Gregory C. Leo

ARYLSULFONATE ESTERS IN SOLID PHASE ORGANIC SYNTHESIS. I. CLEAVAGE WITH AMINES, THIOLATE, AND IMIDAZOLE

Abstract The arylsulfonate ester functionality connecting an alkyl chain to a polystyrene resin is cleaved with neat volatile primary or secondary amines to give secondary or tertiary amines, respectively, in high yields and purity. Non-volatile secondary amines, thiols, and imidazole also cleave the alkyl chain efficiently to afford the expected products which can be readily purified by an ion-exchange resin work-up method.

N-GLUCURONIDATION OF THE PLATELET-DERIVED GROWTH FACTOR RECEPTOR TYROSINE KINASE INHIBITOR 6,7-(DIMETHOXY-2,4-DIHYDROINDENO(1,2-C)PYRAZOL- 3-YL)-(3-FLUORO-PHENYL)-AMINE BY HUMAN UDP-GLUCURONOSYLTRANSFERASES

The potential cancer therapeutic agent, 6,7-(dimethoxy-2, 4-dihydroindeno[1,2-c]pyrazol-3-yl)-(3-fluoro-phenyl)-amine (JNJ-10198409), formed three N-glucuronides that were positively identified by liquid chromatography-tandem mass spectrometry and NMR as N-amine-glucuronide (Glu-A), 1-N-pyrazole-glucuronide (Glu-B), and 2-N-pyrazole-glucuronide (Glu-C). All three N-glucuronides were detected in rat liver microsomes, whereas only Glu-A and -B were found in monkey and human liver microsomes. In contrast to common glucuronides, Glu-B was completely resistant to β-glucuronidase. Kinetic analyses revealed that glucuronidation of JNJ-10198409 in human liver microsomes exhibited atypical kinetics that may be described by a two-site binding model. For the high affinity binding, Km values were 1.2 and 5.0 μM, and Vmax values were 2002 and 2403 nmol min–1 mg–1 for Glu-A and Glu-B, respectively. Kinetic constants of low affinity binding were not determined due to low solubility of the drug. Among the human UDP-glucuronosyltransferases (UGTs) tested, UGT1A9, 1A8, 1A7, and 1A4 were the most active isozymes to produce Glu-A; for the formation of Glu-B, UGT1A9 was the most active enzyme, followed by UGT1A3, 1A7, and 1A4. Glucuronidation of JNJ-10198409 by those UGT1A enzymes followed classic Michaelis-Menten kinetics. In contrast, no glucuronides were formed by all UGT2B isozymes tested, including UGT2B4, 2B7, 2B15, and 2B17. Collectively, these results suggested that glucuronidation of JNJ-10198409 in human liver microsomes is catalyzed by multiple UGT1A enzymes. Since UGT1A enzymes are widely expressed in various tissues, it is anticipated that both hepatic and extrahepatic glucuronidation will likely contribute to the elimination of the drug in humans. Additionally, conjugation at the nitrogens of the pyrazole ring represents a new structural moiety for UGT1A-mediated reactions.

Bioactivation of 4-methylphenol (p-cresol) via cytochrome P450-mediated aromatic oxidation in human liver microsomes.

It has previously been proposed that 4-methylphenol (p-cresol) is metabolically activated by oxidation of the methyl group to form a reactive quinone methide. In the present study a new metabolism pathway is elucidated in human liver microsomes. Oxidation of the aromatic ring leads to formation of 4-methyl-ortho-hydroquinone, which is further oxidized to a reactive intermediate, 4-methyl-ortho-benzoquinone. This bioactivation pathway is fully supported by the following observations: 1) one major and two minor glutathione (GSH) adducts were detected in microsomal incubations of p-cresol in the presence of glutathione; 2) a major metabolite of p-cresol was identified as 4-methyl-ortho-hydroquinone in microsomal incubations; 3) the same GSH adducts were detected in microsomal incubations of 4-methyl-ortho-hydroquinone; and 4) the same GSH adducts were chemically synthesized by oxidizing 4-methyl-ortho-hydroquinone followed by the addition of GSH, and the major conjugate was identified by liquid chromatography-tandem mass spectrometry and NMR as 3-(glutathione-S-yl)-5-methyl-ortho-hydroquinone. In addition, it was found that 4-hydroxybenzylalcohol, a major metabolite derived from oxidation of the methyl group in liver microsomes, was further converted to 4-hydroxybenzaldehyde. In vitro studies also revealed that bioactivation of p-cresol was mediated by multiple cytochromes P450, but CYP2D6, 2E1, and 1A2 are the most active enzymes for formation of quinone methide, 4-methyl-ortho-benzoquinone, and 4-hydroxybenzaldehyde, respectively. Implications of the newly identified reactive metabolite in p-cresol-induced toxicity remain to be investigated in the future.

