chemical structure. Sugar-based polymers (chitosan, dextran, etc.) are a great
example of biocompatible materials. Increasing the biodegradability is of course
an alternative way to limit toxicity (for instance, by incorporation of acid
labile groups such as β-amino esters and ortho esters) because the byproducts of
degradation can be eliminated by the body via natural pathways.
It is important to keep following criteria in mind for efficient polymer-mediated
gene delivery: efficient compaction of genetic material (size <200 nm); stability
of the polyplexes under physiological conditions (i.e., presence of salts, pH 7.4)
because particles that precipitate under these conditions are not suitable for
in vivo applications; high uptake by cells and intracellular release; and efficient
transfection without inducing cytotoxicity. Moreover, biodegradability and
targeting of the polyplexes are important properties for in vivo applications.
2 Polycation/DNA Complexes
2.1 Water-Soluble Polycations
2.1.1 Strong Polyelectrolytes
Strong polyelectrolytes are salts of quaternary ammonium cations (alkyl),
pyridinium, or imidazolium with an anion. Charge neutralization is usually
achieved at or close to a 1:1 stoichiometry for strong polycations and DNA.
Containing Aromatics or Having a Charged Backbone
In the pioneering work of Izumrudov and Zhiryakova, the shorter the PEVP (Fig. 3a)
the less resistant was the polyplex to the addition of salts; this effect was much more
pronounced for chain lengths between 10 and 100 than above [122]. Interestingly,
the stability of the PEC was virtually independent of the length of the nucleic acid in
the studied region (500 bp, DNA from salmon testes; 10,000 bp, calf thymus DNA).
The longer the substituent (methyl, ethyl, and propyl), the longer was the distance
between charges of DNA and quaternized PVP because of the shielding, and the less
was the complexation efficiency. Also, a decrease in charge density (PEVP-β,
quaternization degree β ¼ 23 or 46%) led to a decrease in PEC stability and a
decrease in the critical salt concentration (salt concentration at which half of the
EtBr molecules are intercalated in DNA-free sites).
Given that poly(1-vinylimidazole) (PVIm) has a pK a of around 5.5, this polymer
does not complex DNA at physiological pH. Thus, Allen et al. quaternized
the imidazole ring with various substituents such as bromoethanol in order to
obtain permanently charged imidazolium-containing copolymers [107]. As the
quaternization degree was increased, fewer sites were available for protonation,
132
A. Bertin
example of biocompatible materials. Increasing the biodegradability is of course
an alternative way to limit toxicity (for instance, by incorporation of acid
labile groups such as β-amino esters and ortho esters) because the byproducts of
degradation can be eliminated by the body via natural pathways.
It is important to keep following criteria in mind for efficient polymer-mediated
gene delivery: efficient compaction of genetic material (size <200 nm); stability
of the polyplexes under physiological conditions (i.e., presence of salts, pH 7.4)
because particles that precipitate under these conditions are not suitable for
in vivo applications; high uptake by cells and intracellular release; and efficient
transfection without inducing cytotoxicity. Moreover, biodegradability and
targeting of the polyplexes are important properties for in vivo applications.
2 Polycation/DNA Complexes
2.1 Water-Soluble Polycations
2.1.1 Strong Polyelectrolytes
Strong polyelectrolytes are salts of quaternary ammonium cations (alkyl),
pyridinium, or imidazolium with an anion. Charge neutralization is usually
achieved at or close to a 1:1 stoichiometry for strong polycations and DNA.
Containing Aromatics or Having a Charged Backbone
In the pioneering work of Izumrudov and Zhiryakova, the shorter the PEVP (Fig. 3a)
the less resistant was the polyplex to the addition of salts; this effect was much more
pronounced for chain lengths between 10 and 100 than above [122]. Interestingly,
the stability of the PEC was virtually independent of the length of the nucleic acid in
the studied region (500 bp, DNA from salmon testes; 10,000 bp, calf thymus DNA).
The longer the substituent (methyl, ethyl, and propyl), the longer was the distance
between charges of DNA and quaternized PVP because of the shielding, and the less
was the complexation efficiency. Also, a decrease in charge density (PEVP-β,
quaternization degree β ¼ 23 or 46%) led to a decrease in PEC stability and a
decrease in the critical salt concentration (salt concentration at which half of the
EtBr molecules are intercalated in DNA-free sites).
Given that poly(1-vinylimidazole) (PVIm) has a pK a of around 5.5, this polymer
does not complex DNA at physiological pH. Thus, Allen et al. quaternized
the imidazole ring with various substituents such as bromoethanol in order to
obtain permanently charged imidazolium-containing copolymers [107]. As the
quaternization degree was increased, fewer sites were available for protonation,
132
A. Bertin