Immunopharmacological activities of a synthetic counterpart of a biosynthetic lipid A precursor molecule and of its analogs.
Synthetic lipid A analogs (compounds 404 through 406) were examined for their immunopharmacological activities. These compounds had two amide-bound and two ester-bound (R)-3-hydroxytetradecanoyl groups at the C-2 and C-2' and the C-3 and C-3' positions, respectively, of beta (1-3)glucosamine disaccharide. In all of the in vitro assays, these synthetic compounds exhibited high activities comparable to those of a reference lipid A prepared from Escherichia coli O8:K27 Re-mutant strain F515. The compounds activated the clotting enzyme cascade of the horseshoe crab, activated the human complement via the classical pathway, caused polyclonal B-cell activation, stimulated the phagocytosis of sheep erythrocytes by murine peritoneal macrophages, and enhanced the migration of human polymorphonuclear leukocytes. They also increased the thymidine uptake of splenocytes of BALB/c nu/nu and C3H/HeN mice but not those of C3H/HeJ (a nonresponder to lipopolysaccharide). A dephosphorylated derivative, compound 403, was barely active in all of the above assays except for the enhancement of polymorphonuclear leukocyte migration. However, compounds 404 through 406 were feeble in pyrogenicity and could not prepare the local Shwartzman reaction, although they were very lethal to galactosamine-loaded mice. Therefore, synthetic lipid A analogs described here were fully immunopharmacologically active in in vitro assays, but all of them were far less active than natural E. coli F515 lipid A regarding the biological activities characteristic of endotoxic lipopolysaccharides and lipid A's. The high lethal toxicity of compound 406 (1,4'-bisphosphate) to the galactosamine-loaded mice may not reflect its real toxicity to normal mice. In all activities examined, compound 406 was quite comparable to a biosynthetic lipid A precursor, a natural counterpart of compound 406. The immunopharmacological activities of these newly synthesized lipid A analogs, especially compound 406, were much stronger than those of compounds that had been synthesized earlier by using the originally proposed model of the lipid A structure. The findings described in this report justify the acylation pattern of a disaccharide backbone of lipid A, revised on the basis of recent analytical studies. The low in vivo endotoxic activities of the present lipid A analogs are most probably due to the fact that the kinds of acyl groups were different from those of the complete lipid A from E. coli, although there were no differences in the acylation positions on the disaccharide backbone.