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Biomedical subjects

H B Broughton

Publications and source records attributed to H B Broughton.

16 recordsLinked to original sources
PubMed2000

Development of peptide 3D structure mimetics: rational design of novel peptoid cholecystokinin receptor antagonists.

The two hormones cholecystokinin and gastrin share the same C-terminal sequence of amino acids, namely Gly(29)-Trp(30)-Met(31)-Asp(32)-Phe(33)-NH(2). Nevertheless, this congruence has not precluded using this structure to develop selective ligands for either CCK(1) or CCK(2) receptors. Manipulation of the hydrophobic residues at positions 31 and 33 gave a series of CCK(1) tripeptide antagonists, typified by N-t-BOC-Trp-2-Nal-Asp-2-(phenyl)ethylamide (pK(B) 6.8 +/- 0.3). Molecular modeling was used to identify the bioactive conformation of these CCK(1)-selective compounds and prompted the design of new peptoid structures. We aimed to maintain the conformation of the parent series by exploiting patterns of hydrogen-bonding and pi-stacking interactions present in the original molecule, rather than introducing additional covalent bonds. The prototype, N-(succinyl-D-Asp-2-phenylethylamido)-L-Trp-2-(2-naphthyl)ethylami de, was a potent and selective CCK(1) antagonist (pK(B) 7.2 +/- 0.3). Furthermore, the new series showed patterns of biological activity that mirrored those of the parent tripeptides. These compounds contain elements of both peptide primary and secondary structure and represent a novel approach to designing peptidomimetics. Interesting results were obtained from comparing models of a representative tripeptide CCK(1) antagonist with a conformation of CCK(30)(-)(33) that others have proposed to be responsible for its activity at the CCK(2) receptor. The results suggest that CCK(1) and CCK(2) receptors recognize enatiomeric dispositions of the Trp(30) indole, Asp(32) carboxylic acid, and C-terminal phenyl groups arrayed about a common backbone configuration. This "functional chirality" may underpin the mechanism by which these closely related receptor systems bind CCK(30)(-)(33) and explain patterns of selectivity observed with optical isomers of a number of peptoid and nonpeptide ligands.

Animals↗
PubMed2000

A method for including protein flexibility in protein-ligand docking: improving tools for database mining and virtual screening.

Second-generation methods for docking ligands into their biological receptors, such as FLOG, provide for flexibility of the ligand but not of the receptor. Molecular dynamics based methods, such as free energy perturbation, account for flexibility, solvent effects, etc., but are very time consuming. We combined the use of statistical analysis of conformational samples from short-run protein molecular dynamics with grid-based docking protocols and demonstrated improved performance in two test cases. Our statistical analysis explores the importance of the average strength of a potential interaction with the biological target and optionally applies a weighting depending on the variability in the strength of the interaction seen during dynamics simulation. Using these methods, we improved the num-top-ranked 10% of a database of drug-like molecules, in searches based on the three-dimensional structure of the protein. These methods are able to match the ability of manual docking to assess likely inactivity on steric grounds and indeed to rank order ligands from a homologous series of cyclooxygenase-2 inhibitors with good correlation to their true activity. Furthermore, these methods reduce the need for human intervention in setting up molecular docking experiments.

Combinatorial Chemistry Techniques↗
PubMed1999

3-[3-(Piperidin-1-yl)propyl]indoles as highly selective h5-HT(1D) receptor agonists.

Several 5-HT(1D/1B) receptor agonists are now entering the marketplace as treatments for migraine. This paper describes the development of selective h5-HT(1D) receptor agonists as potential antimigraine agents which may produce fewer side effects. A series of 3-[3-(piperidin-1-yl)propyl]indoles has been synthesized which has led to the identification of 80 (L-772,405), a high-affinity h5-HT(1D) receptor full agonist having 170-fold selectivity for h5-HT(1D) receptors over h5-HT(1B) receptors. L-772,405 also shows very good selectivity over a range of other serotonin and nonserotonin receptors and has excellent bioavailability following subcutaneous administration in rats. It therefore constitutes a valuable tool to delineate the role of h5-HT(1D) receptors in migraine. Molecular modeling and physical properties have been utilized to postulate the binding conformation of these compounds in the receptor cavity.

