Potent Platelet-activating Factor Receptor Antagonist Research Articles

Involvement of endogenous leukotriene B4 and platelet‐activating factor in polymorphonuclear leucocyte recruitment to dermal inflammatory sites in rats

A critical role for leukotriene B(4) (LTB(4)) and/or platelet-activating factor (PAF) in regulating polymorphonuclear cell (PMN) trafficking to inflammatory sites has been reported in a number of experimental inflammatory models. In vitro, newly synthesized LTB(4) and PAF were shown to act in an autocrine/paracrine or intracrine fashion to enhance intracellular arachidonic acid availability and leukotriene biosynthesis. This suggested potentially cooperative effects of these lipid mediators in regulating PMN extravasation. The present study aimed to elucidate whether endogenous LTB(4) and PAF may both act to regulate plasma extravasation and PMN trafficking to inflammatory sites in experimental inflammation. With this aim, we have used selective and potent PAF and LTB(4) receptor antagonist pretreatments in dermal and pulmonary inflammation models in rats. Our results show additive inhibitory effects of dual LTB(4) and PAF receptor blockade in either PAF- or LTB(4)-elicited cutaneous PMN accumulation compared to single-drug administration. Furthermore, the combined administration of the drugs inhibited the PMN accumulation induced by the chemically unrelated soluble agonists tumour necrosis factor-alpha and C5a. Finally, in a model of pulmonary inflammation induced by the intravenous injection of Sephadex beads, lung neutrophilia was reduced by 63% following the administration of LTB(4) and PAF antagonists, in contrast with the lack of effect of single drug administration. Our results strongly support a role of both endogenous LTB(4) and PAF in regulating PMN trafficking to inflammatory sites in various experimental conditions.

Amelioration of experimental acute pancreatitis with a potent platelet-activating factor antagonist.

The effect of a potent platelet-activating factor (PAF) antagonist, BB-882, on an experimental model of acute pancreatitis induced in male Wistar rats by a technique of microvascular ischaemia was studied. A single intraperitoneal injection of BB-882 (5 mg/kg) 30 min after induction of the disease in 12 animals significantly reduced (P < 0.001) the rise in the level of serum amylase (mean 2477 (range 2100-3280) units/l) compared with that in 12 control animals (mean 3928 (range 2800-5900) units/l) and significantly improved (P < 0.001) the mean pancreatic histology score (5.0 (range 3-10) versus 12.3 (range 8-18) in controls). PAF is a biologically active ether phosphorylcholine synthesized in cell membranes and a potent inflammatory mediator. Pancreatic tissue levels of this compound are increased in experimental acute pancreatitis and pretreatment with PAF receptor antagonists can ameliorate the progression of this disease. BB-882 alters the early course of experimental pancreatitis and may have a clinical therapeutic role.

Preparation of 7-substituted ginkgolide derivatives: potent platelet activating factor (PAF) receptor antagonists.

Ginkgolides are structurally unique constituents of Ginkgo biloba extracts and are known antagonists of the platelet-activating factor (PAF) receptor. Ginkgolide C is 25-fold less potent than ginkgolide B as a PAF receptor antagonist, due to the presence of the 7beta-OH. Recently, we found that 7alpha-fluoro ginkgolide B was equipotent to ginkgolide B underlining the critical importance of the 7-position of ginkgolides for PAF receptor activity. Herein we describe the synthesis of a series of ginkgolide B derivatives with modifications at the 7-position and the pharmacological evaluation of these derivatives as assayed by cloned PAF receptors. In two cases nucleophilic attack on a 7beta-O-triflate ginkgolide B did not lead to the expected products, but gave rise to two unprecedented ginkgolide derivatives, one with a novel rearranged skeleton. Furthermore, standard reduction of 7alpha-azido ginkgolide B did not give the expected primary amine, but instead yielded alkylated amines depending on the solvent employed. Pharmacological testing with cloned PAF receptors showed that ginkgolides with 7alpha-substitutents had increased affinity compared to 7beta-substituents, in particular 7alpha-chloro ginkgolide B, the most potent nonaromatic ginkgolide derivative described to date with a K(i) value of 110 nM.

