Threshold and retreatment parameters of NPe6 photodynamic therapy in retinal and choroidal vessels

BACKGROUND AND OBJECTIVE: To determine the threshold fluence for producing choroidal and retinal vascular occlusion with mono-l-aspartyl chlorin e6 (N Pe6) photodynamic therapy (PDT) during primary treatment and the effect of retreatment. METHODS: Primary treatment: Rats, rabbits, and monkeys underwent NPe6 PDT t o determine the threshold fluences for choroidal and retinal vessel occlusi on. The threshold was determined by analyzing fluorescein angiograms for ar eas of nonperfusion. Retreatment: Dutch-belted rabbits underwent NPe6 PDT f ollowed by fluorescein angiography. Rabbits were retreated one week later a t the same parameters. RESULTS: Fluence levels and vascular damage thresholds were always higher f or retinal than for choroidal vascular occlusion. Retreatment caused choroi dal vessel closure at all tested fluences but retinal capillaries closed on ly at a fluence > 17.7 J/cm(2). CONCLUSION: NPe6 PDT has a lower threshold to occlude choroidal vessels tha n retinal vessels. The cumulative effect of retreatment does not damage ret inal vessels unless the threshold is exceeded during a single retreatment s ession.