Biopharmaceutical considerations on antihistamine effects of topically administered emedastine

Antihistamine effects of emedastine applied topically with three vehicles varying in their polarities were investigated in rats. The pharmacological effect of emedastine differed greatly depending on its concentration, treatment time, and vehicle. The antihistamine effect reached a plateau after approximately 2 h of exposure, and the potency of emedastine decreased in the following order by vehicle: isopropyl myristate >geraniol >glycerin. Using a two-layer diffusion model and penetration parameters reported previously (Harada et al., Biol Pharm Bull 23:1224-1228, 2000), transdermal fluxes of emedastine at each time point were calculated. When antihistamine effect of emedastine was plotted against calculated in vitro transdermal flux, not concentration applied, their relationship was sigmoidal and common regardless of the vehicles used. In conclusion, the antihistamine effect of emedastine applied topically varied greatly depending on vehicle, fundamentally due to the difference in skin permeability. The transdermal flux of the drug appears to be a good measure of its pharmacological effect.