# Xanthone Derivatives in the Fight against Glioblastoma and Other Cancers
## 2 Antineoplastic Activity of Xanthone Derivatives
### Gambogic Acid  
Gambogic acid (**5**. Figure 3) is the major active ingredient of Gamboge (resin) from the _Garcinia hanburryi_ tree and is used in traditional Chinese medicine [30]. It has been tested for cytotoxic activity with the use of various cell lines, including those within CNS. Gambogic acid induces ROS generation, which contributes to the anticancer activity of this compound on **glioblastoma multiforme T98G** cell lines (anti-proliferative activity at **300 nM, MTT assay**). The generation of ROS in cancerous cells results in the induction of apoptosis by activating caspase-8, -9, and -3, and down-regulating the expression of Bcl-2 as well as increasing the release of the cytochrome c apoptosis-inducing factor (AIF). Co-treatment with antioxidants neutralizes the cytotoxic effect of Gambogic acid, which confirms the involvement of ROS in its mechanism of action [30]. There are more written reports regarding the interference of Gambogic acid in the antioxidant system of the cell. In our study, it decreased significantly the activity of the pivotal antioxidant enzyme--superoxide dismutase (SOD). The observed effect was opposite to the results obtained for \(\alpha\)-Mangostin [4; 5]. ROS can affect cells in two ways, depending on the concentration. While slightly increased, they promote cell proliferation and differentiation. However, a large increase in the ROS concentration leads to oxidative damage and cell apoptosis. Both phenomena may be supported/prevented by the mechanisms developed by cancer cells [5]. For example, the level of the mitochondrial form of SOD (MnSOD) may be decreased, leading to accumulation of the superoxide anion, which stimulates cell growth. As the tumor cells develop, MnSOD expression increases, the catalase and the glutathione peroxidase expression decrease, and the overall ROS levels constantly increase. Therefore, both the anti- and pro-oxidant activities do not preclude the anti-cancer effect of the compound. Gambogic acid (GA) also causes the apoptosis of rat **C6 glioma** cells in a concentration-dependent manner by the mitochondrial pathway of apoptosis [49]. The research was performed with the use of **rat brain microvascular endothelial cells (rBMEC)** and apoptosis was induced via mitochondrial Bax, Bcl-2, caspase-3, caspase-9, and cytochrome c release [49]. Bcl-2 and Bax are two kinds of apoptosis-related proteins and caspase-3 is the regulator downstream of Bcl-2 and Bax [30; 50; 51]. As a result, the expression of Bcl-2 was decreased, while Bax and caspase-3 increased, thus causing the reduction in the ratio of Bcl-2 to Bax [49; 52]. It had an antineoplastic effect on **rat C6 glioma cells**. Secondly, Gambogic acid passes the blood-brain barrier, which could make this compound very useful in glioblastoma therapy. The in vivo study also revealed that an intravenous injection of GA once a day for two weeks could significantly reduce tumor volumes by the process of anti-angiogenesis and apoptosis of glioma cells. In addition to the previously noted possible mechanisms of action, there are several less common molecular targets associated with gambogic acid. Transferrin receptor 1 (TfR), is a transmembrane protein whose expression is increased in rapidly dividing cells--in cancers such as glioma, as well as colon and pancreatic cancers. It has been proven that Gambogic acid binds to TfR, inducing the signal leading to rapid apoptosis [53]. The effect of gambogic acid on another interesting target--phosphoinositide kinase-3 (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling pathway--was examined in human **U251 glioma cells** (compound cytotoxic in MTT assay at 5 nM). Gambogic acid decreased the phosphorylation of P38, AKT, and mTOR. Additionally, it reduced the phosphorylation of ribosomal protein precursors (Pre), the insulin-like  
Figure 3: Gambogic acid (**5**) chemical structure. growth factor I (IGF-1), and the upstream binding factor (UBF).