Development of a Composite Layered Double Hydroxide-liposomal System for Gene Delivery

Abstract

In recent years, gene therapy has attracted much attention in treating a wide range of severe diseases, including cancers. Unfortunately, naked genetic materials are rapidly degraded by the ubiquitous nucleases in the body and their negative charge property hinders crossing cell membrane to the cytoplasm. Thus, an efficient delivery system is urgently sought for desired gene therapeutic efficacy. Up to date, a wide variety of vectors have been examined for gene delivery, including viral and non-viral ones. Non-viral vectors, including polymers, cell penetrating peptides (CPPs), liposomes, and inorganic nanoparticles (NPs), are clinically promising due to their safety. It is well known that every delivery system has its own unique merits and inherent shortfalls, hence hybrid delivery systems are expected to merge their advantages while avoid their drawbacks. Inorganic layered double hydroxide (LDH) acts as an excellent drug/gene delivery vehicle due to its protection to the loaded drugs/genes and efficient cellular uptake by various mammalian cells. However, aggregation of LDH NPs in serum or culture medium limits the effective application in vivo. On the other hand, liposome is a promising lipid-based system which can protect the hydrophilic genes/drugs in the aqueous core or hydrophobic drugs between the lipid bilayers, and it is colloidally stable in the circulatory blood flow. Unfortunately, its disadvantages are also obvious, such as low transfection efficiency and inefficient endosome escape. This PhD project aims to develop a hybridised delivery system by forming small LDH (sLDH)-liposome composites. Considering encapsulating LDH NPs in the vesicles of lipid bilayers, small Mg-Al-LDH NPs were first prepared by a non-aqueous method with the Z-average diameter size of ~ 40 nm. This method requires co-precipitation of magnesium and aluminium nitrate solution with sodium hydroxide in methanol, followed by LDH slurry collection and re-suspension in methanol. The methanol suspension is then heated in an autoclave, followed by separation via centrifugation and thorough washing with deionised water. The NPs are finally dispersed in deionised water into homogeneous aqueous suspension after 4-6 day standing at room temperature. The prepared sLDH NPs have the Z-average size of 35-50 nm, the number-average size of 14-30 nm and the polydispersity index (PdI) of 0.19-0.25, with the colloidal suspension stable for at least 1 month when stored at fridge (2-8 °C) or ambient (22-25 °C) temperature. Engineered sLDH with the Z-average size of ~40 nm and normal LDH with the Z-average size of ~100 nm (large LDH, denoted as L-LDH in this thesis) were then compared in transfection efficiency to human colon cancer HCT-116 cells. Using fluorescein Isothiocyanate (FITC) to label LDH NPs and as a model anionic drug (where FITC is intercalated in the interlayers of LDH NPs), we found that 40 nm sLDH and 100 nm L-LDH have the similar cellular uptake rate based on the equivalent particle number concentration, which means in the size range of 35 to 100 nm, LDH NP size does not significantly affect the cellular uptake rate. Note worthily, a critical particle number concentration was found for both sLDH and L-LDH, below which the cellular uptake is in linear proportion to the concentration, while above which, the cellular uptake is not further improved. When delivering genetic materials to cancer cells (where DNA is adsorbed on the surfaces of LDH NPs), sLDH is far superior to L-LDH at low LDH:DNA mass ratio (e.g. 5:1). This is mainly attributed to full loading of DNA by sLDH and higher sLDH particle number concentration. At the high mass ratio (40:1), where both sLDH and L-LDH can completely bind and protect DNA, sLDHs are able to transport 2-fold DNA to the cells just because of the higher sLDH particle concentration. Finally, sLDH-liposome composites were prepared by the hydration of freeze-dried matrix (HFDM) method. This composite system exhibits good colloidal stability both in water and in cell culture medium, with the Z-average particle size ~ 200 nm, which is appropriate for cellular uptake. It is also proved for the composite system to completely bind/protect DNA at LDH:DNA mass ratio = 20:1, no matter how DNA is loaded in the composite system. About 3-time higher efficiency is observed in delivery of DNA to HCT-116 cells by the sLDH-liposome composite system compared to sLDH only. In general, the sLDH-liposome composite system shows higher cellular delivery efficiency than either sLDH or liposome only, good colloidal stability and low cytotoxicity, so it could be a promising gene delivery system.