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Avian influenza virus: the threat of a pandemic highlights the urgency for new vaccine production methods
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Caterpillar cells may not be the most obvious choice of incubator for
a new influenza vaccine. But it is one of the many novel approaches on
trial in Europe and the US aimed at avoiding the need for the millions
of hens’ eggs currently required to satisfy global demand for flu
vaccine. And, with the threat of a flu pandemic making daily headlines,
there is no doubting the urgency of the research.
GlaxoSmithKline, Sanofi Pasteur, Solvay and Chiron are among the more
familiar vaccine manufacturers with well advanced alternatives to flu
vaccines from eggs. But the need for a radical rethink of flu vaccine
production is encouraging smaller firms such as Dutch firm, Nobilon,
and Britain’s PowderMed to line up alongside the bigger players.
For decades, flu viruses have been grown in fertilised hens’ eggs,
and the infected allantoic fluid harvested. The extracted virus is purified,
inactivated and treated to produce a whole virus, split or subunit vaccine.1
The choice of virus is determined by the World Health Organization, according
to the prevailing influenza infection that is circulating when companies
go into production early each year. Flu vaccines in current use are trivalent,
containing recent variants of the haemagglutinin (H) and neuraminidase
(N) antigens on the surface of the type A virus, and the type B virus.
As the production process takes about six months, companies using egg-based
methods are limited in how quickly they can respond to the need for a
different vaccine if an unexpected strain of flu virus hits the UK.
Alan Hay, director of the World Influenza Centre at the National Institute
for Medical Research, London, explains that, even before the current
concerns about a flu pandemic, increasing difficulty in getting recent
strains of flu virus to grow in hens’ eggs has encouraged the search
for alternatives.
“There is a lot of momentum behind cell culture and it looks as
though vaccine production will go in that direction, but you still need
to grow
the virus in approved cell lines, and that isn’t always easy,” he
says.
As the name suggests, cell culture technology involves the injection
of flu virus into mammalian (usually dog kidney) or insect cells, which
are encouraged to multiply by optimal growing media in larger and larger
fermenters. The infected cells are then harvested and flu virus extracted,
purified and inactivated. Cell culture technology has the potential for
much greater flexibility than egg systems, with cells kept frozen in
banks, so that companies can respond quickly to requirements for vaccines
against new flu viruses. It has been estimated that cell culture technology
may cut six to eight weeks off the lead time for flu vaccine production
from eggs.
However, it is not all plain sailing. Even when a flu virus is growing
nicely in mammalian or insect cells in the laboratory, scaling up production
takes time.
No flu vaccine made from cell culture techniques is currently being marketed,
although Solvay Pharmaceuticals was granted a licence to sell its tissue
culture flu vaccine, Influvac TC, in the Netherlands in 2001. But the
complexity of the production process meant that building of the cell
culture plant did not start until 2003, with validation procedures currently
being completed, and plans to start production in the first half of 2006.
Clinical trials of an H5N1 vaccine are also scheduled for 2006, ready,
if needed, for the 2006–07 influenza season.
Dutch human vaccine company, Nobilon, is using mammalian cell culture
technology originally developed by parent firm, Akzo Nobel, for animal
vaccines, for a new human flu vaccine which is also expected to go on
trial next year.
“Cell culture enables you to work in a smaller, more contained
way than with hens’ eggs, and it is easier to get consistent efficacy,
quality and purity. You just need to get an optimal growth rate for the
virus
that isn’t too fast or too slow,” says Nobilon director,
Bram Van Dijck.
Having recently received European Good Manufacturing Practice (GMP) status
for its human vaccines, Nobilon is planning trials of an H5N1 vaccine
and two other vaccines with WHO-determined viral strains, all for next
year. One of these will use a live attenuated virus for administration
by nasal spray instead of injection.
Mid 2006 should also see the start of the first significant clinical
trials of a novel DNA flu vaccine being developed by Oxford-based immunotherapy
company, PowderMed. Although it still has a lot to prove, a DNA-based
vaccine has huge potential advantages over egg and possibly even cell
culture methods, in the event of a pandemic.
Clive Dix, PowderMed’s chief executive officer, predicts that the
company will be able to supply the 120 million doses likely to be needed
to immunise the UK population against a flu pandemic in just three months,
once a new factory is built in four years’ time.
The principle of a DNA vaccine is relatively straightforward. Viral DNA
is inserted into a plasmid, purified and injected. The viral DNA is transcribed
in the host cell and the expressed viral protein triggers an immune response.
But, as Dr Dix explains, early clinical trials of DNA vaccines proved
disappointing. While DNA vaccines triggered immune responses when injected
into the thin muscles of small animals, they did not trigger sufficient
response when injected into the much bigger muscles of adult human volunteers.
Where PowderMed believes it has the edge is in using the PowderJect delivery
device to put its haemagglutinin DNA vaccine not into muscle cells but
into the antigen-presenting dendritic cells of the epidermis that can
facilitate a T-lymphocyte response.2
“The device uses gold particles to carry the DNA into the dendritic
cells where the viral protein is then made. These dendritic cells are
then
carried to the lymph nodes where the viral protein is detected as foreign
and triggers an immune response,” explains Dr Dix.
The technology has been tested with DNA vaccines against hepatitis B
and herpes, and a preliminary phase I study in 12 healthy adults volunteers
showed rising levels of seroprotection against viral haemagglutinin,
H3, with increasing doses of vaccines, resulting in 100 per cent protection,
56 days after immunisation with the highest dose.
PowderMed is currently carrying out further preclinical toxicology studies
before starting more extensive clinical studies by the middle of 2006.
Although its studies to date have focused on H3, the same DNA and delivery
technology could be used for an H5 flu vaccine.
Possibly the most radical vaccine currently on the starting block ignores
both the H and N proteins of conventional flu vaccines, and targets the
ion channel protein, M2, of influenza A. In August, Cambridge vaccine
company, Acambis, announced it is to collaborate with the Flanders Interuniversity
Institute for Biotechnology to generate what is being called a “universal” flu
vaccine that would not be subject to the genetic shift and drift of traditional
HN-containing vaccines.
A major component of the new vaccine will be the M2e extracellular domain
of the M2 protein, which is known to be conserved across all influenza
A strains. Recent research reported possible methods of optimising the
effectiveness of M2e in a protective vaccine.3
Dr Hay agrees that something with the potential for such broad spectrum
protection against flu viruses would indeed be the “Holy Grail” that
scientists have been searching for. But he stresses that, even if M2e
does live up to expectations, a commercial vaccine is probably some way
off.
If there really is a flu pandemic just a few months away, we are likely
to be relying on the limited number of doses of H5N1 vaccine that companies
have recently scrambled together by more traditional methods — fertilised
eggs and possibly cell culture — and Roche’s much vaunted
influenza treatment, Tamiflu.
References
1. UK Health Departments. Pandemic Flu. UK
Influenza Pandemic Contingency Plan, October 2005.
2. Dean HJ, Haynes J, Schmaljohn C. The role of particle-mediated DNA
vaccines in biodefense preparedness. Advanced Drug Delivery Reviews 2005;57:1315–42.
3. De Filette M, Min Jou W, Birkett A, Lyons K, Schultz B, Tonkyro A
et al. Universal influenza A vaccine: optimization of M2-based constructs.
Virology 2005;337:149–61. |
The
Pharmaceutical Journal