98th Annual Meeting DOG 2000

V 695

Photodynamic Treatment of Experimental Choroidal Melanoma by Multiphoton Activation of Melanin Precursors

M. Krause1, E. A. Wachter2, J. Xiong1, E. S. Gragoudas1, L. H. Y. Young1

Purpose: Treatment of small choroidal melanoma located near the optic disc and fovea remains difficult. A variety of photodynamic therapy (PDT) regimens for treatment of pigmented choroidal melanoma using different photosensitizers have been successfully tested in our laboratory. The present study evaluated a new modality of PDT. Two or more photons in the near- infrared or infrared can activate photosensitizers that normally require a single photon of UV or visible light. Such alternative activation modes allow high spatial confinement and deep tissue penetration of light. For tissue with high amounts of melanin, as found in pigmented melanoma, biochemical precursors of melanin can be converted into phototoxic products upon exposure to UV-light. Such conversion is also possible via multiphoton excitation using near-infrared light.

Methods: Experimental pigmented choroidal melanomas were established in New Zealand albino rabbits by implantation of B16F10 melanoma cells into the subchoroidal space. A high repetition-rate (120 MHz) mode- locked Nd:YLF laser emitting femtosecond laser pulses at 1047 nm was used for PDT of the melanomas by targeting endogenous melanin precursors. No additional photosensitizing agents were administered.

Results: 16 of 19 tumors (84%) were eradicated by a single treatment. Regrowth was detected in three animals after 10d. No metastases were detected in liver or lung of treated animals.

Conclusions: The data indicate that successful PDT of pigmented choroidal melanoma is possible by multiphoton excitation without additional administration of exogenous phototoxic agents. Thicker choroidal melanomas and different light dosages are included in ongoing studies.

1Massachusetts Eye and Ear Infirmary, Department of Ophthalmology, Harvard Medical School, Boston, MA, USA
Photogen Technologies, Inc., Knoxville, TN, USA
Support: NIH EY 10975, Mass.Lions Eye Res. Fund, Photogen Technologies, Inc., DFG Grant Kr 1918/1-1,1918/1-2