Chapter 4
Haemophilus influenzae Type b Conjugate Vaccine with a Synthetic Capsular Polysaccharide Antigen: Chemical View Downloaded by GEORGETOWN UNIV on September 7, 2015 | http://pubs.acs.org Publication Date: July 2, 2008 | doi: 10.1021/bk-2008-0989.ch004
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Vicente Verez Bencomo , René Roy , Maria C. Rodriguez , Annete Villar , Violeta Fernandez-Santana , Ernesto Garcia , Yury Valdes , Lazaro Heynngnezz , Ivan Sosa , and Ernesto Medina 1
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Center for the Study of Synthetic Antigens, Faculty of Chemistry, University of Havana, Ciudad Habana, Cuba 10400 Department of Chemistry, Université du Québec à Montréal, P.O. Box 8888, Succ. Centre-Ville, Montréal, Québec H3C 3P8, Canada Center for Genetic Engineering and Biotechnology, Apdo 6162, Cubanacan, Playa, Ciudad Habana, Cuba 10600
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Glycoconjugate vaccines containing capsular polysaccharides were developed in the last 20 years against Haemophilus influenzae type b. They provided effective prophylaxis against meningitis and pneumonia, especially amongst infants. Despite recent advancements in synthetic carbohydrate chemistry, the technology for using a synthetic antigen in a commercial vaccine remains a formidable challenge. Major chemical problems to be solved and the strategies employed to produce the first vaccine containing a synthetic carbohydrate antigen that becomes commercially available are described.
© 2008 American Chemical Society
In Carbohydrate-Based Vaccines; Roy, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
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Introduction Modern vaccines are increasingly composed of well-defined molecules isolated by costlyfractionation/purificationprocedures from living organisms obtained by either growing the relevant pathogens or after cloning a nonpathogenic organism that over-expresses the relevant molecules (7). Carbohydrates constitute important components of several relevant antigens. In bacteria, capsular or lipopolysaccharides represent the major antigens, while in parasites, virus, and cancer cells, glycoproteins or glycolipids comprise the other key antigens. Carbohydrate-based vaccines have long been the mainstream of vaccine development. In the past and at least in two different occasions, the advent of polysaccharide-based vaccines represented relevant contributions in the fight against group A and C meningococcal meningitis. In the seventies, important epidemics were controlled due to this achievement (2). The use of capsular polysaccharides for combating other infectious diseases, especially in infants, were not as successful (5). A typical example is found in infections caused by Haemophilus influenzae type b (Hib), one of the main causes of meningitis and pneumonia in infants worldwide. Vaccinology made another tremendous step forward with the introduction of conjugate vaccines. The conjugation of polysaccharides to proteins confers the ability to induce protective antibody responses in infants otherwise unresponsive to capsular polysaccharide alone (4). Application of this technology to Haemophilus influenzae type b essentially eradicated this disease in industrialized countries wherein vaccination programs were inserted. However, several circumstances that include the high cost of conjugate vaccines complicated the global coverage of vaccination in other countries. As a consequence, poor countries still experience more than 600 000 infant's death every year from Hib-pneumoniae or meningitis because of the lack of more available vaccines (5). The initial success of conjugate vaccines was an important stimulus for attempting the use of synthetic Hib-oligosaccharides in place of the natural polysaccharide. The chemical synthesis of antigens has many advantages over other modern approaches for the manufacturing of vaccines. It avoids the use of pathogenic cells and therefore the products are free of infectious materials or pathogen-derived molecules. They can be economically produced in large scale. The final products are usually better molecularly defined pharmaceuticals. However, despite these facts, none of the major human vaccines are actually made from synthetic carbohydrate antigens (6). Synthetic chemistry is usually less efficient than bacteria in producing the antigens needed in the manufacturing of vaccines. Thus, the development of suitable technologies for synthesizing the carbohydrate antigens represents a formidable challenge.
In Carbohydrate-Based Vaccines; Roy, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
73 Anti-Hib conjugate vaccines
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Actual anti-Hib vaccines are composed of either the capsular polysaccharide or its oligosaccharide fragments isolated from bacterial growth. Hence, the molecular weights of effective commercial vaccines cover a wide range of sizes, rangingfromthe intact capsular polysaccharide to relatively small oligosaccharide fragments (7) containing 5-15 ribosyl-ribitol-phosphate re peating units. Despite the enormous differences in composition, the mechanism of action is quite similar and therefore no major differences were found in clinical trials that could be associated to the size of the polysaccharide (8).
η Figure 1. Structure of the repeating unit of the poly-(ribosyl-ribitol-phosphate) (PRP), the capsular polysaccharide of Haemophilus influenzae type b.
Studies conducted in laboratory animals have shown that smaller fragments containing 3-4 repeating units were sufficient for the induction of an efficacious anti-Hib immune response (9). Consequently, the initial synthetic strategies targeted antigens from 3 to 10 repeating units long.
Classical synthesis of PRP-fragments As can be seen from the structure of the capsular polysaccharide (Figure 1), ribosyl-ribitol repeating units are connected by phosphodiester linkages. The retrosynthetic strategy for such structure is represented in Figure 2. The ideal combination of permanent and temporary protecting groups included benzyl and allyl ethers, respectively. The use of acetyl or other acyl protecting groups are not recommended because the required basic conditions employed for their removal could promote migrations of the phosphodiester linkages in the final product.
In Carbohydrate-Based Vaccines; Roy, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
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