Structure-activity analysis of a class of orally active hydroxamic acid inhibitors of leukotriene biosynthesis

Synthesis, Evaluation of Enzyme Inhibition and Redox Properties of Potential Dual COX-2 and 5-LOX Inhibitors

Various dual inhibitors of COX-2 and 5-LOX enzymes have been developed so far in order to obtain more effective and safer anti-inflammatory drugs. The aim of this study was to design and synthesize new dual COX-2 and 5-LOX inhibitors, and to evaluate their enzyme inhibition potential and redox properties. Thirteen compounds (1-13) were designed taking into account structural requirements for dual COX-2 and 5-LOX inhibition and antioxidant activity, synthesized, and structurally characterized. These compounds can be classified as N-hydroxyurea derivatives (1, 2 and 3), 3,5-di-tert-butylphenol derivatives (4, 5, 6, 7 and 13), urea derivatives (8, 9 and 10) and "type B hydroxamic acids" (11 and 12). COX-1, COX-2 and 5-LOX inhibitory activities were evaluated using fluorometric inhibitor screening kits. The evaluation of the redox activity of newly synthesized compounds was performed in vitro in the human serum pool using redox status tests. The prooxidative score, the antioxidative score and the oxy-score were calculated. Seven out of thirteen synthesized compounds (1, 2, 3, 5, 6, 11 and 12) proved to be dual COX-2 and 5-LOX inhibitors. These compounds expressed good COX-2/COX-1 selectivity. Moreover, dual inhibitors 1, 3, 5, 11 and 12 showed good antioxidant properties.

Recent approaches towards one-carbon homologation-functionalization of aldehydes

One-carbon homologation-functionalization in organic synthesis is a quite challenging and difficult task in terms of atom economy, ease of reaction, selectivity and number of steps involved. Due to the reactivity associated with most classes of carbonyls, these groups have always attracted a great deal of attention from synthetic chemists to transform them into various functionalities. In this context various researchers developed new methods for one-carbon extension-functionalization of carbonyls that serve as effective synthetic methodologies and are widely used in target-oriented and natural product synthesis. On account of the vast applicability associated with these transformations, herein we seek to summarize and highlight the important synthetic achievements in this advancing arena for various one-carbon homologation cum functionalization reactions of aldehydes and deep dive into some modern approaches adopted by organic chemists.

Synthesis and structure-activity relationships of novel 5-(hydroxamic acid)methyl oxazolidinone derivatives as 5-lipoxygenase inhibitors

Oxazolidinone hydroxamic acid derivatives were synthesised and evaluated for inhibitory activity against leukotriene (LT) biosynthesis in three in vitro cell-based test systems and on direct inhibition of recombinant human 5-lipoxygenase (5-LO). Thirteen of the 19 compounds synthesised were considered active ((50% inhibitory concentration (IC₅₀) ≤ 10 µM in two or more test systems)). Increasing alkyl chain length on the hydroxamic acid moiety enhanced activity and morpholinyl-containing derivatives were more active than N-acetyl-piperizinyl derivatives. The IC₅₀ values in cell-based assay systems were comparable to those obtained by direct inhibition of 5-LO activity, confirming that the compounds are direct inhibitors of 5-LO. Particularly, compounds PH-249 and PH-251 had outstanding potencies (IC₅₀ < 1 µM), comparable to that of the prototype 5-LO inhibitor, zileuton. Pronounced in vivo activity was demonstrated in zymosan-induced peritonitis in mice. These novel oxazolidinone hydroxamic acid derivatives are, therefore, potent 5-LO inhibitors with potential application as anti-allergic and anti-inflammatory agents.

Targeting Metalloenzymes for Therapeutic Intervention

Metalloenzymes are central to a wide range of essential biological activities, including nucleic acid modification, protein degradation, and many others. The role of metalloenzymes in these processes also makes them central for the progression of many diseases and, as such, makes metalloenzymes attractive targets for therapeutic intervention. Increasing awareness of the role metalloenzymes play in disease and their importance as a class of targets has amplified interest in the development of new strategies to develop inhibitors and ultimately useful drugs. In this Review, we provide a broad overview of several drug discovery efforts focused on metalloenzymes and attempt to map out the current landscape of high-value metalloenzyme targets.

