Bioactivities of Chitosan and Its Derivatives - Chitosan Based Hydrogels: Functions and Applications - page 161

OH

OH

O

O

O

O

HO

HO

NH

NH 2

1-x

x

OH

O

29

HO

Figure 3.26
Deoxycholic acid-modified chitosan. (Adapted from Kim, Y. H., Gihm, S. H., and Park, C. R. 2001. Bioconj Chem
12: 932-938.)

Polymeric micelles can evade scavenging by the MPS because of their hydrophilic sur-
face and generally small size (10-100 nm) [218]. As a result of the “enhanced permeability
and retention (EPR) effect,” the polymeric micelles largely accumulate in tumor tissue
[219]. The pH of tumor tissue is mildly acidic compared with that of healthy tissue. Linkers
that respond to pH changes are mainly used for small drug conjugation, such as N - cis -
aconityl acid [220], hydrazone linkage [221], and so on. The ideal polymer-drug conjugate
should be stable in blood circulation and in healthy tissues at pH 7.2, but hydrolytically
degradable and releasing small drug in the mildly acidic environment of target tumor
cells (pH 5-6). The conjugate of DOX attached via the hydrazone bond to N -(2-hydroxypropyl)
methacrylamide (HPMA) copolymer showed a much higher antitumor activity in vivo
than the free drug [221].
Drug resistance and multidrug resistance (MDR) to current chemotherapeutic agents
account for the failure of human cancer chemotherapy. MDR can be the result of a variety
of mechanisms that have not been unveiled completely [222]. One of the main causes of
MDR is linked to the overexpression of P-gp in tumors. P-gp belongs to the ATP-binding
cassette (ABC) superfamily of transporters [223], which acts as an energy-dependent drug
efflux pump. P-gp expression impairs the response to chemotherapy, and the expression
levels increase as tumors become drug resistant [224]. Many agents have been investigated
to reverse P-gp-mediated MDR. Researchers have proposed that several polymer micelles
can overcome drug resistance. The micelles were assumed to reduce the ATP production
in MDR cells, while sensitive cells had no noticeable change in ATP production [225].
HPMA copolymers containing doxorubicin could overcome MDR in human acute lym-
phoblastic T-cells, leukemia cell lines, and mouse leukemia cell lines [226].
MCF-7 and MCF-7/Adr cells were seeded at a density of 10 5 cells/μL plate and grown
for 24 h. Certain amounts (drug content was 5 μg/mL) of DOX ⋅ HCl and DOX-COS-SA
were added, and the cells were further incubated for 4 and 8 h, respectively. After washing
the cells with PBS three times, cellular uptake was observed by fluorescence microscopy.
In vitro antitumor activities of DOX-COS-SA were evaluated for the determination of
cytotoxicities of DOX-COS-SA using MCF-7 and MCF-7/Adr as model tumor cells, and
DOX HCl solution as a control. Using the MTT method, the 50% cellular growth inhibi-
tions (IC 50 ) for DOX HCl solution, COS-SA micelles, and DOX-COS-SA micelles with dif-
ferent drug loadings against MCF-7 and MCF-7/Adr were determined, and are shown in
Table 3.5. The variation of cell viability with the drug concentration of DOX HCl solution