Structure-activity relationship of 2-oxoamide inhibition of group IVA cytosolic phospholipase A2 and group V secreted phospholipase A2

Identification of a Dual Inhibitor of Secreted Phospholipase A2 (GIIA sPLA2) and SARS-CoV-2 Main Protease

The development of novel agents to combat COVID-19 is of high importance. SARS-CoV-2 main protease (M^pro) is a highly attractive target for the development of novel antivirals and a variety of inhibitors have already been developed. Accumulating evidence on the pathobiology of COVID-19 has shown that lipids and lipid metabolizing enzymes are critically involved in the severity of the infection. The purpose of the present study was to identify an inhibitor able to simultaneously inhibit both SARS-CoV-2 M^pro and phospholipase A₂ (PLA₂), an enzyme which plays a significant role in inflammatory diseases. Evaluating several PLA₂ inhibitors, we demonstrate that the previously known potent inhibitor of Group IIA secretory PLA₂, GK241, may also weakly inhibit SARS-CoV-2 M^pro. Molecular mechanics docking and molecular dynamics calculations shed light on the interactions between GK241 and SARS-CoV-2 M^pro. 2-Oxoamide GK241 may represent a lead molecular structure for the development of dual PLA₂ and SARS-CoV-2 M^pro inhibitors.

N-Acylated and N-Alkylated 2-Aminobenzothiazoles Are Novel Agents That Suppress the Generation of Prostaglandin E2

The quest for novel agents to regulate the generation of prostaglandin E₂ (PGE₂) is of high importance because this eicosanoid is a key player in inflammatory diseases. We synthesized a series of N-acylated and N-alkylated 2-aminobenzothiazoles and related heterocycles (benzoxazoles and benzimidazoles) and evaluated their ability to suppress the cytokine-stimulated generation of PGE₂ in rat mesangial cells. 2-Aminobenzothiazoles, either acylated by the 3-(naphthalen-2-yl)propanoyl moiety (GK510) or N-alkylated by a chain carrying a naphthalene (GK543) or a phenyl moiety (GK562) at a distance of three carbon atoms, stand out in inhibiting PGE₂ generation, with EC₅₀ values ranging from 118 nM to 177 nM. Both GK510 and GK543 exhibit in vivo anti-inflammatory activity greater than that of indomethacin. Thus, N-acylated or N-alkylated 2-aminobenzothiazoles are novel leads for the regulation of PGE₂ formation.

The design and discovery of phospholipase A2 inhibitors for the treatment of inflammatory diseases

AREAS COVERED

This review article summarizes the most important synthetic PLA₂ inhibitors developed to target each one of the four major types of human PLA₂ (cytosolic cPLA₂, calcium-independent iPLA₂, secreted sPLA₂, and lipoprotein-associated Lp-PLA₂), discussing their in vitro and in vivo activities as well as their recent applications and therapeutic properties. Recent findings on the role of PLA₂ in the pathobiology of COVID-19 are also discussed.

EXPERT OPINION

Although a number of PLA₂ inhibitors have entered clinical trials, none has reached the market yet. Lipoprotein-associated PLA₂ is now considered a biomarker of vascular inflammation rather than a therapeutic target for inhibitors like darapladib. Inhibitors of cytosolic PLA₂ may find topical applications for diseases like atopic dermatitis and psoriasis. Inhibitors of secreted PLA₂, varespladib and varespladib methyl, are under investigation for repositioning in snakebite envenoming. A deeper understanding of PLA₂ enzymes is needed for the development of novel selective inhibitors. Lipidomic technologies combined with medicinal chemistry approaches may be useful tools toward this goal.

The Alpha Keto Amide Moiety as a Privileged Motif in Medicinal Chemistry: Current Insights and Emerging Opportunities

Over the years, researchers in drug discovery have taken advantage of the use of privileged structures to design innovative hit/lead molecules. The α-ketoamide motif is found in many natural products, and it has been widely exploited by medicinal chemists to develop compounds tailored to a vast range of biological targets, thus presenting clinical potential for a plethora of pathological conditions. The purpose of this perspective is to provide insights into the versatility of this chemical moiety as a privileged structure in drug discovery. After a brief analysis of its physical-chemical features and synthetic procedures to obtain it, α-ketoamide-based classes of compounds are reported according to the application of this motif as either a nonreactive or reactive moiety. The goal is to highlight those aspects that may be useful to understanding the perspectives of employing the α-ketoamide moiety in the rational design of compounds able to interact with a specific target.