Ginkgolide derivatives for photolabeling studies: preparation and pharmacological evaluation.

The terpene trilactones (TTLs), ginkgolides and bilobalide, are structurally unique constituents of Ginkgo biloba extracts, which exhibit various neuromodulatory properties. Although the TTLs are believed to be responsible for some of these effects, the specific interactions with targets in the central nervous system remain to be elucidated on a molecular level. Ginkgolides are known antagonists of the platelet-activating factor (PAF) receptor. Herein, we describe the first examination of the binding of native TTLs and their derivatives to the cloned PAF receptor, confirming that of the TTLs, ginkgolide B is the most potent PAF receptor antagonist. Ginkgolide derivatives carrying photoactivatable and fluorescent groups for the purpose of performing photolabeling have been prepared and evaluated using the cloned PAF receptor. These studies have shown that ginkgolide derivatives with aromatic photoactivatable substituents are potent PAF receptor antagonists with K(i) values of 0.09-0.79 microM and hence excellent ligands for clarifying the binding of ginkgolides to PAF receptor by photolabeling studies. Ginkgolide derivatives incorporating both fluorescent and photoactivatable groups still retained binding affinity to the PAF receptor and hence should be promising ligands for photolabeling and subsequent sequencing studies.

Platelet-activating factor antagonism and streptokinase-induced hypotension in clinical acute myocardial infarction

Continuing efforts are being made to improve thrombolytic therapy for acute myocardial infarction (AMI). The rate of streptokinase (SK) infusion is commonly limited by the hypotension that is induced. If this could be avoided, an accelerated regimen of SK could be given, analagous to that used for other thrombolytic agents such as alteplase. The mechanism of the SK-induced hypotension is unknown, but there is some evidence that platelet-activating factor (PAF) plays a role. The potent PAF receptor antagonist lexipafant (10 mg) (n=35), or matching placebo (n=36), was administered intravenously over 5 min, in a randomized double-blinded protocol, to consecutive patients about to receive SK for AMI; all but three had inferior MI, because of the preference for strategies other than SK therapy in patients with anterior MI. The rate of infusion of SK was set to give 1.5x10(6) units over 30 min, anticipating significant hypotension. Blood pressure fell sharply over the first 10 min of SK administration. The maximum fall in systolic blood pressure occurred between 8 and 12 min, and averaged 43+/-28 mmHg (mean+/-S.D.) and 40+/-26 mmHg in patients given placebo and lexipafant respectively. Systolic pressure having fallen to <90 mmHg, according to protocol the SK administration rate was reduced in 21 of 36 (58%) of patients given placebo and in 19 of 35 (54%) of patients given lexipafant. The total SK dose was given to all subjects over 40.3+/-17.5 and 40.2+/-13.4 min for the placebo and lexipafant groups respectively. Peak and time integrals of creatine kinase were not different in the two groups. There were no adverse effects attributable to lexipafant. It is concluded that the PAF receptor antagonist lexipafant has no significant effect on SK-induced hypotension and does not facilitate an accelerated regimen of administration.

The Effect of Lexipafant on Bacterial Translocation in Acute Necrotizing Pancreatitis in Rats

