Synthesis, biological evaluation, and molecular docking studies of aldotetronic acid-based LpxC inhibitors.

In order to develop novel inhibitors of the bacterial deacetylase LpxC bearing a substituent to target the UDP binding site of the enzyme, a series of aldotetronic acid-based hydroxamic acids was accessed in chiral pool syntheses starting from 4,6-O-benzylidene-d-glucose and l-arabinitol. The synthesized hydroxamic acids were tested for LpxC inhibitory activity in vitro, revealing benzyl ether 17a ((2S,3S)-4-(benzyloxy)-N,3-dihydroxy-2-[(4-{[4-(morpholinomethyl)phenyl]ethynyl}benzyl)oxy]butanamide) as the most potent LpxC inhibitor. This compound was additionally tested for antibacterial activity against a panel of clinically relevant Gram-negative bacteria, bacterial uptake, and susceptibility to efflux pumps. Molecular docking studies were performed to rationalize the observed structure-activity relationships.

Synthesis, biological evaluation, and molecular docking studies of deoxygenated C-glycosides as LpxC inhibitors.

The bacterial deacetylase LpxC is a promising target for the development of novel antibiotics being selectively active against Gram-negative bacteria. In chiral pool syntheses starting from d- and l-ribose, a series regio- and stereoisomeric monohydroxytetrahydrofuran derivatives was synthesized and tested for LpxC inhibitory and antibacterial activities. Molecular docking studies were performed to rationalize the obtained structure-activity relationships. The (2S,3R,5R)-configured 3-hydroxytetrahydrofuran derivative ent-8 ((2S,3R,5R)-N,3-Dihydroxy-5-(4-{[4-(morpholinomethyl)phenyl]ethynyl}phenyl)tetrahydrofuran-2-carboxamide) was found to be the most potent LpxC inhibitor (Ki = 3.5 µM) of the synthesized series of monohydroxytetrahydrofuran derivatives and to exhibit the highest antibacterial activity against E. coli BL21(DE3) and the D22 strain.

Antibacterial activity of xylose-derived LpxC inhibitors - Synthesis, biological evaluation and molecular docking studies.

LpxC inhibitors represent a promising class of novel antibiotics selectively combating Gram-negative bacteria. In chiral pool syntheses starting from D- and L-xylose, a series of four 2r,3c,4t-configured C-furanosidic LpxC inhibitors was obtained. The synthesized hydroxamic acids were tested for antibacterial and LpxC inhibitory activity, the acquired biological data were compared with those of previously synthesized C-furanosides, and molecular docking studies were performed to rationalize the observed structure-activity relationships. Additionally, bacterial uptake and susceptibility to efflux pump systems were investigated for the most promising stereoisomers.

Synthesis and biological evaluation of triazolyl-substituted benzyloxyacetohydroxamic acids as LpxC inhibitors.

The bacterial deacetylase LpxC is a promising target for the development of antibiotics selectively combating Gram-negative bacteria. To improve the biological activity of the reported benzyloxyacetohydroxamic acid 9 ((S)-N-hydroxy-2-{2-hydroxy-1-[4-(phenylethynyl)phenyl]ethoxy}acetamide), its hydroxy group was replaced by a triazole ring. Therefore, in divergent syntheses, triazole derivatives exhibiting rigid and flexible lipophilic side chains, different configurations at their stereocenter, and various substitution patterns at the triazole ring were synthesized, tested for antibacterial and LpxC inhibitory activity, and structure-activity relationships were deduced based on docking and binding energy calculations.

Proline-based hydroxamates targeting the zinc-dependent deacetylase LpxC: Synthesis, antibacterial properties, and docking studies.

The Zn2+-dependent deacetylase LpxC is an essential enzyme in Gram-negative bacteria, which has been validated as antibacterial drug target. Herein we report the chiral-pool synthesis of novel d- and l-proline-derived 3,4-dihydroxypyrrolidine hydroxamates and compare their antibacterial and LpxC inhibitory activities with the ones of 4-monosubstituted and 3,4-unsubstituted proline derivatives. With potent antibacterial activities against several Gram-negative pathogens, the l-proline-based tertiary amine 41g ((S)-N-hydroxy-1-(4-{[4-(morpholinomethyl)phenyl]ethynyl}benzyl)pyrrolidine-2-carboxamide) was found to be the most active antibacterial compound within the investigated series, also showing some selectivity toward EcLpxC (Ki = 1.4 µM) over several human MMPs.