Animals↗
PubMed1999

Fluorination of 3-(3-(piperidin-1-yl)propyl)indoles and 3-(3-(piperazin-1-yl)propyl)indoles gives selective human 5-HT1D receptor ligands with improved pharmacokinetic profiles.

It has previously been reported that a 3-(3-(piperazin-1-yl)propyl)indole series of 5-HT1D receptor ligands have pharmacokinetic advantages over the corresponding 3-(3-(piperidin-1-yl)propyl)indole series and that the reduced pKa of the piperazines compared to the piperidines may be one possible explanation for these differences. To investigate this proposal we have developed versatile synthetic strategies for the incorporation of fluorine into these ligands, producing novel series of 4-fluoropiperidines, 3-fluoro-4-aminopiperidines, and both piperazine and piperidine derivatives with one or two fluorines in the propyl linker. Ligands were identified which maintained high affinity and selectivity for the 5-HT1D receptor and showed agonist efficacy in vitro. The incorporation of fluorine was found to significantly reduce the pKa of the compounds, and this reduction of basicity was shown to have a dramatic, beneficial influence on oral absorption, although the effect on oral bioavailability could not always be accurately predicted.

Administration, Oral↗
PubMed1997

5-(Piperidin-2-yl)- and 5-(homopiperidin-2-yl)-1,4-benzodiazepines: high-affinity, basic ligands for the cholecystokinin-B receptor.

The design, synthesis, and biological activity of a series of high-affinity, basic ligands for the cholecystokinin-B receptor are described. The compounds, which incorporate a piperidin-2-yl or a homopiperidin-2-yl group attached to C5 of a benzodiazepine core structure, are substantially more basic (e.g., 9d, pKa = 9.48) than previously reported antagonists based on 5-amino-1,4-benzodiazepines (e.g., 5, pKa = 7.1) and have improved aqueous solubility. In view of their basicity, it would be tempting to speculate that the present series of compounds might be binding to the CCK-B receptor in their protonated form. Compounds such as 9d, e and 10d showed high affinity for this receptor (IC50 < 2.5 nM) and very good selectivity over CCK-A (CCK-A/CCK-B > 2000), even as the racemates. Additionally, a significantly improved in vivo half-life was observed for a selection of compounds compared to the clinical candidate L-365, -260 (1).

Animals↗
PubMed1997

4-Heterocyclylpiperidines as selective high-affinity ligands at the human dopamine D4 receptor.

5-(4-Chlorophenyl)-3-(1-(4-chlorobenzyl)piperidin-4-yl)pyrazole (3) was identified from screening of the Merck sample collection as a human dopamine D4 (hD4) receptor ligand with moderate affinity (61 nM) and 4-fold selectivity over human D2 (hD2) receptors. Four separate parts of the molecule have been examined systematically to explore structure-activity relationships with respect to hD4 affinity and selectivity over other dopamine receptors. It was found that the 4-chlorophenyl group attached to the pyrazole is optimal, as is the 4-substituted piperidine. The lipophilic group on the basic nitrogen is more amenable to change, with the optimal group found to be a phenethyl. The aromatic heterocyle can be altered to a number of different groups, with isoxazoles and pyrimidines showing improved affinities. This heterocycle can also be advantageously alkylated, improving the selectivity of the compounds over D2 receptors. It is hypothesized that the conformation around the bond joining the aromatic heterocycle to the piperidine is important for D4 affinity, based on crystal structures of isoxazoles (29 and 30) and on a conformationally constrained compound (28). Putting all the favorable changes together led to the discovery that 5-(4-chlorophenyl)-4-methyl-3-(1-(2-phenylethyl)piperidin-4-yl)iso xazole (36) is a nanomolar antagonist at human dopamine D4 receptors with > 500-fold selectivity over hD2 and > 200-fold selectivity over hD3. Compound 36 is an antagonist of hD4 receptors with good oral bioavailability of 38%, a half life of 2 h, and brain levels 10-fold higher than plasma levels.

