Overview: PAF Receptor Antagonists: Recent Advances

Pathophysiology of pulmonary complications of acute pancreatitis

Acute pancreatitis in its severe form is complicated by multiple organ system dysfunction, most importantly by pulmonary complications which include hypoxia, acute respiratory distress syndrome, atelectasis, and pleural effusion. The pathogenesis of some of the above complications is attributed to the production of noxious cytokines. Clinically significant is the early onset of pleural effusion, which heralds a poor outcome of acute pancreatitis. The role of circulating trypsin, phospholipase A2, platelet activating factor, release of free fatty acids, chemoattractants such as tumor necrosis factor (TNF)-alpha, interleukin (IL)-1, IL-6, IL-8, fMet-leu-phe (a bacterial wall product), nitric oxide, substance P, and macrophage inhibitor factor is currently studied. The hope is that future management of acute pancreatitis with a better understanding of the pathogenesis of lung injury will be directed against the production of noxious cytokines.

Potential new therapies for the treatment of acute pancreatitis

The treatment of acute pancreatitis has remained virtually unchanged for the past 50 years, in large part due to a poor understanding of the initial intracellular events. Furthermore, there is a lack of knowledge regarding the mediator(s) responsible for the progression of the disease from local pancreatic inflammation to a systemic inflammatory disease, as well as the mediator(s) responsible for distant organ dysfunction and failure. With recent advances in the pathophysiology of pancreatitis, in particular the role of the inflammatory mediators interleukin-1 beta, tumour necrosis factor alpha and platelet-activating factor, the potential for new effective therapies has been realised. At present, a number of inflammatory mediator antagonists are being tested in humans, with the hope that we may soon develop a specific treatment for a disease, which thus far, has none.

Early treatment with lexipafant, a platelet-activating factor-receptor antagonist, is not sufficient to prevent pulmonary endothelial damage after intestinal ischaemia and reperfusion in rats

BACKGROUND

Intestinal ischaemia-reperfusion can lead to pulmonary injury characterised by increased macromolecular leakage and leukocyte sequestration. Important mediators of ischaemia-reperfusion-associated injury include polymorphonuclear granulocytes and platelet-activating factor.

AIM

To investigate the potential therapeutic inhibition of platelet-activating factor in intestinal ischaemia-reperfusion associated pulmonary injury, by use of a potent platelet-activating factor-receptor antagonist, lexipafant.

METHODS

Rats were subjected to 30 minutes of intestinal ischaemia followed by 3 or 12 hours reperfusion. Lexipafant or saline was given intraperitoneally after 30 minutes reperfusion.

RESULTS

Increased leakage of radiolabelled human serum albumin was found in the lungs after intestinal ischaemia followed by 3 or 12 hours reperfusion. Administration of lexipafant did not significantly prevent the increased leakage. Pulmonary myeloperoxidase content increased after intestinal ischaemia-reperfusion, indicating polymorphonuclear granulocyte sequestration through the pulmonary endothelium. The increase in interleukin-1beta seen after 3 hours reperfusion was partly reversed by lexipafant.

CONCLUSIONS

Pulmonary injury occurred following intestinal ischaemia-reperfusion, characterised by increased leakage of radiolabelled albumin over the endothelial barrier; correlating with increased pulmonary myeloperoxidase-content, implying involvement of polymorphonuclear granulocytes in the pathogenesis of remote organ injury after intestinal ischaemia-reperfusion. Lexipafant did not significantly decrease severity of pulmonary damage.

The potential role of therapeutic cytokine manipulation in acute pancreatitis

The central, detrimental role of the inflammatory cytokines IL-1 and TNF and the biologically active phospholipid PAF in the pathogenesis of AP has been established over the past 8 years. A number of antagonists to these mediators have been used successfully in the laboratory setting and are currently being examined in prospective randomized trials. The effectiveness of any antagonist depends not only on its ability to block the effects of the inflammatory mediators but also on its administration early enough in the course of the pancreatitis before pancreatic necrosis or organ dysfunction.

The platelet-activating factor antagonist BB-882 does not improve tissue oxygen extraction in endotoxic shock

PURPOSE

We investigated whether BB-882, a novel potent PAF antagonist, could influence systemic and pulmonary hemodynamics and oxygen extraction capabilities during an acute reduction in blood flow induced by cardiac tamponade after endotoxin challenge.

