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

Abstract

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.