Background: This study explores the therapeutic potential of radiodynamic therapy (RDT), a combination of the photosensitizer 5-aminolevulinic acid (5-ALA) administration and X-ray irradiation, for high-grade glioma (HGG). The research aims to verify the RDT efficacy in both normoxic and hypoxic environments, examine its mechanisms, and assess its impact on the tumor micro-immune environment to address resistance to RDT. Methods: Glioma cell lines U87MG and U251MG were used in experiments in vitro. The cells were divided into four groups with or without 5-ALA and X-ray exposure. Results: Results demonstrated that RDT was effective under normoxia (20% O2), increasing reactive oxygen species (ROS) production and significantly decreasing U87MG cell viability in a 5-ALA concentration-dependent manner at 2 Gy and 6 Gy. However, under hypoxic conditions (3% O2) or long-term 3% O2 exposure, the RDT effect was not significant compared to controls. The study also found that RDT under normoxia influenced immune reaction-related gene expression, while under hypoxia, it primarily affects genes related to epithelial–mesenchymal transition (EMT). Further analysis revealed that RDT reduces the secretion of soluble PD-L1, a marker of immune checkpoint inhibition, in a 20% O2 environment. Additionally, RDT suppressed the vascular endothelial growth factor (VEGF), an angiogenesis marker, under 3% O2 conditions. RDT also reduced the secretion of colony-stimulating factor -1 (CSF-1), a differentiation inhibitory marker for macrophages, in a 20% O2 environment. Conclusion: In conclusion, this study provides evidence that RDT, combining 5-ALA and X-ray irradiation, has potential as a therapeutic strategy for HGG, especially under normoxic conditions. It may also offer benefits under hypoxia, particularly in inhibiting angiogenesis. The study also highlights the importance of understanding the role of oxygen levels in the efficacy of RDT and its potential impact on immune responses, angiogenesis, and macrophage differentiation in the tumor microenvironment. Further research is needed to fully elucidate the underlying mechanisms and optimize RDT for clinical application.
Ishikawa et al. (Mon,) studied this question.