A3, a minor adduct with a protonated molecular ion at m/z 453, is proposed to be 3-glutathionyl-S-3-methyloxindole. We also discovered that 3MI is a mechanism-based inactivator of CYP2A13, given that it produced a time-, cofactor-, and 3MI concentration-dependent loss of activity toward 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, with a relatively low KI value of ∼10 μM and a kinact of 0.046 min−1. Thus, CYP2A13 metabolizes 3MI through multiple bioactivation pathways, and the process can lead to a suicide inactivation of CYP2A13.

Discovery of a novel series of quinolone α7 nicotinic acetylcholine receptor agonists

High throughput screening led to the identification of a novel series of quinolone α7 nicotinic acetylcholine receptor (nAChR) agonists. Optimization of an HTS hit (1) led to 4-phenyl-1-(quinuclidin-3-ylmethyl)quinolin-2(1H)-one, which was found to be potent and selective. Poor brain penetrance in this series was attributed to transporter-mediated efflux, which was in turn due to high pKa. A novel 4-fluoroquinuclidine significantly lowered the pKa of the quinuclidine moiety, reducing efflux as measured by a Caco-2 assay.

Simultaneous bioanalysis of a phosphate prodrug and its parent compound using a multiplexed LC–MS method

BACKGROUND Bioanalytical support of drug-discovery efforts increasingly requires more complex multiple component analysis, including the bioanalysis of drugs, prodrugs and metabolites. Just as the physiochemical properties of these components may differ widely from each other, optimal LC and MS conditions, including polarity, can also vary greatly among the analytes of interest, thus presenting significant challenges during quantitative LC-MS-based bioanalysis. A single compromised method for the determination of all analytes may sacrifice sensitivity or chromatographic conditions for one analyte in order to achieve adequate results for another. Manually switching between assay conditions to analyze samples under separately optimized conditions for individual compounds can be time consuming. RESULTS The method presented here addresses the problem of differential analyte optimization using a multiplexed approach for simultaneous quantitative bioanalysis of multiple analytes in the same sample, employing a mixed mode of both turbulent- and laminar-flow chromatography. CONCLUSION The approach is illustrated with the quantitation of a lipophilic drug and its hydrophilic phosphate ester prodrug in a biological matrix under individually optimized LC-MS conditions.

Phenacetin Pharmacokinetics in CYP1A2-Deficient Beagle Dogs

Phenacetin is widely used as an in vitro probe to measure CYP1A2 activity across species. To investigate whether phenacetin can be used as an in vivo probe substrate to phenotype CYP1A2 activity in dogs, beagle dogs previously genotyped for a single nucleotide polymorphism that yields an inactive CYP1A2 protein were selected and placed into one of three groups: CC (wild-type), CT (heterozygous), or TT (homozygous mutants). The dogs were dosed with phenacetin orally at 5 and 15 mg/kg and intravenously at 15 mg/kg. Plasma samples were analyzed by liquid chromatography-tandem mass spectrometry, and phenacetin and its primary metabolite, acetaminophen, were monitored. After intravenous dosing, all groups showed similar exposure of phenacetin irrespective of genotype. After oral dosing at 15 mg/kg, the exposure of phenacetin in CC and CT dogs was similar, but phenacetin exposure was 2-fold greater in TT dogs. Exposure of the metabolite, acetaminophen, was similar in all groups; however, the mean acetaminophen/phenacetin ratio in TT dogs was 1.7 times less than that observed in CC dogs. Similar trends between the groups of dogs with respect to phenacetin exposure were also observed after a lower 5 mg/kg p.o. dose of phenacetin; however, a proportionally greater amount of acetaminophen was generated. Although oral exposure of phenacetin was 2-fold higher and acetaminophen exposure was 2-fold lower in CYP1A2-deficient (TT) dogs, these results were considered modest and suggest that phenacetin is not a selective or robust in vivo probe to measure CYP1A2 enzyme activity in the dog.

Effects of BMS-902483, an α7 nicotinic acetylcholine receptor partial agonist, on cognition and sensory gating in relation to receptor occupancy in rodents.

Abstract The &agr;7 nicotinic acetylcholine receptor is thought to play an important role in human cognition. Here we describe the in vivo effects of BMS‐902483, a selective potent &agr;7 nicotinic acetylcholine receptor partial agonist, in relationship to &agr;7 nicotinic acetylcholine receptor occupancy. BMS‐902483 has low nanomolar affinity for rat and human &agr;7 nicotinic acetylcholine receptors and elicits currents in cells expressing human or rat &agr;7 nicotinic acetylcholine receptors that are about 60% of the maximal acetylcholine response. BMS‐902483 improved 24 h novel object recognition memory in mice with a minimal effective dose (MED) of 0.1 mg/kg and reversed MK‐801‐induced deficits in a rat attentional set‐shifting model of executive function with an MED of 3 mg/kg. Enhancement of novel object recognition was blocked by the silent &agr;7 nicotinic acetylcholine receptor agonist, NS6740, demonstrating that activity of BMS‐902483 was mediated by &agr;7 nicotinic acetylcholine receptors. BMS‐902483 also reversed ketamine‐induced deficits in auditory gating in rats, and enhanced ex vivo hippocampal long‐term potentiation examined 24 h after dosing in mice. Results from an ex vivo brain homogenate binding assay showed that &agr;7 receptor occupancy ranged from 64% (novel object recognition) to ˜90% (set shift and gating) at the MED for behavioral and sensory processing effects of BMS‐902483.