Biotransformation of Geldanamycin and 17-Allylamino-17-Demethoxygeldanamycin by Human Liver Microsomes: Reductive versus Oxidative Metabolism and Implications

Comparative metabolite profiling of geldanamycin and 17-allylamino-17-demethoxygeldanamycin (17AAG) using human liver microsomes in normoxia and hypoxia was conducted to understand their differential metabolic fates. Geldanamycin bearing a 17-methoxy group primarily underwent reductive metabolism, generating the corresponding hydroquinone under both conditions. The formed hydroquinone resists further metabolism and serves as a reservoir. On exposure to oxygen, this hydroquinone slowly reverts to geldanamycin. In the presence of glutathione, geldanamycin was rapidly converted to 19-glutathionyl geldanamycin hydroquinone, suggesting its reactive nature. In contrast, the counterpart (17AAG) preferentially remained as its quinone form, which underwent extensive oxidative metabolism on both the 17-allylamino sidechain and the ansa ring. Only a small amount (<1%) of 19-glutathione conjugate of 17AAG was detected in the incubation of 17AAG with glutathione at 37°C for 60 min. To confirm the differential nature of quinone-hydroquinone conversion between the two compounds, hypoxic incubations with human cytochrome P450 reductase at 37°C and direct injection analysis were performed. Approximately 89% of hydroquinone, 5% of quinone, and 6% of 17-O-demethylgeldanamycin were observed after 1-min incubation of geldanamycin, whereas about 1% of hydroquinone and 99% of quinone were found in the 60-min incubation of 17AAG. The results provide direct evidence for understanding the 17-substituent effects of these benzoquinone ansamycins on their phase I metabolism, reactivity with glutathione, and acute hepatotoxicity.

Thrombin receptor-activating peptides (TRAPs) : Investigation of bioactive conformations via structure-activity, spectroscopic, and computational studies

The thrombin receptor (PAR-1) is an unusual transmembrane G-protein coupled receptor in that it is activated by serine protease cleavage of its extracellular N-terminus to expose an agonist peptide ligand, which is tethered to the receptor itself. Synthetic peptides containing the agonist motif, such as SFLLRN for human PAR-1, are capable of causing full receptor activation. We have probed the possible bioactive conformations of thrombin receptor-activating peptides (TRAPs) by systematic introduction of certain conformational perturbations, involving alpha-methyl, ester psi(COO), and reduced-amide psi(CH2N) scans, into the minimum-essential agonist sequence (SFLLR) to probe the importance of the backbone conformation and amide NH hydrogen bonding. We performed extensive conformational searches of representative pentapeptides to derive families of putative bioactive structures. In addition, we employed 1H NMR and circular dichroism (CD) to characterize the conformational disposition of certain pentapeptide analogues experimentally. Activation of platelet aggregation by our pentapeptide analogues afforded a structure-function correlation for PAR-1 agonist activity. This correlation was assisted by PAR-1 receptor binding data, which gauged the affinity of peptide ligands for the thrombin receptor independent of a functional cellular response derived from receptor activation (i.e. a pure molecular recognition event). Series of alanine-, proline-, and N-methyl-scan peptides were also evaluated for comparison. Along with the known structural features for PAR-1 agonist peptides, our work adds to the understanding of peptide topography relative to platelet functional activity and PAR-1 binding. The absolute requirement of a positively charged N-terminus for strong agonist activity was contradicted by the N-terminal hydroxyl peptide psi(HO)S-FLLR-NH2. The amide nitrogen between residues 1 and 2 was found to be a determinant of receptor recognition and the carbonyl groups along the backbone may be involved in hydrogen bonding with the receptor. Position 3 (P3) of TRAP-5 is known to tolerate a wide variety of side chains, but we also found that the amide nitrogen at this position can be substituted by an oxygen, as in SF-psi(COO)-LLR-NH2, without diminishing activity. However, this peptide bond is sensitive to conformational changes in that SFPLR-NH2 was active, whereas SF-NMeL-LR-NH2 was not. Additionally, we found that position 3 does not tolerate rigid spacers, such as 3-aminocyclohexane-1-carboxylic acid and 2-aminocycloalkane-1-carboxylic acid, as analogues 1A, 1B, 2A, 2B, 3, 4, 5A and 5B lack agonist activity. On the basis of our results, we suggest that an extended structure of the agonist peptide is principally responsible for receptor recognition (i.e. binding) and that hydrophobic contact may occur between the side chains of the second (Phe) and fourth (Leu) residues (i.e. P2-P4 interaction).

