Synthesis of Neisseria meningitidis serogroup A carba analogues as hydrolytically stable antigens for antimeningococcal glycoconjugate vaccines

Synthesis of Neisseria meningitidis serogroup A carba analogues as hydrolytically stable antigens for antimeningococcal glycoconjugate vaccines Ludovic Auberger, Jacopo Enotarpi, Jeroen Codeé, Roberto Adamo, Laura Polito, Luigi Lay The Gram-negative encapsulated bacterium Neisseria meningitidis type A (MenA) is a major cause of meningitis in developing countries, especially in the sub-Saharan region of Africa [1]. The development and manufacture of an efficient glycoconjugate vaccine against MenA is largely hampered by the poor stability in water of the natural capsular polysaccharide (CPS)[2], composed of (1→6)-linked 2-acetamido-2-deoxy-α-D-mannopyranosyl phosphate repeating units, with acetyl substituents. As a consequence, most of MenA glycoconjugates currently available have been licensed as lyophilisates. The availability of MenA polysaccharide mimics resistant to hydrolysis, however, is highly attractive for the development of a more stable glycoconjugate vaccine in liquid formulation. To this end, we envisaged that the replacement of the endocyclic ring oxygen with a methylene group to get a carbocyclic analogue will lead to the loss of the acetalic character of the phosphodiester and consequently to the enhancement of its chemical stability [3], [4]. Furthermore, the 3-O-acetylation aims to strengthen the immunological profile of our neo-glycoconjugates, structurally designed even closer to the natural CPS MenA oligomer, partially acetylated at the position 3-OH with a rate of 75-95% [5]. Thus, we describe our synthetic approaches to 3-O-acetylated phosphodiester-linked carba oligomers of N-acetyl mannosamine (the repeating unit of MenA CPS, containing up to 8 repeating units. The increased stability of the synthetic carba oligomers was first confirmed by an accelerated stability study, then selected fragments were conjugated to CRM197 (a diphtheria toxin mutant) as a protein carrier. Finally, the immunological profile of the resulting neo-glycoconjugates will be carefully investigated, with the purpose to highlight the effect of the carbohydrate chain length and of the 3-O-acetylation on the immunoactivity [6]. [1] Tan L. K. K.; Carlone G. M.; Borrow R. Advances in the development of vaccines against Neisseria meningitidis. N. Engl. J. Med. 2010, 362, 1511-1520. [2] Frasch, C. E. Production and control of Neisseria meningitidis vaccines. Adv. Biotechnol. Processes 1990, 13, 123-145. [3] Gao, Q.; Zaccaria, C.; Tontini, M.; Poletti, L.; Costantino, P.; Lay, L. Synthesis and preliminary biological evaluation of carba analogues from Neisseria meningitidis A capsular polysaccharide. Org. Biomol. Chem. 2012, 10, 6673-6681. [4] Gao Q.; Tontini M.; Brogioni G.; Nilo A.; Filippini S.; Harfouche C.; Polito L.; Romano M. R.; Costantino P.; Berti F.; Adamo R.; Lay L. ACS Chem. Biol. 2013, 8, 2561-2567. [5] Berry DS.; Lynn F.; Lee C-H.; Frasch CE.; Bash MC. Effect of O-Acetylation of Neisseria Meningitis serogroup A capsular polysaccharide on development of functional immune responses, Infection and Immunity, 2002, 70(7), 3707-3713. [6] This project has received funding from the H2020-MSCA-ITN-2015 “Glycovax” under grant agreement No 675671.