Biomedical Engineering Reference
In-Depth Information
of the pDNA-bound polymers via cleavable linkers. For example, pseudoden-
drimers containing low molecular weight oligoethylenimine and biodegradable
diacrylate esters [ 218 , 219 ] can deliver pDNA with high efficiency and low
cytotoxicity in vitro and in vivo upon systemic delivery in a murine tumor model.
Gene expression (luciferase or the therapeutically more relevant sodium iodide
symporter NIS) was primarily detected at the subcutaneous tumor site [ 218 - 220 ].
Stability of these polyplexes however was limited. Lateral stabilization by
crosslinking surface amines via a bifunctional crosslinker resulted in improved
stability. Best gene transfer results were obtained when a bioreducible disulfide-
containing crosslinker was used for caging [ 109 ].
Electrostatic binding of siRNA or oligonucleotides (ONs) to cationic polymers
is weaker than pDNA binding, due to the lower number of negative charges in
the short phosphate backbones. To overcome this hurdle, ONs and siRNAs can be
covalently bound to their carriers [ 53 ]. This can also be performed in a biores-
ponsive reversible way. For example, disulfide bonds, which can be easily cleaved
in the cytosol due to the reducing environment inside the cell, were applied for ON
conjugation with a pH-specific membrane-disruptive carrier [ 54 ] for the synthesis
of dynamic siRNA polyconjugates [ 56 ], and for siRNA conjugated to a PLL that
was modified with an endosomolytic peptide [ 57 ]. The covalent polyplexes are not
cleavable by heparin in concentrations at which analogous electrostatic complexes
are dissociated. Only combinations of heparin with a reducing reagent were able to
release siRNA from the novel conjugates [ 57 ].
4 Therapeutic Strategies
Since the design of the first targeted polyplexes more than 20 years ago [ 97 , 134 ],
numerous efforts have been made to develop polyplexes for use in medical
products, both in pharmacological animal studies and in human studies. Therapeu-
tic modalities include ex vivo treatment of isolated human patient cells, localized
in vivo treatments, and - currently the most challenging delivery scenario - in vivo
targeted intravenous delivery.
4.1 Ex Vivo and Localized Therapies
First clinical human gene therapy trials with polyplexes were performed using
cancer vaccines based on autologous patient tumor cells. These were modified
ex vivo with interleukin-2 pDNA. To obtain high level transfection rates of
patient's primary tumor cells, Tf-PLL/pDNA polyplexes linked with inactivated
endosomolytic adenovirus particles were applied [ 221 ]. Polymer-based in vivo
human gene transfer studies were performed with PEGylated PLL polyplexes,
delivering CFTR pDNA to the airway epithelium of cystic fibrosis patients [ 222 ],
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