gms | German Medical Science

Objective: Emitting red light and the generation of reactive oxygen species are two physical reactions of protoporphyrin IX (PPIX) upon irradiation with different wavelength of light. These two mechanisms are widely used either as an intraoperative tumor marker or to directly “attack” different kind of tumor entities. But what happens if you combine intracellular PPIX accumulation with another sort of photons, with X-rays or standard radiotherapy. Does 5-aminolevulinic acid (5-ALA), despite its effect as a lightsensitizer, have a radiosensitizing effect? To address this hypothesis we investigated the combination of 5-ALA treatment and radiotherapy of different intensities in a human glioblastoma cell line and thereafter in glioblastoma slice cultures.

Method: The human glioblastoma cell line U373 was treated with and without 100 μg/μL 5-ALA for 6 hours. Irradiation was performed with 6MeV photons of a linear accelerator (Varian Medical Systems, Palo Alto, CA, USA) and a dose rate of 4:8 Gy/min. The cells were irradiated with 2.5, 4, 5 and 7:5 Gy. Clonogenic abilities were tested in a methylcellulose based colony formation assay (CFA) in different cell densities depending on the received radiation. Colonies were counted, plating efficiencies and surviving fractions were calculated. Treated/untreated conditions were compared and statistical significance was calculated using the paired Student’s T-Test.

Results: Doses of 4 and 5 Gy in combination with 5-ALA lead to a significant decrease in the surviving fraction in U373 glioma cell line in contrast to the not treated cells (4Gy vs 4Gy+ALA p=0.007 and 5Gy vs 5Gy+ALA p=0.03). However, the surviving fraction of the 5-ALA control condition was also significantly reduced compared to the untreated cells (p=0.026).

Conclusions: As we showed before, 5-ALA treatment may reduce the proliferation rate of target cells, which can explain the observed effects. To exclude an anti-proliferative effect of 5-ALA the experiments will be repeated with glioma slice cultures treated with/without 5-ALA and different radiation intensities, followed by life/dead cell assays. So far the hypothesis that 5-ALA may radiosensitize human glioma cells is not fully answered and further experiments are currently in progress.