Bioactivities of Chitosan and Its Derivatives - Chitosan Based Hydrogels: Functions and Applications - page 163

The carboxymethyl chitosan (CMC) value is one of the important characteristics for
polymeric micelles as a drug delivery carrier. A low CMC value means that the copolymer
can form micelles under highly diluted conditions. DOX conjugation can improve the
hydrophobicity of CSO-SA, which caused the decrease in CMC value for DOX-CSO-SA.
After DOX conjugation, the micellar size was bigger in comparison with CSO-SA, which
might be caused by the increased DOX content in the micelles. The reduction of amino
groups after DOX conjugation led to lower zeta potential of DOX-CSO-SA. The zeta poten-
tial of these micelles was still larger than 30 mV in deionized water, which means that the
micelles were stable in colloidal dispersion.
The cis -aconityl linkage was introduced as a pH-sensitive linkage for release of DOX in
a macromolecule drug. The elevation of pH results in decreased liberation of the drug
from all the DOX-CSO-SA conjugates with different drug contents. The faster drug release
from DOX-CSO-SA under weak acid conditions might be favorable for targeting the deliv-
ery of antitumor drugs [228], because of the lower pH of tumor tissue. The varied drug
release behavior among DOX-CSO-SA with different drug contents might relate to the
varied micellar size of DOX-CSO-SA. The micellar size is a key factor affecting the surface
area in contact with the dissolution medium. Moreover, the larger the area in contact with
the medium, the faster the drug release from the micelles.
Various lines of evidence strongly support the fact that expression of ABC transporters
on the cell membrane, such as P-gp, is associated with drug resistance in cancer. P-gp func-
tions as an energy-dependent efflux pump to remove cytotoxic agents from drug-resistant
cells. The DOX uptakes of DOX-CSO-SA micelles were increased with incubation time in
both MCF-7 and MCF-7/Adr cells. DOX-CSO-SA micelles may enter the cells via endocy-
tosis, which is not a P-gp-dependent pathway.
Nano-sized polymeric carriers have been extensively applied, because they enhanced the
drug's solubility and stability in vivo by encapsulating the hydrophobic camptothecin (CPT)
into nano-sized drug carriers, wherein CPT is encapsulated into hydrophobic cores and the
drug carriers are covered with hydrophilic and biocompatible polymer shells [229,230].
Furthermore, nano-sized polymeric carrier-encapsulated CPTs have exhibited a prolonged
circulation time in vivo by avoiding the RES, and the prolonged circulation time of polymeric
carriers allows the encapsulated CPT to extravasate and accumulate into tumor tissue, wherein
a disorganized vasculature and defective vascular architecture develop, which is called an
enhanced permeability and retention (EPR) effect in tumor tissue [231,232]. Therefore, this
passive targeting of CPT-encapsulated nano-sized polymeric carriers to solid tumors has
enhanced the drug's therapeutic efficacy and potentially decreased severe toxic effects.
Preparation of CPT-encapsulated and nano-sized carriers : 100 mg of aglycol chitosan
5β-cholanic acid (HGC) conjugate (Figure 3.28) was mixed in 5 mL of DMSO and CPT solu-
tions (10, 20, and 30 mg in 2 mL of DMSO). The solution was vigorously stirred for 12 h at
room temperature and dialyzed for 2 days in distilled water by using a dialysis tube with
an MW cutoff of 12,000-14,000 (Spectrum, Rancho Dominquez, CA). After dialysis for 2
days, the solution was centrifuged at 10,000 g for 30 min to remove free CPT. The superna-
tant was filtered with 0.8 μm of syringe filter and lyophilized to give a white powder.
Physicochemical properties of CPT-encapsulated HGC : The critical aggregation of concentra-
tion value of HGC nanoparticles was 0.047 mg/mL, which was lower than those of LMW
surfactants (e.g., 2.3 mg/mL for sodium dodecyl sulfate in water). Thus, HGC nanoparti-
cles could maintain their nanoscale particle structure at the diluted concentration in the
body. As HGC nanoparticles possess hydrophobic inner cores, the nanoparticles are stable
and may encapsulate hydrophobic drugs. Because CPT is water insoluble, CPT is easily
encapsulated into HGC nanoparticles by the dialysis method. Thus, we assumed that HGC