Stereoselectivity in the rhodium(II) acetate catalysed cyclopropanations of 2-diazo-1-indanone with styrenes

Abstract The rhodium(II) acetate catalysed cyclopropanation reactions of 2-diazo-1-indanone 4 with various substituted styrenes 5 have been investigated. The cyclopropane diastereomer 6a bearing a trans relationship between the carbonyl and the aryl ring was in all cases the predominant isomer and the ratio of stereosiomers almost constant over a range of styrene substituents. Styrenes bearing electron-donating substituents gave slightly better stereoselectivity in favour of the trans isomer. These results are substantiated by a mechanistic proposal.

EPSP synthase inhibitor design I. Conformations of enzyme bound shikimate-3-phosphate and 5-enolpyruvoylshikimate-3-phosphate using TRNOE

Abstract The conformations of S3P, 1 and EPSP, 2 bound to E. Coli EPSP synthase have been determined using two dimensional transfer NOE (2D TRNOE) measurements. Both S3P and EPSP bound to the enzyme retain the half-chair conformation with the phosphate group in an axial position similar to “free” S3P in solution. 4 This cis enol proton of bound EPSP is in close proximity to H 4 on the ring. The conformations of S3P, 1 and EPSP, 2 bound to E . Coli EPSP synthase have been determined using 2d transfer NOE experiments.

Automatic Analysis of Quantitative NMR Data of Pharmaceutical Compound Libraries

In drug discovery, chemical library compounds are usually dissolved in DMSO at a certain concentration and then distributed to biologists for target screening. Quantitative (1)H NMR (qNMR) is the preferred method for the determination of the actual concentrations of compounds because the relative single proton peak areas of two chemical species represent the relative molar concentrations of the two compounds, that is, the compound of interest and a calibrant. Thus, an analyte concentration can be determined using a calibration compound at a known concentration. One particularly time-consuming step in the qNMR analysis of compound libraries is the manual integration of peaks. In this report is presented an automated method for performing this task without prior knowledge of compound structures and by using an external calibration spectrum. The script for automated integration is fast and adaptable to large-scale data sets, eliminating the need for manual integration in ~80% of the cases.

Can Untargeted Metabolomics Be Utilized in Drug Discovery/Development?

Untargeted metabolomics is a promising approach for reducing the significant attrition rate for discovering and developing drugs in the pharmaceutical industry. This review aims to highlight the practical decision-making value of untargeted metabolomics for the advancement of drug candidates in drug discovery/development including potentially identifying and validating novel therapeutic targets, creating alternative screening paradigms, facilitating the selection of specific and translational metabolite biomarkers, identifying metabolite signatures for the drug efficacy mechanism of action, and understanding potential drug-induced toxicity. The review provides an overview of the pharmaceutical process workflow to discover and develop new small molecule drugs followed by the metabolomics process workflow that is involved in conducting metabolomics studies. The pros and cons of the major components of the pharmaceutical and metabolomics workflows are reviewed and discussed. Finally, selected untargeted metabolomics literature examples, from primarily 2010 to 2016, are used to illustrate why, how, and where untargeted metabolomics can be integrated into the drug discovery/preclinical drug development process.

NMR three‐dimensional solution structure of the serine protease inhibitor cyclotheonamide A

The nmr solution conformation of cyclotheonamide A (CtA) was determined in aqueous media. The data produced 15 distance and 10 torsional constraints which were used to generate conformations using restrained simulated annealing (SA) and distance geometry/simulated annealing (DG/SA) calculations. Two different calculation protocols were performed to ensure proper sampling of conformational space and even though the torsional restraints were input differently, both calculation methods yielded the same conformation of CtA. In the structure calculations, all solutions of the Karplus equation were sampled simultaneously using the restrained SA protocol and large ranges were used for the dihedral restraints in the DG/SA protocol because all solutions to the Karplus equation could not be sampled simultaneously. The solution conformation was also compared to the solid state x-ray conformations of CtA bound to thrombin and trypsin. The conformation of the residues important for active site binding (d-Phe, h-Arg, and Pro) are nearly identical in aqueous solution and solid state with largest differences at the a-Ala and v-Tyr residues. CtA appears to be preordered in structure and does not undergo a significant conformational change upon binding to the enzyme active site.