Anti-inflammatory, antioxidant and analgesic amides

The synthesis of some new aryl acetic acids and amides and a pharmacochemical study and quantitative structure-activity relationships (QSAR) on them are described. The compounds were screened for their biological activity using the carrageenin induced rat paw oedema model and a significant inhibition of oedema occurred (44.1-80.1%) at a concentration of 0.01 mmol/1 kg. The analgesic activity, based on the inhibition of acetic acid-induced writhing in rats was also found to be significant. The compounds were found to interact with the stable free radical 1,1-diphenylhydrazyl DPPH and with DMSO (for hydroxyl radicals). The compounds were screened for radical scavenging activity with the xanthine/xanthine oxidase system for O2-* and for inhibition of soybean lipoxygenase (LOX). The results are discussed in terms of the structural and physicochemical characteristics of the compounds.

Additions of Azomethine Ylides to Fullerene C(60) Assisted by a Removable Anchor

The addition of nitrile oxides to [60]fullerene, leading to isoxazolinofullerenes, can be reversed using reducing agents such as Mo(CO)(6) or DIBALH. Thus, this reaction can be used, in principle, for protection/deprotection of [60]fullerene or for solubilization purposes. The tether-controlled tandem addition of nitrile oxides and azomethine ylides provides mainly cis-1 patterns. The determination of the structure of bisadducts was obtained by NMR spectroscopy with the help of HMQC, HMBC, and NOEDS techniques. The isoxazoline moiety could be removed using Mo(CO)(6) leaving a fulleropyrrolidine derivative.

N-(5-substituted) thiophene-2-alkylsulfonamides as potent inhibitors of 5-lipoxygenase

Compound 4k N-[5-(4-fluoro)phenoxythien-2-yl]methanesulfonamide is representative of a new class of potent inhibitors of 5-lipoxygenase (5-LO). These versatile compounds exhibit dose-dependent inhibition of 5-LO with IC50s ranging from 20-100 nM in the rat basophilic leukemia (RBL-1) cell homogenate assay and submicromolar IC50s in both the RBL-1 and human peripheral blood leukocyte (PBL) whole cell assays. Compound 4k also showed significant anti-inflammatory activity in the adjuvant arthritic rat at an oral dose of 3 mg/kg.

Azole phenoxy hydroxyureas as selective and orally active inhibitors of 5-lipoxygenase

Azole phenoxy hydroxyureas are a new class of 5-lipoxygenase (5-LO) inhibitors. Structure-activity relationship studies have demonstrated that electronegative substituents on the 2-phenyl portion of the oxazole tail increased the ex vivo potency of these inhibitors. Similar substitutions on the thiazole analogs had only minor contribution to the ex vivo activity. The trifluoromethyl-substituted oxazole 24 was the best compound of the oxazole series in both the ex vivo (6 h pretreated rats) and in vivo (3 h pretreated rats) RPAR assay with ED50 values of approximately 1 and 3.6 mg/kg, respectively, but was weakly active in the allergic guinea pig assay. Oxazole 50 was equally active in both the RPAR and guinea pig in vivo models and was similar to zileuton. The unsubstituted thiazole 52 was the best compound of the thiazole series, by inhibiting the leukotriene B4 biosynthesis in the RPAR assay (3 h pretreated rats) by 99%, at an oral dose of 10 mg/kg, and the bronchoconstriction in the allergic guinea pig by 50%, at an intravenous dose of 10 mg/kg. Oxazole 24 demonstrated high and selective 5-LO inhibitory activity in the in vitro assays, with IC50 values ranging from 0.08 microM in mouse macrophages to 0.8 microM in human peripheral monocytes to 1.2 microM in human whole blood. This activity was selective for 5-LO, as concentrations up to 15 microM in mouse macrophages did not affect prostaglandin formation. Oxazole 59 was the most active inhibitor in the human monocyte assay with an IC50 value of 7 nM.

