Skin permeation of physostigmine from fatty acids-based formulations: evaluating the choice of solvent

This study was conducted to gain an understanding of the enhancement mechanism of fatty acids in skin permeation of physostigmine (PHY) by using a series of fatty acids and two solvents of opposing lipophilicity (propylene glycol (PG) and mineral oil (MO)). Interaction between fatty acid and drug was proven using NMR and conductivity measurements that showed a dependence on type of solvent used. Permeation flux of physostigmine from mineral oil-based formulations to skin was increased as solubility of physostigmine in mineral oil was enhanced in the presence of fatty acids having a longer chain. Thus, the dominant role of fatty acids in mineral oil was to increase solubility of physostigmine in the formulations that increased the driving force for physostigmine permeation through skin. As for propylene glycol, enhancement caused by fatty acids was attributed to their ability to increase the lipophilicity of formulation and to disrupt the lipid bilayers within the stratum corneum (SC). In conclusion, fatty acids enhancement for drug permeation across the skin was found to be dependent on the solvent used. Among various formulations here, oleic acid in mineral oil yielded fast permeation of PHY with a short lag time, which may be a good vehicle for transdermal delivery of PHY.