TRADITIONAL MANUFACTURING METHODS NO LONGER STACK UP
To understand how viral vaccines are manufactured today, we can refer to the pioneering work from the past two centuries,
when both cells and viruses were studied in glass Petri dishes (German biologist Julius Richard Petri, 19th century) and glass
Roux bottles (German biologist Wilhem Roux, 19th century).
When capacity was needed to produce larger quantities of viruses, the most obvious way to proceed was to multiply the number
of these devices. As time went on, Petri dishes became tissue culture flasks and then cell factories or cell stacks. The Roux
bottles then turned into roller bottles. Until recently, these were still made of glass; today they are made of plastics—mainly
polystyrene or polyethylene terephthalate.
The use of such devices is quite straightforward. As scaling-up is strictly linear, no process development is needed. Increased
capacity is obtained by a multiplication of the number of devices and a linear multiplication of footprint, operators, and
operations. For some time, this approach was quite successful and used to produce both human and animal vaccines.
For other vaccines, like the polio vaccine (human), or the foot-and-mouth disease vaccine (animal), the need was so great
that in the mid-1970s it appeared the multiplication of roller bottles was no longer a viable near-term option.
To meet the changing needs, large companies like GlaxoSmithKline, Merial, Intervet, Merck, and Sanofi invested in a different
approach, combining large-scale, stirred-tank bioreactors, already proven for bacterial and suspension cell growth, with
oligosaccharide-based microbeads to provide the adherent cells with a substrate to adhere to. This approach allowed a dramatic
decrease in both the footprint and the number of operations by increasing the ratio surface per volume. It was a very clever
and effective shift for the time.
However, a different set of ongoing challenges has emerged in the present time, mainly related to manufacturing scale. The
most crucial challenge has to do with speed: a sufficiently high mixing speed is required for proper mixing distribution,
but the mixing speed, however, must not be too fast to avoid negative effects on the process.
Microbeads must be in suspension to create a homogeneous distribution of surface into the volume of the bioreactor. In addition,
microbeads and cells need to physically meet for the attachment process to occur. The high mixing speed enables this contact,
but the microbeads are fragile (made of dextran) and have the mechanical properties of hydrogel. Moreover, the beads and cells
need to come together for a sufficient amount of time to attach, therefore, mixing speed must be optimized to enable this
Because the scale-up of mixing is nonlinear to a large extent, it is easy to predict the difficulty in finding a new process
that also offers control. In addition to this difficulty, the multiplication factor of cells on microcarriers lies typically
in the range of five- to 15-fold, which makes several "preculture" steps necessary before cells can be seeded into the final-scale
bioreactor. Typically, a 1000-L bioreactor will be seeded with cells from a 200-L bioreactor, which was seeded with cells
from a 25-L bioreactor to meet scale. In practice, it means that engineers have to develop a process at 1000 L, a process
at 200-L and a process at 25-L. For each volume, the cells need to be detached from the microcarriers in one bioreactor to
seed the next bioreactor, which presents a challenge. In fact, many companies decided to give up on realizing a new process
because of the lack of consistency and reliability in the outcome.