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C.A. Mullenax, D.J. Tate, Jr., D.A. Newsome, C.F. Walker; Reactions of Timolol Maleate Gel Forming Solutions Subjected to Iontophoretic Currents Vary by Manufacturer . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4460.
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Purpose: Testing was undertaken to determine whether particular ophthalmic pharmaceuticals were suitable for use in ocular iontophoresis. Although active ingredients are identical in different companies’ formulations, inactive ingredients may not be. Timolol maleate, the active ingredient in an ocular hypotensive medication available from multiple vendors, was tested in gel-forming solutions from two companies to compare responses to iontophoretic current levels. Methods: An in vitro test cell applied specific, measurable, uniform current densities to 1 mL of test fluid at room temperature. Timoptic-XE 0.25% (Merck) and Timolol GFS 0.5% (Falcon) were tested, each undergoing 7 distinct electrical current densities ranging from 10.5 µA/cm2 to 20.8 mA/cm2 applied as constant direct current for 2 minutes. pH was measured with litmus paper. Fluid samples were subsequently analyzed via high-performance liquid chromatography (HPLC) to verify drug and electrode chemical stability. Results: Timoptic-XE began converting to gel at 1 mA applied current, and the portion of converted solution increased at higher currents. Timolol GFS did not form gel. Additionally, the pH of the Merck fluid increased from 6.5 to 9.7 in conjunction with gel formation; the Falcon product remained at 7.0 for all current levels. HPLC retention times for both formulations were similar and consistent through all applied currents with peak times averaging 6.8 and 14.0 minutes for Timolol GFS, 6.6 and 14.2 minutes for Timoptic-XE. Conclusions: Although the active chemical composition of the formulation did not vary greatly between these two different drug solutions, their reactions to applied current varied both in gel formation and in pH. Both formulations maintained chemical integrity as indicated by the absence of electrode corrosion or drug breakdown products in the HPLC analysis. Timolol GFS is preferable to Timoptic-XE for ocular iontophoresis due to its lesser reaction to the applied currents.
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