The delivery of nucleic acid-based molecules in human cells is a highly researched
method for the treatment of multiple diseases, including monogenic diseases and
cancers. As with most in vitro transitions, in vivo transfection is technically complicated.
Some factors that should be considered include DNA/RNA stability, delivery efficiency
into the body, and tissue/organ specificity.
In vivo transfection of DNA and RNA is usually based on non-viral or viral vectors.
Non-viral vectors for DNA and RNA transfer are generally not as effective as
virus-based systems, but they are particularly suitable because they do not require
biological safety considerations. There are currently many commercially produced in
vivo transfection reagents that can deliver DNA or RNA into living animals.
Cationic lipid-mediated transfection
Cationic lipid-mediated transfection (also known as lipofection) is the most widely used
non-viral gene delivery method. When mixed with nucleic acids, lipid-based
preparations will charge-dependently form a lipid complex structure around the nucleic
acid and protect it from extracellular or intracellular nuclease damage. Neutral lipids
(DOPE) and cholesterol are usually included as "auxiliary lipids" in liposome
formulations to improve transfection efficiency and particle stability. The incorporation
of fusogenic DOPE into cationic liposomes can significantly improve endosomal
escape through membrane fusion and destabilization, resulting in higher transfection
efficiency.
So far, lipofection has been successfully carried out by local administration or
intravenous injection in vivo to deliver nucleic acid in lung, muscle, brain, skin, and
various tumors.
Polymer-based transfection
The ability to functionalize and hybridize makes cationic polymers an attractive tool in
the gene delivery domain. Among them, the most widely studied cationic polymer
is polyethyleneimine (PEI). This aziridine polymer with high amino density has a strong
buffer potential in a wide pH range. This characteristic is a key factor for the effective
release of nucleic acids from endosomes during transfection.
PEI has two forms, branched and linear, and is used to transfect a variety of cell
types in vitro. Among them, linear PEI is widely used for tumor treatment in vivo. The
transfection efficiency of the two forms of PEI largely depends on their molecular
weight, and the most commonly used polymers are 25 kDa and 22 kDa, respectively.
However, the dense positive charge often makes them cytotoxic and
non-biodegradable. As a transfection agent, although low molecular weight PEI (LMW
PEI<10 kDa) is not as efficient as its larger variant (HMW PEI), it is less toxic.
