Drug Targeting Organ-Specific Strategies

1.3.3 Cytoplasmic Delivery
The majority of drugs exert their action in the cytoplasm of the cell where their target enzymes
are located. Consequently, they need to pass from one of the compartments of the endocytotic
pathway into the cytoplasm. The endosomal and lysosomal membranes can be
destabilized using fusogenic peptides derived from viruses, cyclodextrins and polyethyleneimine
[39]. pH-sensitive liposomes or polymers become fusion competent at the acidic
pH of the endosomes and subsequently release their contents into the cytoplasm. Particularly
for the delivery of DNA into cells, this approach seems appropriate and quite successful
[40]. Bacterial components such as listeriolysin O and alpha-haemolysin can form pores in
phagosomal or plasma membranes [39]. It remains, however, to be established whether these
components can be exploited for directing drug-targeting preparations in vivo to specific cellular
compartments.
Schwarze et al. reported on the development of a recombinant fusion protein consisting of
the protein transduction domain of HIV-derived TAT and the 120-kDa β-galactosidase. The
TAT protein was able to deliver the large molecular weight protein to the interior of the cells
in vitro. Interestingly, the enzymatic activity of intracellularly delivered β-galactosidase
peaked about 2 h later than did the intracellular concentration. This likely reflects a slow
posttransduction refolding of the protein by intracellular chaperones. Intraperitoneal injection
of the fusion protein in mice resulted in delivery of the biologically active fusion protein
to all tissues, including the brain [41]. Similarly, the Herpes simplex virus tegument protein
VP22 is able to deliver proteins into the cytoplasm of cells [42]. Both approaches may prove
useful to enhance the delivery of e.g. enzymes for pro-drug protocols.
1.3.4 Nuclear Targeting
1.3 Intracellular Routing of Drug-Carrier Complex 9
The three major obstacles to DNA accessibility in the nucleus of the target cells are low uptake
across the plasma membrane, inadequate release of DNA with limited stability, and lack
of nuclear targeting. Delivery systems of the future need to fully accommodate all steps in the
internalization and targeting routing in order to effectively guide the DNA into the nucleus.
Due to space limitations, a complete overview of recent advances in this field will not be provided.
For more in-depth reading, the reader is referred to a recent concise review by Luo
and Saltzman on novel strategies to accomplish optimal gene delivery [43].
In short, increased targeting and uptake of DNA by the cell using better delivery systems
is the basis for overcoming the plasma membrane hurdle. Increased stability of the DNA
once inside the cell can be achieved by chloroquine or branched cationic polymers which facilitate
the early release of the DNA from the endosomal pathway. Furthermore, bacterial
subunits and adenoviral capsids are capable of bypassing the endosomes, although it should
be realized that these approaches may be significantly hampered by inherent toxicity and/or
immunogenicity. Stabilization of the DNA in the ‘hostile’ environment of the cytosol can be
achieved using PEG, PEG-poly-l-lysine block copolymers and others [43]. Intermediate stability
of DNA/delivery system interactions is most likely the prerequisite to achieve optimal
liberation of DNA molecules once they have become available within the cytoplasm. To