Advanced Systems for Skin Delivery of CyanocobalaminA Model Molecule for > 1 kDa drugs

Resumen

The bioavailability optimization of drugs provides a second opportunity to drugs, which for several reasons, cannot exert an optimal therapeutic effect. The skin has been used as a platform to deliver drugs since ancient times. Nowadays, it is still considered as one of the main alternatives to the oral route. However, as an interface between the body and the environments, the outer skin layer (stratum corneum) prevents the entrance of substances. This opposition is of major importance for large molecules, and 500 Da is consider de maximum size diffuse through the skin structure naturally. Re-formulation of drugs in advanced drug delivery systems (ADDS) is commonly needed to bypass the stratum corneum barrier and produce an optimised therapeutic effect. The main objective of this thesis was to develop different ADDS that allowed the diffusion of large molecules through the skin. As a model molecule, cyanocobalamin (vitamin B12) was selected, due to its high molecular weight (1355 Da) and hydrophobicity, what hinders its transdermal absorption. For this, lipid vesicles and polymeric microneedles were used to encapsulate and increase cyanocobalamin permeability though the skin. Among all the lipid vesicles developed over the years, conventional liposomes, transfersomes and ethosomes were chosen and characterised towards those properties predictive of an enhanced diffusion. Specifically, transfersomes and ethosomes showed the best features, as they were homogeneous populations of small size (< 200 nm) and flexible structures. Freeze-drying was used to solve the long-term stability issues that usually affect lipid vesicles, and as an intermediate step that allows their incorporation in an alternative delivery system, the dissolving microneedles. In vitro and ex vivo studies confirmed the increase in drug absorption when cyanocobalamin was incorporated in the ADDS when compared to a standard aqueous solution. As consequence of their conception, microneedles were able to deliver transdermally the drug after a moderate period, suggesting their suitability to be used as an alternative to parenteral administration. Lipid vesicles, which showed longer latency times to achieve quantifiable dermal concentrations of drug, were able to localize the drug delivery in the outer skin layers improving the penetration in comparison to the standard solution. Cyanocobalamin nitric oxide scavenger behaviour makes possible to ameliorate skin inflammatory disorders such as atopic dermatitis or eczema when applied topically. The feasibility of lipid vesicles for topical treatments was assessed by in vivo delayed-type hypersensitivity model. Cyanocobalamin-loaded transfersomes significantly reduced the increase in ear thickness in comparison to the untreated group, proving their effectiveness. In addition, histological findings such as reduced epidermal hyperplasia, dermis thicken, and leukocyte infiltration confirmed the skin restoring effect of transfersomes.