Differential inhibition of human secretory and cytosolic phospholipase A2

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.

Determination of arachidonic acid by on-line solid-phase extraction HPLC with UV detection for screening of cytosolic phospholipase A2α inhibitors

An on-line solid-phase extraction (SPE)-liquid chromatographic method with ultraviolet detection at 200nm for screening of inhibitors of cytosolic phospholipase A(2)α (cPLA(2)α) was developed and validated. cPLA(2)α was isolated from porcine platelets. Enzyme activity was determined by measuring the release of arachidonic acid from a phospholipid substrate using automated on-line sample clean up on a trap column followed by isocratic back-flush elution on a RP18 analytical column. While the use of a conventional RP18 column for trapping the analyte led to peak broadening only after a few runs due to pollution of the column by binding of components present in the enzyme preparation, the application of a turbulent flow column (TurboFlow Cyclone™) resulted in sharp peaks even after a plurality of injections. Interestingly, for sample introduction a turbulent flow of the mobile phase produced by high flow rates was not necessary to maintain good peak shapes. The same result could also be achieved applying low flow rates (0.5 mL/min). Several known cPLA(2)α inhibitors were used to validate the test system.

Fast method for monitoring phospholipase A2 activity by liquid chromatography-electrospray ionization mass spectrometry

A new liquid chromatography-electrospray ionization mass spectrometry (LC-ESI-MS) method for the fast determination of phospholipase A(2) (PLA(2)) activity has been developed. For the first time, the method allows the parallel detection of glycerophosphatidylcholine (GroPCho) as PLA(2) substrate as well as of its products fatty acid (FA) and lyso-GroPCho. ESI-MS was carried out in negative ion mode, detecting the FA as [M-H](-) ions and the lyso-GroPCho and GroPCho as acetate adducts [M+Ac](-). Utilizing a fast gradient on a short C(5)-modified silica gel column with 3 microm particles, five GroPChos, five FAs and six lyso-GroPChos could be separated according to their chain length in less than 3 min. A very high average chromatographic efficiency of 41,200 theoretical plates (plate height 0.5 microm) was achieved for the separation of the GroPChos. The method was applied for monitoring the release of arachidonic acid (20:4 FA) and 1-stearoyl-lyso-sn-GroPCho (18:0 GroPCho) from unilamellar vesicles of 1-stearoyl-2-arachidonoyl-sn-GroPCho (18:0/20:4 GroPCho). With a limit of detection of 0.5 pmol (total amount injected on column) for the FAs and lyso-GroPChos and 1.5 pmol for the GroPChos as well as a linear range of 1.5 decades, the method has proven to be suitable for the monitoring of different secretory PLA(2) (sPLA(2)) conversions. Furthermore, it was applied to screen a small library of PLA(2) inhibitors for their activity towards sPLA(2) type V and snake venom of Bothrops moojeni. In both cases, active samples could be directly identified. With its short analysis time, its high chromatographic efficiency and the parallel detection of substrate and all products, the developed LC-ESI-MS method is well suited for the analysis of PLA(2) activity.

