The use of cationic liposomes to deliver nucleic acids has shown great promise;
however, their therapeutic potential is greatly limited by their in vivo instability. Upon
systemic administration, cationic liposomes are prone to plasma protein adsorption,
leading to RES recognition and rapid clearance from blood circulation. Surface
modification using hydrophilic polymers, usually PEG, is known to prolong the
circulation half-lives of liposomes in vivo. PEG is required for increased liposome
stability, but it is undesirable once at a target tissue since its presence significantly
inhibits the release of the encapsulated agent. The development of cleavable PEG that
can be removed in response to a specific trigger once at the target site may be one
strategy to overcome this problem.
Here, we report the development of PEGylated peptide-lipid conjugates sensitive to the
proteolytic enzymes found in tumour cells, HLE and MMP-2. The synthesis was carried
out by conjugating an enzyme substrate peptide between PEG and a lipid, enabling a
series of enzyme-sensitive PEGylated peptide-lipid conjugates to be used for liposomal
nucleic acid delivery. Although enzymatic degradation of the peptide linker resulting in
the detachment of PEG was not observed with the analytical techniques used, the in
vitro result showed enhanced pDNA transfection efficiency by the nanoparticles
containing the PEGylated peptide-lipid conjugates in response to the enzymes,
compared to the controls. Physicochemical characterisation showed the nanoparticles
have small diameters and high nucleic acid encapsulation efficiency. The nanoparticles
with lower zeta potential were found to exhibit enhanced stability in serum and
minimum toxicity.
Further investigation in a siRNA system revealed the ability of the PEGylated peptidelipid
nanoparticles to significantly knockdown the target protein in response to
enzymatic activation. Thus, the results from these studies demonstrated the possibility
of using the enzyme-sensitive PEGylated peptide-lipid conjugates to improve nucleic
acid delivery.