Inhibition of vascular smooth muscle cells by photodynamic therapy is effective and depends on intracellular Photofrin

Photodynamic therapy (PDT) is a promising treatment for vascular restenosis. Determining the site of the photosensitiser responsible for cytotoxicity during PDT is essential for planning treatment strategies. Vascular smooth muscle cells (VSMCs) were cultured from 6 individuals using substrate attached explant techniques and transferred into microtitire plates at 50,000 cells/well. Cells were arranged in 3 groups; the first group was incubated with 2 (mu) gml-1 of Photofrin for 48 hours, followed by polychromatic light illumination of 3 Jcm-2. In the second group the extracellular Photofrin was removed by washing cells just prior to illumination, and in the third group Photofrin was only added immediately prior to illumination. Results are expressed as mean percentage cell survival+/- SEM. Cells were minimally affected by extracellular Photofrin alone (91.9+/- 10.7). Photofrin in the intracellular and (intra+extracellular) compartments reduced VSMCs survival to (7.3+/- 4.9) and (8.8+/- 2.6). The mechanism of ablation appears to be due to activation of the intracellular rather than residual Photofrin in the extracellular medium