The delivery model challenge.
The stem-cell manufacturing issue is further confused by an unclear model for delivery of cellular therapies to the patient
and poorly defined large-scale manufacturing requirements. There are currently numerous competing cell types for many therapeutic
applications. Autologous therapies, in which the stem cell is sourced from the patient, manipulated, and then returned to
the patient, may require a type of production facility different from allogeneic therapies, in which a stem cell from a donor
is manipulated, expanded, and banked in order to treat a large population. The autologous therapy will require a facility
that can handle multiple individual samples without cross-contamination, potentially on a walk-in schedule, and is therefore
unlikely to be amenable to the conventional concept of batch manufacture. The allogeneic therapies will require bulk manufacture
and cell banking, potentially achievable in a single campaign. The location of the manufacture will influence the specification
of the manufacturing equipment. For example, the manufacture of some therapies may have to be partly or completely conducted
at small scale on the hospital premises (e.g., to remove cryopreservant, to allow quality control, or for some noncommercial
autologous applications). The scale of production is also confused by scarce availability of data from dosing studies. Stem-cell
doses for many therapies are not yet defined even to an order of magnitude. It is suggested that cell number requirements
will vary between 104 for very specific, small niche applications such as inner-ear hair cell regeneration, and up to 109 for cardiovascular therapies or 1012 for hematological therapies.
The scalable culture environment challenge. Different human stem cells require a plethora of different environments for proliferation and differentiation, potentially
including mechanical stimuli or flow conditions, variable gas tensions, chemical gradients, 3D frameworks, and supporting-cell
paracrine signaling. This raises an issue for the concept of a generic stem-cell manufacturing solution. These requirements
are far more varied between stem-cell types and demanding for given cell types, than the biotechnology industry has had to
negotiate to achieve scaled microbial or CHO growth. Although it is possible that a manufacturing solution could be programmable
to control some of these criteria, it is unlikely (as with scale of production) to be a one-size-fits-all solution. Interesting
questions remain surrounding the possibility of adapting cells to different, more large-scale amenable, culture conditions
(similar to CHO adaptation to nonadherent growth) while retaining safety and therapeutic efficacy. Recent attempts to do this
with human embryonic stem cell (hES) lines have raised controversy regarding the genetic stability of the cells in the new
culture systems (6); it is not yet clear how far cell culture adaptation can be taken.
The state of the manufacturing industry:
Conventional bioprocess versus complex advanced models
Despite these challenges and lack of clarity, the rise of an industry based on the delivery of a viable stem-cell product
has created an urgent requirement for scaled bioprocesses to produce stem cells. Addressing the challenges requires an interdisciplinary
approach pulling expertise from the conventional bioprocess, manufacturing engineering, and cell-biology communities.
There are three key approaches to the current production of stem cells. Production on tissue culture plastic is commonly carried
out in T-flasks or well plates and is the simplest system of growing stem cells in monolayers. It is the usual method employed
in small research laboratories, and due to the low cost and ease of use, many therapies have been developed using this technology.
However, the flasks suffer limited capacity and poor potential for on-line monitoring and control. Attempts to address the
scale issue have been made through the introduction of multilayered triple or hyper flasks and more space efficient systems
such as Nunc's (Rochester, NY) "Cell Factory."
Despite the control and monitoring limitations, these technologies seem to perform perfectly adequately for some stem-cell
types, and as a result several therapies have reached or are approaching the clinic using T-flask-based cell manufacture.
Beyond simple tissue culture plastic systems, there are numerous novel bioreactor approaches of varying degrees of complexity
and success. These are often custom-made multifiber or biomimetic material systems that aim to provide a highly controlled
niche environment for stem-cell expansion or differentiation.