Comparison of phototoxicity mechanism between pulsed and continuous wave irradiation in photodynamic therapy

A study has been conducted in which HeLa cells are incubated with hematopor phyrin derivative (HpD) for 1 h ( 1 mu g/ml of HpD in PBS) to compare the u se of continuous wave (CW) and pulsed laser ( 10 Hz repetition rate and 7-9 ns pulse width) light for photodynamic therapy. Cytotoxic effects on the c ells are evaluated by the 3-(4,5-dimethyl-2-thiazolyl)-2-5-diphenyl-2H-tetr azolium bromide (MTT) assay and the fluorescein diacetate (FDA)/propidium i odide (PI) stain method using a flow cytometer. The type of cell death is e stimated by analysis of the DNA content and observation of the nuclear morp hology. The cytotoxicity ratio of cells irradiated by pulsed laser light is estimated to be lower than that for CW laser light. The viability of cells that received pulsed laser light gradually decreases, whereas no significa nt changes are found in the cells irradiated with CW laser light with the e lapse of post-irradiation time. The type of cell death differs between the pulsed and CUI laser light irradiations. These findings suggest that the cy totoxic efficacy of the excitation light source is displayed by the differe nce in the type of cell death, namely apoptosis or necrosis. (C)1999 Elsevi er Science S.A. All rights reserved.