Amino hydroxamic acids as potent inhibitors of leukotriene A4 hydrolase

Leukotriene A4 hydrolase is a zinc-containing enzyme which catalyzes the hydrolysis of LTA4 to LTB4, a proinflammatory mediator. The enzyme also exhibits an aminopeptidase activity. Due to its biological importance, it is of considerable interest to develop selective inhibitors of this enzyme. The design and synthesis of a number of potent beta-amino hydroxylamine and amino hydroxamic acid inhibitors are described here. It was found that having a free amine was essential for high activity. Hydroxylamines were found to be about an order of magnitude less potent than their analogous hydroxamic acids. Our investigation of amino hydroxamic acids as inhibitors of leukotriene A4 hydrolase has led to the development of hydroxamates 16 and 17, which are among the most potent inhibitors found to date. These, compounds were found to be competitive inhibitors with Ki values of 1.6 nM and 3.4 nM respectively, against the peptidase activity. Inhibitor 16 has an IC50 value of < or = 0.15 microM against the epoxide hydrolase activity and is also potent against the production of LTB4 by isolated polymorphonuclear leukocytes (PMNL) activated with ionophore A23187 (IC50 approximately 0.3 microM).

Synthesis, antioxidant properties, biological activity and molecular modelling of a series of chalcogen analogues of the 5-lipoxygenase inhibitor DuP 654

2-Phenylsulfenyl- (1b), 2-phenylselenenyl- (1c) and 2-phenyltellurenyl-1-naphthol (1d) were prepared and their antioxidative properties evaluated in comparison with 2-benzyl-1-naphthol (1a; DuP 654). 2-Phenyltellurenyl-1-naphthol had a significantly lower (1.00 V versus SCE) oxidation potential than the other three compounds (1.24, 1.27 and 1.25 V, respectively, versus SCE for compounds 1a, 1b and 1c) as determined by cyclic voltammetry. In contrast to the other materials, compound 1d was able to catalyze the reduction of hydrogen peroxide in the presence of thiols as stoichiometric reducing agents. The organotellurium compound was also the most efficient inhibitor of azo-initiated peroxidation of linoleic acid in a two-phase model system. Ab initio geometry optimization at the 3-21G(*) level revealed infinitesimal changes in the molecular conformations of the carbon, sulfur, selenium and tellurium analogues. As judged by their ability to inhibit stimulated LTB4 biosynthesis in human neutrophils, compounds 1a-1d all turned out to be highly potent 5-lipoxygenase inhibitors with IC50-values ranging from 0.40 microM for 2-benzyl-1-naphthol (1a) to 0.063 microM for 2-phenyltellurenyl-1-naphthol (1d).

Novel bishydroxamic acids as 5-lipoxygenase inhibitors

Two series of novel bishydroxamic acids 2 and 3 (types A and B) were synthesized and tested for inhibition of 5-lipoxygenase from rat basophile leukemia (RBL) cells. Both series were potent inhibitors of the isolated enzyme but only the type B reverse hydroxamic acids possessed significant oral activity. The most potent compound, orally, was 3a, [IC50 = 270 nM; ED50 = 1.86 mg/kg], which compares favorably with the clinically useful 5-lipoxygenase inhibitor, zileuton. Unlike known hydroxamic acid inhibitors, the oral activity in this series appears to be associated with the second hydroxamic acid group. The corresponding monohydroxamic acids retained inhibitor potency, in vitro, with reduced oral activity in a mouse zymosan peritonitis model. Compound 4e [IC50 = 7 nM], a monohydroxamic acid derivative related to 3a, is among the most potent inhibitors of the isolated enzyme yet to be reported.

The properties of A-69412: a small hydrophilic 5-lipoxygenase inhibitor

A compound which inhibits leukotriene biosynthesis could be clinically useful in treating several allergic and inflammatory diseases. One site for such inhibition is at the enzyme 5-lipoxygenase. Most inhibitors of this enzyme thus far described are poorly bioavailable. A-69412 is a small, relatively hydrophilic compound of the N-hydroxyurea class, which exhibits minimal plasma protein binding (6-12%). The compound was found to be a potent long-acting inhibitor of leukotriene formation in vivo in the rat (oral ED50 = 5 mg/kg) and ex vivo in several species. In addition, the compound exhibits excellent bioavailability in dogs and monkeys with a relatively long elimination half-life in both the species (6 and 3 h, respectively). The biochemical activity and pharmacological profile of A-69412 indicates its potential utility in asthma and ulcerative colitis, and possibly other inflammatory and allergic conditions.

