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CA Mullenax, DA Newsome, CF Walker; Resistivity of Ophthalmic Pharmaceuticals Suitable for Ocular Iontophoresis . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1848.
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Purpose: This testing was undertaken to determine whether particular ophthalmic pharmaceuticals were suitable for use in ocular iontophoresis based on electrical resistivity. Tested pharmaceutical classes included commercially available dilators, constrictors, topical and injectable anesthetics, and ocular hypotensive medications. Methods: An in vitro test cell was created to allow application of specific, measurable, uniform current densities to 1 mL of test fluid at room temperature. Each pharmaceutical underwent 7 distinct electrical current densities ranging from 1.2x10-5 to 1.3x10-3 A/cm2 which were applied as constant direct current (DC) for 2 minutes. Delivered current was measured by high-resolution ammeter. Test fluid resistivity was calculated from the test circuit resistance, applied voltage, measured circuit current, and test cell geometry. Fluid samples were subsequently analyzed via high-performance liquid chromatography (HPLC) to verify drug and electrode chemical stability. Results: When subjected to electrical charges ranging from 1.2x10-3 to 1.2x10-1 Coulombs, drug solution resistivity decreased with increasing applied current for all tested pharmaceuticals including the saline control. This decrease followed a general linear trend when plotted as log resistivity versus log current density, with drug resistivity ranging from 1.1x105 Ohm-cm at the smallest applied currents to 4.4x103 Ohm-cm at the largest. HPLC retention times remained consistent through all applied currents, indicating no electrode corrosion or drug breakdown products. Conclusion: Results show that, while it does not vary greatly between different commercially-available pharmaceutical solutions, drug resistivity is a function of applied current. Since the active drug molecule in the studied pharmaceuticals was at most 5% of the total mass of the solution, we believe that the overwhelming presence of saline as the base for solution caused this homogeneity. All tested commercially available ocular drugs performed as equally good candidates for iontophoresis as judged by solution resistivity alone.
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