Quality by Design and capillary electrophoresis in the development of thymoquinone-loaded vesicular systems for topical treatment of vitiligo

Vitiligo is a hypopigmentation disorder caused by the loss of melanocyte activity for melanin pigment generation. Recent studies have demonstrated that thymoquinone (TQ), a natural bio-active compound extracted from the seeds of Nigella Sativa, produces melanin stimulatory effects leading to skin darkening. A new and innovative vesicular formulation has been optimized for the topical treatment of vitiligo using Quality by Design (QbD) principles. Critical quality attributes were represented by vesicles size (size), polydispersity index (PDI) and encapsulation efficiency of TQ (EE%). Size and PDI were measured using dynamic light scattering, while EE% was determined with indirect method analyzing the free TQ by a fast capillary electrophoresis method. QbD scouting phase has made it possible to select the formulation components, to classify factors in different categories (formulation, process, method, environment) and to select the critical process parameters to be further studied by experimental design. Ethosomes composed of phospholipid, ethanol and water resulted the suitable vesicular carriers for topical delivery of TQ. Experimental design was efficiently used for rapid and systematic product development. A first screening phase was followed by response surface methodology. The contour plots were drawn and the design space (DS) was defined. The DS was visualized by means of probability maps and included all the operative conditions which led to fulfill the requirements of the quality product with a degree of probability π≥90%. Experimental results, applying working point conditions, highlighted the good predictivity of the model and ex vivo studies performed by Franz diffusion cells confirmed that the TQ-loaded ethosomal system improved the penetration of the active compound into the skin.