Kinetics and mechanism of degradation of Zileuton, a potent 5-lipoxygenase inhibitor

Zileuton (N-(1-benzo[b]thien-2-ylethyl)N-hydroxyurea) is a powerful 5-lipoxygenase inhibitor. The chemical degradation of Zileuton and related hydroxyurea derivatives was studied in aqueous solutions as a function of pH and temperature. The pH profile for the degradation of Zileuton shows an acid-catalyzed region at pH values below 2, water hydrolysis of the protonated form at pH values from 3 to 8, and water hydrolysis of the unprotonated form at pH values greater than 9. Hydrolysis of the hydroxyurea moiety to give the hydroxylamine derivative represents the main degradation pathway for Zileuton. This product, however, is not stable and is present at low concentrations at pH values below 6 and not observed at pH values greater than 7. Further decomposition of the hydroxylamine derivative leads to the observed degradation products. Air oxidation to the isomeric oximes accounts for the observed products at pH values greater than 7. Hydrolysis of the oximes to the ketone derivative accounts for the observed products at pH values 2 to 6. Parallel decomposition pathways to the alcohol derivative were noted under strongly acidic conditions, pH 0 to 2.

Designing therapeutically effective 5-lipoxygenase inhibitors

Metabolism of arachidonic acid by the enzyme 5-lipoxygenase leads to the formation of a group of biologically active lipids known as leukotrienes. Peptidoleukotrienes are powerful bronchoconstrictor agents while leukotriene B4 is a potent chemotactic agent for a variety of leukocytes. In view of these properties, leukotrienes have been proposed as important mediators in allergic and inflammatory disorders, and inhibitors of 5-lipoxygenase, by blocking leukotriene synthesis, have therapeutic potential in a range of diseases including arthritis and asthma. This review by Rodger McMillan and Ed Walker summarizes the biology of leukotrienes and the current knowledge of the mechanism of 5-lipoxygenase, providing a framework for consideration of the discovery, development and clinical status of drugs in the three major classes of 5-lipoxygenase inhibitors: 'redox' inhibitors, iron ligand inhibitors and 'non-redox' inhibitors.

The discovery and development of zileuton: an orally active 5-lipoxygenase inhibitor

The enzyme 5-lipoxygenase is a key target in the effort to discover drugs which inhibit the pathophysiology associated with the formation of leukotrienes. The research efforts of these laboratories have focused on the discovery of direct enzyme inhibitors of 5-lipoxygenase. In particular, compounds with hydroxamate or N-hydroxyurea functionalities have proven to be potent inhibitors of leukotriene biosynthesis in vitro and more importantly in vivo. One of these compounds, zileuton (N-(1-benzo-[b]-thien-2-ylethyl)-N-hydroxyurea) has been shown recently to be an effective leukotriene inhibitor in man. The critical approaches and breakthroughs in the discovery and development of zileuton are described. In addition, some recent results with zileuton in animals and man are detailed.

