Lipopolysaccharides (LPS) of gram-negative bacteria are recognized by the innate immune system, this is due to activation of Toll-like receptor 4 (TLR4). More specifically the Lipid A moiety of LPS activates TLR4, which occurs through dimerization with lymphocyte antigen 96 (MD-2) and leads to a release of various cytokines. Synthetic (derivatives of) Lipid A show promise as an immune-modulator for treatment of sepsis but also as adjuvant for vaccine-therapy. Elucidation of the crystal structure of the heterodimer of TLR4 with MD-2 with Escherichia coli (E. coli) Lipid A has shown that the acyl-group on the amine of the reducing end is important for dimerization [1,2]. Therefore, we were compelled to investigate modulation of this dimerization (and thereby the innate immune system) through this acyl chain.
The presented work describes the synthesis of E. coli Lipid A derivatives with shorter acyl chains on the C-2 amine, as well as their monophosphoryl Lipid A (MPL) variants which are known to induce a tempered immune response (Figure 1). The devised synthesis is highly convergent with protecting groups chosen that are deprotected under similar conditions to limit the number of deprotection steps. The synthesis employed features preinstalled acyl chains on the C-3 and C-3’ as well as the preinstalled phosphate on C-4’ in the monosaccharide stage. Both amines were orthogonally protected, the C-2’ position was protected as a 2,2,2-trichloroethoxycarbamate (Troc) because the improved neighbouring group participation (NGP) properties in comparison with the acylated amine. The selective deprotection of the C-2 amine allows for derivatization to occur at one of the final stages and a single and therefore more robust synthesis route. The option to make monophosphoryl lipid A is also available by protection of the anomeric centre as a silyl ether which can be transformed into a phosphate.
- Park, B. S.; Song, D. H.; Kim, H. M.; Choi, B.-S.; Lee, H.; Lee, J.-O. Nature 2009, 458, 1191-1195.
- Artner, D.; Oblak, A.; Ittig, S.; Garate, J. A.; Horvat, S.; Arrieumerlou, C.; Hofinger, A.; Oostenbrink, C.; Jerala, R.; Kosma, P.; Zamyatina, A. ACS Chem. Biol. 2013, 8, 2423-2432.