In vitro evaluation of Radachlorin sensitizer for photodynamic therapy

This paper reports the evaluation of a new photosensitizer, Radachlorin in comparison with one of its well known components but used solely, Chlorin e(6). The photodynamic properties and cell uptake and localisation of the two drugs were compared. In vitro studies were conducted on human adenocarcinoma cells (HT29) and lung carcinoma cell line (A549). Both dyes showed an absorption maximum between 640 and 650 nm, but those absorption peaks are enhanced by interactions with serum, with a shifted maximum at 661 and 664 nm, and much higher absorbance. As Radachlorin is constituted of different products and as photoreactivity is dependent on absorbed light energy, we chose to adapt concentrations so that both drugs had the same absorption at the irradiation wavelength (664 nm) for photoreactivity tests, and express concentrations in optical density at 664 nm. The capacity of the two drugs to generate Reactive Oxygen Species was identical, but on HT29 cells, Radachlorin reaches its optimal LD50 sooner than Chlorin e(6). Radachlorin LD50 on HT29 cells was 0.0251 OD(664 nm) after 2 h and 0.0672 OD(664 nm) for Chlorin e(6) for a 20 J cm(-2) irradiation. Radachlorin gave very similar results on A549 cells, LD50 being 0.05 for 5 J irradiation, and 0.026 for 10 and 20 J cm(-2). Pharmacokinetics using fluorescence showed that, even if Radachlorin quickly crossed HT29 (a human colonic cancer line) cell membrane, cellular distribution evolved from a diffuse cytoplasmic repartition 1 hour after Radachlorin addition to a delimited localisation into organelles all around the nucleus. Radachlorin intracellular fluorescence decreased after 4 h, whereas we did not observe a decrease of Chlorin e(6) intracellular fluorescence for times up to 24 h. In both case, a quick decline was observed as soon as the culture medium was replaced with a drug-free one. Radachlorin appears to be an excellent photosensitizer, with similar phototoxicity to Chlorin e(6) on cell cultures, but with quicker kinetics, which could be an improvement if confirmed on further in vivo studies.