Human Secretary Phospholipase A2 Mutations and Their Clinical Implications

Phospholipases A2 (PLA₂s) belong to a superfamily of enzymes responsible for hydrolysis of the sn-2 fatty acids of membrane phospholipids to release arachidonic acid. PLA₂s are the rate limiting enzyme for the downstream synthesis of prostaglandins and leukotrienes that are the main mediators of inflammation. The extracellular forms of this enzyme are also called the secretary phospholipase A2 (sPLA₂) and are distributed extensively in most of the tissues in the human body. Their integral role in inflammatory pathways has been the primary reason for the extensive research on this molecule. The catalytic mechanism of sPLA₂ is initiated by a histidine/aspartic acid/calcium complex within the active site. Though they are known to have certain housekeeping functions, certain mutations of sPLA₂ are known to be implicated in causation of certain pathologies leading to diseases such as atherosclerosis, cardiovascular diseases, benign fleck retina, neurodegeneration, and asthma. We present an overview of human sPLA₂ and a comprehensive compilation of the mutations that result in various disease phenotypes. The study not only helps to have a holistic understanding of human sPLA₂ mutations and their clinical implications, but is also a useful platform to initiate research pertaining to structure–function relationship of the mutations to develop effective therapies for management of these diseases.

Structure-Activity Relationship Studies of α-Ketoamides as Inhibitors of the Phospholipase A and Acyltransferase Enzyme Family

The phospholipase A and acyltransferase (PLAAT) family of cysteine hydrolases consists of five members, which are involved in the Ca²⁺-independent production of N-acylphosphatidylethanolamines (NAPEs). NAPEs are lipid precursors for bioactive N-acylethanolamines (NAEs) that are involved in various physiological processes such as food intake, pain, inflammation, stress, and anxiety. Recently, we identified α-ketoamides as the first pan-active PLAAT inhibitor scaffold that reduced arachidonic acid levels in PLAAT3-overexpressing U2OS cells and in HepG2 cells. Here, we report the structure–activity relationships of the α-ketoamide series using activity-based protein profiling. This led to the identification of LEI-301, a nanomolar potent inhibitor for the PLAAT family members. LEI-301 reduced the NAE levels, including anandamide, in cells overexpressing PLAAT2 or PLAAT5. Collectively, LEI-301 may help to dissect the physiological role of the PLAATs.

2-Oxoester Phospholipase A2 Inhibitors with Enhanced Metabolic Stability

2-Oxoesters constitute an important class of potent and selective inhibitors of human cytosolic phospholipase A₂ (GIVA cPLA₂) combining an aromatic scaffold or a long aliphatic chain with a short aliphatic chain containing a free carboxylic acid. Although highly potent 2-oxoester inhibitors of GIVA cPLA₂ have been developed, their rapid degradation in human plasma limits their pharmaceutical utility. In an effort to address this problem, we designed and synthesized two new 2-oxoesters introducing a methyl group either on the α-carbon to the oxoester functionality or on the carbon carrying the ester oxygen. We studied the in vitro plasma stability of both derivatives and their in vitro inhibitory activity on GIVA cPLA₂. Both derivatives exhibited higher plasma stability in comparison with the unsubstituted compound and both derivatives inhibited GIVA cPLA₂, however to different degrees. The 2-oxoester containing a methyl group on the α-carbon atom to the oxoester functionality exhibits enhancement of the metabolic stability and retains considerable inhibitory potency.

Hydration thermodynamics of cytosolic phospholipase A2 GIVA predict its membrane-associated parts and its highly hydrated binding site

During biological events, the water molecules associated with the protein are re-oriented to adapt to the new conditions, inducing changes in the system's free energy. The characterization of water structure and thermodynamics may facilitate the prediction of certain biological events, such as the binding of a ligand and the membrane-associated parts of a protein. In this computational study, we calculated the hydration thermodynamics of cytosolic phospholipase A₂ group IV (GIVA cPLA₂) to study the hydration properties of the protein's surface and binding pocket. Hydrophobicity scales and the Grid Inhomogeneous Solvation Theory (GIST) tool were employed for the calculations. The hydrophobic areas of the protein's surface were predicted more accurately with the GIST method rather than with the hydrophobicity scales. Based on this, a model of the protein-membrane complex was constructed. In addition, the calculation revealed the highly hydrated binding pocket that further contribute to our understanding of the ligands' binding. Communicated by Ramaswamy H. Sarma.

Small-molecule inhibitors as potential therapeutics and as tools to understand the role of phospholipases A2

Phospholipase A2 (PLA2) enzymes are involved in various inflammatory pathological conditions including arthritis, cardiovascular and autoimmune diseases. The regulation of their catalytic activity is of high importance and a great effort has been devoted in developing synthetic inhibitors. We summarize the most important small-molecule synthetic PLA2 inhibitors developed to target each one of the four major types of human PLA2 (cytosolic cPLA2, calcium-independent iPLA2, secreted sPLA2, and lipoprotein-associated LpPLA2). We discuss recent applications of inhibitors to understand the role of each PLA2 type and their therapeutic potential. Potent and selective PLA2 inhibitors have been developed. Although some of them have been evaluated in clinical trials, none reached the market yet. Apart from their importance as potential medicinal agents, PLA2 inhibitors are excellent tools to unveil the role that each PLA2 type plays in cells and in vivo. Modern medicinal chemistry approaches are expected to generate improved PLA2 inhibitors as new agents to treat inflammatory diseases.

