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A. Schwartz, L. Joline, E. Snow, S. Francke, C. Marques, R. Bernardes; Stabilizing Indocyanin Green for Use as Quantitative Standards. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2116.
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Indocyanin Green (ICG) is a dye used in ophthalmology to determine vessel development and leakage. However the stability of ICG solutions prepared for clinical use have only been approved for a period of 6 hours by the FDA. Obviously, six hours is not practical with respect of the use of ICG as a quantitative standard, therefore studies were conducted to determine the instability characteristics of ICG and how to achieve stability for long periods of time without changing the fluorescence characteristics of the dye.
Initially, solutions of clinical grade ICG (ICG-PULSION - Medical System AG, Munich, Germany) were made and examined over time to determine their solution and fluorescence stability. Solution stability was determined by examination at 100 - 400X under a microscope. Fluorescence stability was determined by measuring the emission intensity of the ICG at 810 nm. A variety of different surfactants where tested to improve stability.
Solutions of ICG were made with de-ionized water and evaluated over time. The fluorescence intensity of these aqueous solutions was found to drop 80% over 100 hours. Solutions of ICG made in PBS pH 7.4 were found to drop 85% over 100 hours. Ionic surfactants, e.g., sodium dodecyl sulphate, had only minimal success in stabilization, however, non-ionic surfactants, e.g., Tween 20, were found to provide the necessary stability showing no significant (<5%) decrease in emission intensity over 4 months. Microscopic examination revealed that microscopic green crystals appeared suspended in the unstable fluorescent solutions, whereas the ICG in solutions containing 0.1% Tween 20 remained in solution.
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