Experimental studies on endotoxin infusion in human : evaluation of pharmacological immunomodulation by adenosine and nicotinamide

Abstract

Introduction: Septicaemia and the systemic inflammatory response syndrome (SIRS), is a major cause of mortality among patients in the intensive care units. Inflammation and thrombosis involve multicellular phenomena with activation of leukocytes, platelets and endothelial cells resulting in synthesis and release of various mediators e.g. cytokines, nitric oxide (NO), endothelin and neuropeptide Y, complement factors and reactive oxygen species such as superoxide anions (02-) and its metabolites. Endotoxin (lipopolysaccharide) is a major stimulus for triggering the host response with the subsequent production of pro- and anti-inflammatory cytokines such as tumor necrosis factor alpha (TNF-α) and IL- 10.

Aim: The overall aim of this thesis was to study some response patterns of sepsis and potential modulations in an experimental model where human volunteers receive endotoxin. The work could thereby contribute to increased understanding of sepsis-related mechanisms and pharmacological interventions and hopefully attenuate the inflammatory response.

Methods: The studies were performed using endotoxin (2-4 ng/kg BW) intravenously in healthy male volunteers and took place in the intensive care unit at Huddinge University Hospital. All together, 38 volunteers participated in 73 experiments, 32 received endotoxin at one or two occasions. We measured exhaled NO, plasma cGMP, endothelin, neuropeptide Y, haemostatic parameters, complement activation and nitrite/nitrate after endotoxin (E. Coli, Lot G 1) administration of 4 and 2 ng/kg BW respectively. Using double-blind, cross-over design, we investigated the influence of the anti-inflammatory compounds adenosine (in a low continous, intravenous dose, 40 µg/kg/min) as well as nicotinamide (the amide derivate of vitamin B3, in a high oral dose, 4+4 g, 14 and 2 hours preceding the experiment), on inflammatory parameters after endotoxin.

Results: Endotoxin provoked clinical signs of inflammation (fever, headache, increased heart rate and decreased diastolic blood pressure) over a period of 4-6 hours. There were no adverse responses. An early increase in orally exhaled NO concentration was observed and cGMP levels were increased. Cardiac output increased by 100%, systemic vascular resistance decreased by 50%. There was activation of the complement system as well as increased arterial endothelin-1 -like immunoreactivity (ET-1-LI) over time. NPY-like immunoreactivity (NPY-LI) remained unchanged. Endotoxin evoked in vivo activation of platelets, leukocytes and endothelial cells, as well as enhancing platelet-leukocyte aggregate formation and thrombin generation. A modest anti-inflammatory effect by adenosine treatment was observed in this study. Adenosine significantly attenuated the endotoxininduced IL-6 response whereas there was no clear effect on the TNF-α response. After oral nicotinamide treatment, there was no effect on the inflammatory parameters (TNF-α, IL-6, IL-8, IL-10 levels or exhaled NO) at the maximum dose that can be administered without inducing side-effects.

Conclusion: In conclusion, the present human endotoxaemic model exhibited reproducible results, thereby providing a stable and safe model for randomised studies. Adenosine, in a low continuous intravenous dose, showed only modest immunomodulatory properties. Nicotinamide did not have any anti-inflammatory effects when a high oral dose was given to healthy volunteers receiving endotoxin.