Purpose: Emulsion formulations are often used to enable the performance of lipophilic poorly water soluble compounds or to provide sustained release patterns. The development of emulsion formulation is a labor intensive process, which involves constructing ternary phase diagram by evaluating various combinations of oil, surfactant/co-surfactants, and aqueous vehicles. In this study, we have applied an innovative high throughput method for screening and optimization of emulsion formulation using acoustic mixing. Lipophilic compounds with properties suitable to be delivered through emulsion formulation will be used as model compounds.
Methods: Emulsion Composition screening
Partial pseudoternary phase diagram is constructed with different ratios of oils (capryol 90, phosal 50, miglyol 812, olive oil, and safflower oil), surfactants (tween 80, span 20, solutol HS 15, chremophor EL), and buffer (pH=5 50 mM Citrate buffer). The ratios evaluated are targeting for the formulation of o/w emulsions. Samples are prepared in 96 well plate with milling beads at 50% bead loading and mixed via acoustic mixing. Compositions that render stable o/w emulsions are tested for optimal drug loading. The emulsion with highest drug loading is chosen for further characterization and stability evaluation using PLM, DSC, and laser diffraction. Physical stability is monitored by PLM and chemical stability is determined by HPLC. The properties of the emulsions are prepared by acoustic mixing and compared to those from the traditional homogenization process.
Entrapment efficiency of the drug is determined by centrifugation and HPLC.
In vitro release study
In vitro drug release from the emulsion is evaluated by dialysis method. The dialysis bag is incubated at 37 °C for 24 hours in PBS. Samples are collected at various time points, and HPLC is used to determine the amount of drug released.
Results: Multiple oil surfactant ratios result in stable o/w emulsions ranging from micro and nano droplet sizes. Capryol 90/span 20/tween 80/buffer (11.6/11.6/11.6/65) combination yielded stable o/w emulsion with droplet size near 5 micron. Particle size of this formulation was similar when prepared through acoustic mixing or homogenization. Phosal 50/span 20/solutol HS 15/buffer (13.3/13.3/13.3/60) resulted in droplet size near 500 nm. The DSC results indicate the micro-emulsion formulation may be physically unstable above 50 °C. This was consistent with thermal stability result when sample was stored 10 degrees above 50 °C. It was observed that the oil and water phase separation occurred under this storage condition.
The most stable emulsion was chosen to test for maximum drug loading, entrapment study, and in vitro release.
Conclusion: Acoustic mixing is a viable option for high throughput screening of emulsions because of its speed in sample preparation. It eliminates the need for manual manipulation for mixing and allows for evaluation of samples in 96-well plate all at once while using minimal material. Furthermore, manual labor of pipetting various ratios of oil, surfactant, and buffer into the 96 well plate can be minimized through automation. In our study we have successfully screened and optimized emulsion formulations using the acoustic mixer for the model compound. The identified multiple emulsion combinations are physically stable and has similar properties as prepared with homogenizer. In conclusion, acoustic mixing appears to be a viable option for high throughput development of emulsion formulations because of its speed and material sparing characteristic.
Kang-Jye Chou– Genentech