The influence of porosity changes in human epidermal membrane during iontophoresis on the permeability enhancement of a model peptide

Purpose: We tested the hypothesis that the increases in the porosity of the skin during iontophoresis would not significantly increase the transport of peptides due to the small size of electrically induced pores. To investigate this mechanistically, we used human epidermal membrane under constant voltage conditions, applying the Nernst–Planck equation to the transport of a small ionic solute, tetraethylammonium bromide (TEAB), and a model peptide, luteinizing hormone releasing hormone. Methods: Steady-state flux of the drugs was determined under passive conditions and also during iontophoresis using constant DC voltages applied across side-by-side diffusion cells. Electrical conductance measurements were used to monitor the porosity changes that occur during electrical field application. Results: Porosity increases observed in the membrane substantially increased the permeability enhancement of the small ionic solute TEAB. The permeability enhancement was well described by Nernst–Planck model predictions after porosity changes in the membrane were taken into account. Enhancement of luteinizing hormone releasing hormone under identical conditions was much less than TEAB. The porosity increases induced by iontophoresis had little or no effect on the permeability enhancement of the larger molecule. Conclusions: These findings closely parallel those reports that have found electrically induced pores to be significantly smaller than preexisting pores in the human epidermal membrane. The data obtained also support the view that iontophoresis-induced pores, alone, may provide limited benefit for macromolecule transport across the skin.