(141c) CG-MD and CFD Simulation of Continuous Manufacturing of Liposome Formation

Liposomes is one of the most researched nanocarriers for drug delivery applications and have major benefits such as stabilizing therapeutic compounds, enhancing drug uptake, and improving biodistribution to target sites. In brief, liposomes are spherical vesicles comprising amphipathic phospholipids either as a single bilayer or as a concentric series of multiple bilayers surrounding an aqueous core. Drug molecules can be encapsulated in the aqueous core and/or loaded into the lipid bilayer. Conventionally, liposomes are processed and manufactured by batch processing in the pharmaceutical industry. However, some disadvantages of batch processing such as scalability and reproducibility issues have led to alternative approaches. Continuous liposome processing is one approach that provides many benefits, such as higher-throughput, increased productivity and reduced energy requirements. An innovative continuous liposome processing system was developed at UConn, where two liquid flows are mixed in a highly controlled manner via a co-axial turbulent jet, producing monodispersed nanoparticles. Understanding the mixing process and lipid aggregation and liposomal formation on a molecular level will provide better insight to macroscopic attributes such as particle size and liposomal stability. In this work, we studied the effect of intermolecular forces (e.g., dipole-dipole forces) among the various components (ethanol, lipid and water) along with applying the principles of fluid dynamics. Accordingly, we applied a multi-scale computational study of the liposome formation process as a coaxial turbulent jet flow to probe the underlying mechanism and to quantitatively predict the formation of liposomal nanoparticles.

Methods:

Both coarse-grained molecular dynamics (CG-MD), as a micro-scale investigation, and computational fluid dynamics (CFD), as a meso-scale simulation, have been conducted to reveal the detailed mechanism of liposome formation, and also implement multi-scale case studies for the process. The MD simulation trajectories and their analysis were carried out using GROMACS package. We applied MARTINI force-fields for CG-MD simulations along with periodic boundary conditions. In addition, the particle size changes were studied in Non-Equilibrium Molecular Dynamics (NEMD) simulations with different flow shear rates. Large Eddy Simulation (LES) model in COMSOL Multiphysics incorporating energy equation, in CFD simulations, were implemented with a high-resolution mesh in the mixing area. The simulation results were verified with experiments, using flow patterns and temperature profiles.

Results:

From the CG-MD simulations, liposomes can be formed successfully in co-solvent solution. Though the MARTINI force fields (FFs) could not capture the experimental particle size, the liposome formation mechanism can be revealed at different ethanol percentages and different process temperatures. Additionally, the bilayer thickness of formed micelles at different temperatures agrees well with the trend of thickness changes from previous experimental studies. From NEMD simulations, the ratio of particle size changes caused by shear stress matched with previous experimental results.

By refining the CFD element sizes, a mesh independence study was done to manifest the CFD solution’s independence of the mesh resolution. Moreover, an increased formation of vortices led to smaller particles being formed at higher flow rates has been shown from vortex identification studies and probability density functions. Therefore, the multi-scale simulations were found to be accurate when compared to the experimental data and trends.

Conclusions:

Although, it was a challenge to model the formation of large sized liposomes, the results from CG-MD accurately portray liposome internal structure view, bilayer thickness, and formation mechanisms, which are difficult to observe and study through experimental work. Furthermore, the impact of process temperatures and flow rates to the jet flow have been captured by CFD simulations. The modeled MD shear flow indicates the breakdown of particles are consistent with eddies generation in CFD simulations, and further the ratio of particle size changes, in CG-MD, agrees with the measurements from experimental work. The integrative discrete CG-MD and continuum CFD computational modeling of liposome formation were found to be two viable complementary approaches which cover micro-scale and meso-scale, respectively.

Keywords:

Continuous processing, Liposomes, Molecular dynamics, Multiscale, Computational fluid dynamics

Acknowledgements: FDA Grant# 1U01FD005773-01.

Disclaimer: This article reflects the views of the authors and should not be construed to represent FDA’s views or policies.