Evaluation of the new photosensitizer Tookad (WST09) for photodynamic vessel occlusion of the choroidal tissue in rabbits

PURPOSE. To determine the efficacy of Tookad (WST09; NegmaLerads, Magny-Les-Hameaux, France) photodynamic therapy (T-PDT) by evaluating the angiographic and histologic closure of choroidal vessels at different radiance exposures, drug dosages, and intervals between photosensitizer injection and laser application in a rabbit model. METHODS. Chinchilla Bastard rabbits were injected intravenously with three different dye concentrations (2.5, 5, and 10 mg/kg) before application of light. In every group T-PDT was performed at four different times after injection: 5, 15, 30, and 60 minutes with different radiance exposures ranging from 200 to 3 J/cm². Fundus photographs and fluorescein angiograms were obtained 90 minutes after injection. Follow-up angiographies were performed at days 1, 3, 7, and 14 after initial treatment. Histology was performed in selected cases immediately after treatment and on days 1,3, and 7. RESULTS. Immediately after irradiation, most of the visible le-sions were angiographically hyperfluorescent due to damaged vessel endothelium and associated RPE damage. Lesions from high-radiance exposures revealed immediate hypofluorescence, indicating vessel closure. Hypofluorescent lesions appeared mainly during day 1 (all lesions angiographically visible, some hypofluorescent) to day 3 (all lesions hypofluorescent) after treatment. At day 7, ophthalmoscopically visible hyper-pigmentation took place in all lesions. ED₅₀ thresholds for angiographic hypofluorescence determined at day 3 after treatment with 2.5 mg/kg were 18.8 J/cm² (5 minutes), 62.0 J/cm2 (15 minutes), and > 100 J/cm² (30 minutes); with 5 mg/kg, 8.4 J/cm² (5 minutes), 22.8 J/cm² (15 minutes), 54.5 J/cm² (30 minutes), and > 100 J/cm² (60 minutes); and with 10 mg/kg, 11.7 J/cm² (30 minutes) and 54.1 J/cm² (60 minutes). Histology of the angiographically hypofluorescent lesions revealed vessel thrombosis in all groups 1 hour after PDT up to 7 days after treatment. Sparing of photoreceptors indicated selectivity of T-PDT; however, slight damage was partly observable. After 7 days, localized proliferation of the RPE cells was noted and was enhanced 14 days after treatment. CONCLUSIONS. T-PDT has the potential to achieve selective choroidal vessel occlusion with proper parameter selection, such as (1) 2.5 mg/kg, 5 minutes, 100 J/cm²; (2) 5 mg/kg, 5 minutes, 25 J/cm²; or (3) 5 mg/kg, 15 minutes, 50 J/cm²; however, slight damage to the photoreceptors cannot be ruled out. RPE proliferation indicates primary RPE damage due to PDT, also described with the use of all other photosensitizers.