Cell Line↗
PubMed1997

Molecular modeling.

The past year has seen a maturation of molecular modeling, with an increasing number of comparative studies between established methods becoming possible, together with an explosion of new work especially in the areas of combinatorial chemistry and molecular diversity. Traditionally 'difficult' areas such as modeling oligosaccharides look set to join the mainstream in the next few years.

Biopolymers↗
PubMed1996

Controlled modification of acidity in cholecystokinin B receptor antagonists: N-(1,4-benzodiazepin-3-yl)-N'-[3-(tetrazol-5-ylamino) phenyl]ureas.

The design, synthesis, and biological activity of a novel series of CCK-B receptor antagonists (1) which incorporate a tetrazol-5-ylamino functionality attached to the phenyl ring of the arylurea moiety of L-365,260 are described. In these compounds, the acidity of the tetrazole was gradually modified by utilization of simple conformational constraints, and X-ray crystallographic data were obtained to support the conformational depenence of the pK(a) of the aminotetrazoles. Compounds to emerge from the present work such as 1f and 2c,d are among the highest affinity and, in the case of 1f, most selective (CCK-A/CCK-B, 37 000) antagonists so far reported for this receptor. The C(5)-cyclohexyl compound 2c (L-736,380) dose-dependently inhibited gastric acid secretion in anesthetized rats (ID(50), 0.064 mg/kg) and ex vivo binding of [(125)I]CCK-8S in BKTO mice brain membranes (ED(50), 1.7 mg/kg) and is one of the most potent acidic CCK-B receptor antagonists yet described.

Animals↗
PubMed1995

Structure and mechanism of inositol monophosphatase.

Since lithium inhibits IMPase and modulates phosphatidylinositol (PtdIns) cell signalling at therapeutically relevant concentrations (0.5-1.0 mM), IMPase has attracted attention as a putative molecular target for lithium in the treatment of manic depression. IMPase is a homodimer, with each subunit organised in an alpha beta alpha beta alpha arrangement of alpha-helices and beta-sheets, and this type of structure seems crucial to the two-metal catalysed mechanism in which an activated water molecule serves as a nucleophile. Lithium appears to inhibit the enzyme following substrate hydrolysis by occupying the second metal binding site before the phosphate group can dissociate from its interaction with the site 1 metal. The understanding of IMPase structure and the mechanism of substrate hydrolysis and lithium inhibition should be useful in the development of novel inhibitors which may prove clinically useful in the treatment of manic depression.

Amino Acid Sequence↗
PubMed1995

Inositol monophosphatase--a putative target for Li+ in the treatment of bipolar disorder.

Attenuation of the phosphatidylinositol (PI) signal transduction pathway as a consequence of inhibition of inositol monophosphatase (IMPase) has been proposed as the mechanism for the efficacy of Li+ in the treatment of bipolar disorder. Nevertheless, Li+ also affects other aspects of PI signal transduction, and it is therefore not clear whether modulation of PI responses by Li+ can be attributed solely to inhibition of IMPase. However, inhibitors of IMPase mimic the effects of Li+ on some aspects of PI cell signalling, thus highlighting the potential of IMPase as a target for the treatment of bipolar disorder. The recent description of the three-dimensional structure of IMPase in conjunction with site-directed mutagenesis and kinetic studies has led to the elucidation of the enzyme mechanism. These structural and mechanistic data should prove useful in the development of novel inhibitors of IMPase that might ultimately prove useful clinically.

Antimanic Agents↗
PubMed1994

Structural analysis of inositol monophosphatase complexes with substrates.

