Threshold power levels for NPe6 photodynamic therapy

To determine the threshold power levels for producing retinal and choroidal vascular occlusion using mono-L-aspartyl chlorin e6 (NPe6) photodynamic therapy; to evaluate its efficacy with longer intervals between photosensitizer injection and laser application; to determine the elapsed time between light application and appearance of angiographic changes.

Pigmented and nonpigmented rabbits were injected intravenously with 2 mg/kg of NPe6 before laser irradiation of the retina-choroid. Group 1 was treated at increasing power levels; fluorescein angiograms were obtained at each fluence. Group 2 animals were exposed to laser irradiation at 5 minutes, and 1 and 3 hours postinjection to determine (by fluorescein angiography 24 hours post-treatment) if increasing the interval affected outcome. Group 3 animals underwent fluorescein angiography at 30 minutes, 1 hour, 2 hours, and 24 hours posttreatment to document the time between laser application and subsequent vessel closure.

Choroidal vessel occlusion was angiographically evident in all lesions at fluences of > or = 2.65 J/cm2 in pigmented rabbits and at > or = 0.88 J/cm2 in nonpigmented rabbits. Lesion diameter decreased as the time between injection and treatment increased. Vessel occlusion was documented at least 2 hours after treatment.

Choroidal vessel occlusion can occur at very low fluence.