“…For example, lower pH (Chen et al., 2014; Lv et al., 2016; Hong et al., 2017) and higher glutathione (GSH) levels (Cheng et al., 2011; Raza et al., 2018) have been reported in TME. Studies on tumor surface proteins have reported the presence of multiple specific receptors, such as CD44 receptors (Chen et al., 2016, 2017a; Dong et al., 2018), folate receptors (Chen et al., 2017b), glycyrrhetinic acid receptors (Tian et al., 2012; Sun et al., 2017; Wang et al., 2018b; Yan et al., 2018), and transferrin receptors (Zhao et al., 2018). Based on the specificities of tumor cells, formulation workers have designed a range of targeted agents for the targeted delivery of anti-cancer drugs to tumor sites (Wang et al., 2018a, 2018c).…”