β-Lactones: A Novel Class of Ca2+-Independent Phospholipase A2 (Group VIA iPLA2) Inhibitors with the Ability To Inhibit β-Cell Apoptosis

Ca²⁺-independent phospholipase A₂ (GVIA iPLA₂) has gained increasing interest recently as it has been recognized as a participant in biological processes underlying diabetes development and autoimmune-based neurological disorders. The development of potent GVIA iPLA₂ inhibitors is of great importance because only a few have been reported so far. We present a novel class of GVIA iPLA₂ inhibitors based on the β-lactone ring. This functionality in combination with a four-carbon chain carrying a phenyl group at position-3 and a linear propyl group at position-4 of the lactone ring confers excellent potency. trans-3-(4-Phenylbutyl)-4-propyloxetan-2-one (GK563) was identified as being the most potent GVIA iPLA₂ inhibitor ever reported ( X_I(50) 0.0000021, IC₅₀ 1 nM) and also one that is 22 000 times more active against GVIA iPLA₂ than GIVA cPLA₂. It was found to reduce β-cell apoptosis induced by proinflammatory cytokines, raising the possibility that it can be beneficial in countering autoimmune diseases, such as type 1 diabetes.

Highly Potent 2-Oxoester Inhibitors of Cytosolic Phospholipase A2 (GIVA cPLA2)

Cytosolic phospholipase A₂ (GIVA cPLA₂) has attracted great interest as a medicinal target because it initiates the eicosanoid cascade and is involved in a number of inflammatory diseases. As a consequence, the development of potent synthetic inhibitors is of great importance. We have developed highly potent 2-oxoester inhibitors of GIVA cPLA₂ presenting X_I(50) values between 0.000019 and 0.000066. We demonstrate that the 2-oxoester functionality is essential for in vitro inhibitory activity, making these inhibitors useful research reagents. However, their high reactivity results in rapid degradation of the inhibitors in human plasma, limiting their pharmaceutical utility without further modification.

Hydroxamic Acids Constitute a Novel Class of Autotaxin Inhibitors that Exhibit in Vivo Efficacy in a Pulmonary Fibrosis Model

Autotaxin (ATX) catalyzes the hydrolysis of lysophosphatidylcholine (LPC) generating the lipid mediator lysophosphatidic acid (LPA). Both ATX and LPA are involved in various pathological inflammatory conditions, including fibrosis and cancer, and have attracted great interest as medicinal targets over the past decade. Thus, the development of novel potent ATX inhibitors is of great importance. We have developed a novel class of ATX inhibitors containing the zinc binding functionality of hydroxamic acid. Such novel hydroxamic acids that incorporate a non-natural δ-amino acid residue exhibit high in vitro inhibitory potency over ATX (IC₅₀ values 50-60 nM). Inhibitor 32, based on δ-norleucine, was tested for its efficacy in a mouse model of pulmonary inflammation and fibrosis induced by bleomycin and exhibited promising efficacy. The novel hydroxamic ATX inhibitors provide excellent tools for the study of the role of the enzyme and could contribute to the development of novel therapeutic agents for the treatment of fibrosis and other chronic inflammatory diseases.

2-Oxoesters: A Novel Class of Potent and Selective Inhibitors of Cytosolic Group IVA Phospholipase A2

Cytosolic phospholipase A₂ (GIVA cPLA₂) is the only PLA₂ that exhibits a marked preference for hydrolysis of arachidonic acid containing phospholipid substrates releasing free arachidonic acid and lysophospholipids and giving rise to the generation of diverse lipid mediators involved in inflammatory conditions. Thus, the development of potent and selective GIVA cPLA₂ inhibitors is of great importance. We have developed a novel class of such inhibitors based on the 2-oxoester functionality. This functionality in combination with a long aliphatic chain or a chain carrying an appropriate aromatic system, such as the biphenyl system, and a free carboxyl group leads to highly potent and selective GIVA cPLA₂ inhibitors (X_I(50) values 0.00007–0.00008) and docking studies aid in understanding this selectivity. A methyl 2-oxoester, with a short chain carrying a naphthalene ring, was found to preferentially inhibit the other major intracellular PLA₂, the calcium-independent PLA₂. In RAW264.7 macrophages, treatment with the most potent 2-oxoester GIVA cPLA₂ inhibitor resulted in over 50% decrease in KLA-elicited prostaglandin D₂ production. The novel, highly potent and selective GIVA cPLA₂ inhibitors provide excellent tools for the study of the role of the enzyme and could contribute to the development of novel therapeutic agents for the treatment of inflammatory diseases.