Structure and function of an irreversible agonist-ß(2) adrenoceptor complex.

G-protein-coupled receptors (GPCRs) are eukaryotic integral membrane proteins that modulate biological function by initiating cellular signalling in response to chemically diverse agonists. Despite recent progress in the structural biology of GPCRs, the molecular basis for agonist binding and allosteric modulation of these proteins is poorly understood. Structural knowledge of agonist-bound states is essential for deciphering the mechanism of receptor activation, and for structure-guided design and optimization of ligands. However, the crystallization of agonist-bound GPCRs has been hampered by modest affinities and rapid off-rates of available agonists. Using the inactive structure of the human ß(2) adrenergic receptor (ß(2)AR) as a guide, we designed a ß(2)AR agonist that can be covalently tethered to a specific site on the receptor through a disulphide bond. The covalent ß(2)AR-agonist complex forms efficiently, and is capable of activating a heterotrimeric G protein. We crystallized a covalent agonist-bound ß(2)AR-T4L fusion protein in lipid bilayers through the use of the lipidic mesophase method, and determined its structure at 3.5 Å resolution. A comparison to the inactive structure and an antibody-stabilized active structure (companion paper) shows how binding events at both the extracellular and intracellular surfaces are required to stabilize an active conformation of the receptor. The structures are in agreement with long-timescale (up to 30 µs) molecular dynamics simulations showing that an agonist-bound active conformation spontaneously relaxes to an inactive-like conformation in the absence of a G protein or stabilizing antibody.

Synthesis and biological evaluation of enantiomerically pure glyceric acid derivatives as LpxC inhibitors.

Inhibitors of the UDP-3-O-[(R)-3-hydroxymyristoyl]-N-acetylglucosamine deacetylase (LpxC) represent a promising class of novel antibiotics, selectively combating Gram-negative bacteria. In order to elucidate the impact of the hydroxymethyl groups of diol (S,S)-4 on the inhibitory activity against LpxC, glyceric acid ethers (R)-7a, (S)-7a, (R)-7b, and (S)-7b, lacking the hydroxymethyl group in benzylic position, were synthesized. The compounds were obtained in enantiomerically pure form by a chiral pool synthesis and a lipase-catalyzed enantioselective desymmetrization, respectively. The enantiomeric hydroxamic acids (R)-7b (Ki=230nM) and (S)-7b (Ki=390nM) show promising enzyme inhibition. However, their inhibitory activities do not substantially differ from each other leading to a low eudismic ratio. Generally, the synthesized glyceric acid derivatives 7 show antibacterial activities against two Escherichia coli strains exceeding the ones of their respective regioisomes 6.

Synthesis, biological evaluation and molecular docking studies of benzyloxyacetohydroxamic acids as LpxC inhibitors.

The inhibition of the UDP-3-O-[(R)-3-hydroxymyristoyl]-N-acetylglucosamine deacetylase (LpxC) represents a promising strategy to combat infections caused by multidrug-resistant Gram-negative bacteria. In order to elucidate the functional groups being important for the inhibition of LpxC, the structure of our previously reported hydroxamic acid 4 should be systematically varied. Therefore, a series of benzyloxyacetohydroxamic acids was prepared, of which the diphenylacetylene derivatives 28 (Ki=95nM) and 21 (Ki=66nM) were the most potent inhibitors of Escherichia coli LpxC. These compounds could be synthesized in a stereoselective manner employing a Sharpless asymmetric dihydroxylation and a Sonogashira coupling in the key steps. The obtained structure-activity relationships could be rationalized by molecular docking studies.

Synthesis and biological evaluation of flexible and conformationally constrained LpxC inhibitors.

Inhibitors of the UDP-3-O-[(R)-3-hydroxymyristoyl]-N-acetylglucosamine deacetylase (LpxC) represent promising candidates for the development of antibiotics possessing a so far unexploited mechanism of action. In a chiral pool synthesis, starting from the D-mannose derived mannonolactone 4, conformationally constrained C-glycosidic as well as open chained hydroxamic acids with a defined stereochemistry were prepared. Diversity was introduced by performing C­C coupling reactions like the Sonogashira and Suzuki cross-coupling reactions. The biological evaluation of the synthesized compounds revealed that in the case of the C-glycosides a long, linear and rigid hydrophobic side chain is required for antibiotic activity against E. coli. The open chain derivatives show higher biological activity than the conformationally constrained C-glycosides. The morpholinomethyl substituted open chain derivative 43, being the most potent compound presented in this paper, inhibits LpxC with a Ki value of 0.35 µM and represents a promising lead structure.

