There has also been significant investment in the provision of disposable bioreactor systems for vaccine manufacturing, as
they are particularly useful in multiproduct environments, where they reduce the requirement for cleaning-validation activities
(6). They can also be rapidly deployed in emergency situations. Although the use of disposable systems (stirred tank or rocking
platform) is gaining momentum for vaccines manufactured in mammalian cell culture, further development is necessary to provide
the mass transfer required for microbial cultures.
Vaccine purification, in particular purification of whole-cell vaccines or those comprised of complex macromolecules, has
traditionally been relatively inefficient and technically challenging. For example, production of viruses and virus-like particles
(VLPs) has commonly involved a primary purification step by density-gradient centrifugation, a labor-intensive process that
is not readily scalable. An alternative strategy is to apply column chromatography, which until recently has been confined
mainly for use as a polishing step during the production of viruses and VLPs. The binding capacity of conventional porous
chromatography resins for typically large molecules in recombinant vaccines during primary capture is relatively low because
of steric restriction to the active binding channels. This capacity can be increased when using membranes or monolithic columns,
as these have a wider pores and channels, and thus offer a more accessible active chemistry than the porous beads within packed
beds (7). Membranes and monoliths can also be operated at high flow rates, resulting in smaller columns and shorter cycle
times. These combined benefits are making chromatographic purification of certain vaccines an increasingly viable option.
Characterization and formulation.
For many complex vaccines, the true proof of principle can only be gained by clinical testing, which means that in preclinical
development, the product features that influence efficacy may be poorly defined. It is important, however, to elucidate the
physicochemical features of the vaccine as far as practicable. As vaccines are traditionally relatively complex, detailed
characterization has necessitated the development of novel analytical methods such as those based upon spectroscopy and mass
spectrometry (8). These developments, in turn, have provided opportunities to design and evaluate vaccine manufacturing processes
with greater consistency.
Designing formulations that enhance stability upon storage and maintain or optimize the subsequent immune response is of critical
importance during vaccine development. Vaccines, perhaps more than any other class of biopharmaceutical, require storage across
a wide range of temperatures, especially when administered in developing nations or in military environments. Likewise, the
complexity of vaccines often causes them to be relatively unstable in comparison with other pharmaceutical products that have
a low turn over and frequently require a long shelf life.
A more established way of achieving stability is through lyophilization. If it is not possible to maintain stability in a
liquid formulation, then removing the solvent via freeze- or spray-drying represents another way to minimize product degradation. For this reason, many vaccines are lyophilized
before storage and reconstituted in water for injection (WFI) immediately before administration. Vaccine delivery can be further
improved by using technologies that involve controlled product release from microspheres. Spray-drying technology is particularly
suited to vaccines since alum-based adjuvants (see below) are preserved during spray drying, and the resulting powder can
be combined stoichiometrically during the preparation of multi-valent vaccines.
Many vaccines are formulated with adjuvants, which help modulate and stimulate the immune response (9). These compounds bind
to the vaccine and aid retention at the site of injection or delivery to the lymph nodes. As a result, the release of the
antigens to the surrounding tissues is slowed, inducing a stronger immune response than would be generated by the vaccine
alone. The predominant adjuvants are "alum" based, containing a mixture of aluminum salts. However, there are novel adjuvants
under investigation that can provide benefits over alum such as the technology developed by Antigenics (Lexington, MA), QS-21,
an adjuvant derived from tree bark (10). The uptake of new adjuvants may be restricted, as they themselves will require testing
and regulatory approval, similar to the vaccine itself.