DESIGN AND SYNTHESIS OF ENDOSOMOLYTIC CONJUGATED POLYASPARTAMIDE FOR CYTOSOLIC DRUG DELIVERY - Biologically-Responsive Hybrid Biomaterials - page 192

system, since the cationic monomers do not agglutinate even at high
concentrations. Erythrocytes are often agglutinated by surrounding polycations
within minutes, even at very low concentrations. For C18/MPEG/API-g-
PASPAM, no hemolysis occurred above pH 7, while they considerably lysed the
red blood cells at pH 5.5. This result indicates that only the C18/MPEG/API-g-
PASPAM, composed of both imidazole and C18 groups, can cause hemolysis in
API-containing systems. This is because the hydrophobic interaction between the
lipid core and C18 chain played a critical role in membrane fusion, after polymer
binding to the outer shell, by electrostatic interactions between the positively-
charged imidazole groups and negatively-charged lipid bilayers. This membrane
lysis involved two steps: linkage of positively-charged polymers with the
negatively-charged membrane; penetration and rupturing of lipid cores by
hydrophobic interactions.
The blood compatibility is one of the most important properties of drug
delivery carriers, as the blood is the first biological environment they encounter
after intravenous administration. Both poly(aspartamide) systems did not cause
any hemolysis or agglutination at normal pHs, indicating that polyaspartamides
have good blood compatibility under physiological conditions.
8. Endosomal Escape and Cytosolic Delivery of Therapeutic Agents
As mentioned in the introduction, one of the most significant barriers for
cytosolic delivery of therapeutics that work inside the cell is the endosomal
barrier. For therapeutics such as plasmid DNA, anticancer drugs,
oligonucleotides (ODNs), ribozymes, and immunotoxins, their ability to reach
their intracellular target depends on their ability to escape the endosome and into
the cytoplasm.[35-37]
Polycations that have considerable secondary or tertiary amines can provide
buffering effect in an endo/lysosomal pH range and destabilize the endosome by
“proton sponge” effect. The so-called “proton sponge” hypothesis was first
reported by Behr and coworkers [38] and the mechanism is shown in Figure 16.
After macromolecules are trafficked to the endosome via endocytosis, the
endosomal compartmen is acidified through the activity of the membrane-bound
V-ATPase proton pumps. Once cationic polymers bearing unprotonated amino
groups are present in the endosome, they absorb protons that are pumped and
buffer the internal pH of the endosome. Due to repulsion between the protonated
amine groups, the polymer swells and an influx of Cl - counterions occurs in order
to maintain electroneutrality. The influx of protons and Cl - ions increases the