Reverse iontophoresis: Parameters determining electro-osmotic flow. II. Electrode chamber formulation

'Reverse iontophoresis' involves the imposition of an electrical potential gradient across the skin with the goal of non-invasively extracting molecules of biological interest to the body surface. A number of potential clinical chemistry applications (in particular, the monitoring of blood sugar) have been envisaged. The ultimate objective of the research described in this paper is to optimize the electro-osmotic component of reverse iontophoretic extraction. In vitro experiments, using diffusion cells in which both electrode chambers are situated on the epidermal side of hairless mouse skin, were performed. Current (0.56 mA/cm²) was passed for 2 h via Ag/AgCl electrodes and the electro-osmotic extraction of radiolabeled mannitol from the receptor phase (5 mM mannitol in pH 7.4 HEPES-buffered saline (0.14 M)) was measured as a function of the formulations placed in the anodal and cathodal chambers. Mannitol represented a non-ionizable, non-metabolizable model compound, which has been used extensively in previous electrotransport experiments of this kind. It was found that iontophoresis of divalent ions from the anode chamber (i.e. an anode formulation with CaCl₂ or MgCl₂ instead of NaCl) increased electro-osmotic flow from beneath the skin surface towards the anode (i.e. modulation of the normal situation in which the skin's permselectivity dictates net electro-osmosis in the opposite direction). Shielding of the net negative charge on the skin is a possible mechanism for this phenomenon, a hypothesis not inconsistent with the ability of certain lipophilic peptides (e.g, the luteinizing hormone releasing hormone analog, Nafarelin) to achieve a similar effect. In the cathode chamber, to which electro-osmosis predominates, ~3-fold levels of enhancement were achieved by formulating the electrode bathing solution with either 2 mM calcein, 50-300 USP U/ml heparin, or 10 mM EDTA. It is postulated that this augmentation of electrotransport involves either binding of endogenous Ca²⁺ (and hence decreased shielding of the negative charge on the skin), or simply the presence of more negativity in the membrane (or a combination of the two). Overall, the results demonstrate that electro-osmosis can be increased by manipulation of the electrode formulations, and that commonly used pharmaceutical excipients are among those materials which can be used to optimize the transport.