For complexation of mRNA into polyplexes, double-pH-responsive
lipo-xenopeptides (XP), comprising tetraethylene pentamino succinic acid
(Stp) and lipoamino fatty acids (LAFs), were combined with PEGylated
lipids, either DMG-PEG 2 kDa (DMG-PEG) or azido-group-containing
DSPE-PEG 2 kDa (DSPE-PEG-N3), to increase colloidal stability and to
facilitate ligand-mediated targeted mRNA delivery. LAF-XPs mixed with
DMG-PEG at low (1.5% and 3%) molar ratios improved colloidal stability
and retained transfection efficiency. PEGylation also enabled the
formulation of otherwise unstable carrier complexes and prevented
aggregation induced by salt, proteins, and serum. PEGylation of more
positively charged Stp-LAF2 mRNA polyplexes decreased fibrinogen adsorption. More neutral, LAF-rich Stp-LAF4
polyplexes exhibited low fibrinogen binding without PEGylation.
Intravenous administration of these stabilized mRNA complexes
demonstrated enhanced biosafety while preserving transfection
efficiency. DSPE-PEG-N3 was selected for cell targeting after
strain-promoted azide-alkyne cycloaddition (SPAAC)-mediated
click-coupling of DBCO-modified ligands. Higher PEG ratios (10% and 20%)
provided effective shielding but reduced transfection efficiency, a
drawback known as the “PEG dilemma”. Functionalization with an
EGFR-targeting ligand restored transfection in EGFR-positive cell lines
in a ligand-specific manner. High transfection efficiency is consistent
with a lipophilic-to-hydrophilic polarity switch of LAF-XP carriers upon
endosomal protonation, triggering dissociation of the PEG lipids and
deshielding of the polyplex.