Development of a system for evaluating 5-lipoxygenase inhibitors using human whole blood

Discovery of Boolean metabolic networks: integer linear programming based approach

Background

Traditional drug discovery methods focused on the efficacy of drugs rather than their toxicity. However, toxicity and/or lack of efficacy are produced when unintended targets are affected in metabolic networks. Thus, identification of biological targets which can be manipulated to produce the desired effect with minimum side-effects has become an important and challenging topic. Efficient computational methods are required to identify the drug targets while incurring minimal side-effects.

Results

In this paper, we propose a graph-based computational damage model that summarizes the impact of enzymes on compounds in metabolic networks. An efficient method based on Integer Linear Programming formalism is then developed to identify the optimal enzyme-combination so as to minimize the side-effects. The identified target enzymes for known successful drugs are then verified by comparing the results with those in the existing literature.

Conclusions

Side-effects reduction plays a crucial role in the study of drug development. A graph-based computational damage model is proposed and the theoretical analysis states the captured problem is NP-completeness. The proposed approaches can therefore contribute to the discovery of drug targets. Our developed software is available at “http://hkumath.hku.hk/~wkc/APBC2018-metabolic-network.zip”.

Calcium ionophore (A-23187) induced peritoneal eicosanoid biosynthesis: a rapid method to evaluate inhibitors of arachidonic acid metabolism in vivo

The present investigation characterizes calcium ionophore (A-23187) induced peritoneal eicosanoid biosynthesis in the rat. Intraperitoneal injection of A-23187 (20 μg/rat) stimulated marked biosynthesis of 6-keto-PGF_1α (6-KPA), TxB₂, LTC₄ and LTB₄, with no detectable changes on levels of PGE₂. Levels of all eicosanoids decreased rapidly after a peak which was seen as early as 5 min. Enzyme markers of cellular contents of neutrophils and mononuclear cells, MPO and NAG respectively, decreased rapidly after ionophore injection; this was followed by increases after 60 min. Indomethacin, a selective cyclooxygenase inhibitor, and zileuton and ICI D-2138, two selective 5-lipoxygenase inhibitors attenuated prostaglandin and leukotriene pathways respectively. Oral administration of zileuton (20 mg/kg, p.o.) inhibited LTB₄ biosynthesis for up to 6 h suggesting a long duration of pharmacological activity in the rats consistent with its longer half-life. The rapid onset and the magnitude of increases in levels of eicosanoids render the ionophore induced peritoneal eicosanoid biosynthesis a useful model to evaluate pharmacological profiles of inhibitors of eicosanoid pathways in vivo.

LTB4 as marker of 5-LO inhibitory activity of two new N-omega-ethoxycarbonyl-4-quinolones

The supposed 5-LO inhibitory activity of two N-omega-ethoxycarbonyl-4-quinolones was tested determining leukotriene B4 (LTB4) in RBL-1 cell cultures, pretreated with the two compounds of interest. LTB4, obtained by solid-phase extraction (SPE) from cell cultures supernatants, was determined by micellar electrokinetic chromatography (MEKC). The analysis was performed using an uncoated capillary, filled with borate buffer at pH 8.3, containing 12.5 mM SDS as micelles generator. Therefore, following the decreasing of LTB4 it was possible to verify the 5-LO inhibitory activity of two quinolone derivatives. To asses the suitability of the use of LTB4 as marker of the activity of the new compounds, the analysis was repeated using quercetin, a well known 5-LO inhibitor.