Fluorogenic probes to monitor cytosolic phospholipase A2 activity

Functionalization on the alkyl tail of arachidonyl trifluoromethylketone leads to the development of fluorogenic inhibitor and substrate probes of cPLA₂.

2-Oxoamides based on dipeptides as selective calcium-independent phospholipase A2 inhibitors

Calcium-independent phospholipase A₂ (GVIA iPLA₂) has recently attracted interest as a medicinal target. The number of known GVIA iPLA₂ inhibitors is limited to a handful of synthetic compounds (bromoenol lactone and polyfluoroketones). To expand the chemical diversity, a variety of 2-oxoamides based on dipeptides and ether dipeptides were synthesized and studied for their in vitro inhibitory activity on human GVIA iPLA₂ and their selectivity over the other major intracellular GIVA cPLA₂ and the secreted GV sPLA₂. Structure-activity relationship studies revealed the first 2-oxoamide derivative (GK317), which presents potent inhibition of GVIA iPLA₂ (X_I(50) value of 0.007) and at the same time significant selectivity over GIVA cPLA₂ and GV sPLA₂.

2-Oxoamide inhibitors of cytosolic group IVA phospholipase A2 with reduced lipophilicity

Cytosolic GIVA phospholipase A2 (GIVA cPLA2) initiates the eicosanoid pathway of inflammation and thus inhibitors of this enzyme constitute novel potential agents for the treatment of inflammatory diseases. Traditionally, GIVA cPLA2 inhibitors have suffered systemically from high lipophilicity. We have developed a variety of long chain 2-oxoamides as inhibitors of GIVA PLA2. Among them, AX048 was found to produce a potent analgesic effect. We have now reduced the lipophilicity of AX048 by replacing the long aliphatic chain with a chain containing an ether linked aromatic ring with in vitro inhibitory activities similar to AX048.

Liberating Chiral Lipid Mediators, Inflammatory Enzymes, and LIPID MAPS from Biological Grease

In 1970, it was well accepted that the central role of lipids was in energy storage and metabolism, and it was assumed that amphipathic lipids simply served a passive structural role as the backbone of biological membranes. As a result, the scientific community was focused on nucleic acids, proteins, and carbohydrates as information-containing molecules. It took considerable effort until scientists accepted that lipids also "encode" specific and unique biological information and play a central role in cell signaling. Along with this realization came the recognition that the enzymes that act on lipid substrates residing in or on membranes and micelles must also have important signaling roles, spurring curiosity into their potentially unique modes of action differing from those acting on water-soluble substrates. This led to the creation of the concept of "surface dilution kinetics" for describing the mechanism of enzymes acting on lipid substrates, as well as the demonstration that lipid enzymes such as phospholipase A₂ (PLA₂) contain allosteric activator sites for specific phospholipids as well as for membranes. As our understanding of phospholipases advanced, so did the understanding that many of the lipids released by these enzymes are chiral information-containing signaling molecules; for example, PLA₂ regulates the generation of precursors for the biosynthesis of eicosanoids and other bioactive lipid mediators of inflammation and resolution underlying disease progression. The creation of the LIPID MAPS initiative in 2003 and the ensuing development of the lipidomics field have revealed that lipid metabolites are central to human metabolism. Today lipids are recognized as key mediators of health and disease as we enter a new era of biomarkers and personalized medicine. This article is my personal "reflection" on these scientific advances.

Inhibitors of secreted phospholipase A2 suppress the release of PGE2 in renal mesangial cells

The upregulation of PGE2 by mesangial cells has been observed under chronic inflammation condition. In the present work, renal mesangial cells were stimulated to trigger a huge increase of PGE2 synthesis and were treated in the absence or presence of known PLA2 inhibitors. A variety of synthetic inhibitors, mainly developed in our labs, which are known to selectively inhibit each of GIVA cPLA2, GVIA iPLA2, and GIIA/GV sPLA2, were used as tools in this study. Synthetic sPLA2 inhibitors, such as GK115 (an amide derivative based on the non-natural amino acid (R)-γ-norleucine) as well as GK126 and GK241 (2-oxoamides based on the natural (S)-α-amino acid leucine and valine, respectively) presented an interesting effect on the suppression of PGE2 formation.

