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Sangly P Srinivas, Wanachat Chaiyasan, Yueren Wang, Yiran Jiang, Waree Tiyaboonchai, Uday Kompella; Transport of Topical Riboflavin across the Cornea with and without Iontophoresis. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5996.
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Photochemical cross-linking of stromal collagen (CxL) to halt progression of corneal ectasia is being widely accepted as an approach to treat keratoconus. A conventional protocol for CxL consists of irradiation of the cornea with UV-A (365 nm; 2 mW/cm2) concomitant with topical drops of riboflavin (Rf) phosphate (0.1% in a 20% 500 kDa Dextran sulfate) for 30 min. In this study, we have measured the dynamics of transcorneal penetration of Rf following its topical administration with and without iontophoresis.
Freshly excised porcine eyes were used in all experiments, which were conducted at RT. The cathodal iontophoresis was performed for 10 min at a constant current of 0.4 mA. The tip of the cathode was 3 mm in dia. The anode, formed by an 18g SS needle, was inserted into the globe. At the start of iontophoresis, cathode tip was gently pressed against a 4 mm dia. filter paper which was soaked in Rf (0.1% or 1% in 20% dextran sulfate) and placed on the cornea. After iontophoresis, z-scans across the cornea were performed for simultaneous acquisition of Rf fluorescence using a custom-built confocal scanning microfluorometer (CSMF). The excitation was at 470 nm and the emission was collected through a long pass filter (> 520 nm). Area under the curve (AUC) of fluorescence vs. depth was calculated as a measure of total Rf penetration.
With a 40x water-immersion objective (wd = 1.2 mm; NA = 0.75), CSMF provided a depth resolution of ~ 7 microns for trans-corneal florescence. Accumulation of Rf following its topical administration (0.1% Rf) on bare stroma was higher compared to that obtained in the presence of epithelium (AUC w/o Epi ~ 3 to 5x AUC w/Epi; n = 4 eyeballs). Iontophoresis with topical 1% Rf (with intact Epi) resulted in variable but higher AUC compared to that without iontophoresis (AUC w/iontophoresis ~ 3 to 5x AUC w/o iontophoresis; n = 8). Similar experiments with bare stroma produced AUC 10x higher compared to that without iontophoresis (AUC w/iontophoresis ~ 8-10x AUC w/o iontophoresis; n = 10). Histology showed that epithelium was not damaged by iontophoresis even after 10 min. The stromal thickness was unaffected by iontophoresis.
Iontophoresis accelerates Rf loading into the stroma. Specifically, iontophoresis can be turned on and off to regulate Rf to desired levels across the depth of the stroma and in ensuring that Rf does not reach the endothelium.
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