Liposomes & Lipid nanoparticles
Since their discovery in 1965, by Alec D. Bangham, liposomes have been recognised as the drug delivery vehicle of choice. Their biocompatibility results in minimal adverse reactions. Their amphiphilic structure allows encapsulation of both hydrophilic and hydrophobic active pharmaceutical ingredients (APIs). More recently the liposome’s analogous cousin, the lipid nanoparticle, has gained prominence because of its ability to deliver therapeutic payloads, including DNA and mRNA for vaccines. They can both deliver their payload very precisely through treating their surface with proteins allowing highly specific binding to a target cell type.
Liposome & lipid nanoparticle manufacture
Typically, liposomes are manufactured as sterile injectables for delivery to the bloodstream, and release of the drug takes place when the lipid envelope breaks down. Having delivered the liposome to the desired location, controlled release of the API can be achieved by natural breakdown of the liposome, or it can be designed to react to a change in temperature, pH or shear stress. Although liposomes have shown some success in drug product approvals, the limitations identified in the technology have remained almost stagnant over decades and have slowed its market penetration. The most common disadvantages of liposomes arise partly from poor stability under shelf and in-vivo conditions. This is mostly due to potential lipid oxidation and hydrolysis, leakage and loss of hydrophilic cargoes, as well as particle fission and fusion. There are numerous lab scale, and a few large-scale, techniques for liposome production, allowing for size control from around 20nm, up to several microns and composing of one or more bilayers. As the self-assembly of liposomes is based on an interaction between phospholipids and water molecules, control of the interaction between them is key to making structures with the desired morphology, encapsulation efficiency and stability. For liposomes the current state of the manufacturing art is characterised thus:
Fig 2: Nkanga, Bapolisi, Okafor & Krause; 2019
In general, LNPs are similarly formed by condensing lipids from an ethanol solution in water. However, the method by which they are synthesized is critical, because it directly affects both the LNP size and encapsulation efficiency. The properties of individual LNPs strongly depends on local, microscopic, mixing rates, where diffusive transport effects can lead to LNPs with variable compositions. Therefore, rapid mixing of the ethanol–lipid phase with excess water is key for the synthesis of small, uniform LNPs. A note of caution must be added. Many of the techniques also have the potential to degrade the efficacy of APIs due to the application of mechanical stresses (e.g. high shear, sonication, high pressure, etc.), unsuitable chemicals (e.g. volatile organic solvents, etc.), or extremes of pH. This is particularly true in the case of commercially available high temperature methods in which the lipids are heated to around 80°C.
The Micropore difference
Micropore Technologies already provides aseptic membrane devices for the formation PLGA microspheres. Recent work has pointed to other ways of using the membrane technology, as refined micromixers, mixing two miscible liquids together in a controlled manner. Micropore’s approach to both liposomes and lipid nanoparticles is, at room temperature or below, to inject a dispersion of phospholipids and API in a suitable solvent which can be introduced in a controlled manner into a continuously flowing continuous phase. The resulting self-assembly is moderated without high temperature, pressure or shear. As droplets form on the surface of the membrane, gentle shear, provided by the flow of the continuous phase, deforms and detaches the droplet, meanwhile the water miscible solvent partitions into the aqueous phase, leaving the liposomes to form in its wake.
In drug delivery applications using lipid-based carriers, such as liposome and nanoliposome formulations, a Polydispersity Index (PDI) of 0.3 and below is considered to be acceptable and indicates a homogenous population of phospholipid vesicles. The last edition of the FDA’s “Guidance for Industry” concerning liposome drug products emphasises the importance of size and size distribution as “critical quality attributes (CQAs)”, as well as essential components of stability studies of these products. Micropore’s process has several advantages over the most common commercial processes.
Working with Micropore was a very positive experience for us. The speed with which we progressed reflects how user-friendly we found the Micropore equipment.