Computer-aided drug design guided by hydrogen/deuterium exchange mass spectrometry: A powerful combination for the development of potent and selective inhibitors of Group VIA calcium-independent phospholipase A2

Potent and selective inhibitors for phospholipases A₂ (PLA₂) are useful for studying their intracellular functions. PLA₂ enzymes liberate arachidonic acid from phospholipids activating eicosanoid pathways that involve cyclooxygenase (COX) and lipoxygenase (LOX) leading to inflammation. Anti-inflammatory drugs target COX and LOX; thus, PLA₂ can also be targeted to diminish inflammation at an earlier stage in the process. This paper describes the employment of enzymatic assays, hydrogen/deuterium exchange mass spectrometry (DXMS) and computational chemistry to develop PLA₂ inhibitors. Beta-thioether trifluoromethylketones (TFKs) were screened against human GVIA calcium-independent, GIVA cytosolic and GV secreted PLA₂s. These compounds exhibited inhibition toward Group VIA calcium-independent PLA₂ (GVIA iPLA₂), with the most potent and selective inhibitor 3 (OTFP) obtaining an X_I(50) of 0.0002 mole fraction (IC₅₀ of 110nM). DXMS binding experiments in the presence of OTFP revealed the peptide regions of GVIA iPLA₂ that interact with the inhibitor. Molecular docking and dynamics simulations in the presence of a membrane were guided by the DXMS data in order to identify the binding mode of OTFP. Clustering analysis showed the binding mode of OTFP that occupied 70% of the binding modes occurring during the simulation. The resulted 3D complex was used for docking studies and a structure-activity relationship (SAR) was established. This paper describes a novel multidisciplinary approach in which a 3D complex of GVIA iPLA₂ with an inhibitor is reported and validated by experimental data. The SAR showed that the sulfur atom is vital for the potency of beta-thioether analogues, while the hydrophobic chain is important for selectivity. This work constitutes the foundation for further design, synthesis and inhibition studies in order to develop new beta-thioether analogues that are potent and selective for GVIA iPLA₂ exclusively.

Development of Potent and Selective Inhibitors for Group VIA Calcium-Independent Phospholipase A2 Guided by Molecular Dynamics and Structure-Activity Relationships

The development of inhibitors for phospholipase A2 (PLA2) is important in elucidating the enzymes implication in various biological pathways. PLA2 enzymes are an important pharmacological target implicated in various inflammatory diseases. Computational chemistry, organic synthesis, and in vitro assays were employed to develop potent and selective inhibitors for group VIA calcium-independent PLA2. A set of fluoroketone inhibitors was studied for their binding mode with two human cytosolic PLA2 enzymes: group IVA cPLA2 and group VIA iPLA2. New compounds were synthesized and assayed toward three major PLA2s. This study led to the development of four potent and selective thioether fluoroketone inhibitors as well as a thioether keto-1,2,4-oxadiazole inhibitor for GVIA iPLA2, which will serve as lead compounds for future development and studies. The keto-1,2,4-oxadiazole functionality with a thioether is a novel structure, and it will be used as a lead to develop inhibitors with higher potency and selectivity toward GVIA iPLA2.

Development of a potent 2-oxoamide inhibitor of secreted phospholipase A2 guided by molecular docking calculations and molecular dynamics simulations

Inhibition of group IIA secreted phospholipase A2 (GIIA sPLA2) has been an important objective for medicinal chemists. We have previously shown that inhibitors incorporating the 2-oxoamide functionality may inhibit human and mouse GIIA sPLA2s. Herein, the development of new potent inhibitors by molecular docking calculations using the structure of the known inhibitor 7 as scaffold, are described. Synthesis and biological evaluation of the new compounds revealed that the long chain 2-oxoamide based on (S)-valine GK241 led to improved activity (IC50=143 nM and 68 nM against human and mouse GIIA sPLA2, respectively). In addition, molecular dynamics simulations were employed to shed light on GK241 potent and selective inhibitory activity.

Transition metal-free synthesis of α-ketoamides from arylmethyl ketones and alkylphosphoramides

Synthesis of α-ketoamides from arylmethyl ketones and hexalkylphosphoramides using TBAI as the catalyst and TBHP as the oxidant.

ASB14780, an Orally Active Inhibitor of Group IVA Phospholipase A2, Is a Pharmacotherapeutic Candidate for Nonalcoholic Fatty Liver Disease

We have previously shown that high-fat cholesterol diet (HFCD)-induced fatty liver and carbon tetrachloride (CCl4)-induced hepatic fibrosis are reduced in mice deficient in group IVA phospholipase A2 (IVA-PLA2), which plays a role in inflammation. We herein demonstrate the beneficial effects of ASB14780 (3-[1-(4-phenoxyphenyl)-3-(2-phenylethyl)-1H-indol-5-yl]propanoic acid 2-amino-2-(hydroxymethyl)propane-1,3-diol salt), an orally active IVA-PLA2 inhibitor, on the development of fatty liver and hepatic fibrosis in mice. The daily coadministration of ASB14780 markedly ameliorated liver injury and hepatic fibrosis following 6 weeks of treatment with CCl4. ASB14780 markedly attenuated the CCl4-induced expression of smooth muscle α-actin (α-SMA) protein and the mRNA expression of collagen 1a2, α-SMA, and transforming growth factor-β1 in the liver, and inhibited the expression of monocyte/macrophage markers, CD11b and monocyte chemotactic protein-1, while preventing the recruitment of monocytes/macrophages to the liver. Importantly, ASB14780 also reduced the development of fibrosis even in matured hepatic fibrosis. Additionally, ASB14780 also reduced HFCD-induced lipid deposition not only in the liver, but also in already established fatty liver. Furthermore, treatment with ASB14780 suppressed the HFCD-induced expression of lipogenic mRNAs. The present findings suggest that an IVA-PLA2 inhibitor, such as ASB14780, could be useful for the treatment of nonalcoholic fatty liver diseases, including fatty liver and hepatic fibrosis.

