In view of the potential of compound 15 to act as a PDT agent for glioblastoma, we examined its ability to selectively kill glioblastoma cells when they were cultured in the presence of non-cancerous brain cells. We co-cultured the glioblastoma cell line U87-nlsCrimson with human HCN-2 cells as representative non-cancerous neuronal precursor cells. HCN-2 cells were pre-stained with CellMask Green and co-cultured with U87-nlsCrimson cells (Figure ). Under normal co-culture conditions, both HCN-2 and U87 cells were viable, as indicated by fluorescence emission of their respective markers (i.e., CellMask Green for HCN-2, E2Crimson for U87, Figure ). Co-cultures were then incubated with compound 15, irradiated with visible light at 37 J cm -2 and monitored by fluorescence confocal microscopy. Notably, the combination treatment of compound 15 and light induced cell death in most U87 cells -as shown by the lack of fluorescence in the E2Crimson channel-but not in HCN-2 cells, which are less glycolytic and metabolically-active (Figure ). Plot profile analysis confirmed that HCN-2 cells remained intact after the treatment whereas U87 cells were devoid of E2Crimson fluorescence (Figure ). Furthermore, we confirmed that co-cultures which had not been treated with compound 15 or not exposed to light showed no signs of death in any cell type (Figure ). Finally, we analyzed the solubility and phototoxicity profiles of compound 15 and 5-aminolevulinic acid (5-ALA, Figure ), the current gold standard in image-guided surgery for brain tumor resection.