Bacterial translocation (BT) from the gastrointestinal tract to mesenteric lymph nodes (MLNs) and other extra intestinal organs is an important source of infection in acute pancreatitis (AP). Lexipafant (BB-882) is a potent platelet-activating factor receptor antagonist that has an anti-inflammatory effect. To examine whether BB-882 could affect BT in acute necrotizing pancreatitis, 48 male Sprague Dawley rats (250–350 g) were studied. AP was induced in Group I and Group II by pressure injection of 3% taurocholate and trypsin into the common biliopancreatic duct (1 mL/kg of body weight). Group I rats received BB-882 (10 mg/kg, ip qd) and Group II rats received a similar volume of normal saline as a placebo postoperatively for 2 days. Group III and Group IV received BB-882 and placebo, respectively, after an exploratory laparotomy. At 48 hours postoperatively, blood was drawn for culture, serum amylase, and tumor necrosis factor (TNF)-α determinations. Specimens from MLNs, spleen, liver, pancreas, and cecum were harvested for culture of Gram-positive, Gram-negative, and anaerobic bacteria. Quantitative cecal cultures of Gram-positive, Gram-negative, and anaerobic bacteria were obtained. A point scoring system for five histological features that include interstitial edema, inflammatory cellular infiltration, fat necrosis, parenchymal necrosis, and hemorrhage was used to evaluate the severity of pancreatitis. There was no difference in serum amylase levels (2415 ± 127 IU/L versus 2476 ± 170 IU/L), serum TNF-α levels (7820 ± 1396 pg/mL versus 7318 ± 681 pg/mL), and the mean pancreatic histology score (5.9 ± 1.2 versus 6.5 ± 1.1) between Group I and Group II, respectively (P gt 0.05). Seven of 12 Group I rats had BT to MLNs, compared with 11 of 12 rats in Group II (P &gt; 0.05). Five of 12 Group I rats had BT to distant sites such as pancreas, spleen, liver, and/or blood, compared with 11 of 12 rats in Group II (P &lt; 0.05). BB-882 treatment decreases bacterial spread to distant sites, but does not reduce serum amylase levels and serum TNF-α levels or ameliorate pancreatic damage in rats with AP.

Treatment with lexipafant ameliorates the severity of pancreatic microvascular endothelial barrier dysfunction in rats with acute hemorrhagic pancreatitis.

Treatment with lexipafant reduced the severity of pancreatitis-associated endothelial barrier compromise, also associated with a decrease in systemic concentrations of interleukin (IL) 1. Thus, the present findings imply that platelet-activating factor (PAF) may play an important role in the pathogenesis of pancreatic endothelial dysfunction by signaling and triggering the production and release of certain cytokines. Pancreatic capillary endothelial barrier dysfunction is an initial and characteristic feature of acute pancreatic injury and pancreatitis. PAF, a proinflammatory mediator and an intercellular signaling substance, has been considered to be involved in the inflammatory reaction and the systemic endothelial dysfunction of acute pancreatitis. The development of pancreatic capillary endothelial barrier dysfunction was monitored by tissue edema and exudation of plasma albumin into the interstitium, 3 and 12 h after induction of acute pancreatitis by intraductal infusion of 5% sodium taurodeoxycholate in rats. Pancreatic leukocyte recruitment was reflected by measuring myeloperoxidase activity. Serum levels of IL-1 beta and IL-6 were determined by an enzyme-linked immunosorbent assay (ELISA). Pretreatment with lexipafant, a potent PAF receptor antagonist, significantly reduced the pancreatitis-induced increase in pancreatic endothelial barrier dysfunction, pancreatic leukocyte recruitment and serum levels of IL-1 beta, although a difference persisted between animals with sham operation and pancreatitis.

Antiinflammatory activity of the platelet-activating factor receptor antagonist A-85783.

Accumulating evidence suggests an important role for the lipid mediator, platelet-activating factor (PAF), in cutaneous inflammation. In these studies the antiinflammatory effects of the potent and selective lipophilic PAF receptor antagonist A-85783 topically applied to the ventral ears of male Wistar rats were assessed. Intradermal injections of PAF into rat ears resulted in cutaneous inflammation as assessed by both ear thickness measurements and histological evaluation. Pretreatment of the ears with A-85783 resulted in an inhibition of subsequent PAF-induced inflammation. A-85783 treatment also inhibited phorbol myristic acetate-induced cutaneous inflammation, suggesting that the PAF receptor is involved in the etiology of this experimental dermatitis. These findings demonstrate that epicutaneous A-85783 is an appropriate tool to study the role of the PAF receptor in cutaneous inflammation, and suggest the possible clinical utility of this new class of antiinflammatory agents.

