ABSTRACT
Two high resolution, 3D imaging techniques were applied to visualize and characterize sterilizing grade dual-layer filtration of liposomes, enabling membrane structure to be related with function and performance. Two polyethersulfone membranes with nominal retention ratings of 650 nm and 200 nm were used to filter liposomes of an average diameter of 143 nm and a polydispersity index of 0.1. Operating conditions including differential pressure were evaluated. X-ray computed tomography at a pixel size of 63 nm was capable of resolving the internal geometry of each membrane. The respective asymmetry and symmetry of the upstream and downstream membranes could be measured, with pore network modeling used to identify pore sizes as a function of distance through the imaged volume. Reconstructed 3D digital datasets were the basis of tortuous flow simulation through each porous structure. Confocal microscopy visualized liposome retention within each membrane using fluorescent dyes, with bacterial challenges also performed. It was found that increasing pressure drop from 0.07 MPa to 0.21 MPa resulted in differing fluorescent retention profiles in the upstream membrane. These results highlighted the capability for complementary imaging approaches to deepen understanding of liposome sterilizing grade filtration.
ABSTRACT
Liposomes are increasingly being investigated and implemented as injectable drug delivery systems. The preferred method for sterilizing injectable drug formulations using liposomes is to use filtration. However, because of the size of liposomes and their physicochemical properties, this can be challenging with sterilizing-grade filters rated at 0.2 µm. Filter validation studies with injectable liposomes have shown a higher likelihood of premature filter blockage and bacterial penetration compared to other parenteral drug types. Consequently, a greater understanding of the sterilizing filtration of liposomes is required so that appropriate decisions are made concerning the selection and validation of sterilizing-grade filters for these applications. In this work, Lipoid S100 liposomes were produced using a microfluidization technique without any encapsulated drug (empty) to investigate their filtration through a polyethersulfone filter. In order to improve the sterilizing-grade filtration of liposomes, optimization of both the filtration process and the formulation characteristics is important. To show this, the effect of the different filtration conditions/parameters (prefiltration, serial filtration, differential pressure, inlet pressure) and liposome characteristics such as size and size distribution on filtration were examined. For example, by decreasing the size of the liposome from 179.0 to 127.3 nm, the volumetric throughput (L/m2) was increased by more than 40-fold. Or by increasing the differential pressure, the volumetric throughput was improved significantly by more than 18-fold (0.7 to 4.1 bar) and in another experiment by more than 10-fold (0.3 to 2.1 bar). In addition, the benefit of using higher differential pressure on the liposome transmission through various sterilizing-grade membranes was shown.
