Abstract
The bottleneck in delivering drugs to the brain for treatment of diseases related to central nervous system (CNS), lies behind the presence of blood-brain barrier (BBB). This barrier prevents most of the large and a number of small molecules from entering the brain, thus posing a considerable challenge in administration of drugs. The present treatment for CNS diseases involves oral drug delivery that seems to result in limited efficacy of drug therapy and undesired peripheral side effects. While invasive strategies have been proved to increase drug loading into the brain, the procedures can be risky with debilitating side effects, thus unsuitable for long-term treatment. Nevertheless, the use of particulate drug carriers, such as nanoparticles, can be a promising non-invasive strategy to increase drug loading into the brain by masking drug properties, to ascertain higher encapsulation efficacy and to enhance the stability of the drugs. One type of lipid-based nanoparticles, cubosomes, had been investigated in this study to determine its suitability as a drug carrier. By decorating and modifying the surface of cubosomes with specific moieties, several pathways can serve as targets at the BBB. With that, this thesis looked into intraveneous formulation of cubosomes as a drug carrier to cross the BBB, primarily due to several advantages it has to offer. The overarching aim of this thesis, hence, is to assess the hypothesis that the surface of cubosomes can be modified with specific moieties so as to serve as effective drug carriers to enter the brain.
In order to target low-density lipoprotein (LDL) receptors at the BBB, Chapter 3 presents the investigation of phytantriol cubosomes stabilised with BBB-targeting moieties; Tween 80 and Poloxamer 188. These stabilisers serve as cubosome stabilisers and also to target the BBB. Optimum concentrations of Tween 80 and Poloxamer 188 had been determined at 15% w/w (of phytantriol), where the cubosomes formed Im3m and Pn3m internal structures, respectively. The homogeneous cubosomes were formed and no vesicles had been observed with cubosomes stabilised using Tween 80. Nevertheless, in vitro cellular uptake displayed lack of uptake proposed to be from lack of ApoE in the cell culture media, which appeared to prevent the interaction of the cubosomes with LDL-receptors. This indicates the significance of ApoE in the plasma which initially binds to the cubosomes surface decorated with Tween 80 and Poloxamer 188, followed by the interaction with LDL-receptor and internalisation into the brain endothelial cells.
In Chapter 4, cationic cubosomes were investigated as an alternative potential drug carrier to enter the brain. In order to investigate the effects of using single and double chain cationic lipids; cetyltrimethylammonium bromide, CTAB (single chain), 1,2-Dioleoyl-3-trimethylammonium-propane, DOTAP and 1,2-di-O-octadecenyl-3-trimethylammonium propane, DOTMA (double chain) were added into the standard cubosome formulation which were stabilised by Pluronic F127. The concentration of cationic lipids in the cubosome formulation was optimised to avoid instability and to hinder any changes from occurring in the internal structure. The addition of 1.4 mol% cationic lipids maintained the internal structure as Pn3m structure and formed homogenous dispersion. In addition, observation under electron microscopy displayed the presence of vesicles which is believed to be the precursors to cubosome formation. Despite of the associated cellular toxicity risks of using cationic lipids, in vitro study exhibited that the addition of cationic lipids was not toxic at the studied concentration. Incorporation of cationic lipids increased the cellular uptake of the cubosomes, which was due to the electrostatic interaction with the cell membrane, followed by uptake via adsorptive endocytosis pathway.
In order to further determine the potential of using surface modified cubosomes to target the BBB, all the formulations were administered intraveneously into zebrafish larvae in vivo in Chapter 5, where the uptake and the toxicity of cubosomes in the midbrain region had been assessed. There was a significant two-fold uptake of cubosomes from larvae treated with Tween 80 and CTAB cubosomes, while lack of uptake observed in other formulations. The uptake of CTAB cubosomes into the midbrain could be related to the local toxicity effect, as visualised in toxicity studies. Other formulations, nonetheless, did not cause toxicity to the brain and no significant uptake. The uptake of Tween 80 cubosomes was not related to local toxicity and observation under electron microscopy revealed that gold-labelled Tween 80 cubosomes ended up in the brain parenchyma of the larvae. This observation highlights the potential of Tween 80 cubosomes as a drug carrier to target the brain.
In summary, this thesis supported the hypothesis that cubosomes can act as a drug carrier and be surface modified to cross the blood-brain barrier through selected pathways. Despite the lack of uptake in other cubosome formulations, Tween 80 stabilised cubosomes represent a promising approach as a drug carrier to cross the BBB.