Biomedical Engineering Reference
In-Depth Information
These polymers contain a combination of carboxyl groups and hydrophobic alkyl
groups, and are protonated at the endosomal pH range. Upon a decrease in pH,
they increase their hydrophobicity, and penetrate into the endosomal membranes
and disrupt them. The hydrophobicity of the polymers is important for disrupting
lipid membranes. Foster et al. have applied these amphiphilic polymers to nano-
particle delivery systems [
145
]. pH-responsive nanoparticles (180 nm)
incorporating OVA-conjugated poly(propylacrylic acid) (PPAA) (PPAA-OVA)
were evaluated to test whether improved cytosolic delivery of a protein antigen
could enhance CD8
+
CTL and prophylactic tumor vaccine responses.
Nanoparticles containing PPAA-OVA were formed by ionic complexation of
cationic poly(dimethylaminoethyl methacrylate) (PDMAEMA) with the anionic
PPAA-OVA conjugate (PPAA-OVA/PDMAEMA). The PPAA-OVA/
PDMAEMA nanoparticles were stably internalized and could access the MHC
class I pathway in the cytosol by triggering endosome escape. In an EG.7-OVA
mouse tumor protection model, PPAA-OVA/PDMAEMA-immunized mice
delayed tumor growth for nearly 5 weeks, whereas control mice injected with
PBS and free OVA developed tumors in less than 10 days. This response was
attributed to the eightfold increase in production of OVA-specific CD8
+
T-
lymphocytes and an 11-fold increase in production of anti-OVA IgG. However,
these vinyl polymers are not biodegradable and, thus, their molecular weight
presents a limitation for medical applications.
Recently, our group developed novel biodegradable nanoparticles composed
of hydrophobically modified
g
-PGA (
g
-PGA-Phe). The nanoparticles showed a
highly negative zeta potential (
25 mV) due to the ionization of the carboxyl
groups of
g
-PGA located near the surfaces. Protein-encapsulating
g
-PGA-Phe
nanoparticles efficiently delivered proteins from the endosomes to the cytoplasm
in DCs [
146
]. To evaluate their potential applications as membrane disruptive
nanoparticles, the nanoparticles were characterized with respect to their hemo-
lytic activity against erythrocytes as a function of pH. The nanoparticles showed
hemolytic activity with decreasing pH from 7 to 5.5, and were membrane-inactive
at physiological pH. As the pH decreased, the hemolytic activity of the
nanoparticles gradually increased, reaching a peak at pH 5.5. This activity was
dependent on the hydrophobicity of
g
-PGA. The mechanism responsible for the
pH-dependent hemolysis by the nanoparticles involved a conformational change
of
g
-PGA-Phe and corresponding increase in the surface hydrophobicity.
Increased polymer hydrophobicity resulted in increased membrane disruption.
The
g
-PGA-Phe has carboxyl side chain groups, so the p
K
a of the proton of the
carboxyl groups is also a very important factor for the pH sensitivity of the
g
-PGA-Phe [
104
].
It has also been reported that antigen delivery to DCs via PLGA particles
increased the amount of protein that escaped from endosomes into the cytoplasm.
How proteins or peptides encapsulated within PLGA particles become accessible to
the cytoplasm is still not clear. It is suggested that the gradual acidification of
endosomes leads to protonation of the PLGA polymer, resulting in enhanced
hydrophobicity and attachment and rupture of the endosomal membrane [
147
].