5-lipoxygenase inhibitors and their anti-inflammatory activities

A wide variety of agents have been reported as 5-LO inhibitors. The majority of the series appear to be lipophilic reducing agents, including phenols, partially saturated aromatics, and compounds containing heteroatom-heteroatom bonds. Many of these are not selective 5-LO inhibitors, but often affect CO and other LOs as well. In vivo systemic activity for many of these has been, in general, disappointing, probably because of poor bioavailability caused by lipophilicity and metabolic instability (oxidation, and conjugation of phenolic compounds). However, topically a number of agents have shown promise for skin inflammation, with Syntex's lonapalene the most advanced of these. Most results published to date appear more disappointing in the allergy/asthma field. More excitingly, a few structural types are selective 5-LO inhibitors which have shown systemic activity in vivo and in the clinic. Abbott's zileuton (136) appears to be one of the leading compounds in this category, along with other hydroxamates such as BW-A4C (129) from Burroughs-Wellcome. Recent selective non-reducing agents such as Wyeth-Ayerst's Wy-50,295 (143) and the similar ICI compounds such as ICI 216800 (145) also hold promise. The enantiospecific effects of (106) and (145) are especially interesting for the design of new inhibitors. If compounds like these validate the hypothesis that inhibition of 5-LO will have a significant anti-inflammatory effect, a redoubling of effort throughout the industry to find second- and third-generation selective agents may be expected. Part of the difficulty in interpreting and comparing the 5-LO literature is the plethora of test methods and activity criteria. As pointed out in the introduction, inhibition of product release from cells, often stimulated with A23187, has commonly been used to demonstrate 5-LO inhibition. However, this type of assay cannot be assumed to be diagnostic for 5-LO inhibition. Only if specificity for 5-LO product generation and (ideally) activity in cell-free enzymes is also shown should mechanistic interpretations be made. Recently, a new class of compounds was found at Merck which inhibited LT biosynthesis without inhibiting 5-LO, but apparently by a novel, specific mechanism. L-655,240 (169) and L-663,536 (MK-886) (170) were both active in human ISN, with IC50 values in the low micromolar range. Both also orally inhibited GPB (< 1 mg/kg). MK-886 was effective in Ascaris-induced asthma in squirrel monkeys, in rat carrageenan pleurisy, in rat Arthus pleurisy, and (topically) in guinea-pig ear oedema induced by A23187.(ABSTRACT TRUNCATED AT 400 WORDS)

Attenuation of inhaled allergen-induced airway microvascular leakage and airflow obstruction in guinea pigs by a 5-lipoxygenase inhibitor (A-63162)

Leukotrienes, products of the 5-lipoxygenase pathway in the metabolism of arachidonic acid, are potent mediators of airway microvascular leakage and smooth muscle contraction in guinea pigs. We studied the effect of a specific 5-lipoxygenase inhibitor, A-63162, on airway microvascular leakage and airflow obstruction following inhaled ovalbumin in actively sensitized guinea pigs. Airway microvascular leakage was assessed by extravasation of Evans blue dye into airway tissues. Inhaled ovalbumin caused increased lung resistance (RL) with decreased dynamic compliance (CL) and increased extravasation of Evans blue dye at all airway levels. A-63162 reduced the changes in RL and CL at 1.0 mumol/kg (p less than 0.05) but not at lower doses of 0.1 and 0.3 mumol/kg. A-63162 reduced Evans blue dye extravasation in airway tissues at all three doses, however, being most effective in the distal airways (p less than 0.01), and these reductions were greater than those in RL and CL. A 5-lipoxygenase inhibitor thus reduced airway microvascular leakage to a greater extent than airflow obstruction, suggesting that leukotrienes have a larger contribution to changes in airway microvascular permeability than smooth muscle contraction following inhaled allergen challenge in actively sensitized guinea pigs in vivo.

Effect of a 5-lipoxygenase inhibitor on leukotriene generation and airway responses after allergen challenge in asthmatic patients

The effect of a single oral dose (800 mg) of zileuton (A-64077), a specific 5-lipoxygenase inhibitor, on the early and late airway responses to inhaled allergen was studied in a randomised, double blind, placebo controlled, and crossover trial in nine subjects with atopic asthma. Leukotriene generation was also assessed in vivo by measuring urinary leukotriene (LT) E4 excretion, and ex vivo by measuring calcium ionophore stimulated whole blood LTB4 production. Zileuton almost completely inhibited ex vivo LTB4 production but reduced urinary excretion of LTE4 by only about half. There was a trend for the early asthmatic response to be less on the day of zileuton treatment, but this did not reach statistical significance (p = 0.08). The zileuton induced reduction in maximum fall in FEV1 in the early asthmatic response was, however, significantly related to the reduction in urinary LTE4 excretion (r = 0.8), but not to the reduction in LTB4 generation ex vivo. There was no significant change in the allergen induced late asthmatic response, or in the increase in airway responsiveness to methacholine following antigen. The results provide some support for the hypothesis that the cysteinyl leukotrienes have a role in the allergen induced early asthmatic response. More complete in vivo inhibition of 5-lipoxygenase may be needed to produce a significant reduction in airway response to allergen challenge.