Determining selectivity of drugs by quantitative two-dimensional gel analysis. A study of tenidap, piroxicam, and dexamethasone

In vitro pharmacologic measures of drug specificity are well established, i.e. drug interaction with a specific target such as an enzyme, receptor, or ion channel. However, in vitro measures of drug selectivity, defined as effects on secondary targets, are lacking. Two-dimensional gel electrophoresis (2-D gel) was examined as a measure of drug selectivity by comparing the effects of three drugs, tenidap, piroxicam, and dexamethasone, on the synthesis of intracellular proteins in lipopolysaccharide (LPS)-stimulated murine macrophages. A set of 902 35S-methionine-labeled proteins were separated consistently, identified by their coordinates of apparent isoelectric point and molecular weight, and quantified. LPS altered the concentrations of 45 proteins. Tenidap, at 10 microM, affected a total of five proteins (suppressed three; stimulated two), whereas piroxicam, at 10 microM, suppressed two proteins. Dexamethasone at 0.01 microM suppressed eight proteins and stimulated one. Thus, none of the drugs reversed the LPS-induced changes. Two of the eight proteins suppressed by dexamethasone were also suppressed by tenidap and were identified as proIL-1 alpha and proIL-1 beta. Since the subset of affected proteins provided a unique protein "fingerprint" for each drug, the three drugs were mechanistically differentiated by 2-D gel analysis. Compared to LPS (5% affected proteins), all three drugs were selective (< or = 1% affected) with piroxicam > tenidap > dexamethasone. With identification of affected proteins, this technique can provide a useful in vitro assessment of drug selectivity.

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.

Effects of noradrenaline and dopamine infusions on arachidonic acid metabolism in man

We infused noradrenaline (0.025 micrograms/kg/min for 60 min, n=7) and dopamine (3.0 micrograms/kg/min for 60 min, n=6) into healthy male volunteers to study the effects of these catecholamines on in vivo thromboxane A2, prostacyclin and leukotriene E4 production measured as urinary excretions of 11-dehydro-thromboxane (TX) B2, 2,3-dinor-6-keto-prostaglandin (PG) F1alpha and leukotriene (LT) E4, respectively. Plasma noradrenaline and dopamine concentrations were 2.9+/-0.3 and 233+/-17 nmol/l at the endo fo the noradrenaline and dopamine infusions, respectively. Noradrenaline decreased thromboxane production and increased leukotriene production almost two fold. It had hardly any effect on prostacyclin production. Dopamine had no significant effects on any of the variables, however, it had a tendency to increase prostacyclin and leukotriene production. The results indicate that noradrenaline is a more important modulator of arachidonic acid metabolism than dopamine in vivo.

Noradrenaline and dopamine infusions modulate arachidonic acid cyclooxygenase and 5-lipoxygenase pathways ex vivo in man

We have previously demonstrated that adrenaline infusion increases the thromboxane/leukotriene (TX/LT) ratio in whole blood in healthy volunteers. The aim of the present study was to see whether other catecholamines--noradrenaline and dopamine--are also capable of modulating arachidonic acid (AA) metabolism in man. Low doses of noradrenaline (0.025 microgram/kg/min) and dopamine (3.0 micrograms/kg/min), which did not change hemodynamics, were infused for 60 min into healthy male volunteers. Both dopamine and noradrenaline decreased TX synthesis stimulated by spontaneous clotting, but no remarkable effect was seen when calcium ionophore A23187 was used as a stimulus. Dopamine but not noradrenaline increased prostaglandin E2 (PGE2) synthesis in A23187-stimulated whole blood. They both marginally decreased LTB4 formation in A23187-stimulated whole blood. The findings indicate that not only adrenaline but also noradrenaline and dopamine modulate AA metabolism in man.

Cloricromene inhibits leukotriene formation by human polymorphonuclear leucocytes by suppressing arachidonate release from membrane phospholipids