The structures of ternary complexes of human inositol monophosphatase with inhibitory Gd3+ and either D- or L-myo-inositol 1-phosphate have been determined to 2.2-2.3 A resolution using X-ray crystallography. Substrate and metal are bound identically in each active site of the phosphatase dimer. The substrate is present at full occupancy, while the metal is present at only 35% occupancy, suggesting that Li+ from the crystallization solvent partially replaces Gd3+ upon substrate binding. The phosphate groups of both substrates interact with the phosphatase in the same manner with one phosphate oxygen bound to the octahedrally coordinated active site metal and another oxygen forming hydrogen bonds with the amide groups of residues 94 and 95. The active site orientations of the inositol rings of D- and L-myo-inositol 1-phosphate differ by rotation of nearly 60 degrees about the phosphate ester bond. Each substrate utilizes the same key residues (Asp 93, Ala 196, Glu 213, and Asp 220) to form the same number of hydrogen bonds with the enzyme. Mutagenesis experiments confirm the interaction of Glu 213 with the inositol ring and suggest that interactions with Ser 165 may develop during the transition state. The structural data suggest that the active site nucleophile is a metal-bound water that is activated by interaction with Glu 70 and Thr 95. Expulsion of the ester oxygen appears to be promoted by three aspartate residues acting together (90, 93, and 220), either to donate a proton to the leaving group or to form another metal binding site from which a second Mg2+ coordinates the leaving group during the transition state.

Crystallography, X-Ray↗
PubMed1994

Mechanism of inositol monophosphatase, the putative target of lithium therapy.

myo-Inositol monophosphatase (myo-inositol-1-phosphate phosphohydrolase, EC 3.1.3.25) is an attractive target for mechanistic investigation due to its critical role in the phosphatidylinositol signaling pathway and the possible relevance of its inhibition by Li+ to manic depression therapy. The x-ray crystallographic structure of human inositol monophosphatase in the presence of the inhibitory metal Gd3+ showed only one metal bound per active site, whereas in the presence of Mn2+, three ions were present with one being displaced upon phosphate binding. We report here modeling, kinetic, and mutagenesis studies on the enzyme, which reveal the requirement for two metal ions in the catalytic mechanism. Activity titration curves with Zn2+ or Mn2+ in the presence or absence of Mg2+ are consistent with a two-metal mechanism. Modeling studies based on the various x-ray crystallographic structures (including those with Gd3+ and substrate bound) further support a two-metal mechanism and define the positions of the two metal ions relative to substrate. While the first metal ion may activate water for nucleophilic attack, a second metal ion, coordinated by three aspartate residues, appears to act as a Lewis acid, stabilizing the leaving inositol oxyanion. In this model, the 6-OH group of substrate acts as a ligand for this second metal ion, consistent with the reduced catalytic activity observed with substrate analogues lacking the 6-OH. Evidence from Tb3+ fluorescence quenching and the two-metal kinetic titration curves suggests that Li+ binds at the site of this second metal ion.

Amino Acid Sequence↗
PubMed1993

Probing the role of metal ions in the mechanism of inositol monophosphatase by site-directed mutagenesis.

Since inhibition of myo-inositol monophosphatase (EC 3.1.3.25) by lithium ions and the resulting attenuation of phosphatidylinositol cycle activity may be the mechanism by which lithium exerts its therapeutic effect in the treatment of manic depression, it is of great interest to understand the mechanism of the enzyme and how lithium and other metals interact with it. Divalent magnesium is essential for enzyme activity, whereas Li+ and high concentrations of Mg2+ act as uncompetitive inhibitors with respect to substrate. From the recently solved crystal structure of the human enzyme, several amino acid residues in the active site were targeted for mutagenesis studies. Nine single-residue substituted mutants were characterized with regard to catalytic parameters, Mg2+ dependence, and Li+ inhibition. In addition, a terbium fluorescence assay was developed to determine the metal binding properties of the wild-type and mutant enzymes. Although none of these mutations affected Km for substrate substantially, the mutations Glu70-->Gln, Glu70-->Asp, Asp90-->Asn and Thr95-->Ala, in which residues within coordinating distance of the active site metal were modified, all resulted in large reductions in catalytic activity. The position of Glu70 in the crystal structure further suggests that this residue may be involved in activating water for nucleophilic attack on the substrate. The mutations Lys36-->Ile, Asp90-->Asn, Thr95-->Ala, Thr95-->Ser, His217-->Gln, and Cys218-->Ala all resulted in parallel reductions in both lithium and magnesium affinity, suggesting that Li+ and Mg2+ share a common binding site.

Base Sequence↗