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.