Systemic Toxicology and Laser Safety of Laser Targeted Angiography with Heat Sensitive Liposomes

Abstract

Angiography is currently limited by its lack of local and tissue specificity. The dye rapidly fills both the retinal and choroidal vessels and leaks out of the vessels thus hampering visualization of small vascular beds such as occult choroidal neovascularization. We have developed a method of laser targeted delivery based on encapsulating the dye in heat sensitive liposomes, administering the liposomes intravenously and causing them to release their content by noninvasively warming the target tissue with a laser pulse delivered through the pupil. The local release yields a bright fluorescent bolus which selectively highlights retinal or choroidal vessels. A preliminary investigation of the potential side effects of the method is presented. In rats the systemic toxicity of carboxyfluorescein-entrapped liposomes was compared with that of the free dye. No significant difference was found between the two. Non-human primates exposed to repeated laser targeted angiography were monitored over time and no significant side effects were observed. The safety of the laser exposures was assessed by conventional fluorescein angiography and histopathology. Choroidal laser targeted angiography was achieved without damage. Retinal laser targeted angiography was accompanied by mild and local damage in an area remote from the fovea. The study indicates that laser targeted choroidal angiography can be performed safely and holds promise for diseases such as age related macular degeneration with occult choroidal neovascularization. Further improvements are needed to ensure that no side effects accompany retinal laser targeted angiography.

Diode laser-induced thermal damage evaluation on the retina with a liposome dye system

“…Moreover the threshold radiant exposure for a chorioretinal thermal damage reported by McHugh and coworkers [32] is also in accordance with our data. In an experimental study with pigmented rabbit eyes using a diode laser (810 nm), these authors found a threshold radiant exposure of 64 J/cm 2 (P‫001ס‬mW, spot size : 200 m, exposure duration : 200 ms) [32]. This value is close to the lowest radiant exposure (51J/cm 2 − corresponding to power ‫ס‬ 100 mW) leading to a measurable fluorescence in our study (Fig.…”

“…No measurable autofluorescence occurring either from the thermal damage of the chorioretinal layers or from the liposomes could interfer with the fluorescence measurements. The range of power (100-400 mW) used in our study is in agreement with those found in the literature for infrared photocoagulation of the retina in pigmented rabbits [27][28][29][30][31][32]. Moreover the threshold radiant exposure for a chorioretinal thermal damage reported by McHugh and coworkers [32] is also in accordance with our data.…”

“…The range of power (100-400 mW) used in our study is in agreement with those found in the literature for infrared photocoagulation of the retina in pigmented rabbits [27][28][29][30][31][32]. Moreover the threshold radiant exposure for a chorioretinal thermal damage reported by McHugh and coworkers [32] is also in accordance with our data. In an experimental study with pigmented rabbit eyes using a diode laser (810 nm), these authors found a threshold radiant exposure of 64 J/cm 2 (P‫001ס‬mW, spot size : 200 m, exposure duration : 200 ms) [32].…”

“…Other studies show that for infrared lasers the main zone of thermal damage is centered upon the RPE and inner choroid with some extension of tissue damage to the outer retina and mid choroid [29][30][31]. No evidence of diode laser-related thermal damage to vascular elements in the neurosensory retina have as yet been described [32]. These elements would claim for the inner choroid origin of the fluorescence in our model.…”

“…This causes a temperature elevation within the target tissue sufficient to induce protein denaturation and other local tissue changes, resulting in the ophthalmoscopically visible '' laser burn. '' In their histological study, McHugh and coworkers [32] found the primary site of thermal damage to be located within the retinal pigmented epithelium (RPE), and the underlying choroid. Other studies show that for infrared lasers the main zone of thermal damage is centered upon the RPE and inner choroid with some extension of tissue damage to the outer retina and mid choroid [29][30][31].…”