Chitosans have been successfully used as vectors for delivery of genes (gene therapy) since the first
report about 15 years ago [146,147]. Chitosan forms stable complexes with plasmid DNA and can be
used as a vector for the administration of genes to mucosal tissues such as the lungs [148] and
intestinal epithelium [149,150]. There are, however, certain drawbacks connected to the use of high
molecular weight chitosans because of the low solubility at physiological pH, the high viscosity and
the fact that the chitosan complexes often tend to form aggregates. By using CHOS instead of chitosan
these drawbacks may be overcome [16]. Köping-Höggård et al. [16] showed that fully deacetylated
CHOS (DP 24) formed stable complexes with plasmid DNA, and in vitro and in vivo experiments
proved that these CHOS were effective vectors for delivery of genes [16,17]. It has been speculated
that a delicate balance between the stability of the CHOS-DNA-complexes at lower pH-values (around
pH 6) and their instability at higher pH-values (above pH 7) could be the reason for their efficiency
[151]. This has recently been confirmed in detailed studies of how chitosan chemistry can be used to
create an optimal balance between the stability of the complexes and their unpacking [152].