Category: Formulation and Quality
Purpose: Glabridin is a compound that is found in the root extract of Glycyrrhiza glabra. Various biological effects make glabridin highly interesting for dermal use, e.g. neutralizing radical oxygen species (ROS), inhibiting inflammation, and brightening the skin by tyrosinase inhibition. Significant reduction in skin pigmentation induced by UVB radiation was shown on brownish guinea pigs already at 0.5% glabridin. As glabridin is almost insoluble in water (1.5 mg/L) and poorly soluble in oils, a mixture of ethanol/propylene glycol/water (6.5/2.5/1.0) was used in this study to dissolve desired amount of 0.5% glabridin. This solvent however is not suited for the daily facial application.
To improve the skin penetration, the dermal delivery system smartLipids seems promising. smartLipids are the 3rd and newest generation of solid lipid nanoparticles. On skin, they form an occlusive film, not only promoting the skin penetration of loaded active but also increasing the skin hydration and minimizing the adhesion of particulate matters (anti-pollution). Thus, in this study glabridin smartLipid formulation was developed, and production parameters optimized for small lab scale (40 mL), technical (700 mL), and large scale production (a few 100 kg).
Methods: Due to the poor solubility of glabridin in lipids, it was challenging to find a lipid composition enabling a high glabridin loading, solubility study in various oils and solid lipids was performed.
smartLipids were produced by high pressure homogenization in lab scale (40 mL, Micron LAB 40, APV, Germany) at 95°C with varying pressures (200, 300, 400 and 500 bar) and cycle numbers (1-5), to assess optimal production parameters on small scale Formulation was stabilized by ECOCERT surfactants at 1.5%, 2.0% and 3.0%, and preserved by various preservatives at different concentrations.
The production of optimized formulation was transferred to technical (700 mL, Micron LAB 40, continuous production mode) and large scale production (LAB 60). The Micron LAB 60 has a homogenization capacity of 60 kg/hour. Physical storage stability was assessed by photon correlation spectroscopy (PCS), laser diffractometry (LD) and light microscopy.
Results: All tested oils had low glabridin solubilities in the range of 1-3% (w/w) – besides one exception. In lemongrass oil (LGO) glabridin of up to 25% was dissolvable. As the lipid matrix have to be solid at skin temperature two highly melting solid lipids carnauba wax and bees wax were added to glabridin/LGO-solution till melting point of 40°C was reached.
Stabilization of suspension was obtained by Eumulgin SG at 3%. Applying 2 cycles at 200 bar (LAB 40) led to PCS mean particle size of 155 nm with polydispersity index of 0.02. Diameters LD50% and LD99% were measured to be 150 nm and 320 nm, respectively. No larger particles or aggregates were detected under microscope. Less number of cycles or lower pressure led to LD99% in µm-range. In contrast, increasing the cycle number or pressure led to smaller particle sizes being declarable as “nano” after EU cosmetic regulation.
The addition by 1% Microcare PEHG, 1%, 3% and 3% Hydrolite 5, and 15% propylene glycol did not lead to any major changes in particle size (LD99% of 295-350 nm). After 3 months storage, Microcare PEHG was identified as the best preservative (almost unchanged LD99% of 310 nm), even at stress conditions of 40°C.
Best glabridin smartLipid formulation was produced again using the continuous mode of Micron LAB 40, allowing the production 700 mL suspension. As the homogenization valve is identical to the discontinuous LAB 40, identical production parameters led to comparable particle sizes (PCS mean: 164 nm, LD50%: 147 nm, and LD99%: 339 nm).
With LAB 60 having different valves, pressure and cycle number had to be optimized again. Due to the high price of Glabridin a pre-screening was done with unloaded smartLipids. For glabridin containing screening the minimum amount of 3 kg were produced (1,500 USD per batch). By applying 300 bar for one passage desired particle size and distribution were obtained (PCS mean size: 163 nm, LD50%: 149 nm, LD99%: 322 nm). Since only one passage through this homogenizer is needed, production can be run continuously from a feeding vessel. With a homogenization capacity of 60 kg/h industrial large scale production is possible by running of a few hours (e.g. 8 hours correspond to half a ton of smartLipids suspension concentrate).
Conclusion: Glabridin smartLipids were successfully developed with a loading of 12.5% and total glabridin content in the suspension of 2.5%. Using the LAB 60 homogenizer with the optimized production parameters, only one passage is required for production. This allows industrial large scale production of smartLipids concentrate (60 kg/h). The size can be tailor-made depending on the pressure, from about 200 nm (submicron particles) to smaller particles (< nano > according to cosmetic regulations). The glabridin smartLipids concentrates are commercially available. The effective concentration is below 0.05%, thus the concentrate can be diluted by a factor of at least 50-100.