Adding value during vaccine development
As for all biopharmaceuticals, there are key value-adding stages during vaccine development. However, as vaccines are probably
the most disparate of all biological medicines, both in terms of biochemical composition and the methods of production, there
are currently only a limited number of platform manufacturing methods applicable to distinct vaccines. Likewise, there is
not a single expression system suitable for numerous vaccine candidates, as they range from relatively simple polypeptides
to entire cells.
The criteria when choosing an expression system are the same as for any other biopharmaceutical—it is essential that the system
produce a product of consistent safety and quality. It is also necessary to ensure that the yield of the production method
is fully-optimized, as cost-of-goods for vaccines are critically important, particularly for those medicines intended for
supply to developing nations.
The adoption of novel expression systems has to be carefully considered. Although there are benefits in applying innovation
to shorten development times and be first to market, the vaccine industry is traditionally relatively conservative. This is
because the medicines are typically administered to healthy patients, and there is a resulting strong emphasis on product
safety. In addition, the complex nature of many vaccines minimizes the available options for risk-reduction through extensive
As vaccines are typically administered in fewer and smaller doses than are therapeutic drugs, market supply may consist of
grams rather than kilograms of material per annum, diminishing the net requirement for large-scale production. The drive for
large-scale production involving high-titer expression systems has, therefore, been less for vaccine manufacture than for
other biological molecules such as monoclonal antibodies. With all of these considerations, there has been reluctance in the
vaccine industry to adopt novel expression systems, but rather a preference for working with well established systems of a
known and accepted safety profile.
There are examples of economic and regulatory influences, however, that are encouraging the adoption of new expression technologies
for vaccine development. A high profile example is the move from eggs to cell culture in the production of the influenza vaccine
(see Table 1). There is likely to be further adoption of new expression systems, especially those with associated benefits
for reducing overall development times.
Table 1: Drivers to move from eggs to cell culture when manufacturing influenza vaccine.
There are very few expression systems that are specifically targeted toward vaccines, although there are technologies used
in protein expression that are particularly adaptable to vaccine manufacturing. These include the PER.C6 cell line, and the
associated AdVac/Virosome technology, available from Crucell/DSM (Leiden, The Netherlands), and the avian-derived cell lines
from Vivalis (EBx, Nantes, France) and ProBioGen (Berlin, Germany). Another expression technology with potential benefits
is the Pfēnex Expression Technology from Dow (Midland, MI), which has been applied to generate high levels of vaccine antigens.
Insect cells also represent an alternative system for vaccine production, with examples including production of antigen for
the Provenge cellular vaccine (Dendreon, Seattle, WA) and Ceravix (GSK). Protein Sciences Corporation (Meriden, CT). has developed
a patented baculovirus protein expression system (BEVS) for production of proteins and vaccines in variant Sf9 cells.
Rational drug design.
There are few examples of engineered vaccines, probably because the technology is relatively immature and the approaches
of protein engineering are not easily applied to traditionally complex vaccines. There is, however, potential application
of protein engineering as subunit recombinant vaccines are further developed (4). One of the main objectives in the rational
design of conventional biotherapeutics is to minimize immunogenicity, either through humanization, PEGylation, or by reducing
protein aggregation. The opposite is true for vaccines, however, where protein design may be applied to increase the antigenicity
of the molecule.
Process development and advances in vaccine production.
As described, vaccines are complex and diverse biomolecules ranging from recombinant subunit antigens to live organisms.
Correspondingly, therefore, a range of production technologies are required to manufacture sufficient quantities of these
products, including the use of eggs, cell factories, roller bottles, shake/tissue culture flasks, and bioreactors. This range
of production methods can be problematic, as each method requires specific capital expenditure, specialized development approaches,
and operator expertise. As a result, there is a growing trend to lower the number of production platforms by implementing
manufacture in suspension culture bioreactors wherever possible. For vaccines manufactured in mammalian-cell culture, it is
necessary to establish well-characterized expression permissive cell lines that have been adapted to grow in suspension culture,
and (where appropriate) in a serum-free medium. An example of this is the emergence of suspension HEK293 cultures for the
production of adenovirus viral vectors, where the development of chemically defined and serum-free formulations has been influential
in increasing titers and generating fully scalable processes (5). Similarly, microbial systems are being optimized for the
manufacture of vaccines, especially those in which co-expression of two or more proteins is beneficial to provide multivalency.