Antibacterial and antifungal vaccines based on synthetic oligosaccharides - Carbohydrate Chemistry: Chemical and Biological Approaches - page 568

Scheme 1
were also conjugated to synthetic peptides containing potent T-helper
cell determinants and B-cell epitopes. 41 The conjugate provided T-cell
help and the carbohydrate hapten became T-cell dependent. In an infant
rat model, the antibodies raised were protective against Hib infection. A
trimer containing three repeating units (RU) was established as a min-
imal requirement for protection in laboratory animals.
However, a degree of uncertainty concerning the minimal protective
epitope in human and particularly in infants was recognized. 42
The project poses also some challenges for synthetic chemistry. The
critical problems were already outlined in van Boom group's first
synthesis of PRP-fragments. 43 Synthesis using solid phase chemistry 44,45
or polymer-bound solution techniques 46 provided a solution to chain
elongation, one of such critical problems. None of the above mentioned
processes was deemed acceptable for vaccine production, however,
especially when the need for a low final vaccine cost was kept in mind.
For the use of synthetic PRP as a source of antigen in vaccine production,
the general synthetic pathway needed be simplified by a) a simpler route
to a key disaccharide repeating unit, b) a high yielding elongation
process, c) a simple protective-group strategy allowing simple and clean
deprotection, d) a high yield conjugation process.
The introduction in clinical practice of Hib conjugate vaccines in the
nineties further changed the situation. The demand for a new Hib con-
jugate vaccine increased, but that demand was mainly concentrated
in developing countries with no rewards for large multinational com-
panies. On the other hand, the accumulated knowledge simplified clin-
ical trials.
There was an opportunity for the use of a synthetic oligosaccharide
providing a low-cost process was targeted! Our improved synthesis 47
includes a simple synthesis of the disaccharide intermediate 3 from 2 by
selective benzylation, which was followed by a one-pot oligomerization of 4
and 5 using H-phosphonate chemistry (Scheme 2). The process could be
controlled to obtain fragments of different sizes. In the light of the un-
certainty regarding the minimal size of the epitope needed for protection
in infants, 41,42 we decided to select an average size of 8RU in order to avoid
complex studies defining the minimal size in clinical trials with infants.
Another important improvement during the synthesis was the presence of
only benzyl protecting groups. The deprotection of 6 afforded pure syn-
thetic oligosaccharide. Finally, synthetic PRP fragments were armed with a
terminal maleimido group 7 for smooth conjugation to thiolated TT. After
all these improvements, the prototype vaccine 8 was obtained in high yield,