Cloricromene, an antithrombotic agent known to inhibit the release of arachidonic acid (AA) in stimulated human platelets, was tested for its effects on arachidonate release and metabolism in human polymorphonuclear leucocytes (PMNs). Cloricromene dose-dependently suppressed the release of leukotriene B4 (LTB4), as assessed by radioimmunoassay, from both isolated PMNs and human whole blood stimulated with the calcium ionophore A23187 or with serum-treated zymosan (STZ). The inhibitory effect was higher when the concentration of the stimulating agent was weaker. Cloricromene also inhibited dose-dependently the liberation of LTB4, LTC4, LTD4 and 5-hydroxy-6,8,11,14-eicosatraenoic acid as assessed by HPLC in the supernantant of A23187-stimulated PMNs. Finally, the drug was able to suppress the release of [3H]AA from purified human PMNs prelabeled with the radioactive fatty acid and stimulated with either A23187 or with STZ. The A23187-induced decrease in the radioactivity of phosphatidylinositol, the phospholipid class mainly involved in AA release in stimulated PMNs, was also inhibited by cloricromene. Cloricromene suppresses leukotriene formation in human PMNs by reducing AA release from membrane phospholipids, possibly through interference with phospholipase A2 activation; this activity may contribute to the leucocyte-inhibitory effects reported previously for cloricromene.

Evaluation of 5-lipoxygenase inhibitors, zileuton, A-78773 and ICI-D-2138 in an ionophore (A-23187)-induced pleural inflammation model in the rat

Intrapleural injection of A-23187 (10 micrograms), a calcium ionophore, elicited rapid increase in biosynthesis of prostaglandins and leukotrienes in a time-dependent manner. 6-Keto-prostaglandin-F1 alpha (6-KPA) was the principal cyclooxygenase product with modest increases in levels of thromboxane B2 and prostaglandin-E2. Orally administered indomethacin, a selective cyclooxygenase inhibitor, and three selective 5-lipoxygenase inhibitors, zileuton, A-78773 and ICI-D-2138 markedly attenuated respective arachidonate pathways with projected ED50 values of < 1-2 mg/kg. Furthermore, a single oral administration of either ICI-D-2138 or A-78773 (each 20 mg/kg, po) resulted in persistent inhibition of 5-lipoxygenase pathway for up to 24 hr. These results indicate zileuton, A-78773 and ICI-D-2138 to be potent and selective inhibitors of 5-LO and document the utility of A-23187-induced pleural inflammation in evaluating efficacy of inhibitors of arachidonic acid metabolism in vivo.

In-vitro evaluation of 5-lipoxygenase and cyclo-oxygenase inhibitors using bovine neutrophils and platelets and HPLC

Inhibition of 5-lipoxygenase has been determined by monitoring the formation of leukotriene B4 and 5-hydroxyeicosatetraenoic acid in bovine polymorphonuclear leucocytes. For evaluating the inhibition of cyclo-oxygenase two different test systems are presented: the first uses 12-hydroxyheptadecatrienoic acid produced by bovine platelets as an indicator of the cyclo-oxygenase activity; the second test system monitors the prostaglandin E2 formation by bovine platelets. All arachidonic acid metabolites are quantified by reverse-phase HPLC with UV-detection.

A specific assay for leukotriene B4 in human whole blood

Leukotrienes (LTs) are potent mediators of inflammatory and allergic responses, and are present in biological fluids in minute amounts, that is, in the picogram range. The aim of this study was to develop and validate a method for determination of LTB4 synthesized in vitro in human whole blood. Heparinized blood was stimulated with calcium-ionophore A23187 at 37 degrees C. After 30 min cells were separated by centrifugation. LTB4 was analyzed by radioimmunoassay (RIA). When sample preparation was restricted to protein precipitation with acetone, interference was demonstrated by lack of parallelism between standard and sample dilution curves. Purification was, therefore, extended by combinations of the following steps: 1) protein precipitation, 2) lipid extractions, and 3) high-performance liquid chromatography (HPLC). One of two commercially available LTB4 standards was found to contain multiple components, several of which were immunoreactive in RIA. Even for the standard containing pure LTB4, interference was demonstrated by lack of parallelism between sample and standard dilution curves. Testing eight combinations of varying purification steps, we found that only a three-step purification procedure, including 1) solid-phase extraction, 2) protein precipitation at -20 degrees C, and 3) HPLC, was able to eliminate interference in RIA. Using this procedure, the recovery was 78%. Stimulation of whole blood from normal subjects with calcium-ionophore showed optimal LTB4 production at 10 microM ionophore, yielding 6.6 ng LTB4/mL blood.

