Antiendotoxin activity of lipid A analogues: requirements of the chemical structure

Lipid X, a monosaccharide precursor of lipid A, has been found to prevent death in animals given a lethal dose of endotoxin, but the mechanism of this protective effect is unknown. We previously reported that lipid X blocks endotoxin-induced priming of human neutrophils in a manner consistent with competitive inhibition. To determine the molecular requirements for this antiendotoxin activity, we studied several derivatives of lipid X using the neutrophil priming assay. Neutrophil priming was quantitated by measuring stimulated superoxide (O2-) release. The removal of either acyl group from lipid X or even the simple change of the amide to an ester linkage at C2 of the glucosamide ring resulted in a marked loss of antagonism. Monosaccharide analogues, structurally related to native lipid A by the presence of acyloxyacyl side chains, demonstrated marked inhibition of endotoxin-induced priming at low concentrations but an endotoxin-like, priming effect at high concentrations. The addition of a phosphate group at position 4 of the sugar moiety was the only modification studied so far that produced a pure antagonist with increased antiendotoxin activity. Demonstration of these structural requirements for the antiendotoxin activity of lipid A analogues supports the hypothesis that this effect may be mediated via specific cellular binding sites. Lipid X derivatives may be useful for studying the interaction of endotoxin with cells and their antiendotoxin activity may prove beneficial in the treatment of septicemia.