Drug Targeting Organ-Specific Strategies

296 11 Development of Proteinaceous Drug Targeting Constructs
Table 11.4. Advantages and limitations of various expression systems.
Expression system Advantages Disadvantages
E. coli Economical, fast, easy, high yield, Insolubility and misfolding of prowell
characterized genetics, teins, no glycosylation possible,
large number of cloning vectors difference in codon usage between
prokaryotes and eukaryotes
Fungi Economical, fast, easy, high yield, Glycosylation and other postwell
characterized genetics (yeast), translational modifications are often
Insect cells
glycosylation possible, able to
secrete correctly folded and
processed proteins
different to mammalian systems
baculovirus high yield, safe due to restricted Controlled culture conditions are
host range, able to perform most required, expression peaks when
of the post-translational modifications
carried out by mammalian
cells
cells are dying
stable transformants Stable Time consuming, relatively low yield
Mammalian cells Advanced post-translational
modifications, signals for synthesis,
Time consuming, relatively low yield
processing and excretion are
correctly recognized
Complex regulatory system
and mammalian systems can, at least in part, overcome these limitations, but these systems
are more expensive and difficult to manipulate due to complex regulatory systems. The
choice of appropriate expression organism depends on the individual protein and its applications.
In the past few years, the fundamental insights into the mechanisms of production,
stability and cellular locations of proteins have increased greatly.This knowledge will help researchers
working in the field of drug targeting to rationalize their choice of expression systems.
11.8 Recombinant DNA Constructs
11.8.1 Antibody-based Constructs
As stated above, a coding sequence for carrier, homing device and active drug can be designed
together in one fusion construct. However, even if this construct consisted of a small
carrier, a very short recognition sequence as the homing device and a small proteinaceous
drug substance, the final design would encode for a relatively large protein. Due to the size
of the construct, one could expect problems regarding the stable maintenance of the encoding
gene in a certain expression system, in addition to problems with respect to accurate synthesis,
export and folding of the recombinant protein. The smaller the total size of the recombinant
protein, and the smaller the changes made to the construct as compared to a natural
protein, the less likely it will be for these problems to occur.A significant reduction in re-