MATERIALS AND METHODS

Twenty-one anesthetized, ventilated, and endotoxin-shocked (2 mg/kg i.v. Escherichia coli endotoxin) dogs were randomly divided in three groups. One group (N = 7) served as control. A second group (N = 7) received BB-882 as a single bolus dose of 5 mg/kg, 30 minutes before endotoxin administration. A third group (N = 7) received BB-882 as a continuous infusion of 5 mg/kg x h, started 30 minutes after endotoxin. Hemodynamic and gazometric measurements were obtained in all dogs 30 minutes after endotoxin injection and repeated 30 minutes after cardiac filling pressures were restored to baseline by generous saline infusion. Saline infusion rate was then set at 20 mL/kg x h and tamponade was induced by repeated bolus injections of warm saline into the pericardial sac.

RESULTS

Compared with controls, pretreatment with BB-882 attenuated the early endotoxin-induced decrease in arterial pressure (70 +/- 17 v 51 +/- 14 mm Hg, P < .05), cardiac index (118 +/- 29 v 91 +/- 15 mL/ kg x min, P < .05), stroke index (1.0 +/- 0.2 v 0.7 +/- 0.3 mL/kg, P < .05), and left ventricular stroke work index (0.9 +/- 0.3 v 0.4 +/- 0.2 g x m/kg, P < .05), but these effects were not sustained after fluid resuscitation. In contrast, BB-882 post-treatment maintained arterial pressure and improved cardiac performance at lower filling pressures in the later phase of endotoxic shock. BB-882 did not influence pulmonary hemodynamics. Treatment with BB-882 did not influence oxygen extraction at critical oxygen delivery (51.5 +/- 9.9% and 52.8 +/- 13.9% v 46.6 +/- 9.0%, respectively BB-882 pretreatment and post-treatment v control).

CONCLUSIONS

We conclude that in this model of endotoxic shock the administration of BB-882, either before or after endotoxin challenge, has time-related beneficial hemodynamic and cardiac effects but does not improve global oxygen extraction capabilities. The potential benefit of adjunctive treatment with a platelet-activating factor antagonist in sepsis remains doubtful.

Synthesis and pharmacological evaluation of a new class of bicyclic phospholipids, designed as platelet activating factor antagonists

(+/-)-3-Alkoxymethyl-(2-oxabicyclo[3.3.0]octane)-5-yl-methyl-phosp horyl- ethyl-pyridinium [alkyl chain = methyl (5a) and (5b), allyl (6a) and (6b), n-propyl (7a) and (7b) and n-hexyl (8a) and (8b)] derivatives, structurally designed as conformationally restricted platelet activating factor (PAF) antagonists were synthesized in 12-26% overall yield, using ethyl (+/-)-3-hydroxymethyl-5-(2-oxabicyclo [3.3.0] octane) carboxylate (13a,b) as key intermediate. The anti-platelet profile of the new derivatives was evaluated in a PAF-induced aggregation model in rabbit platelet-rich plasma; only compound 8a exhibited a modest activity.

A pharmacophore for high affinity PAF antagonists. II. Hydrophobicity study using the molecular lipophilicity potential

Platelet-activating factor (PAF) is a powerful phospholipid-derived autacoid involved in many physiopathological mechanisms. Many PAF antagonists have been synthesized and evaluated as therapeutic candidates. In a previous report, we have described an electronic pharmacophore of PAF antagonists using the molecular electrostatic potential. In the present study, a molecular lipophilicity potential is used to compare the hydrophobic properties of 49 "heterocyclic sp2 nitrogen' highly potent PAF antagonists, belonging to six structurally different series (nine hetrazepines, five pyrrolo[1,2-c]thiazoles, 14 carboxamides, nine dihydropyridines, nine pyridinyl-thiazolidines and three imidazo[4,5-c]pyridines). Their common features consist of three hydrophilic (HYD2, HY14(3)B and HYD3) and two lipophilic zones (LIP3 and LIP4), defining the lipophilic pharmacophore of the antagonists. This pharmacophore is also characterized by several zone-to-zone distances: HYD3-HYD2 = 1.3 +/- 1.0 A, HY3B-HYD2 = 7.8 +/- 1.1, HYD3-HY3B = 5.1 +/- 1.1 A, LIP4-LIP3 = 5.4 +/- 1.1 A, LIP3-HYD2 = 11.3 +/- 1.6 A, LIP3-HY3B = 5.9 +/- 1.0 A, LIP3-HYD3 = 4.3 +/- 0.9 A, LIP4-HYD2 = 14.7 +/- 1.6 A, LIP4-HY3B = 8.1 +/- 1.2 A and LIP4-HYD3 = 3.9 +/- 1.1 A. These results represent a new step in the determination of a global pharmacophore for PAF antagonists.

