Lipid A and its two chemically stable analogues, wherein the glycosidic phosphoryl groups in lipid A is replaced with 2-(phosphonooxy)ethyl or carboxymethyl groups, have been synthesized by an improved and divergent route via a common allyl glycoside intermediate in which the 4-hydroxy group was protected as a benzyl ether. The total yields were more than 20% for 11 or 12 steps starting from allyl 4,6-O-benzylidene-2-deoxy-2-(trichloroethoxycarbonylamino)-D-glucopyranoside. These synthetic chemically stable analogues induce interleukin-6 and tumor necrosis factor α in human peripheral whole blood cells with potencies comparable to those by natural-type synthetic lipid A. The Limulus activities of both analogues were found to be even stronger than the activity of the natural-type one.

Four tetrahydropyran-2-carboxylic acid derivatives with 3-(tetradec-7-enyloxy)tetradecyl chains instead of 3-(tetradecanoyloxy)tetradecanoyl chains in lipid A were synthesized and their biological activities toward human U937 cells, human whole blood cells and mouse peritoneal resident macrophages were measured. These compounds showed LPS-antagonistic activity toward these three kinds of cells. The IC₅₀ values (nM) (1 M = 1 mol dm⁻³) of these four compounds (21, 21′, 24 and 24′) toward human monoblastic U937 cells were 2.2, 1.0, 0.017 and 0.055, respectively. However, the LPS-antagonistic activities (IC₅₀ values) of these four compounds toward human whole blood cells were only 0.28, 0.21, 0.81 and 0.58 μM, respectively. The IC₅₀ values (μM) toward mouse peritoneal resident macrophages were 2.49, 0.49, 0.91 and 0.69, respectively.

Both endotoxic and antagonistic [³H]-labeled 2-(phosphonooxy)ethyl (PE) analogs of lipid A were synthesized with high purity and high specific radioactivity. Lipid A-binding proteins were detected by using the endotoxic analog of hexaacyl Escherichia coli-type designated [³H] PE-506. The plasma membrane fractions from peritoneal macrophages derived from LPS-responder C3H/HeN mice and LPS-hyporesponder C3H/HeJ mice were separated by SDS-PAGE and transferred onto nitrocellulose membranes. The membranes were then incubated with the [³H] PE-506. Several [³H] PE-506 binding proteins were detected in both C3H/HeN and C3H/HeJ macrophages. Unlabeled hexaacyl lipid A inhibited the interaction between [³H] PE-506 and these proteins. The result suggests that there exist multiple binding sites for lipid A on macrophages. LPS-induced change in the profile of the cell surface lipid A binding proteins was observed in C3H/HeN macrophages, but not in C3H/HeJ macrophages, by preincubation of macrophages with LPS.

The synthesis of regiospecifically ¹³C-labeled compounds of a biosynthetic precursor of lipid A and its analogue with shorter acyl chains is described. D-(6-¹³C)Glucose was converted into a suitably protected glucosamine derivative via 1,6-anhydro-β-D-(6-¹³C)glucose. After coupling with glycosyl donors, the desired compounds were synthesized through a 6-step reaction sequence. The total yields were 1.7% for the biosynthetic precursor, and 6.4% for the short acyl analogue, respectively, for a total of 18 steps from D-(6-¹³C)glucose.

To elucidate the structural requirements for the endotoxic and antagonistic activities of lipid A derivatives, we have focused on the effects of the acyl moieties and acidic groups at the 1- and 4′-positions in the present study. We have synthesized new analogues corresponding to Rubrivivax gelatinosus lipid A, which has a characteristic symmetrical distribution of its acyl groups on its two glucosamine residues with shorter acyl groups (decanoyl groups (C₁₀) and lauryl groups (C₁₂)) than Escherichia coli lipid A’s. Carboxymethyl (CM) analogues in which one of the phosphates was replaced with a CM group were also synthesized with a different distribution of acyl groups. Biological tests revealed that the acyl group distribution in the lipid A analogue, strongly affected its bioactivity. The synthetic Ru. gelatinosus type lipid A showed potent antagonistic activity against LPS, whereas its 1-O-carboxymethyl analogue showed weak endotoxic activity. These results demonstrate that when lipid A has shorter (C₁₀ and C₁₂) hexa-acyl groups, its bioactivity is more easily affected by small structural differences, such as differences in acidic groups or acyl group distribution, and that they can change bioactivity from endotoxic to agonistic or vice versa at this structural boundary for the bioactivity.

Lipopolysaccharide (LPS), the endotoxin of Gram-negative bacteria, is a strong elicitor in the immune system by interacting with lipopolysaccharide-binding protein and CD14 with high specificity. The removal of LPS contamination in protein drug products expressed by bacteria is essential in pharmaceutical products for human use. Although polymyxin B (PMB)-immobilized columns are mainly used for removal of LPS, there are some problems, such as high production cost, and the toxicity of ligands. We synthesized aromatic lipids bearing lysine or arginine at the headgroup. These lipids form a complex with LPS through electrostatic interaction between cationic amino acids and phosphate groups in the lipid A backbone. The resultant complexes induce the structural transition of LPS from a cylindrical structure to a vesicle. Addition of amino-lipid/LPS complexes to RAW264.7 cells, a macrophage-like cell line, decrease the LPS activity. The efficiencies are higher than commonly used cationic compounds, such as dioleoyltrimethylammoniumpropane (DOTAP) and PMB. These results show that amphiphilic lipids with cationic amino acids can be used for deactivation of LPS.

A biosynthetic precursor of lipid A has been synthesized by an improved efficient method. Two appropriately modified acyl-substituted glucosamine units were synthesized from D-glucosamine using (R)-3-(benzyloxy)tetradecanoic acid and then coupled by the Lewis acid-promoted glycosidation via the corresponding trichloroacetimidate. Glycosyl phosphorylation and hydrogenolytic deprotection, followed by purification by liquid–liquid partition chromatography, afforded the target compound in 2.9% total yield through 13 steps from N-Troc-D-glucosamine.

Three anomeric pairs of lipid A-type disaccharides containing a glucose on their reducing end were synthesized, and their LPS-antagonistic activities were measured. The inhibitory activities (IC₅₀) on the LPS-induced TNFα production of these six compounds (16α, 16β, 28α, 28β, 40α, and 40β) toward human whole blood cells were 0.35, 0.42, 2.37, 1.16, 2.89, and 7.70 nM, respectively.