The effect of lexipafant on bacterial translocation in acute necrotizing pancreatitis in rats

Bacterial translocation (BT) from the gastrointestinal tract to mesenteric lymph nodes (MLNs) and other extra intestinal organs is an important source of infection in acute pancreatitis (AP). Lexipafant (BB-882) is a potent platelet-activating factor receptor antagonist that has an anti-inflammatory effect. To examine whether BB-882 could affect BT in acute necrotizing pancreatitis, 48 male Sprague Dawley rats (250-350 g) were studied. AP was induced in Group I and Group II by pressure injection of 3% taurocholate and trypsin into the common biliopancreatic duct (1 mL/kg of body weight). Group I rats received BB-882 (10 mg/kg, i.p. qd) and Group II rats received a similar volume of normal saline as a placebo postoperatively for 2 days. Group III and Group IV received BB-882 and placebo, respectively, after an exploratory laparotomy. At 48 hours postoperatively, blood was drawn for culture, serum amylase, and tumor necrosis factor (TNF)-alpha determinations. Specimens from MLNs, spleen, liver, pancreas, and cecum were harvested for culture of gram-positive, gram-negative, and anaerobic bacteria. Quantitative cecal cultures of gram-positive, gram-negative, and anaerobic bacteria were obtained. A point scoring system for five histological features that include interstitial edema, inflammatory cellular infiltration, fat necrosis, parenchymal necrosis, and hemorrhage was used to evaluate the severity of pancreatitis. There was no difference in serum amylase levels (2415 +/- 127 IU/L versus 2476 +/- 170 IU/L), serum TNF-alpha levels (7820 +/- 1396 pg/mL versus 7318 +/- 681 pg/mL), and the mean pancreatic histology score (5.9 +/- 1.2 versus 6.5 +/- 1.1) between Group I and Group II, respectively (P > 0.05). Seven of 12 Group I rats had BT to MLNs, compared with 11 of 12 rats in Group II (P > 0.05). Five of 12 Group I rats had BT to distant sites such as pancreas, spleen, liver, and/or blood, compared with 11 of 12 rats in Group II (P < 0.05). BB-882 treatment decreases bacterial spread to distant sites, but does not reduce serum amylase levels and serum TNF-alpha levels or ameliorate pancreatic damage in rats with AP.

Apafant (a PAF receptor antagonist) suppresses the early and late airway responses in guinea pigs: a comparison with antiasthmatic drugs

We studied the effects of apafant (WEB 2086 BS), a specific and potent platelet activating factor (PAF) receptor antagonist, on the early and late airway responses in conscious and actively sensitized guinea pigs. An increase in airway resistance (Rs) was seen 1 min after the inhaled antigen challenge (early airway response), followed by another increase in Rs which peaked between 4 and 8 h after the provocation (late airway response). Oral administration of apafant as well as theophylline inhibited both early and late airway responses. Ozagrel, an inhibitor of thromboxane A 2 synthetase, salbutamol, a β 2-adrenoceptor agonist, and dexamethasone significantly inhibited either the early or the late airway response only. Disodium cromoglycate inhibited neither the early nor the late airway response. The results showed that apafant inhibited both the early and late airway responses in sensitized guinea pigs and its effect was comparable or superior to that of anti-asthmatic drugs used clinically.

Treatment with a platelet-activating factor antagonist has little protective effects during endotoxic shock in the dog.

