Feasibility of laser targeted photo-occlusion of ocular vessels

Neovascularisation occurs in many major ocular diseases such as diabetes, age-related macular degeneration, and sickle cell disease. Laser photocoagulation is typically used to obliterate the vessels but it also causes severe damage to adjacent normal tissues. This is a very significant limitation especially in the treatment of choroidal neovascularisation which often covers large areas of the posterior pole and the fovea. A method, laser targeted delivery, has been developed capable of releasing drugs locally and non-invasively in the choroidal or retinal vasculature. This method could be used to target a photo-sensitiser to neovascular membranes and cause their selective occlusion by irradiating them. The targeting properties of the method promise to yield a treatment for neovascularisation that does not damage adjacent tissues and thus preserves vision. The purpose of the present study was to test the feasibility of occluding ocular vessels with this method.

The iris vessels of the albino rat were chosen because the treatment could be assessed unequivocally and followed with time. Aluminium phthalocyanine tetrasulphonate was encapsulated in heat sensitive liposomes and administered systemically. The iris vessels were irradiated with a yellow laser to raise their temperature to 41 degrees C, cause a phase transition in the liposomes and thereby locally release the photosensitiser. The laser was also used to excite the released photosensitiser and cause occlusion. The effect was monitored immediately and for 8 months thereafter. Controls for the effect of the laser and the unencapsulated drug were conducted.

The results demonstrated that occlusion can be achieved and sustained for the period of follow up. The controls showed that the effect was not due to heat or to the activation of the low dose of free drug.

These preliminary findings indicate that laser targeted photo-occlusion is a promising new method for the treatment of neovascularisation.