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1From the Department of Ophthalmology, University of Tokyo School of Medicine, Tokyo, Japan; and the 3Department of Ophthalmology, Yokohama City University School of Medicine, Yokohama, Japan.
| Abstract |
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METHODS. In vitro, rat RGCs were purified by a two-step immunopanning procedure, briefly exposed to ICG (2.5 x 103 mg/L), and irradiated with an endoilluminator for 15 minutes or incubated in the presence of ICG (concentration: 2250 mg/L) without irradiation. The number of viable RGCs was counted after 3 days in culture. In vivo, after rats received an intravitreal injection of 3 µL ICG (0.25 and 2.5 mg/L), the distribution of ICG was observed with a fundus camera, and the number of viable RGCs was examined by a DiI (1,1'-dioctadecyl-1-3,3,3',3'-tetramethylindocarbocyanine perchlorate)retrolabeling technique.
RESULTS. In vitro, a brief exposure to ICG and light did not affect RGC survival. However, ICG reduced the number of viable RGCs in a dose-dependent manner when the cells were exposed for 3 days. In vivo, the dye was initially distributed on the retinal surface and around the optic disc. At day 7, the fluorescence became invisible in the 0.25-mg/L group, whereas the staining of the optic disc contour was evident in the 2.5-mg/L group. The number of viable RGCs decreased significantly in the 2.5-mg/L group 14 days after the injection.
CONCLUSIONS. ICG showed an inherent toxicity to RGCs in a dose-dependent manner. Lower concentration and shorter staining time of ICG should be used for dye-assisted vitrectomy.
After the intraocular administration of ICG, the residual dye is distributed around the macular area and accumulates thereafter at the optic disc.12 13 14 Using anterograde and retrograde labeling techniques in rabbits and rats, a recent study clearly demonstrated that ICG diffuses into retinal ganglion cells (RGCs) and traces axons.15 However, the toxic effects of ICG on RGCs has not been studied. In this study, we investigated the effects of ICG on rat RGCs, both in vitro and in vivo.
| Materials and Methods |
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Preparation of ICG Solution
For in vitro experiments, 25 mg of ICG (Diagnogreen; molecular weight, 774.96; Daiich Pharmaceutical, Tokyo, Japan) was dissolved with 1 mL of distilled water, which was further diluted with distilled water to obtain ICG solutions with concentrations of 0.2, 1.0, 2.5, 5, 10, and 25 x 103 mg/L. These solutions were further diluted (at a dilution of 1:100) with B27 complete medium (described later) to give rise to ICG solutions with concentrations of 2, 10, 25, 50, 100, and 250 mg/L. For in vivo experiments, 25 mg of ICG was dissolved with 1 mL of distilled water, which was further diluted with distilled water to obtain ICG solutions with concentrations of 0.25 x 103 and 2.5 x 103 mg/L. The solutions were further diluted at 1:1000 in physiological saline to obtain 0.25- and 2.5-mg/L ICG solutions. The pH and osmolarity of the ICG solutions used in this study are shown in Table 1 .
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Intravitreous Injection
ICG was injected to the vitreous cavity of right globes, as described previously.21 A general anesthesia was induced with an intraperitoneal injection (1000 µL/kg) of a mixture (5:1) of ketamine hydrochloride (Ketalar; Sankyo, Tokyo, Japan) and xylazine hydrochloride (Celactal; Bayer, Tokyo, Japan). After mydriasis was achieved with a drop of 0.5% tropicamide, a 33-gauge needle was inserted into the midvitreous of the right eye, guided by a stereoscopic microscope, with care taken to avoid lens and retinal injury. A single injection of 3 µL ICG at a concentration of 0.25 or 2.5 mg/L (n = 5 in each group) was completed in 1 minute. For control, 3 µL of physiological saline was injected intravitreously to the right eyes of other animals (n = 5).
