Mechanism study on ion-pair complexes controlling skin permeability: Effect of ion-pair dissociation in the viable epidermis on transdermal permeation of bisoprolol.

Though ion-pair strategy has been widely used in transdermal drug delivery system, knowledge about the molecular mechanisms involved in the skin permeation processes of ion-pair complexes is still limited. In the present study, a homologous series of fatty acids were chosen to form model ion-pair complexes with bisoprolol (BSP) to rule out the influence of functional groups on polar surface area, stability and other physicochemical properties of ion-pair complexes. The ion-pair complexes were characterized by FTIR, thermal analysis, and 1H NMR. The skin permeability of BSP as well as its ion-pair complexes was investigated by in vitro skin permeation experiments then visualized by CLSM. The skin permeability coefficient (kp) of BSP ion-pair complex was negatively related to its n-octanol/water apparent partition coefficient (P'o/w) in the hydrophobic vehicle caprylic/capric triglyceride, (log kp=-1.657-1.229 log P'o/w), suggesting that the instability of ion-pair complexes due to their dissociation in the viable epidermis (VED) played an important role in controlling the skin permeability of BSP, which was further proved by 1H NMR and molecular docking. These findings broadened our understanding about the molecular mechanisms involved in the skin permeation processes of ion-pair complexes.