Synthetic and natural inhibitors of phospholipases A2: their importance for understanding and treatment of neurological disorders

Phospholipases A2 (PLA2) are a diverse group of enzymes that hydrolyze membrane phospholipids into arachidonic acid and lysophospholipids. Arachidonic acid is metabolized to eicosanoids (prostaglandins, leukotrienes, thromboxanes), and lysophospholipids are converted to platelet-activating factors. These lipid mediators play critical roles in the initiation, maintenance, and modulation of neuroinflammation and oxidative stress. Neurological disorders including excitotoxicity; traumatic nerve and brain injury; cerebral ischemia; Alzheimer's disease; Parkinson's disease; multiple sclerosis; experimental allergic encephalitis; pain; depression; bipolar disorder; schizophrenia; and autism are characterized by oxidative stress, inflammatory reactions, alterations in phospholipid metabolism, accumulation of lipid peroxides, and increased activities of brain phospholipase A2 isoforms. Several old and new synthetic inhibitors of PLA2, including fatty acid trifluoromethyl ketones; methyl arachidonyl fluorophosphonate; bromoenol lactone; indole-based inhibitors; pyrrolidine-based inhibitors; amide inhibitors, 2-oxoamides; 1,3-disubstituted propan-2-ones and polyfluoroalkyl ketones as well as phytochemical based PLA2 inhibitors including curcumin, Ginkgo biloba and Centella asiatica extracts have been discovered and used for the treatment of neurological disorders in cell culture and animal model systems. The purpose of this review is to summarize information on selective and potent synthetic inhibitors of PLA2 as well as several PLA2 inhibitors from plants, for treatment of oxidative stress and neuroinflammation associated with the pathogenesis of neurological disorders.

Inhibition of group IVA cytosolic phospholipase A2 by thiazolyl ketones in vitro, ex vivo, and in vivo

Group IVA cytosolic phospholipase A2 (GIVA cPLA2) is the rate-limiting provider of pro-inflammatory mediators in many tissues and is thus an attractive target for the development of novel anti-inflammatory agents. In this work, we present the synthesis of new thiazolyl ketones and the study of their activities in vitro, in cells, and in vivo. Within this series of compounds, methyl 2-(2-(4-octylphenoxy)acetyl)thiazole-4-carboxylate (GK470) was found to be the most potent inhibitor of GIVA cPLA2, exhibiting an XI(50) value of 0.011 mole fraction in a mixed micelle assay and an IC50 of 300 nM in a vesicle assay. In a cellular assay using SW982 fibroblast-like synoviocytes, it suppressed the release of arachidonic acid with an IC50 value of 0.6 μM. In a prophylactic collagen-induced arthritis model, it exhibited an anti-inflammatory effect comparable to the reference drug methotrexate, whereas in a therapeutic model, it showed results comparable to those of the reference drug Enbrel. In both models, it significantly reduced plasma PGE2 levels.

New potent and selective polyfluoroalkyl ketone inhibitors of GVIA calcium-independent phospholipase A2

Group VIA calcium-independent phospholipase A2 (GVIA iPLA2) has recently emerged as an important pharmaceutical target. Selective and potent GVIA iPLA2 inhibitors can be used to study its role in various neurological disorders. In the current work, we explore the significance of the introduction of a substituent in previously reported potent GVIA iPLA2 inhibitors. 1,1,1,2,2-Pentafluoro-7-(4-methoxyphenyl)heptan-3-one (GK187) is the most potent and selective GVIA iPLA2 inhibitor ever reported with a XI(50) value of 0.0001, and with no significant inhibition against GIVA cPLA2 or GV sPLA2. We also compare the inhibition of two difluoromethyl ketones on GVIA iPLA2, GIVA cPLA2, and GV sPLA2.

Selenium dioxide-mediated synthesis of α-ketoamides from arylglyoxals and secondary amines

A facile and expeditious synthetic approach to α-ketoamides 3 is described. A series of α-ketoamides 3 was synthesized via reaction of selenium dioxide-mediated oxidative amidation between arylglyoxals 1 and secondary amines 2, and accelerated with microwave irradiation. Our findings indicate that constrained amines, such as piperazine and piperidine exhibit higher conversions for this transformation. This reaction was explored by synthesizing a series of α-ketoamides 3 from various arylglyoxals with cyclic and acyclic secondary amines.

