Neutral Liposomes
The materials of the neutral liposomes (Table 1) including natural
product phosphatidylcholine (PC), phosphatidyl ethanolamine (PE) and
synthetic product 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE),
1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). The non-charged lipids
can also be referred to as helper lipids, which can stabilize the
bilayer membrane and reduce the toxicity of the charged lipid component,
wherein DOPE and cholesterol are most used. The role of cholesterol has
been described above, the addition of DOPE can interfere with the lipid
membrane, make the inclusion body unstable, promote the release of
nucleic acid substances, and play an auxiliary role in cell osmosis.
Neutral liposomes can be transformed into lipids with different charges
by modifying lipid molecules with different charges. Furthermore, in
targeted tumor delivery, neutral liposomes play an advantage in EPR
effect, and play a priority role in the treatment of solid tumors
[47]. In immunization, using
neutral liposome can induce a Th1 type immune response than cationic
liposomes in the mice model [48]. In delivery, a neutral liposome
loaded with focal adhesion kinase short interference RNA (FAKsiRNA) to
treat ovarian cancer. The Western blotting and immunohistochemical
analysis showed that a single dose of FAKsiRNA-liposome could
effectively reduce the expression of FAK in vivo for up to 4
days, and the average tumor weight decreased significantly (44%-72%)
[49]. In another cervical cancer treatment study, the stability,
cytotoxicity, and cell uptake of targeted delivery of arsenic trioxide
(ATO) to cervical cancer cells by liposomes of different charge types
were investigated. The authors concluded that neutral liposomes prepared
from PC and cholesterol selectively produced a high mortality rate in
HeLa cells and minimal toxicity in control cells. Although ATO
encapsulated by liposomes is less easily absorbed by cells than free
ATO, delivered ATO can reduce the expression of oncogenes and show
reduced toxicity [50].