Antitumor Agents: Taxol and Taxanes – Production by Yew Cell Culture
435
6.7
Culture System
Various culture systems have been tested
for taxane production by cell cultures, both
with free and immobilized cultures (glass
Fber, alginate, etc.). Use of 50% condi-
tioned medium instead of full replacement
of medium has proven useful for the
growth of
T. chinensis
and production of
taxuyunnanine C. Two-phase culture sys-
tems involve the presence of a nonpolar
phase to capture by partitioning the prod-
uct of interest, thereby alleviating feedback
inhibition on biosynthetic enzymes of cells
in the aqueous phase. Product puriFcation
is also facilitated in this system. Accumula-
tion sites for compounds of interest of the
solid resin type (e.g. Amberlite XAD series)
have promoted Taxol
yield in
T. cuspidata
by 40 to 70%.
Organic solvents used in two-phase cul-
ture systems have also been used for taxane
production in cell cultures; application of
10 to 11% (v/v) dibutylphthalate as the
in situ
solvent extraction has improved
Taxol
with minimal deleterious effects
on cell growth and integrity.
Two-stage cultivation, involving a growth
and
a
production
medium,
has
been
largely used for taxane production in cell
cultures. The most frequent strategy in-
volves addition of precursors and elicitors
or the exposure to abiotic factors (such
as temperature shifts) after a certain cell
biomass has been attained.
Semicontinuous
and
perfusion
cul-
tures, which involve medium replacement
throughout the growth cycle, have been
used for Taxol
production studies. In
T. canadensis
, a fourfold increase in accu-
mulation was observed in semicontinuous
cultivation with total cell recycle versus
batch culture. Alginate immobilized cul-
tures and free cells with a nylon-mesh
separator of
T. cuspidata
were cultivated
in continuous medium perfusion mode
a
n
dr
e
s
u
l
t
e
di
na
no
r
d
e
ro
fm
a
g
n
i
-
tude higher Taxol
speciFc production
(0.3 mg gdw
1
day
1
for 40 days) relative
to batch cultivation; however, some degree
of cell growth inhibition owing to a dilution
effect was observed.
7
Scale-up of Taxus Cell Cultures
The scale-up of plant cell cultures remains
a challenge in spite of considerable efforts
to bring this technology to commercially
viable use. In general, as culture systems
move from flask to bioreactor and further
up to large-scale cultivation, productivity
and growth drop considerably. Problems
during scale up include shear stress, con-
trol of gas composition, lack of adequate
mixing of medium and cells, and oxygen
supply. Yew cell cultures have been cul-
tivated in bioreactors, in a large scale, to
a signiFcant degree of success compared
to other species capable of accumulating
industrially interesting chemicals.
The general strategy used for scale up
cultivation of yew cell cultures has been
tocomb
inesomeo
ftheabove
-desc
r
ibed
techniques and adapt their application
to the reactor setting to improve yields.
Some
examples
of
bioreactor
cultures
of yew species are depicted in Table 5.
Taxol
from cell culture–based large-scale
production has been commercialized since
2001 by Samyang Genex (Taejon, Korea).
8
Future Perspectives
Data to date shows that cultured cells
of
Taxus
spp. represent a viable source
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