Fundus Epifluorescence Examination
Twelve, 24, and 48 hours and 7 days after the intravitreous injection, the fundus of the rats was observed with a fundus camera (model TRC50IX; Topcon, Tokyo, Japan) equipped with a 780-nm infrared illumination and ICG filter sets. After anesthesia and mydriasis was achieved, the fundus photographs were taken with a slide glass placed on the rat cornea. All fundus images were obtained under the same illumination intensity and transported into a computer system with the aid of a charged-coupled device camera.
RGC Counting
RGC counting was performed as has been described.21 Seven days after the intravitreous injection of ICG, anesthetized rats were placed in a stereotactic frame, two holes were made in the skull, and 0.2 µL of 5% DiI (1,1'-dioctadecyl-1-3,3,3',3'-tetramethylindocarbocyanine perchlorate) in dimethyl sulfoxide was injected into the superior colliculi of both sides. Each injection was made over 2 minutes with a syringe (Hamilton, Reno, NV). Seven days thereafter, the globes were enucleated, and cornea, lens, and vitreous were removed. Six radial cuts were made at the peripheral retina to the equator, and the retina was separated and mounted on a slide with vitreous side facing up. The number of cell bodies of RGCs was counted. Counts were taken from six circumferential points 1 mm eccentric from the optic nerve of the retinal flat preparation. The counts were averaged to give the count in one eye. RGC counting was performed with the observer masked as to treatment.
Statistics
The Mann-Whitney test was used to compare the number of RGCs in vivo analysis. P < 0.05 was considered statistically significant.
| Results |
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| Discussion |
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Our in vitro experiments demonstrated that dose-dependent ICG-induced damage to RGCs can occur without photoirradiation, with an IC50 of 4.2 x 10-5 M (31 mg/L), suggesting that a lengthy exposure to ICG can damage RGCs. The photodynamic effect of ICG, presumably mediated by radical species, is well studied.23 24 25 In addition, in vitro viability assays have demonstrated that ICG is toxic to several cells, even in the absence of light.23 24 After a brief exposure of ICG at a concentration of 1.3 x 10-3 M (0.1%), the viability of RPE cells was reduced by approximately 25%, as demonstrated by WST-1 assay,23 although this may be related to osmotic effect.26 Similarly, when HaCat cells were treated by ICG at a concentration of 5 x 10-5 M for 24 hours, cell viability was decreased by 18%, according to MTT assay.24 On the contrary, another study demonstrated that a 24-hour exposure of ICG at a dose up to 1 x 10-4 M did not show a toxic effect on U937 cells.25 Taken together, these results suggest that ICG possess an inherent toxicity (dark toxicity) that is cell-typespecific. In accordance with these findings23 24 25 our results imply that RGCs are highly susceptible to ICG.
In vivo, we have shown that ICG is toxic to RGCs at a high dose (2.5 mg/L). Under this condition, the number of RGCs was reduced by 23%. If the dye is distributed evenly in the vitreous cavity (60 µL in rats), the intravitreous concentration of ICG would be 1.7 x 10-7 M. In light of our in vitro experiments, 23% reduction of RGCs occurs in the presence of ICG at a concentration of 2.1 x 10-5 M. Apparent higher toxicity of ICG in vivo compared with that in vitro may be explained as follows. First, our in vivo observation showed that the dye initially diffuses throughout the retina and accumulates to the optic disc. Thus, RGCs may have been exposed to much higher concentrations of ICG than 1.7 x 10-7 M. Second, previous laboratory studies have demonstrated that exposure to ICG alone induces ILM detachment and disruption of the ILM in cadaveric eyes.27 Although we found no apparent light microscopic abnormalities in vivo experiments under the present conditions, yet undetermined damage to the retinal structure may have caused secondary degeneration of the RGCs. Finally, it is possible that ICG exerted a photodynamic effect.