Platelet-activating factor (PAF) is a potent vasoactive and inflammatory lipid mediator which has been implicated in the hemodynamic alterations of endotoxemia and sepsis. Different PAF receptor antagonists have been shown to attenuate the systemic and pulmonary disturbances of sepsis, but they were generally administered before the injection of endotoxin and their effects have not been consistent. To examine the effects of BB-882, a novel potent PAF receptor antagonist, on general hemodynamics and regional flow distribution in a canine endotoxic shock model, 14 anesthetized and ventilated dogs received 2 mg/kg of Escherichia coli endotoxin intravenously (i.v.) followed by generous fluid resuscitation. Thirty minutes later, the dogs received either BB-882 (n = 7) as a continuous i.v. infusion with hourly increasing doses (2, 5, and 10 mg/kg.h, respectively) or a corresponding amount of saline (n = 7). The administration of BB-882 resulted in a dose-dependent reduction in cardiac output and an increase in systemic and pulmonary vascular resistance. Mesenteric and renal flow were not different from control values whereas femoral blood flow progressively decreased. Another group of 7 dogs received 5 mg/kg i.v. bolus of BB-882 30 min before endotoxin. Pretreatment significantly increased mesenteric blood flow by about 50% but did not show any significant hemodynamic effects. This study demonstrates that the administration of a PAF receptor antagonist following endotoxic shock in fluid resuscitated dogs does not offer significant hemodynamic benefit. Pretreatment with BB-882 at the dose used only enhanced mesenteric perfusion. These findings do not support beneficial effects of PAF receptor antagonists in septic shock.

Pharmacology of ABT-491, a highly potent platelet-activating factor receptor antagonist

ABT-491 (4-ethynyl- N, N-dimethyl-3-[3-fluoro-4-[(2-methyl-1 H-imidazo-[4,5- c]pyridin-1-yl)methyl]benzoyl]-1 H-indole-1-carboxamide hydrochloride) is a novel PAF (platelet-activating factor) receptor antagonist with a K i for inhibiting PAF binding to human platelets of 0.6 nM. Binding kinetics of ABT-491 to the PAF receptor is consistent with a relatively slow off-rate of the antagonist when compared to PAF. Inhibition of PAF binding is selective and is correlated with functional antagonism of PAF-mediated cellular responses (Ca 2+ mobilization, priming, and degranulation). Administration of ABT-491 in vivo leads to potent inhibition of PAF-induced inflammatory responses (increased vascular permeability, hypotension, and edema) and PAF-induced lethality. Oral potency (ED 50) was between 0.03 and 0.4 mg/kg in rat, mouse, and guinea-pig. When administered intravenously in these species, ABT-491 exhibited ED 50 values between 0.005 and 0.016 mg/kg. An oral dose of 0.5 mg/kg in rat provided >50% protection for 8 h against cutaneous PAF challenge. ABT-491 administered orally was also effective in inhibiting lipopolysaccharide-induced hypotension (ED 50=0.04 mg/kg), gastrointestinal damage (0.05 mg/kg, 79% inhibition), and lethality (1 mg/kg, 85% vs. 57% survival). The potency of this novel antagonist suggests that ABT-491 will be useful in the treatment of PAF-mediated diseases. © 1997 Elsevier Science B.V.

The effect of the selective PAF antagonist CIS-19 on PAF- and antigen-induced bronchoconstriction, microvascular leakage and bronchial hyperreactivity in guinea-pigs.

