Objective: Platelet-activating factor (PAF) and structurally-related oxidized phospholipids are proinflammatory mediators in systemic inflammatory states such as severe sepsis. The enzyme platelet-activating factor acetylhydrolase (PAF-AH) rapidly degrades PAF and oxidized phospholipids into inactive metabolites. Reduced PAF-AH activity has been observed in patients with severe sepsis and may contribute to their systemic inflammatory response and organ dysfunction. A previous clinical trial with recombinant human PAF-AH (rPAF-AH, Pafase) suggested that this treatment may decrease 28-day all-cause mortality in patients with severe sepsis. The current study was undertaken to confirm this result.
Design: A prospective, randomized, double-blind, placebo-controlled, multicenter, international trial.
Setting: One hundred forty-six intensive care units from nine countries.
Patients: Approximately 2,522 patients were planned to be enrolled ≤12 hrs after the onset of severe sepsis. Eligible patients were randomized to receive either rPAF-AH 1.0 mg/kg or placebo administered intravenously once daily for five consecutive days.
Measurements and Main Results: The study was terminated based on the recommendation of an independent data and safety monitoring committee after the second of three planned interim analyses, and the enrollment of 1,425 patients. rPAF-AH treatment was well tolerated among the 1,261 patients included in the interim analysis (643 rPAF-AH and 618 placebo), but did not decrease 28-day all-cause mortality compared with placebo (25% for rPAF-AH vs. 24% for placebo; relative risk, 1.03; 95% confidence interval, 0.85-1.25; p = .80). There were no statistically significant differences between treatment groups in any of the secondary efficacy end points. The overall incidence of adverse events was similar among rPAF-AH and placebo-treated patients, and no rPAF-AH-treated patients developed antibodies to PAF-AH.
Conclusions: rPAF-AH was well tolerated and not antigenic, but did not decrease 28-day all-cause mortality in patients with severe sepsis.
Platelet-activating factor (PAF) is a potent proinflammatory phospholipid molecule generated by leukocytes, myeloid cells, platelets, mast cells, and endothelial cells. PAF has remarkably diverse biological activities due to the presence of the PAF receptor in many tissues and its capacity to alter target cell activity at multiple transcriptional, translational, and posttranslational levels (1-4). PAF synthesis is up-regulated in response to a variety of exogenous and endogenous stimuli, and increased concentrations of PAF can contribute to the deleterious effects of systemic inflammation in the pathogenesis of severe sepsis (5-9). The synthesis and degradation of PAF and its receptor are highly regulated by an integrated set of synthetic and degradation pathways (4, 10, 11).
In addition to PAF, oxidatively modified phospholipids that are structurally similar to PAF have the capacity to bind to the PAF receptor (10, 12, 13). These bioactive lipids may up-regulate the expression of the endotoxin receptor Toll-like receptor-4 on macrophages (14) and this may provide a stimulus for sustaining ongoing signal transduction, resulting in propagation of the inflammatory response (15).
In experimental models, PAF administration induces increased pulmonary vascular permeability, pulmonary vasoconstriction, and neutrophil infiltration into the alveolar capillary membrane (11, 16). Increased concentrations of PAF have been measured in the bronchoalveolar lavage fluid from patients with acute respiratory distress syndrome (ARDS) (17), and elevated concentrations of PAF have been found in the systemic circulation of patients with septic shock (18). Transgenic mice that overexpress the PAF receptor have an increased risk of mortality after exposure to bacterial endotoxin (19). In animal models of acute lung injury and systemic inflammation, treatment with a variety of different PAF receptor antagonists have been shown to limit the extent of injury (9, 20-22). Pretreatment of healthy subjects with a PAF-receptor antagonist decreased LPS-induced symptoms but had no effect on plasma concentrations of tumor necrosis factor-α, interleukin-1β, or interleukin-6 (23). Clinical trials with PAF receptor antagonists in patients with sepsis or septic shock have not improved survival (24-26).
PAF acetylhydrolase (PAF-AH) is a member of the phospholipase A2 family of enzymes (27, 28). The extracellular form of PAF-AH is a secreted plasma protein that serves to inactivate PAF and oxidized phospholipids that can produce PAF-like effects (10, 28). The extracellular, secreted form of human PAF-AH is constitutively expressed and found in human plasma at concentrations ranging between 0.5 and 1.0 μg/mL (29). This enzyme accounts for all of the PAF-inhibitory activity found in human blood, limiting the normal serum half-life of PAF to only a few minutes (28, 29). The circulating concentrations of this enzyme are diminished in sepsis as a consequence of endotoxin and cytokine-mediated reduction in PAF-AH gene transcription and possibly inactivation by oxidant injury (30-32).
The gene for the extracellular isoform of human PAF-AH has been expressed in Escherichia coli (12). This recombinant human enzyme (rPAF-AH, Pafase; ICOS, Bothell, WA) hydrolyzes PAF and oxidized phospholipids. The administration of rPAF-AH improves outcome in experimental models of inflammation and endotoxemia (10, 33-35). In an opossum biliopancreatic duct ligation model of severe acute pancreatitis, administration of rPAF-AH 2 days after duct ligation significantly decreased the concentration of PAF in lung and pancreatic tissue and reduced the extent of pancreatitis-associated acute lung injury compared with control (35). The therapeutic rationale for the administration of rPAF-AH in severe sepsis is to increase PAF-AH activity in the presence of generalized inflammation and coagulation. The therapeutic potential for this strategy was supported by the results from a phase II trial of rPAF-AH in 127 patients with severe sepsis (36). A phase III trial was undertaken to confirm these results in patients at risk for ARDS and mortality from severe sepsis.