Biomedical Engineering Reference
In-Depth Information
than 1 million infected patients die each year from the associated complications of liver cirrhosis
and/or liver cancer. Prior to its approval, hepatitis B vaccines consisted of HBsAg purifi ed directly
from the blood of hepatitis B sufferers. When present in blood, HBsAg exists not in monomeric
form, but in characteristic polymeric structures that display a diameter of 22 µm. Production of
hepatitis B vaccine by direct extraction from blood suffered from two major disadvantages:
•
The supply of fi nished vaccine was restricted by the availability of infected human plasma.
•
The starting material will likely be contaminated by intact, viable hepatitis B viral particles
(and perhaps additional viruses, such as HIV). This necessitates introduction of stringent pu-
rifi cation procedures to ensure complete removal of any intact viral particles from the product
stream. A fi nal product QC test to confi rm this entails a 6-month safety test on chimpanzees.
The HBsAg gene has been cloned and expressed in a variety of expression systems, including
E. coli
,
S. cerevisiae
and a number of mammalian cell lines. The product used commercially is
produced in
S. cerevisiae
. The yeast cells are not only capable of expressing the gene, but also as-
sembling the resultant polypeptide product into particles quite similar to those found in the blood
of infected individuals. This product proved safe and effective when administered to both animals
and humans. An overview of its manufacturing process is presented in Figure 13.10.
Various other companies have also produced recombinant HBsAg-based vaccines. SmithKline
Beecham secured FDA approval for such a product (tradename Engerix-B) in 1989 (Figure 13.11 and
Box 13.4). Subsequently, SmithKline Beecham have also generated various combination vaccines in
which recombinant HBsAg is a component. Twinrix (tradename), for example, contains a mixture of
inactivated hepatitis A virus and recombinant HBsAg. Tritanrix, on the other hand, contains diphtheria
and tetanus toxoids (produced by traditional means), along with recombinant HBsAg. Dukoral is the
tradename given to an additional recombinant protein-containing vaccine now on the market. Indicated
for active immunization against disease caused by
Vibrio cholerae
(serogroup 01), the product contains
recombinant cholera toxin subunit B and four whole (heat- or formalin-inactivated)
V. cholerae
strains.
It seems likely that many such (recombinant) subunit vaccines will gain future regulatory ap-
proval. One such example is that of
Bordetella pertussis
subunit vaccine.
B. pertussis
is a Gram-
negative coccobacillus. It is transmitted by droplet infection and is the causative agent of the upper
respiratory tract infection commonly termed 'whooping cough'.
13.4.3 Peptide vaccines
An alternative approach to the production of subunit vaccines entails their direct chemical syn-
thesis. Peptides identical in sequence to short stretches of pathogen-derived polypeptide antigens
can be easily and economically synthesized. The feasibility of this approach was fi rst verifi ed in
the 1960s, when a hexapeptide purifi ed from the enzymatic digest of tobacco mosaic virus was
found to confer limited immunological protection against subsequent administration of the intact
virus. (The hexapeptide hapten was initially coupled to bovine serum albumin, used as a carrier
to ensure an immunological response.)
Similar synthetic vaccines have also been constructed that confer immunological protection
against bacterial toxins, including diphtheria and cholera toxins. Although coupling to a carrier is
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