Structural study of lipid bicontinuous cubic phases for cellular delivery
Lipid bicontinuous cubic phase is fundamentally interesting phase. Initially it is considered as an intermediate structure between lamellar and hexagonal phases due to its small phase space and complicated structure. With the development of structure characterization technologies, especially X-ray diffraction, it is found that this intermediate structure has cubic symmetry, and can be categorized into several different space groups including Diamond (Pn3m), Primitive (Im3m), and Gyroid (Ia3d). The lipid bicontinuous cubic phases thus have three-dimensional symmetry, high surface area per volume, and highly ordered pore and membrane network. As the building block of the cubic phases is lipid bilayer, it is also possible to incorporate both hydrophilic and hydrophobic molecules of interest in the bulk cubic phase system. The structural characteristics of bicontinuous cubic phases have been utilized for protein crystallization and sensing applications. Lipid bicontinuous cubic phases also have a fusogenic property due to its resemblance of the fused structure. A simple argument by Helfrich on membrane elasticity and thermodynamics can rationalize that the lowered free energy cost of fused bilayer structure formation is the source of the fusogenic property. Lipids have been used as a cellular delivery material for more than decades due to its nontoxicity and resemblance of vesicle transport systems in the cell, such as exosome. Primary formulation of lipid based delivery system is vesicle, called liposome. By modulating lipid composition in the liposome formulation, drug release kinetics can be optimized. There are few drug delivery systems based on liposome which already have obtained FDA approval and commercialized. However, its low efficiency compared to virus based delivery system still limits their cellular delivery potential. Membrane charge density is another important parameter that affect cellular delivery efficiency. In general, positively charged liposomes have more chance to fuse with endosomal membrane via Coulombic attraction. The increase of fusion events leads to higher delivery efficiency, however, induce higher toxicity with negatively charged cells. In this regards, new strategy or formulation of lipid based structure is in great needs for futuristic nanomedicine fields. Utilizing lipid bicontinuous cubic phase can be a solution for low efficiency issue in liposomal cellular delivery system. The challenge, however, is lied on making sub-micrometer lipid particles with internal bicontinuous cubic phase. Current cubosome formulation methods can be classified as top-down and bottom-up approach. Top-down approach fragments bulk lipid bicontinuous cubic phase system into particles via sonication or homogenization. Naturally, random shear stress and more than enough energy are delivered to the system, and produce polydisperse particles with some portion of liposomes. The size distribution of sonicated lipid particles is found to follow Weibull extremal probability distribution. Bottom-up approach, on the other hand, is more controlled method. In general lipids in volatile solvent are mixed with large amount of water, producing emulsions. Upon evaporation of volatile solvent, lipids in emulsion assemble into cubosomes. The resultant cubosomes are still polydisperse, typically have bigger than 0.1 polydispersity index due to the initial broad emulsion size distribution. Here we approach the cellular delivery strategy with structural understanding. Specifically, topologically active cubic phase particles are prepared via both low-temperature sonication and improved bottom-up approach. The demonstration of cellular delivery is performed with siRNA as delivery material. When delivering nucleic acid materials via lipid based vehicles, it was found that the efficiency limiting step is the endosomal escape process. As endosomal escape process is governed by membrane fusion event, siRNA delivery efficiency can represent fusion efficiency. Throughout the study, we show internal structure engineering is also an important designing handle for efficient cellular delivery. Based on the structural understanding, we also introduce topology trigger delivery system where cubic phase system can be transformed into hexagonal phase with spatiotemporal control. This novel phase trigger delivery system is investigated in bulk phase system for its feasibility. Lastly, we also report an important finding about lipid self-assembly. We found a simple methodology that produce bicontinuous cubic phase with extremely large unit cell parameter. The combination of 1. Charged lipid 2. PEGylated lipid and 3. Cubic phase forming lipid are capable of forming highly charged lipid domains that induce super-swelling. Upon incubation for 6 weeks at room temperature, we also found the super-swelled cubic phase becomes a single crystal exceeding volume of 1 mm3. The unit cell dimension is well-beyond the previous predictions and theories. We also show demonstration of encapsulation of large biological solute using 10 nm gold nanoparticles to underpin the importance of our finding.
Lipids, Bicontinuous cubic, Self-assembly, Drug delivery, siRNA delivery, Single crystal, Molecular single crystals
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign