Activatable Silicon-Xanthene Dye for Selective PDT of Glioblastoma

Photodynamic therapy (PDT) offers a promising complementary strategy for the treatment of glioblastoma multiforme (GBM); however, achieving selective activation in tumor tissue and maintaining efficacy under hypoxic conditions remain significant limitations. In this study, we present the synthesis and functional evaluation of Gal-SiX, an enzymatically activatable Si-xanthene photosensitizer designed to address these challenges. Prepared through an improved 10-step synthetic route, Gal-SiX displays a clear turn-on fluorescence and absorbance response upon β-galactosidase activation and generates reactive oxygen species efficiently in aqueous media. Mechanistic studies revealed that Gal-SiX enables both Type I and Type II PDT pathways, an advantageous feature for GBM, where oxygen availability is restricted. In vitro assays conducted on U87MG glioblastoma cells and L929 healthy fibroblasts demonstrated meaningful selectivity, with IC50 values of 3.30 μM and 7.19 μM, respectively. Gal-SiX also showed minimal dark toxicity (>80 μM) and potent light-induced cytotoxicity, yielding a phototoxicity index of 24.8 in glioblastoma cells. Confocal imaging and MTT assays consistently demonstrated its activation and PDT efficacy. Overall, this work introduces the first activatable Si-xanthene–based PDT agent for glioblastoma and provides the first evidence that the Si-xanthene scaffold can support dual Type I/II phototoxicity. These results underscore Gal-SiX’s potential as a selective PDT platform for addressing the unique constraints of GBM biology.