ObjectivesEndotoxin is one of the principal mediators of Gram-negative septic shock. Pretreatment with monophosphoryl lipid A, a hydrolyzed derivative of endotoxin from Salmonella minnesota R595, induces endotoxin tolerance and nonspecific resistance to infection in experimental animals. The present clinical trial was undertaken to test the response to monophosphoryl lipid A in humans and the ability of monophosphoryl lipid A to attenuate the response of normal human volunteers to U.S. Reference Ec-5 endotoxin.DesignProspective, double-blind, randomized, controlled trial.SettingClinical research center.PatientsForty-four healthy volunteers.InterventionsIn part 1 of the study, 29 volunteers were randomized in varying ratios to receive vehicle control or monophosphoryl lipid A intravenously in a double-blind dose escalation trial. In part 2 of the study, 12 volunteers were randomized to receive either monophosphoryl lipid A (20 micro gram/kg) or vehicle control and, 24 hrs later, all 12 volunteers were challenged with U.S. Reference Ec-5 endotoxin (20 units/kg intravenous, bolus injection). Systemic response to endotoxin challenge was evaluated and compared between the monophosphoryl lipid A and vehicle control-pretreated subjects.Measurements and Main ResultsIn part 1 of the study, subjective effects and increases in cytokine levels were not observed until a dose of 10 micro gram/kg of monophosphoryl lipid A was administered. Six volunteers receiving a maximum dose of 20 micro gram/kg experienced mild-to-moderate symptoms that did not require therapy. Moderate increases in temperature, heart rate, and tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, and IL-8 release were observed. IL-1 alpha and IL-1 beta were not detected but a significant increase in IL-1 receptor antagonist was observed.In part 2 of the study, monophosphoryl lipid A pretreatment reduced the number of volunteers who experienced one or more subjective complaints after endotoxin administration (3/6 vs.6/6; p equals .09). The febrile response and tachycardic response to endotoxin were significantly reduced by pretreatment with monophosphoryl lipid A. Monophosphoryl lipid A-pretreated volunteers demonstrated significantly reduced concentrations of TNF-alpha after endotoxin challenge, as compared with subjects treated with vehicle control (84 plus minus 76 vs. 244 plus minus 128 pg/mL; p less than .05). IL-6 concentrations (100 plus minus 91 vs. 268 plus minus 171 pg/mL; p less than .05) and IL-8 concentrations (136 plus minus 86 vs. 632 plus minus 323 pg/mL; p less than .05) elicited by endotoxin challenge were also significantly reduced by monophosphoryl lipid A pretreatment.ConclusionsData indicate that monophosphoryl lipid A, in a dose 10,000 times that of endotoxin, used in experimental pyrogenicity trials, is well tolerated in human volunteers. Pretreatment of normal human volunteers with monophosphoryl lipid A attenuated the systemic response to bacterial endotoxin. These data support further clinical testing of monophosphoryl lipid A for the prevention or amelioration of the severe sequelae of sepsis.(Crit Care Med 1995; 23:9-17)