Charge-reversible lipid derivative: A novel type of pH-responsive lipid for nanoparticle-mediated siRNA delivery.

RNA interference induced by small interfering RNA (siRNA) is a promising strategy for the treatment of various intractable diseases including cancer. Lipid nanoparticles (LNP) composed of ionizable lipids and siRNA are known as a leading siRNA delivery system. However, LNPs composed of conventional ionizable lipids will be aggregated in the physiological environment because of loss of ionization. Therefore, the inclusion of hydrophilic polymer-conjugated lipids such as polyethylene glycol (PEG)-conjugated lipid is required to improve the LNP stability. Herein, we synthesized a novel charge-reversible lipid derivative, dioleoylglycerophosphate-diethylenediamine conjugate (DOP-DEDA). The surface of LNP composed of DOP-DEDA (DOP-DEDA LNP) was constantly ionized and positively charged at pH 6.0, almost neutral at pH 7.4, and negatively charged at pH 8.0. Importantly, DOP-DEDA LNP were stable in the physiological milieu without PEG-conjugated lipid. DOP-DEDA LNP disrupted the red blood cells only under the low-pH condition in a hemolysis assay, suggesting that the interaction between DOP-DEDA LNP and biological membranes is pH-dependent. DOP-DEDA LNP encapsulating siRNA against polo-like kinase 1 (siPLK1) highly suppressed the expression of PLK1 mRNA and its protein. The cellular uptake of DOP-DEDA LNP was increased in an apolipoprotein E3 (apoE3) dose-dependent manner. In addition, DOP-DEDA LNP was taken up into cancer cells via both clathrin- and caveola-mediated endocytosis pathways. These findings indicate that LNP composed of this charge-reversible lipid should be a highly stable and potent siRNA delivery vector.