Inhibitors of Cytosolic Phospholipase A2α as Anti-inflammatory Drugs

Arachidonic acid derivatives, like prostaglandins and leukotrienes, as well as the alkyl-ether phospholipid platelet-activating factor (PAF) are highly active substances with diverse biological actions. Elevated levels of these lipid mediators in response to a variety of stimuli have been implicated in the pathology of many inflammatory diseases. The rate-limiting step in the generation of prostaglandins, leukotrienes and PAF, respectively, is the cleavage of the sn-2-ester of membrane phospholipids by a phospholipase A2 (PLA2). Among the superfamily of PLA2 enzymes, cytosolic PLA2α (cPLA2α, also referred to as group IVA PLA2) is thought to play the primary role in this biochemical reaction. Therefore, inhibition of cPLA2α activity is an attractive approach to the control of inflammatory disorders.

In this chapter the main groups of cPLA2α inhibitors are described and the problems associated with the development of clinical active drug candidates are discussed. Furthermore, in-vivo data obtained with such compounds in pre-clinical animal models of inflammation will be presented.

Potent and selective 2-oxoamide inhibitors of phospholipases A2 as novel medicinal agents for the treatment of inflammatory diseases

Phospholipases A2 (PLA2s) are enzymes that are capable of catalyzing the hydrolysis of the sn-2 ester bond of glycerophospholipids, releasing free fatty acids, including arachidonic acid (AA), and lysophospholipids. Both products are precursor signaling molecules involved in inflammation. Among the various PLA2s, cytosolic GIVA cPLA2 is considered a major target for inflammatory diseases, while secreted GIIA sPLA2 is involved in cardiovascular diseases. We have developed lipophilic 2-oxoamides based on (S)-γ- or δ-amino acids as potent and selective inhibitors of GIVA cPLA2, which present interesting in vivo anti-inflammatory activity. 2-Oxoamides based on natural α-amino acids are selective inhibitors of GIIA sPLA2. The mode of binding of 2-oxoamides with either GIVA cPLA2 or GIIA sPLA2 has been studied by various techniques.

Binding conformation of 2-oxoamide inhibitors to group IVA cytosolic phospholipase A2 determined by molecular docking combined with molecular dynamics

The group IVA cytosolic phospholipase A(2) (GIVA cPLA(2)) plays a central role in inflammation. Long chain 2-oxoamides constitute a class of potent GIVA cPLA(2) inhibitors that exhibit potent in vivo anti-inflammatory and analgesic activity. We have now gained insight into the binding of 2-oxoamide inhibitors in the GIVA cPLA(2) active site through a combination of molecular docking calculations and molecular dynamics simulations. Recently, the location of the 2-oxoamide inhibitor AX007 within the active site of the GIVA cPLA(2) was determined using a combination of deuterium exchange mass spectrometry followed by molecular dynamics simulations. After the optimization of the AX007-GIVA cPLA(2) complex using the docking algorithm Surflex-Dock, a series of additional 2-oxoamide inhibitors have been docked in the enzyme active site. The calculated binding affinity presents a good statistical correlation with the experimental inhibitory activity (r(2) = 0.76, N = 11). A molecular dynamics simulation of the docking complex of the most active compound has revealed persistent interactions of the inhibitor with the enzyme active site and proves the stability of the docking complex and the validity of the binding suggested by the docking calculations. The combination of molecular docking calculations and molecular dynamics simulations is useful in defining the binding of small-molecule inhibitors and provides a valuable tool for the design of new compounds with improved inhibitory activity against GIVA cPLA(2).

Phospholipase A2 superfamily members play divergent roles after spinal cord injury

Spinal cord injury (SCI) results in permanent loss of motor functions. A significant aspect of the tissue damage and functional loss may be preventable as it occurs, secondary to the trauma. We show that the phospholipase A(2) (PLA(2)) superfamily plays important roles in SCI. PLA(2) enzymes hydrolyze membrane glycerophospholipids to yield a free fatty acid and lysophospholipid. Some free fatty acids (arachidonic acid) give rise to eicosanoids that promote inflammation, while some lysophospholipids (lysophosphatidylcholine) cause demyelination. We show in a mouse model of SCI that two cytosolic forms [calcium-dependent PLA(2) group IVA (cPLA(2) GIVA) and calcium-independent PLA(2) group VIA (iPLA(2) GVIA)], and a secreted form [secreted PLA(2) group IIA (sPLA(2) GIIA)] are up-regulated. Using selective inhibitors and null mice, we show that these PLA(2)s play differing roles. cPLA(2) GIVA mediates protection, whereas sPLA(2) GIIA and, to a lesser extent, iPLA(2) GVIA are detrimental. Furthermore, completely blocking all three PLA(2)s worsens outcome, while the most beneficial effects are seen by partial inhibition of all three. The partial inhibitor enhances expression of cPLA(2) and mediates its beneficial effects via the prostaglandin EP1 receptor. These findings indicate that drugs that inhibit detrimental forms of PLA(2) (sPLA(2) and iPLA2) and up-regulate the protective form (cPLA2) may be useful for the treatment of SCI.

