Expression of Endotoxic Activities by Synthetic Monosaccharide Lipid A Analogs with Alkyl-branched Acyl Substituents

Synthetic monosaccharide lipid A analogs with alkyl-branched acyl substituents instead of the usual ester-branched acyl substituents were investigated for their biological activities. The activities were compared with those of a representative synthetic monosaccharide lipid A analog with an ester branch (GLA-60) and synthetic complete lipid A (506) to estimate the role of the attaching mode of the branched side chains for expression of endotoxic activities. Among the analogs with alkyl branches, GLA-146 and GLA-147, which have C12 and C14 alkyl side chains, respectively, showed strong endotoxic activities. These analogs exhibited comparable or stronger activities than those of GLA-60 in murine macrophage activation activities to induce mediators such as tumor necrosis factors, interleukin 6, and nitric oxide and in mitogenic activity towards murine spleen cells; however, these activities were weaker than the respective activities of 506. With respect to lethal toxicity to galactosamine-sensitized mice, the analogs showed stronger activity than that of GLA-60 and activity closer to that of 506. With respect to adjuvant activity, no significant activity was observed in the analogs, while the activities of GLA-60 and 506 were strong. When lipopolysaccharide-resistant C3H/HeJ mice were used, the activities described above were not observed either for the analogs under investigation nor for GLA-60 and 506. These findings indicate that the ester type of branch in lipid A and its analogs does not play an indispensable role in the expression of various endotoxic activities. However, it may play some role in the expression of adjuvant activity and in lowering the level of toxicity.