MMP9 and central nervous system cancer: This on-demand releasestrategy takes advantage of the elevated MMP-9 levels typically foundin the GBM tumor microenvironment, where several signaling pathwayslead to secretion and increase of MMPs in gliomas., Indeed, GBM cells are known to secrete high levels of MMP-9, whichplays a critical role in tumor progression, invasion, and metastasisformation by modulating the extracellular matrix. Once the nanofiber has crossed the cell membrane, elevatedlevels of MMP-9 cleave the linker, triggering the release of BrP covalentlybound to TPP+ in the cytoplasm.