Two-photon photodynamic therapy and its potential application to age related macular degenerations

Abstract

Photodynamic therapy (PDT) using verteporfin is widely used for treatment of age related macular degeneration (AMD). Due to non-perfect selectivity of the drug accumulation in the neovasculature some collateral damage to healthy tissue arises during the treatment. Damage to healthy structures in the eye is always a concern because of a high probability of reducing visual acuity. Two-photon (2-g) photodynamic therapy potentially offers much higher treatment selectivity than its one-photon (1-g) counterpart. By utilizing focused light for 2-g excitation, treatment volumes on the order of microliters can be achieved thus maximizing localized insult to abnormal blood vessels and sparing healthy tissue. We propose that 2-g photodynamic therapy will be valuable in the treatment of choroidal neovascularization secondary to age related macular degeneration as well as other conditions. To ascertain feasibility of 2-g photodynamic therapy we measured 2-g spectrum and cross sections of verteporfin (80 GM at 940 nm, 1 GM = 10 -50 cm 4 s/photon), chlorin e6 (14 GM at 800 nm) and tetrasulfonated aluminum phthalocyanine (140 GM at 900 nm) and investigated their in vitro efficiency under 2-g excitation. Only verteporfin demonstrated cell kill under the used irradiation parameters (average light intensity 9.1 mW, wavelength 850 nm, total light dose 6900 J/cm 2 ). Dorsal skinfold window chamber model in mouse was used to test efficiency of 2-g PDT with verteporfin in vivo . Although we were able to induce photodynamic damage to a blood vessel using 1-g excitation, 2-g excitation resulted in no visible damage to irradiated blood vessel. The most probable reason is low efficiency of verteporfin as a 2-g photosensitizer. We also report 2-g spectrum of new photosensitizer, HCC4 (4300 GM at 830 nm), specifically designed for efficient 2-g excitation.