Changes in Antigen Expression on Human FME Melanoma Cells after Exposure to Photoactivated Hematoporphyrin Derivative1

Exponentially growing melanoma cells of the line FME were incubated with hematoporphyrin derivative (HpD) for 1 and 18 h and subsequently exposed to light in the presence of HpD. Quantitative changes in the expression of the melanoma-associated surface antigen p250 recognized by the monoclonal antibody 9.2.27 were studied by flow cytometry. Treatment with HpD and light resulted in no immediate changes in the antigen expression. However, a few hours after light exposure a significant reduction in antigen expression was observed. For cells incu bated with HpD for 1 h, the minimum expression of the antigen was observed 6 h after the irradiation, and the duration of the reduced expression was almost dose independent. On the other hand, the duration of the reduced antigen expression increased strongly with light dose for cells incubated with HpD for 18 h. In both cases antigen expression decreased exponentially with the product of drug concentration and light dose, indicating that there is no rapid mechanism by which the cells can repair the damage which leads to reduced antigen expression. Days were needed before the cells expressed a normal level of the antigen. A slight overshoot of the level of antigen expression above that for untreated cells was observed 2-5 days after light exposure depending on the incubation conditions with HpD and the light dose. At a given cell survival level (>0.1), the decrease in antigen expression was more pronounced on cells incubated with HpD for 1 h than on cells incubated with the drug for 18 h.