A previous study demonstrated that the toxic effects of an ICG solution on RPE is related to osmotic effects of the solvent.26 Our in vitro experiments used ICG solutions with osmolarities ranging from 189 to 210 mOsM. These osmolarities were low compared with those used in former studies, in which the osmolarity of the ICG solutions was approximately 275 to 290 mOsM. This is presumably because the osmolarity of the solvent medium (i.e., neurobasal medium18 : 201 mOsM in this study) is lower compared with the solvent medium used in previous studies (
290300 mOsM).28 Under the current in vitro conditions, the toxic effect on RGCs was observed in a concentration-dependent, but not osmolarity-dependent manner. In addition, osmolarity of the ICG solutions used in vivo experiments was not reduced compared with the physiological saline. Taken together, it is likely that the toxic effect of ICG solution to RGCs is dose-dependent and cannot be related to hypo-osmolarity.
Our results raise the possibility that ICG-induced RGC damage can occur after ICG-assisted vitrectomy. Although our in vivo experiments do not mimic the clinical situation, the dye initially was distributed on the retinal surface and transported toward the optic disc. Although the dye is immediately washed after ILM staining during vitrectomy, persistent retinal staining is observed,12 13 14 suggesting that a significant amount of the dye resides in the eye. It is impossible to know the actual concentration of the residual dye; however, the concentration of the retinal ICG may be higher in a patients eye than was observed our in vitro experiments, because the fluorescence is apparently brighter in patients after ICG-assisted vitrectomy compared with those in rats in the present experiments.
Another important question is whether ICG exerts a photodynamic effect on RGCs during ICG-assisted vitrectomy. In vitro, we showed that the number of viable RGCs was not decreased after a brief exposure to ICG at a concentration of 2.5 x 103 mg/L and to a standard endoillumination for 15 minutes. It should be noted that a previous study demonstrated that ICG can induce apoptosis in human RPE cells under an irradiation condition identical with ours.19 Because the ICG concentration used herein (2.5 x 103 mg/L) is comparable to the clinically used concentrations (0.04%0.5%)1 2 3 4 5 6 and the light dose was maximal, we believe that an ICG-induced photodynamic effect on RGCs is unlikely to happen during surgery.
In conclusion, our results demonstrate that ICG was toxic in long exposure of 72 hours to RGCs both in vivo and in vitro at concentrations lower than clinically used and that ICG-induced RGC damage can occur theoretically during and after ICG-assisted vitrectomy. One thing that deserves special mention is that some investigators have reported ICG-related adverse effects.4 5 6 Previous investigators have reported a possible toxicity of ICG on the retina. However, ICG was not toxic in brief exposure of 20 minutes in our experimental model. Using human donor eyes, previous investigators have demonstrated that the inner retina is severely damaged when ILM is stained with ICG and illuminated by a light source with the emission beyond 620 nm,27 a wavelength overlapping the absorption band of ICG.28 In addition, several laboratory studies have demonstrated that ICG can cause retinal pigment epithelial damage in vitro.19 23 Together with the present study, these results suggest that the damages can occur in retina when higher concentration of ICG is injected into the vitreous cavity. However, it is uncertain whether ICG-induced RGC damage occurs in every clinical situation. It is particularly noteworthy that several investigators have reported a favorable visual functional outcome free of ICG-related adverse effects after ICG-assisted vitrectomy.8 9 10 11 In vitro, a long exposure to ICG did not affect the RGC survival when the dye was applied at concentrations lower than 10 mg/L (0.001%). Thus, it seems important to reduce the residual dye in patients eyes. Lower concentration and shorter staining time should be used in ICG-assisted vitrectomy.
| Footnotes |
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Submitted for publication September 17, 2003; revised October 22 and November 12, 2003; accepted November 23, 2003.
Disclosure: A. Iriyama, None; S. Uchida, None; Y. Yanagi, None; Y. Tamaki, None; Y. Inoue, None; K. Matsuura, None; K. Kadonosono, None; M. Araie, None
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C.
1734 solely to indicate this fact.
Corresponding author: Yasuo Yanagi, Department of Ophthalmology, University of Tokyo School of Medicine, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan; yanagi-tky{at}umin.ac.jp.
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