# Xanthone Derivatives in the Fight against Glioblastoma and Other Cancers
## 1 Introduction
### Characteristics of Medicinal Chemistry Research Regarding Brain Cancers
Many alterations in the progress of gliomas [11] have been observed, such as an increase in the activation and/or overexpression of growth factors and receptors (EGF, TGF, and EGFR), as well as increased mutations of tumor suppressor genes (e.g., p. 53, phosphatase and tensin homolog deleted on chromosome 10, PI3K/AKT--intracellular signaling proteins). Development of therapeutic agents targeting these altered signaling cascades may provide alternative strategies, in addition to conventional treatment procedures. On the contrary, cell death is regulated by mitochondrial signaling involving anti-apoptotic proteins such as Bcl-2 (B-cell lymphoma 2) and Bcl-XL (Bcl-2-like protein) and pro-apoptotic proteins such as Bax (Bcl-2-associated X protein), Bid (BH3-interacting domain death agonist), and Bad (Bcl-2-associated death promoter, BCL-2 antagonist of cell death). The pro-apoptotic proteins activate caspase-3, -8, and -9 and induce the cleavage of caspase substrate proteins including poly-ADP ribosyl polymerase (PARP), which conducts the last step of the apoptotic cell signaling pathway [30]. Research on new antineoplastic agents relies on cell viability assays, which are useful tools in screening a large number of chemicals for their cytotoxicity to various cell lines. We can divide them into dye exclusion, colorimetric, fluorometric, luminometric, and flow cytometric assays. Among them, the most commonly used ones are colorimetric assays, where biochemical markers are measured spectrophotometrically to determine the metabolic activity of the cells [31]. The NR (neutral red, 3-amino-7-dimethylamino-2-methyl-phenazinehydrochlorid) [32], CV (crystal violet), and SRB (sulforhodamine B) assays rely on the uptake of the dye by the viable cells and its retention within lysosomes (NR) or proteins and DNA (CV). In the case of SRB, the dye binds to proteins in acidic conditions, while rising pH enables its dissolution [31]. Among commonly used tetrazolium salts, MTT (3-(4,5-Dimethyl-2-thiazol)-2,5-diphenyl-2 tetrazolium bromide) [33] was the first to be developed. The test relies on the reduction of yellow salt into purple insoluble formazan in the reaction catalyzed by the dehydrogenase in metabolically active viable cells. The remaining tetrazolium assays are variations of the MTT test, characterized primarily by the water/medium solubility of the resulting product, which simplifies the test methodology. These include the following: XTT (2,3-Bis-(2-methoxy-4-nitro-5-sulfophenyl)-2\(H\)-tetrazolium-5-carboxanilid) [34], MTS (tetrazolium inner salt, 3-(4,5-Dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2\(H\)-tetrazolium) [33], WST-1 (2-(4-Isodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2\(H\)-tetrazolium, monosodium salt) [35], and WST-8 (2-(2-Methoxy-4-nitro phenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2\(H\)-tetrazolium monosodium salt) [36]. One of the tetrazolium derivatives (Iodonitrotetrazolium or 2-(4-Iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2\(H\)-tetrazolium) is also utilized in the LDH (lactate dehydrogenase) assay. In contrast to the above-mentioned assays, LDH is based on necrotic/apoptotic/damaged cells detection--the enzyme is released into culture media due to loss of cell membrane integrity and reduces the reagent to formazan dye [37]. In this paper, we review the role of several natural and synthesized xanthone derivatives in the treatment of glioblastoma and other cancers, using the results of cell viability assays performed with the use of various cell lines as a basis and--if available--in vivo or clinical studies. Only compounds cytotoxic to central nervous system cell lines are included. However, we decided to discuss their activity against other cancer cell lines in order to obtain a holistic picture of their pharmacological profile. In some cases, this approach also allows us to show the multi-stage process of their development as CNS antineoplastic compounds.