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Light-Induced Retinal Vascular Damage by Pd-porphyrin Luminescent Oxygen Probes

¹From the Swiss Federal Institute of Technology (EPFL), the Institute of Environmental Engineering, Lausanne, Switzerland; the ³Institute of Research in Ophthalmology, Sion, Switzerland; and the ⁴Geneva University Eye Clinic, Geneva, Switzerland.

PURPOSE. The phosphorescence lifetime of certain metalloporphyrins dissolved in a physiological medium provides an optical signature for local oxygen concentration (pO₂). This effect is used for measuring physiological pO₂ levels in various tissues. However, the phosphorescence quenching of certain metalloporphyrin triplet states by oxygen also creates singlet oxygen, which is highly reactive and capable of inducing tissue damage. In the current study, the Pd-meso-tetra(4-carboxyphenyl) porphyrin dye (PdTCPP) was simultaneously used as an oxygen sensor and a photosensitizer. Phototoxicity was assessed in the eye fundus and correlated with tissue oxygenation, drug–light dose, and severity of tissue damage.

METHODS. The kinetics of photochemical oxygen depletion during PdTCPP excitation was measured in vivo on the optic disc of piglets by phosphorescence lifetime imaging. Blood–retinal barrier breakdown and tissue damage were assessed by confocal and electron microscopy.

RESULTS. For a retinal irradiance of 5 mW/cm² at 532 nm and an injected PdTCPP dose of 20 mg/kg, the mean phosphorescence lifetime measured at the optic disc increased from 100 to 600 µs within 8 minutes of continuous illumination. This corresponds to a decrease of pO₂ from 25 to 0 mm Hg, induced by a light dose of only 2.4 J/cm². An exposure time of 6 minutes (1.8 J/cm²) generated an increase in phosphorescence lifetime from 100 to 400 µs, corresponding to a decrease in pO₂ from 25 to 4 mm Hg. This caused edema in all retinal layers, whereas irradiation of 2 minutes (0.6 J/cm²) damaged blood vessels and induced edema in the inner nuclear layer only. Heavy redistribution of occludin occurred after a 30-minute exposure time (9 J/cm²).

CONCLUSIONS. PdTCPP is potentially phototoxic under certain experimental conditions and can induce damage in peripapillary retina and optic nerve head after light exposure. The severity of tissue damage correlates with the phosphorescence measurements.

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