Pyrrolidine in Drug Discovery: A Versatile Scaffold for Novel Biologically Active Compounds.

The five-membered pyrrolidine ring is one of the nitrogen heterocycles used widely by medicinal chemists to obtain compounds for the treatment of human diseases. The great interest in this saturated scaffold is enhanced by (1) the possibility to efficiently explore the pharmacophore space due to sp3-hybridization, (2) the contribution to the stereochemistry of the molecule, (3) and the increased three-dimensional (3D) coverage due to the non-planarity of the ring-a phenomenon called "pseudorotation". In this review, we report bioactive molecules with target selectivity characterized by the pyrrolidine ring and its derivatives, including pyrrolizines, pyrrolidine-2-one, pyrrolidine-2,5-diones and prolinol described in the literature from 2015 to date. After a comparison of the physicochemical parameters of pyrrolidine with the parent aromatic pyrrole and cyclopentane, we investigate the influence of steric factors on biological activity, also describing the structure-activity relationship (SAR) of the studied compounds. To aid the reader's approach to reading the manuscript, we have planned the review on the basis of the synthetic strategies used: (1) ring construction from different cyclic or acyclic precursors, reporting the synthesis and the reaction conditions, or (2) functionalization of preformed pyrrolidine rings, e.g., proline derivatives. Since one of the most significant features of the pyrrolidine ring is the stereogenicity of carbons, we highlight how the different stereoisomers and the spatial orientation of substituents can lead to a different biological profile of drug candidates, due to the different binding mode to enantioselective proteins. We believe that this work can guide medicinal chemists to the best approach in the design of new pyrrolidine compounds with different biological profiles.

Bioactive pyrrole-based compounds with target selectivity.

The discovery of novel synthetic compounds with drug-like properties is an ongoing challenge in medicinal chemistry. Natural products have inspired the synthesis of compounds for pharmaceutical application, most of which are based on N-heterocyclic motifs. Among these, the pyrrole ring is one of the most explored heterocycles in drug discovery programs for several therapeutic areas, confirmed by the high number of pyrrole-based drugs reaching the market. In the present review, we focused on pyrrole and its hetero-fused derivatives with anticancer, antimicrobial, and antiviral activities, reported in the literature between 2015 and 2019, for which a specific target was identified, being responsible for their biological activity. It emerges that the powerful pharmaceutical and pharmacological features provided by the pyrrole nucleus as pharmacophore unit of many drugs are still recognized by medicinal chemists.

Structure-Based Design, Synthesis, and Biological Evaluation of Triazole-Based smHDAC8 Inhibitors.

Schistosomiasis is a neglected tropical disease caused by parasitic flatworms of the genus Schistosoma, which affects over 200 million people worldwide and leads to at least 300,000 deaths every year. In this study, initial screening revealed the triazole-based hydroxamate 2 b (N-hydroxy-1-phenyl-1H-1,2,3-triazole-4-carboxamide) exhibiting potent inhibitory activity toward the novel antiparasitic target Schistosoma mansoni histone deacetylase 8 (smHDAC8) and promising selectivity over the major human HDACs. Subsequent crystallographic studies of the 2 b/smHDAC8 complex revealed key interactions between the inhibitor and the enzyme's active site, thus explaining the unique selectivity profile of the inhibitor. Further chemical modifications of 2 b led to the discovery of 4-fluorophenoxy derivative 21 (1-[5-chloro-2-(4-fluorophenoxy)phenyl]-N-hydroxy-1H-1,2,3-triazole-4-carboxamide), a nanomolar smHDAC8 inhibitor (IC50 =0.5 µM), exceeding the smHDAC8 inhibitory activity of 2 b and SAHA (vorinostat), while exhibiting an improved selectivity profile over the investigated human HDACs. Collectively, this study reveals specific interactions between smHDAC8 and the synthesized triazole-based inhibitors and demonstrates that these small molecules represent promising lead structures, which could be further developed in the search for novel drugs for the treatment of schistosomiasis.

Relationships between the structure of 6-allyl-6,8-diazabicyclo[3.2.2]nonane derivatives and their sigma receptor affinity and cytotoxic activity.

