[Endogenous porphyrins for photodynamic therapy of cataracts in vitro]

Photodynamic therapy is based on accumulation of a photosensitizing substance in tissues and excitement by light of a suitable wavelength. Because of high levels of locally absorbed energy this process may also enable the selective destruction of proliferating lens epithelial cells. Delta-aminolaevulinic acid (ALA) is a natural precursor of hemoglobin. An accumulation of porphyrin derivates occurs following ingestion of ALA, of which one, protoporphyrin IX (P IX), is a potent photosensitizer.

Bovine lens epithelial cells were removed from the anterior lens capsule and cultivated in Waymouth/Ham's F12 (1:2) with 10% serum. Cells of an epithelial tumor cell line were cultivated. Cultures were incubated with various concentrations of ALA and irradiated with an argon-pulsed dye laser (630 nm).

Fluorescence typical for porphyrin was detectable following incubation with 200 micrograms/ml ALA. Irradiation of such cultures with 10 J/cm2 resulted in death of the cells; 75% of the cells were lethally damaged at 3 J/cm2. Addition of P IX to the medium at a concentration of 50 micrograms/ml resulted in cell death at the low energy of 1 J/cm2. No lethal effect was observed following laser treatment without addition of ALA or P IX.

Our results show for the first time that photodynamic destruction of lens epithelial cells can be achieved by a photosensitizer, which is the product of cellular metabolism.