TKT and brain cancer: Previously, proof-of-concept studies used the biocompatible (mPEG-TK-COOH) (Swierczewska et al., 2015; Regmi et al., 2019) to create an ROS-responsive mPEG-TK conjugate with a fluorescent model drug (Cy5), and demonstrated a stimulus-responsive release of this dye only in brain cancer cells (C6 rat GBM cells) and not in healthy brain cells (rat astrocytes) (Oddone et al., 2019).