Inhibition of secreted phospholipases A₂ by 2-oxoamides based on α-amino acids: Synthesis, in vitro evaluation and molecular docking calculations

Group IIA secreted phospholipase A₂ (GIIA sPLA₂) is a member of the mammalian sPLA₂ enzyme family and is associated with various inflammatory conditions. In this study, the synthesis of 2-oxoamides based on α-amino acids and the in vitro evaluation against three secreted sPLA₂s (GIIA, GV and GX) are described. The long chain 2-oxoamide GK126 based on the amino acid (S)-leucine displayed inhibition of human and mouse GIIA sPLA₂s (IC₅₀ 300nM and 180nM, respectively). It also inhibited human GV sPLA₂ with similar potency, while it did not inhibit human GX sPLA₂. The elucidation of the stereoelectronic characteristics that affect the in vitro activity of these compounds was achieved by using a combination of simulated annealing to sample low-energy conformations before the docking procedure, and molecular docking calculations.

Potent and selective fluoroketone inhibitors of group VIA calcium-independent phospholipase A2

Group VIA calcium-independent phospholipase A(2) (GVIA iPLA(2)) has recently emerged as a novel pharmaceutical target. We have now explored the structure-activity relationship between fluoroketones and GVIA iPLA(2) inhibition. The presence of a naphthyl group proved to be of paramount importance. 1,1,1-Trifluoro-6-(naphthalen-2-yl)hexan-2-one (FKGK18) is the most potent inhibitor of GVIA iPLA(2) (X(I)(50) = 0.0002) ever reported. Being 195 and >455 times more potent for GVIA iPLA(2) than for GIVA cPLA(2) and GV sPLA(2), respectively, makes it a valuable tool to explore the role of GVIA iPLA(2) in cells and in vivo models. 1,1,1,2,2,3,3-Heptafluoro-8-(naphthalene-2-yl)octan-4-one inhibited GVIA iPLA(2) with a X(I)(50) value of 0.001 while inhibiting the other intracellular GIVA cPLA(2) and GV sPLA(2) at least 90 times less potently. Hexa- and octafluoro ketones were also found to be potent inhibitors of GVIA iPLA(2); however, they are not selective.

Phospholipases A2 and inflammatory responses in the central nervous system

Phospholipases A2 (PLA2s) belong to a superfamily of enzymes responsible for hydrolyzing the sn-2 fatty acids of membrane phospholipids. These enzymes are known to play multiple roles for maintenance of membrane phospholipid homeostasis and for production of a variety of lipid mediators. Over 20 different types of PLA2s are present in the mammalian cells, and in snake and bee venom. Despite their common function in hydrolyzing fatty acids of phospholipids, they are diversely encoded by a number of genes and express proteins that are regulated by different mechanisms. Recent studies have focused on the group IV calcium-dependent cytosolic cPLA2, the group VI calcium-independent iPLA2, and the group II small molecule secretory sPLA2. In the central nervous system (CNS), these PLA2s are distributed among neurons and glial cells. Although the physiological role of these PLA2s in regulating neural cell function has not yet been clearly elucidated, there is increasing evidence for their involvement in receptor signaling and transcriptional pathways that link oxidative events to inflammatory responses that underline many neurodegenerative diseases. Recent studies also reveal an important role of cPLA2 in modulating neuronal excitatory functions, sPLA2 in the inflammatory responses, and iPLA2 with childhood neurologic disorders associated with brain iron accumulation. The goal for this review is to better understand the structure and function of these PLA2s and to highlight specific types of PLA2s and their cross-talk mechanisms in these inflammatory responses under physiological and pathological conditions in the CNS.

Location of inhibitors bound to group IVA phospholipase A2 determined by molecular dynamics and deuterium exchange mass spectrometry

An analysis of group IVA (GIVA) phospholipase A(2) (PLA(2)) inhibitor binding was conducted using a combination of deuterium exchange mass spectrometry (DXMS) and molecular dynamics (MD). Models of the GIVA PLA(2) inhibitors pyrrophenone and the 2-oxoamide AX007 docked into the protein were designed on the basis of deuterium exchange results, and extensive molecular dynamics simulations were run to determine protein-inhibitor contacts. The models show that both inhibitors interact with key residues that also exhibit changes in deuterium exchange upon inhibitor binding. Pyrrophenone is bound to the protein through numerous hydrophobic residues located distal from the active site, while the oxoamide is bound mainly through contacts near the active site. We also show differences in protein dynamics around the active site between the two inhibitor-bound complexes. This combination of computational and experimental methods is useful in defining more accurate inhibitor binding sites and can be used in the generation of better inhibitors against GIVA PLA(2).