Design, synthesis and immunological evaluation of novel monosaccharide TLR4 agonists

Monophosphoryl lipid A (MPLA), a Toll-like Receptor 4 (TLR4) agonist, has been utilized as vaccine Adjuvant in several approved human vaccines. However, the complex disaccharide structure of MPLA presents significant challenges for chemical synthesis. To address this limitation, we designed a series of glucosamine-based alkyl 4-phosphate (GAP) compounds. These are monosaccharide lipid A analogues, synthesized by replacing the reducing glucosamine disaccharide of MPLA with an N-diaminoalkyl glycoside unit. Three novel TLR4 agonists, GAP210, GAP212 and GAP214, were successfully synthesized by modulating the length of the secondary acyl chains at the C-2 and C-3 positions of the non-reducing sugar, and were subsequently evaluated as vaccine adjuvants. The results revealed that the length of the secondary acyl chains significantly influenced immune activation. Notably, GAP214-adjuvanted group exhibited 2.5-fold higher IgG level than the positive TLR4 Agonist control after three immunizations. These findings identify the GAP analogues as effective TLR4 agonists and promising candidates for vaccine Adjuvant development.

Immunostimulant; Lipid A analogues; MPLA; TLR4 agonist; Vaccine adjuvant.