We investigated the effects of a novel platelet-activating factor (PAF) receptor antagonist, CIS-19 [cis-2-(3, 4-dimethoxyphenyl)-6-isopropoxy-7-methoxy-1-(N-methyl-formamido)-1, 2, 3, 4-tetrahydronaphthalene], on PAF-, histamine-, substance P- and antigen-induced bronchoconstriction and microvascular leakage, as well as PAF- and antigen-induced bronchial hyperreactivity to methacholine in urethane-anesthetized guinea-pigs. Administration of CIS-19 (0.5-5 mg/kg, i.v.) inhibited the increase in lung resistance induced by PAF (30 ng/kg, i.v.) in a dose-dependent manner, but failed to inhibit the increase induced by histamine (30 micrograms/kg, i.v.) or substance P (6.5 micrograms/kg, i.v.). CIS-19 (5 mg/kg, i.v.) did not inhibit the increase in lung resistance induced by ovalbumin (2 mg/kg, i.v.) in actively sensitized guinea-pigs. PAF (30 ng/kg, i.v.)-induced microvascular leakage, measured by the extravasation of Evans blue dye, was dose-dependently inhibited by CIS-19 (0.5-5 mg/kg, i.v.) in the trachea, main bronchi and intrapulmonary airways, but it did not affect histamine (30 micrograms/kg, i.v.)- or substance P (6.5 micrograms/kg, i.v.)-induced microvascular leakage at all airway levels. CIS-19 (2.5 and 5 mg/kg) did not affect ovalbumin (2 mg/kg, i.v.)-induced microvascular leakage in all airway levels in actively sensitized guinea-pigs, CIS-19 (2.5 and 5 mg/kg, i.v.) significantly inhibited PAF-induced enhancement of the bronchial response to methacholine, but had no effect on ovalbumin (0.05 mg/kg, i.v.)-induced bronchial hyperreactivity in actively sensitized guinea-pigs. It is concluded that CIS-19 is a potent PAF receptor antagonist which inhibits PAF- but not antigen-induced bronchoconstriction, microvascular leakage and bronchial hyperreactivity. These results suggest that PAF plays little or no role in early airway responses following antigen challenge.

Role of platelet-activating factor on cardiovascular dysfunction in postischemic shock in pigs.

This study aimed at evaluating the role of platelet-activating factor (PAF) on cardiovascular dysfunction in postischemic shock in pigs. Sixteen pigs were randomly allocated to two groups of eight each. Their aorta was clamped above the celiac axis for 45 min and then declamped. The animals were studied for 2 h after declamping. They were given a continuous infusion of Hartmann's solution 6.75 ml/kg/h throughout the experiment. The experimental group was given a potent specific PAF receptor antagonist 15 min before reperfusion (BB-882 1 mg/kg bolus followed by continuous infusion of 1 mg/ kg/h till the end of the experiment). The control group was given vehicle instead. Reperfusion in the control group caused prolonged hypotension (mean arterial pressure (SEM): 29 (1) mm Hg, immediately after declamping, compared with 74 (3) at baseline), an increase in pulmonary vascular resistance (491.6 (51.5) dyn.s.cm-5, 2 h after declamping, compared with 274.2 (19.4) dyn.s.cm-5 at baseline), a reduction in cardiac output (1.75 (0.15) liters/min, 2 h after declamping, compared with 2.8 (0.21) liters/min at baseline), hyperglycemia (13.7 (0.8) mmol/l, immediately after declamping, compared with 6.26 (0.6) mmol/l at baseline), and lactic acidemia (11.28 (0.5) mmol/l, immediately after declamping, compared with 4.55 (0.67) mmol/l at baseline). BB-882 did not improve any of these variables. PAF does not play a major role on cardiovascular dysfunction in postischemic shock in pigs.

Role of platelet-activating factor antagonism in posthemorrhage septic shock in pigs.

To study the role of platelet-activating factor (PAF) antagonism in posthemorrhage septic shock in pigs. Experimental study. Twelve anesthetized pigs were bled, kept with a mean arterial pressure of 30 mm Hg for 30 minutes, and then resuscitated with 50 mL/kg of isotonic saline. A continuous infusion of Escherichia coli endotoxin 36 micrograms/kg/hour was given intravenously for 3.5 hours starting 30 minutes after resuscitation. The animals were divided into two groups of six each. One group received I mg/kg of BB-882 (a potent specific PAF receptor antagonist) as a bolus during resuscitation, followed by a continuous infusion of BB-882 1 mg/kg/hour. The other group received vehicle alone. The measured variables were blood temperature, heart rate, intravascular pressures, cardiac output, systemic and pulmonary vascular resistance, lung-thorax compliance, blood gases, hemoglobin oxygen saturation, packed cell volume and blood sugar and serum lactic acid concentrations. The group treated with BB-882 had significantly higher intracardiac pressures and cardiac output, and had less increase in systemic vascular resistance. The BB-882 group had significantly less lactic acidemia than the control group (p < 0.05, analysis of variance appropriate for repeated measurement design). BB-882 had no effect on endotoxin-induced hypoxia or reduced lung-thorax compliance. PAF antagonism reduced the increase in systemic vascular resistance, improved cardiac output, and reduced lactic acidemia in posthemorrhage septic shock in pigs, but it did not improve hypoxia or reduced lung-thorax compliance.

