Monophosphoryl lipid A analogues with varying 3-O-substitution: synthesis and potent adjuvant activity Original Research Article

Structurally defined immunostimulatory adjuvants play important roles in the development of new generation vaccines. Here described are the syntheses of three monophosphoryl lipid A analogues (1–3) with different substitution at 3-O-position of the reducing sugar and their potent immunostimulatory adjuvant activity. The syntheses involve the preparation of glycosylation acceptors benzyl 3,4-di-O-benzyl-2-deoxy-2-[(R)-3-tetradecanoyloxytetradecanamido]-β-d-glucopyranoside (16) and benzyl 3-O-allyl-4-O-benzyl-2-deoxy-2-[(R)-3-tetradecanoyloxytetradecanamido]-β-d-glucopyranoside (17). The glycosylation reactions between the donor 4,6-di-O-benzylidene-2-deoxy-2-(2′,2′,2′-trichloroethoxycarbonylamino)-α-d-glucopyranosyl trichloroacetimidate (21) and acceptors 16 and 17 provide the desired β-(1→6)-linked disaccharides 22 and 23, respectively. Selective reductive ring opening of the 4,6-di-O-benzylidene group, installation of a phosphate group to the 4′-hydroxyl group, and the final global debenzylation produce the designed monophosphoryl lipid A analogues 1–3. All three synthetic analogues induce antigen specific T-cell proliferation and interferon-gamma (IFN-γ) production in ex vivo experiments with a totally synthetic liposomal vaccine system. The immunostimulatory potency of compound 1–3 is in the same order of magnitude as that of the detoxified natural lipid A product isolated from Salmonella minnesota R595 (R595 lipid A). The substituent at the 3-O-position of the reducing sugar does not have much effect on the adjuvant activity of monophosphoryl lipid A analogues. The preliminary lethal toxicity study indicates that the 3-O-acylated hepta-acyl monophosphoryl lipid A may not be more toxic than its 3-O-deacylated hexa-acyl analogue.