Phospholipase A2 inhibitors in development

To date, three isoforms of phospholipase A2 (PLA2) have been identified. Of these, the two Ca2+-dependent isoforms, secretory (sPLA2) and cytosolic phospholipase A2 (cPLA2), are targets for new anti-inflammatory drugs. The catalytic mechanisms and functions of the third isoform, Ca2+-independent cytosolic phospholipase A2 (iPLA2), are unknown at present. sPLA2 and cPLA2 are both implicated in the release of arachidonic acid and prophlogistic lipid mediators. However, recent findings provide evidence that cPLA2 is the dominant isoform in various kinds of inflammation, such as T-cell-mediated experimental arthritis. A triple function of PLA2-derived lipid mediators has been suggested: causing immediate inflammatory signs, involvement in secondary processes, e.g., superoxide free radical (O2) generation, apoptosis, or tumour necrosis factor-alpha (TNF-alpha)-cytotoxicity, and controlling the expression and activation of pivotal proteins implicated in inflammation and cell development, e.g., cytokines, adhesion proteins, proteinases, NF-kappaB, fos/jun/AP-1, c-Myc, or p21ras. In the past, research predominantly focused on the development of sPLA2 inhibitors; however, present techniques enable discrimination of cPLA2, sPLA2, and iPLA2, and specific inhibitors of each of the three isoforms are likely to appear soon. Over the last decade, between 40 and 50 sPLA2 inhibitors have been described; and the list is growing. However, of these, few have the potential for clinical success, and those that do are predominantly active site-directed inhibitors, e.g., BMS-181162, LY311727, ARL-67974, FPL67047, SB-203347, Ro-23-9358, YM-26734, and IS-741. At present, there are no likely clinical candidates emerging from the ranks of cPLA2 and iPLA2 inhibitors in development. Indications for which PLA2 inhibitors are being pursued include, sepsis, acute pancreatitis, inflammatory skin and bowel diseases, asthma, and rheumatoid arthritis. The three main obstacles to the successful development of PLA2 inhibitors include, insufficient oral bioavailability, low affinity for the enzyme corresponding to low in vivo efficacy and insufficient selectivity.

HPLC assay with UV spectrometric detection for the evaluation of inhibitors of cytosolic phospholipase A2

A non-radioactive spectrometric assay for the evaluation of inhibitors of cytosolic phospholipase A(2) (cPLA(2)) is described. The enzyme was isolated from human platelets applying anion exchange chromatography. Sonicated covesicles consisting of 0.2mM 1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphocholine and 0.1mM 1,2-dioleoyl-sn-glycerol were used as enzyme substrate. The cPLA(2) activity was determined by measuring the arachidonic acid released by the enzyme with reversed-phase HPLC and UV detection at 200 nm after cleaning up the samples by solid phase extraction. Two known cPLA(2) inhibitors were used to validate the test assay.

Biaryl diacid inhibitors of human s-PLA2 with anti-inflammatory activity

Twenty-four hydrophobic dicarboxylic acids are described which were evaluated as inhibitors of 14 kDa human platelet phospholipase A2 (HP-PLA2). In general, biarylacetic acid derivatives were found to be more active than biaryl acids or biarylpropanoic acids. More potent inhibitors were obtained when hydrophobic groups were attached to the biaryl acid nucleus using an olefin linkage as compared to an ether linkage. Compounds with larger hydrophobic groups were usually more potent inhibitors of HP-PLA2. Five of the compounds disclosed in this report (2, 4, 28, 36b and 36i) were found to possess significant anti-inflammatory activity in a phorbol ester induced mouse ear edema model of chronic inflammation.

Phospholipase A2: its role in ADP- and thrombin-induced platelet activation mechanisms

ADP and thrombin are two of the most important agonists of platelet aggregation--a cellular response that is critical for maintaining normal hemostasis. However, aberrant platelet aggregation induced by these agonists plays a central role in the pathogenesis of cardiovascular and cerebrovascular diseases. Agonist-induced primary or secondary activation of phospholipases leads to generation of the second messengers that participate in biochemical reactions essential to a number of platelet responses elicited by ADP and thrombin. Phospholipase A2 (PLA2) has been linked to cardiovascular diseases. However, the mechanism(s) of activation of PLA2 in platelets stimulated by ADP and thrombin has remained less well defined and much less appreciated. The purpose of this review is to examine and compare the molecular mechanisms of activation of PLA2 in platelets stimulated by ADP and thrombin.

Structure-activity relationships of (4-acylpyrrol-2-yl)alkanoic acids as inhibitors of the cytosolic phospholipase A2: variation of the substituents in positions 1, 3, and 5

