Combination chemotherapy potentiates 5-aminolevulic acid-mediated photodynamic therapy killing of patient-derived glioma stem-like cells in vitro

Abstract Background 5-aminolevulinic acid photodynamic therapy (5-ALA PDT) is a promising therapy for glioblastoma. 5-ALA induces tumor-selective accumulation of protoporphyrin IX (PPIX). Photoactivation of PPIX induces reactive oxygen species generation and tumor cell death. Glioma stem-like cells (GSC) are hypothesized to be resistant to standard therapies and contribute to glioblastoma (GBM) recurrence. Differential PPIX accumulation in GSC could lead to treatment resistance. We investigated if chemotherapeutics currently utilized in GBM treatment, temozolomide (TMZ) and bis-chloroethylnitrosurea (BCNU), could potentiate 5-ALA PDT killing of GSC. Methods Patient-derived GSC lines were established. PPIX accumulation was assessed with flow cytometry. 5-ALA metabolic enzyme mRNA expression was measured with qRT-PCR. BCNU or TMZ was co-administered with 5-ALA prior to laser irradiation. The effect of combination therapy was evaluated using Loewe combination index analysis. Results Three GSC lines were exposed to 5-ALA in vitro. Peak PPIX fluorescence levels differed significantly between two cell lines (“high-” and “low-fluorescing” GSC); there were no significant differences in 5-ALA metabolism enzyme expression between these. PPIX fluorescence correlated with GSC susceptibility to 5-ALA PDT. TMZ did not affect 5-ALA PDT efficacy. BCNU and 5-ALA PDT worked synergistically in low-fluorescing, but not in high-fluorescing, GSC. In high-fluorescing GSC, BCNU upregulated heme-oxygenase 1 (HMOX1) expression, an enzyme that promotes PPIX catabolism. Conclusions We report the effect of combination chemotherapy and 5-ALA PDT on GSC viability, demonstrating the potential for BCNU and 5-ALA PDT to work synergistically. We speculate that HMOX1 upregulation by BCNU could mitigate this effect.