Investigating impacts of surface charge on intraocular distribution of intravitreal lipid nanoparticles

In this paper, the effect of surface charge of intravitreal lipid nanoparticles (LNPs) on their intraocular accumulation and distribution was quantitatively and systematically studied. The retinal distribution of LNPs delivering model drug (siRNA) with optimal surface charge was visualized as well. LNPs and siRNA-loaded LNPs (siLNPs) were prepared with ethanol-injection method. C57BL/6 Mice with the age of 6-8 weeks were used for the animal experiments. Quantitative accumulation in the eye and respective intraocular distribution of the intravitreally injected LNPs with different surface charge were examined after predesignated time points. The fluorescence intensity of the fluorescent labeled siRNA was also quantified based on the confocal microscope images. The release of siRNA from siLNPs was studied in mimicked vitreous environment at 37 °C with the presence of 50% serum protein. We successfully prepared six types of LNPs had relatively uniform size around 70 nm and varied in zeta potential ranging from -30 mV to +50 mV. Negative, neutral and slightly positive charged LNPs were cleared much faster than strongly positive charged LNPs from the mice's eyes. After 6 h, LNPs with zeta potential of +35 mV had highest ratio of retinal to vitreous accumulation and penetrated through the entire layer of the retina. The siLNPs could successfully deliver the siRNA to the ganglion cells and inner plexiform layer of the retina. After 24 h, majority of LNPs and siLNPs were cleared, suggesting that the LNPs with pegylated surface in the absence of any targeting ligand were not internalized by retinal cells. The mechanism of how the surface charge of nanoparticles affects their intraocular retention and distribution was intensively discussed. The results suggested that intravitreally injected LNPs with optimal surface charge around +35 mV can distribute and penetrate the retina, which could be further modified as promising nanocarriers for retinal delivery.