Derivatives of 3-(1,3,5-trimethyl-4-octadecanoylpyrrol-2-yl)propionic acid (1) and (1,3,5-trimethyl-4-octadecanoylpyrrol-2-yl)acetic acid (4) were prepared and evaluated for their ability to inhibit the cytosolic phospholipase A2 of intact bovine platelets. While replacement of one of the methyl groups in position 1, 3, or 5 of the acetic acid 4 by a benzyl residue did not influence the inhibitory potency significantly, the introduction of a dodecyl chain led to compounds which even enhanced the enzymatic activity. Stepwise elongation of the alkyl substituent in position 1 showed that the ability to inhibit the enzyme was lost when the alkyl chain exceeded a length of five carbons in case of compound 1 or six carbons in case of compound 4. Introduction of a polar functional group at the end of the 1-alkyl chain of these inactive pyrroles, however, restored or even elevated inhibitory potency. The most preferable of the polar terminal functions investigated was the carboxylic acid moiety. 6-[2-(2-Carboxyethyl)-4-dodecanoyl-3,5-dimethylpyrrol-1-yl]hexanoi c acid (65c) and 6-[2-(carboxymethyl)-4-dodecanoyl-3,5-dimethylpyrrol-1-yl]nonanoic acid (66f) were the synthesized inhibitors with the greatest potency. With IC50 values of 3.4 and 3.3 microM, respectively, they were about 3-fold more active than the standard cPLA2 inhibitor arachidonyl trifluoromethyl ketone (IC50: 11 microM).

Synthesis, biological evaluation, and structure-activity relationships of 3-acylindole-2-carboxylic acids as inhibitors of the cytosolic phospholipase A2

3-Acylindole-2-carboxylic acid derivatives were prepared and evaluated for their ability to inhibit the cytosolic phospholipase A2 of intact bovine platelets. To define the structural requirements for enzyme inhibition, the carboxylic acid group, the acyl residue, and the moiety in position 1 were systematically modified. Furthermore, different substituents were introduced into the phenyl part of the indole. Replacement of the carboxylic acid group in position 2 of the indole with an acetic or propionic acid substituent led to a decrease of inhibitory potency. Enzyme inhibition was optimal when the acyl residue in position 3 had a length of 12 or more carbons. Conformational restriction of the acyl residue did not influence activity. Introduction of alkyl chains at position 1 of the indole with 8 or more carbons resulted in a loss of activity. However, replacing the omega-methyl group of such compounds with a carboxylic acid moiety was found to increase inhibitory potency significantly. Among the tested indole derivatives, 1-[2-(4-carboxyphenoxy)ethyl]-3-dodecanoylindole-2-carboxyli c acid (29b) had the highest potency. With an IC50 of 0.5 microM it was about 20-fold more active than the standard cPLA2 inhibitor arachidonyl trifluoromethyl ketone (IC50: 11 microM).

3-(3,5-Dimethyl-4-octadecanoylpyrrol-2-yl)propionic acids as inhibitors of 85 kDa cytosolic phospholipase A2

3-(1,4-Diacylpyrrol-2-yl)propionic acids were designed as inhibitors of cytosolic phospholipase A2. Enzyme inhibition was assayed by evaluation of calcium ionophore A23187-induced arachidonic acid release from bovine platelets. While the synthesized bisacyl compound 3-[3,5-dimethyl-4-octadecanoyl-1-(3-phenylpropionyl)pyrrol-2-yl] propionic acid was inactive at 33 microM, the related monoacylated 3-(3,5-dimethyl-4-octadecanoylpyrrol-2-yl)-propionic acid and 3-(1,3,5-trimethyl-4-octadecanoylpyrrol-2-yl)-propionic acid proved to be inhibitors of cytosolic phospholipase A2(IC50: 24 microM and 13 microM, respectively).

Native peptide inhibition. Specific inhibition of type II phospholipases A2 by synthetic peptides derived from the primary sequence

The binding of low molecular weight type II phospholipase A2 (EC) to membrane surfaces and hydrolysis of phospholipid are thought to involve the formation of a hydrophobic channel into which a single substrate molecule diffuses before cleavage. The floor and right side of the channel are provided by hydrophobic residues 2, 5, and 9 of an amphipathic amino-terminal helix. The channel is postulated to form via a conformational change in this helix and inward movement of a hydrophobic flap (residue 69 side chain). We show that the amino-terminal tryptic peptide of human type II phospholipase A2 forms a noncovalent complex with the tryptic peptide from residues 70-74 of the enzyme. Further, the 70-74-peptide sequence (FLSYK) dose-dependently inhibits phospholipid hydrolysis in a mixed micelle assay. This native peptide inhibition also occurred with type II enzymes from Crotalus durissus and Crotalus atrox, which have different amino acid sequences at the amino terminus as well as different 70-74 regions of the molecules. Despite significant conservation of tertiary structure among the enzymes, inhibition by each peptide is specific to the enzyme from which the peptide sequence is derived. We propose that these native peptides inhibit enzyme activity via a sequence-specific, noncovalent interaction with the amino-terminal residues of the enzyme, thereby preventing the conformational change on binding to the micelle interface. These experiments demonstrate a new method for specific inhibition of phospholipase A2 which, in principle, would be applicable to other biologically active polypeptides and proteins.