Angioplasty triggers intracoronary leukotrienes and lipoxin A4. Impact of aspirin therapy

BACKGROUND

Percutaneous transluminal coronary angioplasty (PTCA) is a widely used and important method of reperfusing coronary arteries. However, it is also associated with serious complications such as acute reocclusion and accelerated restenosis. The factors as well as the mechanisms involved in PTCA-associated complications remain to be fully elucidated. Because peptidoleukotrienes and lipoxins are potent vasoactive compounds, the formation of which is not inhibited by aspirin (ASA) treatment in vitro, it is possible that these eicosanoids are involved in PTCA-associated untoward events. To test this, we determined the intracoronary levels of peptidoleukotrienes and lipoxin A4 (LXA4) as well as thromboxane (TX) and 5S,12S-dihydroxyeicosatetraenoic acid (5S,12S-DiHETE; a product of double dioxygenation) after plaque rupture and evaluated the impact of ASA therapy.

METHODS AND RESULTS

PTCA was performed on 12 patients with coronary artery disease, six undergoing ASA therapy and six without ASA therapy, for at least 2 weeks before PTCA. By means of a technique that permitted sampling of intracoronary blood at the plaque site in situ, samples were taken immediately before and 10 seconds after initiation of plaque rupture. Lipoxygenase (LO)-derived products, including LXA4 and 5S,12S-DiHETE, and a marker of cyclooxygenase activity, i.e., TXB2, were quantitated after extraction and chromatography using deuterium-labeled internal standards and electron capture negative ion chemical ionization mass spectrometry. Peptidoleukotrienes (LTC4 and LTD4) were quantitated after reverse-phase high-performance liquid chromatography coupled with radioimmunoassay. Intracoronary blood taken before PTCA showed no detectable levels of these eicosanoids (the minimum limits of detection were within the picomole range). In contrast, each of these LO products was detected after PTCA. Patients undergoing ASA treatment showed elevated levels of each LO product examined compared with those not receiving ASA. Eicosanoid levels were (mean +/- SEM): LTC4, 7.10 +/- 1.22 ng/ml (ASA) versus 0.48 +/- 0.10 ng/ml; LTD4, 4.92 +/- 0.56 ng/ml (ASA) versus 1.17 +/- 0.48 ng/ml; LXA4, 24.98 +/- 4.11 ng/ml (ASA) versus 15.83 +/- 2.43 ng/ml; 5S,12S-DiHETE, 19.47 +/- 3.98 ng/ml (ASA) versus 11.98 +/- 1.83 ng/ml; TXB2, complete blockage (ASA) versus 31.04 +/- 7.38 ng/ml (p less than 0.05 for LTC4 and LTD4). To distinguish between dilatation of whole blood versus dilatation of whole blood and atheroma for contribution of eicosanoids, we also monitored their formation in Gore-tex grafts. Upon balloon inflation, TXB2 was generated, but LO products were not detected. In contrast, injection of platelet- and leukocyte-directed agonists within the graft led to both peptidoleukotriene and lipoxin formation.

CONCLUSIONS

The results indicate that PTCA triggers the intraluminal release of peptidoleukotrienes and LXA4 and that ASA therapy enhances their appearance in intracoronary blood. In addition, they provide direct evidence for LO products (LTC4, LTD4, and LXA4) in a local milieu in vivo. Moreover, the presence of the double dioxygenation product 5S,12S-DiHETE (a potential marker of 5- and 12-LO interactions) suggests that transcellular metabolic events can contribute to eicosanoid formation in vivo.