A series of bridged piperazine derivatives was prepared and the affinity toward sigma(1) and sigma(2) receptors by means of radioligand binding assays as well as the inhibition of the growth of six human tumor cell lines was investigated. All possible stereoisomers of the 2-hydroxy, 2-methoxy, 2,2-dimethoxy, 2-oxo, and 2-unsubstituted 6,8-diazabicyclo[3.2.2]nonanes were prepared in a chiral pool synthesis starting with (S)- and (R)-glutamate. A Dieckmann analogous cyclization was the key step in the synthesis of the bicyclic framework. The configuration in position 2 was established by a diastereoselective LiBH(4) reduction and subsequent Mitsunobu inversion. Structure-affinity relationships demonstrate that substituents in position 2 decrease sigma(1) receptor affinity which might be due to unfavorable interactions with the sigma(1) receptor protein. Without a substituent in position 2 high sigma(1) affinity was obtained (23a ((+)-(1S,5S)-6-allyl-8-(4-methoxybenzyl)-6,8-diazabicyclo[3.2.2]nonane): K(i)=11 nM). Experiments with six human tumor cell lines showed a weak but selective growth inhibition of the human small cell lung cancer cell line A-427 by the methyl ethers ent-16b (IC(50)=18.9 microM), 21a (IC(50)=16.4 microM), ent-21a (IC(50)=20.4 microM), and 21b (IC(50)=27.1 microM) and the unsubstituted compounds 23a and 23b (42% inhibition at 20 microM).

Relationships between the structure of 6-substituted 6,8-diazabicyclo[3.2.2]nonan-2-ones and their sigma receptor affinity and cytotoxic activity.

A series of 2-oxo-6,8-diazabicyclo[3.2.2]nonane derivatives was prepared and the affinity towards sigma(1) and sigma(2) receptors was investigated by means of radioligand binding assays as well as their inhibition of the growth of six human tumor cell lines was studied. Starting from the enantiopure bicyclic ketones 3 and ent-3 bridged piperazines with different residues in position 6 were synthesized. The N-6 allyl protective group was removed by a RhCl(3) catalyzed double bond isomerization and subsequent hydrolysis of the resulting enamide 8. After acetalization the secondary amide 10 was alkylated and arylated. Structure affinity relationships show that a relatively large substituent, which has not necessarily to be an aromatic one, is required in position 6 for high sigma(1) receptor affinity (e.g., 12 and ent-12 with a dimethylallyl residue: K(i)=20 nM and 17 nM). Furthermore, it was shown that substituents that reduce the basicity of N-6 led to a severe decrease in sigma(1) affinity. Growth inhibition experiments with six human tumor cell lines revealed that the allyl and benzyl substituted 6,8-diazabicyclo[3.2.2]nonan-2-one derivatives 5, ent-5 and ent-14 are able to selectively inhibit the growth of the bladder cancer cell line 5637.

Chiral Pool Synthesis, Biological Evaluation and Molecular Docking Studies of C-Furanosidic LpxC Inhibitors.

Inhibitors of the bacterial deacetylase LpxC are a promising class of novel antibiotics, being selectively active against Gram-negative bacteria. To improve the biological activity of reported C-furanosidic LpxC inhibitors, the stereochemistry at positions 3 and 4 of the tetrahydrofuran ring was varied. In chiral pool syntheses starting from d-gulono-γ-lactone and d-ribose, a series of (3S,4R)-configured dihydroxytetrahydrofuran derivatives was obtained, of which the (2S,5S)-configured hydroxamic acid 15 ((2S,3S,4R,5S)-N,3,4-trihydroxy-5-(4-{[4-(morpholinomethyl)phenyl]ethynyl}phenyl)tetrahydrofuran-2-carboxamide) was found to be the most potent LpxC inhibitor (Ki =0.4 µm), exhibiting the highest antibacterial activity against E. coli BL21 (DE3) and the D22 strain. Additionally, molecular docking studies were performed to rationalize the obtained structure-activity relationships.

Characterization of Histone Deacetylase 8 (HDAC8) Selective Inhibition Reveals Specific Active Site Structural and Functional Determinants.