3-(Octadecanoylaminomethyl)indole-2-carboxylic acid derivatives and 1-methyl-3-octadecanoylindole-2-carboxylic acid as inhibitors of cytosolic phospholipase A2

3-(1-Acylaminooctadecyl)indole-2-carboxylic acids and 3-(1-acylaminooctadecyl)-1-methylindole-2-carboxylic acids were designed and synthesized as inhibitors of cytosolic phospholipase A2. Enzyme inhibition was assayed by evaluation of calcium ionophore A23187-induced arachidonic acid release from bovine platelets. While compounds with 1-octadecanoylaminooctadecyl groups in position 3 of the indole were inactive inhibition data for 3-[1-(3-phenylpropionylamino)octadecyl]indole-2-carboxylic acids could not be evaluated because of lysis of the platelets. However 3-(octadecanoylaminomethyl)indole-2-carboxylic acid derivatives and 1-methyl-3-octadecanoylindole-2-carboxylic acid proved to be inhibitors of cytosolic phospholipase A2. The most active inhibitor was the latter compound with an IC50 of 8 microM.

Regulation of phospholipase A2 enzymes: selective inhibitors and their pharmacological potential

The area of PLA2 research has grown immensely over the past 20 years. There is a better understanding of the kinetics, or factors that affect the kinetics, of the different forms of PLA2. New forms of PLA2 are being discovered, such as the cPLA2, which fit the role of an intracellularly regulated enzyme. Multiple forms of PLA2 tend to complicate the elucidation of the cellular mechanisms that regulate AA release and the subsequent eicosanoid production. Because of the factors that affect PLA2 kinetics and the unknown nature of the PLA2 that regulates AA release (there may be more than one), it has been difficult to design or isolate specific inhibitors. This review discussed selected classes of inhibitors because these have generated the most intense research in the field. There is a multitude of structurally diverse compounds reported in the literature that have been reported to be inhibitors of PLA2 in vitro and some have been reported to have anti-inflammatory activity (Wilkerson, 1990; Connolly and Robinson, 1993a). It is clear from a brief survey of the literature that the bulk of PLA2 inhibitors have topical anti-inflammatory activity. This may be due to the nature of these inhibitors: because they are hydrophobic they may be more readily absorbed in the skin whereas when given orally they may not be absorbed. To data, manoalide has been clinically evaluated in man and a new Bristol-Myers Squibb retenoid derivative may enter clinical trials for psoriasis (BMS-181162 (XVI)); however, there are no PLA2 inhibitors on the market or significantly advanced in clinical development (Table III). This indicates the lack of understanding of this enzyme for the development of relevant inhibitors, which is related to the lack of understanding of the relevant PLA2 that regulates AA release and eicosanoid biosynthesis. The concept of regulation of eicosanoid biosynthesis by PLA2 inhibition and decreased AA availability still remains a viable therapeutic approach for the treatment of inflammatory diseases. The proof of this concept has not been obtained because of the complex nature of PLA2 and the multiple forms of PLA2 in the cell. Clinical results with cyclooxygenase inhibitors and recent clinical results with inhibitor of 5-lipoxygenase demonstrate that if inhibition of PLA2 results in reduction in both lipid mediators, a good anti-inflammatory compound should result. The added advantage of PLA2 inhibitors would be the reduction of PAF levels; however, the clinical results with potent and specific PAF antagonists has been less encouraging about the potential benefits of reduction in PAF levels.(ABSTRACT TRUNCATED AT 400 WORDS)