The effect of hypoxia on photodynamic therapy with 5-aminolevulinic acid in malignant gliomas

Background: Glioblastoma (GBM) is a high-grade, poor prognosis tumor that is resistant to standard treatment. The presence of a small number of glioma stem cells (GSCs) surviving in the harsh microenvironment is responsible for their refractoriness. This study aimed to investigate the effect of a hypoxic environment on the sensitivity of GSCs to photodynamic therapy with 5-aminolevulinic acid (ALA-PDT).Materials and methods: Six human GSC lines, Mesenchymal types HGG13, HGG30, HGG1123, and Proneural types HGG146, HGG157, HGG528, were divided into two groups: normoxia (O2 21%)-cultured cells (Normoxia-GSCs), and hypoxia (O2 5%)-cultured cells (Hypoxia-GSCs). To compare the effects of different oxygen partial pressures on photoporphyrin IX (PpIX) biosynthetic activity, PpIX biosynthetic enzyme and transporter expression levels were examined by qRT-PCR; the intracellular PpIX concentration was determined using flow cytometry. Addi-tionally, the sensitivity of these two groups of cells to ALA-PDT was evaluated in vitro.Results: Hypoxia-GSCs showed higher mRNA levels of FECH (ferrochelatase), which is required for iron synthesis to convert PpIX to heme, compared with Normoxia-GSCs. Flow cytometry revealed that the accumulation of PpIX in Hypoxia-GSCs reduced upon incubation with ALA. However, Hypoxia-GSCs showed less reduction in sensi-tivity to ALA-PDT than Normoxia-GSCs.Conclusion: Hypoxia-GSCs had lower intracellular PpIX accumulation than Normoxia-GSCs due to increased gene expression of FECH, and that their sensitivity to ALA-PDT was reduced less, despite accumulating lower con-centrations of PpIX. ALA-PDT is a potentially effective therapy for hypoxia-tolerant GSCs that exist in hypoxia at 5% oxygen concentration.

Automated summary

This study investigated the effect of a hypoxic environment on the sensitivity of glioma stem cells (GSCs) to photodynamic therapy with 5-aminolevulinic acid (ALA-PDT). The results showed that hypoxia-GSCs had lower intracellular PpIX accumulation and less reduction in sensitivity to ALA-PDT compared to normoxia-GSCs.