Effects of cyclooxygenase inhibitors on ex vivo cysteinyl-leukotriene production by whole human blood allowed to clot spontaneously. Comparison to stimulated blood

In ex vivo experiments we have investigated the effects of single oral administrations of aspirin (100 or 1000 mg) or ibuprofen (200 or 1200 mg) on the formation of immunoreactive thromboxane (TX) B2 and cysteinyl-leukotrienes (LT) by whole human blood in vitro. Whole human blood was allowed to clot spontaneously and in addition, in the presence of the chemotactic tripeptide f-Met-Leu-Phe (FMLP, 1 microM), or the non-physiological ionophore A 23187 (10 microM). Oral pretreatment with aspirin or ibuprofen led to dose-dependent inhibition of the TXB2 production in the presence of any of the stimuli used. After drug pretreatment the cysteinyl-LT production was not significantly enhanced in blood stimulated with FMLP or allowed to clot spontaneously as compared to values prior to oral drug administration. However, pronounced inhibition of the cyclooxygenase pathway was accompanied by an enhanced cysteinyl-LT formation when ionophore A 23187 was used as a stimulus. This could also be shown in experiments where the cyclooxygenase inhibitors were added in vitro. The higher oral dose of aspirin reduced the ex vivo cysteinyl-LT production in spontaneously clotting blood. By reverse phase HPLC immunoreactive cysteinyl-LT were characterized as a mixture of LTD4 and LTE4 as well as small amounts of LTC4. The data show that in the presence of therapeutic cyclooxygenase inhibitors no enhanced formation of cysteinyl-LT occurs in human blood in vitro after physiological stimuli such as contact activation of the clotting cascade or FMLP.

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)

Inhibition of 5-lipoxygenase product formation and polymorphonuclear cell degranulation by tenidap sodium in patients with rheumatoid arthritis

We studied the effect of tenidap sodium, a new antiinflammatory/antirheumatic drug (120 mg/day for 7 days), on eicosanoid production and neutrophil degranulation in patients with rheumatoid arthritis. Endogenous prostaglandin E2 levels and ex vivo production of leukotriene B4 (LTB4) were measured in synovial fluid samples obtained at baseline and 1 week later. We measured peripheral blood polymorphonuclear cell (PMN) degranulation following surface-bound IgG stimulation, a possible 5-lipoxygenase product-mediated event, by determining lactoferrin and elastase release into the culture fluid. We found decreased levels of endogenous prostaglandin E2 as measured by radioimmunoassay, and decreased ex vivo production of LTB4 by PMN as measured by high performance liquid chromatography, in synovial fluid samples from patients who took tenidap. Release of the granule proteins lactoferrin and elastase was decreased in PMN obtained from patients receiving tenidap, as well as in the PMN incubated in vitro with tenidap. Improvement in clinical measures paralleled the biochemical changes. The unique 5-lipoxygenase inhibitory property of tenidap, as measured by LTB4 production and degranulation, suggests that it may have clinical activity which differentiates it from nonsteroidal antiinflammatory drugs.

Antigen-dependent leukotriene synthesis and histamine release from IgG1 passively-sensitized guinea pig lungs ex vivo: relationship between serum levels of antigen-specific IgG1 and mediator synthesis/release

Naive guinea-pigs were passively sensitized with varying amounts of affinity column purified, homologous, anti-ovalbumin IgG1 (anti-OA IgG1) and then examined for a) the capacity of lung tissue to release mediators (histamine and LTB4/LTD4) in response to antigen-challenge ex vivo and b) the attendant circulating levels of anti-OA IgG1. Intraperitoneal administration of anti-OA IgG1 (0.125-0.75 mg/kg) to guinea-pigs facilitated the synthesis of LTB4 (8-25 ng/g lung) and LTD4 (18-80 ng/g) and the release of histamine (1-7 ug/g) from lung tissue after exposure to 10 micrograms/ml of ovalbumin for 20 min ex vivo. Peak levels of mediators were found using 0.5 mg/kg anti-OA IgG1 with an ED50 = 0.35 mg/kg. LTD4/LTB4 synthesis and histamine release were both antigen concentration- and time-dependent, and LT synthesis was observable in non-perfused lungs and in lungs perfused free of blood. Maximum sensitization occurred at 1-2 days post i.p. administration of anti-OA IgG1 and was maintained up to 7 days. Measurement of anti-OA IgG1 using an enzyme-linked immunosorbent assay demonstrated that circulating antibody levels were 2-6 micrograms/ml at the doses which caused sensitization. The level of anti-OA IgG1 found in passively sensitized animals was at least 100-fold less than that found in actively-sensitized guinea-pigs despite the similar magnitude in LTD4/LTB4 synthesized and the amount of histamine released. Using purified antibody, the results demonstrate that in guinea-pigs, IgG1 can play a prominent role in regulating lung LT synthesis and histamine release, and that microgram per ml circulating levels of this antibody are sufficient to sensitize naive lungs.