Attenuation of endotoxin-induced pathophysiology by a new potent PAF receptor antagonist.

The role of platelet-activating factor (PAF) as a mediator of endotoxin-induced pathophysiology has been studied in several animal models with conflicting results. We evaluated the effect of a new, potent, and specific PAF receptor antagonist, ABT-299 (Abbott Laboratories) against endotoxin (lipopolysaccharide; LPS)-induced cardiopulmonary dysfunction in a porcine model. In initial experiments, the potency of ABT-299 was confirmed in vitro by its ability to inhibit PAF-induced porcine platelet aggregation at an IC50 of .047 +/- .01 microM, and in vivo by the ability of low doses (.12 mg/kg + .03 mg/kg/h) to block the cardiopulmonary pathologic response to exogenous PAF infusion. To evaluate the effect of ABT-299 administration during endotoxemia, pigs were randomly assigned to one of three groups: controls (n = 7), LPS (n = 9), or ABT-299 + LPS (n =7). ABT-299 was given at 1.0 mg/kg from -0.5 to 0 h plus .3 mg/kg/h from 0 to 6 h. LPS was given at .5 micrograms/kg/hr from 0 to 6 h. ABT-299 reduced the early LPS-induced fall in cardiac index and stroke volume, pulmonary hypertension and vasoconstriction, bronchoconstriction, and hypoxemia. Administration of LPS resulted in 44% mortality (before 6 h), which was blocked by ABT-299. Results with this antagonist indicate that PAF contributes to endotoxin-induced cardiopulmonary dysfunction in the pig, and is associated with mortality in this model.

BB‐882 is a Potent Antagonist of the Haemodynamic Changes Induced by Platelet‐Activating Factor in Pigs

The effects of bolus doses (0.2-2 micrograms) of platelet-activating factor (PAF) on systemic and pulmonary arterial pressures were tested in five juvenile pigs, and were demonstrated to have a biphasic effect. There was an initial drop in the mean systemic arterial pressure within seconds, followed by an increase within 1-2 min., whereas mean pulmonary arterial pressure only rose. The response was dose-related and did not include tachyphylaxis. The pressures returned to baseline within 10 min. After confirming these results a novel specific PAF receptor antagonist (BB-882) was given as a 1 mg/kg bolus followed by a continuous infusion of 2 mg/kg/hr, and a third dose response curve was repeated with ten-fold higher PAF doses (2-20 micrograms). BB-882 effectively counteracted these effects. Six pigs were given a continuous infusion of BB-882 33 mg/kg/hr for 5 hr and were compared with another group of five pigs given vehicle only. In this high dose BB-882 did not affect the intravascular pressures. These results indicate that BB-882 is a potent PAF receptor antagonist in juvenile pigs.

The specific binding of the platelet-activating factor (PAF) receptor antagonist WEB 2086 and the benzodiazepine flunitrazepam to rat hepatocytes