Metal-dependent histone deacetylases (HDACs) are key epigenetic regulators that represent promising therapeutic targets for the treatment of numerous human diseases. Yet the currently FDA-approved HDAC inhibitors nonspecifically target at least several of the 11 structurally similar but functionally different HDAC isozymes, which hampers their broad usage in clinical settings. Selective inhibitors targeting single HDAC isozymes are being developed, but precise understanding in molecular terms of their selectivity remains sparse. Here, we show that HDAC8-selective inhibitors adopt a L-shaped conformation required for their binding to a HDAC8-specific pocket formed by HDAC8 catalytic tyrosine and HDAC8 L1 and L6 loops. In other HDAC isozymes, a L1-L6 lock sterically prevents L-shaped inhibitor binding. Shielding of the HDAC8-specific pocket by protein engineering decreases potency of HDAC8-selective inhibitors and affects catalytic activity. Collectively, our results unravel key HDAC8 active site structural and functional determinants important for the design of next-generation chemical probes and epigenetic drugs.

LpxC inhibitors: a patent review (2010-2016).

INTRODUCTION: The Zn2+-dependent deacetylase LpxC is an essential enzyme of lipid A biosynthesis in Gram-negative bacteria and a promising target for the development of antibiotics selectively combating Gram-negative pathogens. Researchers from industry and academia have synthesized structurally diverse LpxC inhibitors, exhibiting different LpxC inhibitory and antibacterial activities. Areas covered: A brief introduction into the structure and function of LpxC, showing its suitability as antibacterial target, along with the structures of several reported LpxC inhibitors, is given. The article reviews patents (reported between 2010 and 2016) and related research publications on novel small-molecule LpxC inhibitors. Emphasis is placed on structure-activity relationships within the reported series of LpxC inhibitors. Expert opinion: The performed analysis of patents revealed that the current search for novel LpxC inhibitors is focused on small molecules, sharing common structural features like a Zn2+-chelating group as well as a highly lipophilic side-chain. However, despite the promising preclinical data of many of the reported compounds, besides the recently withdrawn clinical candidate ACHN-975, no other LpxC inhibitor has entered clinical trials. The lack of clinical candidates might be related with undesired effects caused by the common structural elements of the LpxC inhibitors.

Insights into the Zinc-Dependent Deacetylase LpxC: Biochemical Properties and Inhibitor Design.

The bacterial enzyme UDP-3-O-[(R)-3-hydroxymyristoyl]-N-acetylglucosamine deacetylase (LpxC), catalyzing the first committed step of lipid A biosynthesis, represents a promising target in the development of novel antibiotics against Gram-negative bacteria. Structure, catalytic reaction mechanism and regulation of the Zn2+-dependent metalloamidase have been intensively investigated. The enzyme is required for growth and viability of Gram-negative bacteria, displays no sequence homology with any mammalian protein, but is highly conserved in Gram-negative bacteria, thus permitting the development of Gram-negative selective antibacterial agents with limited off-target effects. Several smallmolecule LpxC inhibitors have been developed, like the substrate analog TU-514 (12a), the aryloxazoline L-161,240 (13w), the sulfonamide BB-78485 (23a), the N-aroyl-L-threonine derivative CHIR-090 (24a), the sulfone-containing pyridone LpxC-3 (43e), and the uridine-based inhibitor 1-68A (47a), displaying diverse inhibitory and antibacterial activities. Most of these compounds share a Zn2+-binding hydroxamate moiety attached to a structural element addressing the hydrophobic tunnel or the UDP binding site. The butadiynyl derivative ACHN-975 (28) is the first LpxC inhibitor entering clinical trials.

Chiral pool synthesis and biological evaluation of C-furanosidic and acyclic LpxC inhibitors.

Inhibitors of the bacterial deacetylase LpxC have emerged as a promising new class of Gram-negative selective antibacterials. In order to find novel LpxC inhibitors, in chiral-pool syntheses starting from d-mannose, C-furanosides with altered configuration in positions 2 and/or 5 of the tetrahydrofuran ring were prepared in stereochemically pure form. Additionally, the substitution pattern in positions 3 and 4 of the tetrahydrofuran ring as well as the structure of the lipophilic side chain in position 2 were varied. Finally, all stereoisomers of the respective open chain diols were obtained via glycol cleavages of properly protected C-glycosides. The biological evaluation of the synthesized hydroxamic acids revealed that in case of the C-glycosides, 2,5-trans-configuration generally leads to superior inhibitory and antibacterial activities. The relief of the conformational strain leading to the respective open chain derivatives generally caused an increase in the inhibitory and antibacterial activities of the benzyloxyacetohydroxamic acids. With Ki-values of 0.35 µm and 0.23 µm, the (S,S)-configured open-chain derivatives 8b and 8c were found to be the most potent LpxC inhibitors of these series of compounds.