Addressing the need for biomarker liquid chromatography/mass spectrometry assays: A protocol for effective method development for the bioanalysis of endogenous compounds in cerebrospinal fluid

RATIONALE Research on disorders of the central nervous system (CNS) has shown that an imbalance in the levels of specific endogenous neurotransmitters may underlie certain CNS diseases. These alterations in neurotransmitter levels may provide insight into pathophysiology, but can also serve as disease and pharmacodynamic biomarkers. To measure these potential biomarkers in vivo, the relevant sample matrix is cerebrospinal fluid (CSF), which is in equilibrium with the brains interstitial fluid and circulates through the ventricular system of the brain and spinal cord. Accurate analysis of these potential biomarkers can be challenging due to low CSF sample volume, low analyte levels, and potential interferences from other endogenous compounds. METHODS A protocol has been established for effective method development of bioanalytical assays for endogenous compounds in CSF. Database searches and standard-addition experiments are employed to qualify sample preparation and specificity of the detection thus evaluating accuracy and precision. RESULTS This protocol was applied to the study of the histaminergic neurotransmitter system and the analysis of histamine and its metabolite 1-methylhistamine in rat CSF. CONCLUSIONS The protocol resulted in a specific and sensitive novel method utilizing pre-column derivatization ultra high performance liquid chromatography/tandem mass spectrometry (UHPLC/MS/MS), which is also capable of separating an endogenous interfering compound, identified as taurine, from the analytes of interest.

Biaryls as potent, tunable dual neurokinin 1 receptor antagonists and serotonin transporter inhibitors

Depression is a serious illness that affects millions of patients. Current treatments are associated with a number of undesirable side effects. Neurokinin 1 receptor (NK1R) antagonists have recently been shown to potentiate the antidepressant effects of serotonin-selective reuptake inhibitors (SSRIs) in a number of animal models. Herein we describe the optimization of a biaryl chemotype to provide a series of potent dual NK1R antagonists/serotonin transporter (SERT) inhibitors. Through the choice of appropriate substituents, the SERT/NK1R ratio could be tuned to afford a range of target selectivity profiles. This effort culminated in the identification of an analog that demonstrated oral bioavailability, favorable brain uptake, and efficacy in the gerbil foot tap model. Ex vivo occupancy studies with compound 58 demonstrated the ability to maintain NK1 receptor saturation (>88% occupancy) while titrating the desired level of SERT occupancy (11-84%) via dose selection.

Ion suppression and cannulation locking solutions

Ion suppression is a common concern when utilizing liquid chromatography-tandem mass spectrometry for quantitation of analytes in biological samples. Ion suppression can cause the analytical signal for the analyte and/or the internal standard to be reduced compared with prepared analytical standards, leading to erroneous quantitation values for the desired analyte of interest. While it has become commonplace to note ion suppression due to the dosing vehicle in in vivo experiments, we have observed a similar phenomenon of ion suppression due to the components of the locking solution used to keep the cannula patent in certain rodent experiments. During one such typical bioanalysis of a drug candidate dosed to a cannulated rodent, significant ion suppression (∼60%) was observed for the structural analogue internal standard, which led to this investigation that revealed the cannula locking solution as the source of the ion suppression.

Bioanalysis of (1R,4R,5S,6R)-4-amino-2-oxabicyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY379268) in rat plasma using derivatization liquid chromatography/mass spectrometry.

BACKGROUND (1R,4R,5S,6R)-4-amino-2-oxabicyclo[3.1.0]hexane-4,6-dicarboxylic acid, also known as LY379268, a group II metabotropic glutamate receptor agonist, has been widely used in neuroscience as a model compound in studies evaluating antipsychotic drugs for the treatment of schizophrenia. MATERIALS & METHODS So far, no reports describing methods of the bioanalysis of LY379268 have been published. Here, a novel method is presented for determining LY379268 in rat plasma employing precolumn derivatization with pentafluorobenzoyl chloride reagent coupled to liquid chromatography/mass spectrometry. CONCLUSION Chemical derivatization of a low-molecular-weight and highly polar molecule yields a derivative that is retained on a reversed-phase liquid chromatography column with improved tandem mass spectrometric response.

Analysis of L-serine-O-phosphate in cerebrospinal spinal fluid by derivatization-liquid chromatography/mass spectrometry.

L-serine-O-phosphate (L-SOP), the precursor of L-serine, is a potent agonist against the group III metabotropic glutamate receptors (mGluRs) and, thus, is of interest as a potential biomarker for monitoring modulation of neurotransmitter release. So far, no reports are available on the analysis of L-SOP in cerebrospinal fluid (CSF). Here a novel method is presented to determine L-SOP levels in CSF employing precolumn derivatization with (5-N-succinimidoxy-5-oxopentyl)triphenylphosphonium bromide (SPTPP) coupled to liquid chromatography/mass spectrometry (derivatization-LC/MS, d-LC/MS).