A pharmacophore for high affinity PAF antagonists. I. Electronic model using molecular electrostatic potential

PAF is a powerful phospholipid-derived autacoid involved in many physio-pathological mechanisms. Many PAF antagonists have been synthesized and assayed for therapeutic purposes. In this study, molecular electrostatic potential is used to compare the electronic properties of 48 'heterocyclic sp2 nitrogen' highly potent PAF antagonists, belonging to six series (nine hetrazepines, five pyrrolo[1,2-c]thiazoles, 14 carboxamides, nine dihydropyridines, nine pyridinylthiazolidines and two imidazo[4,5-c]pyridines). Their common features consist of three main electronegative zones (A, B1 and B2) describing the electronic pharmacophore of these ligands. The high affinity of these PAF antagonists seems to be related to this electronegative system A-B(x), which is characterized by three distances A-B1 (9.3 +/- 1.0 A), A-B2 (13.4 +/- 0.7 A) and B1-B2 (4.9 +/- 0.9 A). Moreover, B1 and B2 may surround a common anchorage point in the binding site of the receptor.

Platelet-activating factor

Platelet-activating factor (PAF) is a proinflammatory lipid. It has been strongly implicated in the regulation of pancreatic exocrine secretion and in the local and systemic events which occur in acute pancreatitis. PAF antagonists, when given as pretreatment, ameliorate the severity of experimental acute pancreatitis by reducing serum amylase, oxidative injury, morphological changes, polymorph infiltration, pulmonary damage, and exudative levels of PAF in blood and peritoneal fluid. The novel, potent PAF antagonist, lexipafant, can ameliorate microvascular-induced acute pancreatitis after induction of the disease. It also reduces lung injury by preventing increased pulmonary capillary permeability. In a double-blind, randomized, Phase II, clinical study of lexipafant in human acute pancreatitis, a clinical benefit was found, as indicated by a significant reduction in an organ failure score measured during 72 h of infusion. In addition, organ failure recovered in seven of 12 severe acute pancreatitis patients treated with lexipafant, but recovered in only two of 11 patients given placebo. New organ failure developed in a further two patients on placebo. The results of these studies indicate that lexipafant is a potential therapy for the treatment of human acute pancreatitis. A multicentre, Phase III, UK study in patients with severe acute pancreatitis is currently underway.

Randomized, double-blind phase II trial of Lexipafant, a platelet-activating factor antagonist, in human acute pancreatitis

The aims of the study were to determine whether the platelet-activating factor antagonist Lexipafant could alter the clinical course and suppress the inflammatory response of human acute pancreatitis. In a double-blind, placebo-controlled study 83 patients were randomized to receive Lexipafant 60 mg intravenously for 3 days, or placebo. Clinical progression was assessed by daily Acute Physiology And Chronic Health Evaluation (APACHE) II score and organ failure score (OFS). The magnitude of the inflammatory response on days 1-5 was assessed by serial measurement of interleukin (IL) 8, IL-6, E-selectin, polymorphonuclear elastase-alpha1-antitrypsin (PMNE-alpha 1-AT), and C-reactive protein (CRP). At entry, patients receiving Lexipafant (n = 42) or placebo (n = 41) were matched for age and sex, aetiology, APACHE II score and OFS. The disease was classified as severe in 29 patients (APACHE II score eight or more). There was a significant reduction in the incidence of organ failure (P = 0.041) and in total OFS (P = 0.048) at the end of medication (72 h). During this time seven of 12 patients with severe acute pancreatitis who had Lexipafant recovered from an organ failure; only two of 11 with severe acute pancreatitis who had placebo recovered from an organ failure and two others developed new organ failure. Lexipafant treatment significantly reduced serum IL-8 (P = 0.038), and IL-6 declined on day 1. Plasma PMNE-alpha 1-AT complexes peaked on day 1; the gradual fall to baseline over 5 days observed in controls did not occur in patients given Lexipafant. No effect was observed on serum CRP. This study provides a rationale for further clinical trials with the potent PAF antagonist Lexipafant in human acute pancreatitis.

A Monte Carlo pharmacophore generation procedure: application to the human PAF receptor

A novel pharmacophore definition procedure is described, which uses a Monte Carlo method to superimpose molecules. Pharmacophore space is searched by a technique similar to high temperature annealing. Subsequent refinement of candidate pharmacophores by energy minimization produces low-energy conformations that may be involved in receptor binding. The method has been applied to compounds that bind to the human platelet-activating factor (PAF) receptor. Alternative binding site models for the PAF receptor are presented and discussed.