In vitro killing of human malignant mesothelioma by photodynamic therapy

Photodynamic therapy was investigated as a potential new modality for the treatment of human malignant mesothelioma (HMM) utilizing the H-MESO-1 HMM cell line and the photosensitizing agent, Photofrin-II (PF-II). Uptake of PF-II by H-MESO-1 was documented by incubating H-MESO-1 cells with PF-II and measuring the fluorescence at 625 nm following excitation at 400 nm. Cytotoxicity of photodynamic therapy was determined by incubating H-MESO-1 cells (2 × 10⁵) in microtiter plates for 24 hr with concentrations of PF-II varying from 0 to 10 pg/ml. The wells were exposed to gold vapor laser light (628 nm) in doses ranging from 0 to 24,000 J/m². Twenty-four hours following treatment, [³h]thymidine (1 μCi) was added to each well. Cells were harvested 24 hr later and counted for tritium incorporation. Five replicates were performed for each combination of light and drug. Peak absorption of PF-II by HMESO-1 was reached within 8 hr. Maximal doses of light alone caused minimal cell killing. PF-II without light was cytotoxic only at the highest concentrations. However, the combination of PF-II at concentrations at or above 2.5 μg/ml and light produced a significant increase in cytotoxicity. These data demonstrate that photodynamic therapy can effectively kill human malignant mesothelioma cells in vitro.