Orally administered tolfenamic acid inhibits leukotriene synthesis in isolated human peripheral polymorphonuclear leukocytes

Special interest has been focused on the development of dual inhibitors of the cyclo-oxygenase and lipoxygenase pathways of arachidonic acid metabolism. In contrast to other classic NSAIDs, some fenamates in clinically achievable concentrations have been shown to inhibit synthesis of 5-lipoxygenase products in vitro. In the present work, we studied the effect of orally administered tolfenamic acid (600 mg) on Ca ionophore A 23187 -induced leukotriene synthesis in isolated human polymorphonuclear leukocytes. Leukotriene production was reduced in all 14 subjects studied, the mean inhibition of LTB4 synthesis being 16 +/- 3% and that of LTC4 33 +/ 7%. The inhibition correlated positively with serum acid concentrations. We suggest that inhibition of leukotriene synthesis is an additional mechanism of the anti-inflammatory, antimigraine and antidysmenorrhoeic effects of tolfenamic acid, and a possible explanation for its rare gastric and bronchoconstrictive side-effects.

Radioimmunoassay of LTB4 in plasma from different species: a cautionary note

In clinical and pre-clinical research the pharmacodynamics of selective 5-lipoxygenase and dual 5-lipoxygenase/cyclo-oxygenase inhibitors may be studied by direct RIA of plasma LTB4. Although immunoreactive LTB4 in plasma from A23187 stimulated human blood has the characteristics of authentic LTB4 our results show, particularly in mice and rats, that exposure to A23187 produces large quantities of 12-HETE. Since in different species the levels of 12-HETE increase with platelet concentration we suggest that the 12(S)-HETE is produced by platelet lipoxygenase. However, we do not rule out the possibility that a proportion of 12-HETE may exist as the (R)-stereoisomer. The latter has greater potential for interference in the direct RIA of LTB4. Biosynthesis of 12-HETE may be measured either by RPHPLC/U.V. abs. (8) or by RIA (9) and LTB4 by a more specific antibody described in this report. We conclude that the combined ex vivo RIA of plasma TXB2, LTB4 and 12-HETE has utility in determining the selectivity of inhibitors of arachidonate metabolism and in distinguishing between selective 5-lipoxygenase inhibitors which interact directly with the enzyme and anti-oxidant or free radical scavenging types which may be less specific.

Spectrophotometric monitoring of lipoxygenase and cyclooxygenase pathway activity using ionophore stimulated whole blood

We have employed clotting human blood stimulated with ionophore to develop a system for measuring cyclooxygenase, 5-lipoxygenase, and 12-lipoxygenase pathway products released into the serum fraction. In a single chromatographic run, 5-HETE, 12-HETE, 12-OH-heptadecatrienoic acid (HHT), LTB4, 20-OH-LTB4, and 20-COOH-LTB4 are quantitated by UV monitoring after separation by HPLC. The kinetics of product formation/release of all fatty acid products into the serum show an apparent lag of approximately 2 min, after which time the amounts of HHT, 5-HETE, and LTB4, respectively, plateau at 10 min while 12-HETE increases over a 60 min period. The system is responsive to standard cyclooxygenase and lipoxygenase inhibitors, and is of value of evaluating prospective blockers of AA metabolism in a whole blood setting.