Thieno-triazolodiazepines WEB 2086 and BN 50739 have been described as the potent PAF receptor antagonists. Binding of radiolabeled [ 3H]WEB 2086 has been widely employed to characterize PAF receptors in different cells. In a search for a PAF receptor in isolated rat hepatocytes, we discovered that the binding of [ 3H]WEB to rat hepatocytes was highly specific but had a relatively low affinity with a K d of 113 nM and B max of 0.65 pmol/10 6 cells in freshly isolated cell suspension and K d of 1.65 μM and B max of 2.0 pmol/plate in cultured hepatocytes. No consistent specific binding of [ 3H]PAF itself was found in the same cell preparations. The binding of [ 3H]flunitrazepam in the presence of the peripheral type of benzodiazepine receptor antagonist Ro 5-4864 was saturated and exhibited a K i of 3.8 nM and B max of 3.5 pmol/plate. The central type of benzodiazepine receptor antagonist clonazepam also competed for the [ 3H]flunitrazepam binding, however with a much lower affinity. Various antagonists inhibited the binding of [ 3H]WEB 2086 with a rank order BN 50739⪢Ro 5-4864≥clonazepam. Interestingly, bicuculline, a specific antagonist of GABA(A) recognition sites, also significantly reduced the binding of [ 3H]WEB 2086. The binding of [ 3H]flunitrazepam was inhibited with a rank potency BN 50739⪢WEB 2086. Taken together, these findings suggest that the specific binding of PAF receptor antagonists WEB 2086 and BN 50739 in rat hepatocytes does not involve PAF receptors and occurs via peripheral benzodiazepine and, possibly GABA(A) receptor sites.

Importance of platelet activating factor in anaphylaxis-induced t-PA and PAI-1 increase in the rabbit

Summary Single injections of ovalbumin to presensitized rabbits dramatically increased the circulating activities of plasma tissue-type plasminogen activator (t-PA) and plasminogen activator inhibitor (PAI-1) and induced a severe thrombocytopenia. These effects occurred in a time- and dose-dependent manner, the maximum circulating levels of t-PA and PAI-1 being observed 30 min after administration of the antigen. In order to determine the importance of platelet activating factor (PAF) in this phenomenon, SR 27417, a potent and selective PAF receptor antagonist, was administered prior to antigen challenge. The intravenous administration of SR 27417 strongly inhibited the effects of ovalbumin, showing that t-PA and PAI-1 increase resulting from the anaphylaxis reaction depend on PAF release. SR 27417 also strongly inhibited ovalbumin-induced thrombocytopenia. The mechanism of action of PAF leading to t-PA and PAI-1 increase during anaphylaxis is still unclear, but might be considered as a contributing mechanism during the allergic reaction.

Regional haemodynamic effects of platelet activating factor in the rat

The effects of platelet activating factor (PAF) on haemodynamics in the absence and presence of the potent PAF receptor antagonist TCV-309 {3-bromo-5[ N-phenyl- N-[2-[[2-(1,2,3,4-tetrahydro-2-isoquinolyl-carbonyloxy)ethyl]carbamoyl]ethyl]carbamoyl]-1-propylpyridinium nitrate} were studied by the microsphere technique in pentobarbitone-anaesthetized rats. I.v. infusion of the low dose PAF (0.05 μg kg −1 min −1) did not significantly alter mean arterial pressure, cardiac output or total peripheral resistance but increased arterial conductances in the stomach, intestine, caecum and colon and reduced conductance in the spleen. I.v. infusion of the high dose of PAF (0.3 μg kg −1 min −1) markedly reduced mean arterial pressure (−53 mm Hg) and cardiac output (−62%) and insignificantly increased total peripheral resistance. Arterial conductances in the lungs, stomach, intestine, caecum and colon, kidneys and spleen were reduced and those in the heart and muscle were increased. TCV-309 (10 μg kg −1) abolished all changes in arterial pressure, cardiac output and total peripheral resistance and arterial conductances elicited by either the low or the high dose of PAF. The results show that a non-hypotensive dose of PAF caused vasodilatation of the gastrointestinal organs and vasoconstriction of the spleen. A high dose of PAF which markedly decreased arterial pressure and cardiac output caused vasodilation of the heart and muscle and vasoconstriction of the lungs (bronchial), gastrointestinal organs, kidneys and spleen. All haemodynamic changes were blocked by TCV-309 indicating the involvement of PAF receptors.