Photodynamic therapy (
PDT) involves the use of
laser or noncoherent light energy with photosensitizing
dyes to induce a cytotoxic reaction in the target cells, resulting in cell injury and/or death. In this study, we have examined
laser-induced
phototoxicity in normal human skin fibroblasts and HT-1080
fibrosarcoma cells incubated with
aluminum phthalocyanine tetrasulfonate (AlPcS) in vitro. The culture,
laser, and
photosensitizer parameters were varied in attempts to establish the conditions for differential cytotoxicity between normal and malignant human fibroblasts. Biochemical assays, as a measure of cytotoxicity, included [3H]
thymidine incorporation (an index of DNA replication), [35S]
methionine incorporation (a measure of
protein synthetic activity), and the MTT assay (an indirect index of mitochondrial activity). In the absence of
laser irradiation, AlPcS was non-toxic to both cell lines in concentrations up to 25 micrograms/ml.
Laser light alone at 675 nm (the absorption maximum of AlPcS) had no effect on the cells at energy densities up to 16 J/cm2. In the presence of 3 or 10 micrograms/ml of AlPcS, both cell lines demonstrated marked energy-dependent toxicity. If an 8-h or a 24-h "efflux" period in AlPcS-free medium was allowed to take place prior to
laser irradiation, normal fibroblasts were much less sensitive to
PDT, whereas
fibrosarcoma cells still exhibited a marked degree of toxicity. The results indicate that, under appropriate treatment conditions, AlPcS is capable of preferentially sensitizing a malignant mesenchymal cell line, while sparing its non-malignant normal cell counterpart.