Anticancer Efficacy of Antibacterial Quinobenzothiazines

The antitumor potency of a series of designed and prepared antibacterial quinobenzothiazines was evaluated against different types of human cancer cell lines, such as glioblastoma SNB-19, lung adenocarcinoma A549 and breast cancer T47D, and the activities of the compounds were compared to cisplatin and doxorubicin. 9-Propoxy-5-methyl-12H-quino[3,4-b][1,4]benzo- thiazinium chloride (4a), 9-allyloxy-5-methyl-12H-quino[3,4-b][1,4]benzothiazinium chloride (4d) and 11-benzyloxy-5-methyl-12H-quino[3,4-b][1,4]benzothiazinium chloride (4l) were the most active compounds; their IC50 values against all three cell lines ranged from 5.3 to 9.3 µM. The effective derivatives showed no cytotoxic effect up to 100 µM on normal human dermal fibroblasts (NHDFs). To explore the structure–activity relationship, the effect of the type/nature and position of the substituents on the tetracyclic quinobenzothiazine system on the anticancer activity was investigated. Additionally, the receptor-dependent approach was used to specify the mutual ligand–enzyme (bio)compositions that might be potentially valid for the antitumor characteristics of new quinobenzothiazine derivatives. In particular, the molecular docking procedure was applied for the most potent agents against the human breast cancer line T47D in order to obtain comprehensive knowledge about the aromatase–inhibitor binding mode. The docking study revealed that some regularities in the spatial atomic distribution and nonbonding interactions (e.g., hydrophobic patterns) can be observed for the most active molecules. The surface of the electron-rich aromatic rings of 4d and 4l molecules could also contribute to π–π stacking interactions with protoporphyrin IX (HEM) as well as to the formation of π–cation interactions with the adjacent iron cofactor.