Controlling the pH dependent transition between monoolein Fd3m micellar cubosomes and hexosomes using fatty acetate...

Cubosomes made from the inverse micellar cubic mesophase (I₂) with Fd3m symmetry possess a unique structure of closely packed inverse micelles. These have prospective functionality in sustained drug release. In this study, we hypothesised that similar to fatty acids, various fatty acetate compounds can induce the formation of micellar Fd3m cubosomes in monoolein (MO) nanoparticles. They are different to micellar cubosomes made of MO and a fatty acid, which are pH responsive and can transition from an Fd3m phase to an inverse hexagonal phase (H₂) as pH increases. We hypothesised that by co-doping a fatty acetate and fatty acid into MO, precise control of the Fd3m-H₂ phase transition pH in nanoparticles can be achieved.

Five unsaturated fatty acetates with hydrocarbon chain lengths between 18 and 24 were added to MO at a weight ratio of 0.45 - 0.60 to form nanoparticles. The nanoparticles were prepared using high-throughput formulation and characterised with synchrotron small angle X-ray scattering (SAXS). MO nanoparticles doped with vaccenyl acetate and vaccenic acid were used to demonstrate the fine control over Fd3m-H₂ phase transition pH.

Micellar cubosomes (Fd3m phase) were found in MO nanoparticles doped with fatty acetates. The Fd3m structure was stable in a wide pH range of 2.6 - 8 and at temperatures up to 45 °C. In MO nanoparticles doped with the acetate/acid mixture, the Fd3m-H₂ phase transition pH was tuned between pH 5 and pH 7 by adjusting the ratio of vaccenyl acetate and vaccenic acid. As a H₂ phase generally offers faster drug release than an Fd3m phase, the pH responsive lipid nanoparticles developed here may find application in orally administrated formulation, where the vehicles must pass a low pH environment in the stomach before reaching neutral pH in the blood.