REVERSE IONTOPHORESIS - PARAMETERS DETERMINING ELECTROOSMOTIC FLOW .2. ELECTRODE CHAMBER FORMULATION

'Reverse iontophoresis' involves the imposition of an electrical poten tial gradient across the skin with the goal of non-invasively extracti ng molecules of biological interest to the body surface. A number of p otential clinical chemistry applications (in particular, the monitorin g of blood sugar) have been envisaged. The ultimate objective of the r esearch described in this paper is to optimize the electro-osmotic com ponent of reverse iontophoretic extraction. In vitro experiments, usin g diffusion cells in which both electrode chambers are situated on the epidermal side of hairless mouse skin, were performed. Current (0.56 mA/cm(2)) was passed for 2 h via Ag/AgCl electrodes and the electro-os motic extraction of radiolabeled mannitol from the receptor phase (5 m M 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 chambe rs. Mannitol represented a non-ionizable, non-metabolizable model comp ound, which has been used extensively in previous electrotransport exp eriments of this kind. It was found that iontophoresis of divalent ion s from the anode chamber (i.e. an anode formulation with CaCl2 or MgCl 2 instead of NaCl) increased electro-osmotic flow from beneath the ski n surface towards the anode (i.e. modulation of the normal situation i n 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 inconsi stent with the ability of certain lipophilic peptides (e.g. the lutein izing hormone releasing hormone analog, Nafarelin) to achieve a simila r effect. In the cathode chamber, to which electro-osmosis predominate s, similar to 3-fold levels of enhancement were achieved by formulatin g 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 o f electrotransport involves either binding of endogenous Ca2+ (and hen ce decreased shielding of the negative charge on the skin), or simply the presence of more negativity in the membrane (or a combination of t he two). Overall, the results demonstrate that electro-osmosis can be increased by manipulation of the electrode formulations, and that comm only used pharmaceutical excipients are among those materials which ca n be used to optimize the transport.