Effect of photofrin II and light energy on retinoblastoma-like cells in vitro. Dose-response relationships, effect of light dose rate and recovery ratio.

A clonogenic survival assay on retinoblastoma-like cells (EXP-5 cells) was applied in vitro to assess the effect of photodynamic therapy at relative low concentrations of photofrin II and high doses of light energy. The cellular concentration of photofrin II was estimated by spectrofluorimetry. The cellular content of photofrin II increased with the concentration in the medium (up to 3 micrograms/ml) and the incubation time (up to 24 h) in cells first incubated in photofrin II for 24 h and then in photofrin-II-free medium for 24 h. There was an obvious dose-response relationship between cell damage and photodynamic therapy both for the cellular photofrin II content and the total amount of light energy delivered. The dose rate of light energy had no influence on the cell damage. The cells had a recovery factor of only 1.34, suggesting a low repair of sublethal damage. An approximately linear isoeffect curve at the 10% survival level was described as a function of photofrin II concentration and light energy, suggesting a predictable reversible relationship between them. The results suggest that retinoblastoma cells are very sensitive to photodynamic therapy.