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A new method for modifying the structure of tetracyclic quinobenzothiazinium derivatives has been developed, allowing introduction of various substituents at different positions of the benzene ring. The method consists of reacting appropriate aniline derivatives with 5,12-(dimethyl)thioquinantrenediinium bis-chloride. A series of new quinobenzothiazine derivatives was obtained with propyl, allyl, propargyl and benzyl substituents in 9, 10 and 11 positions, respectively. The structure of the obtained compounds was analyzed by 1H and 13C NMR (HSQC, HMBC) and X-ray analysis. All the compounds were tested against reference strains Staphylococcus aureus ATCC 29213 and Enterococcus faecalis ATCC 29212, and representatives of multidrug-resistant clinical isolates of methicillin-resistant S. aureus (MRSA) and vancomycin-resistant E. faecalis (VRE). In addition, all the compounds were evaluated in vitro against Mycobacterium smegmatis ATCC 700084 and M. marinum CAMP 5644. 9-Benzyloxy-5-methyl-12H-quino [3,4-b][1,4]benzothiazinium chloride (6j), 9-propoxy-5-methyl-12H-quino[3,4-b][1,4]benzothiazinium chloride (6a) and 9-allyloxy-5-methyl-12H-quino[3,4-b][1,4]benzothiazinium chloride (6d) demonstrated high activity against the entire tested microbial spectrum. The activities of the compounds were comparable with oxacillin, tetracycline and ciprofloxacinagainst staphylococcal strains and with rifampicin against both mycobacterial strains. Compound 6j had a significant effect on the inhibition of bacterial respiration as demonstrated by the MTT assay. The compounds showed not only bacteriostatic activity, but also bactericidal activity. Preliminary in vitro cytotoxicity screening of the compounds performed using normal human dermal fibroblasts (NHDF) proved that the tested compounds showed an insignificant cytotoxic effect on human cells (IC50 > 37 µM), making these compounds interesting for further investigation. Moreover, the intermolecular similarity of novel compounds was analyzed in the multidimensional space (mDS) of the structure/property-related in silico descriptors by means of principal component analysis (PCA) and hierarchical clustering analysis (HCA), respectively. The distance-oriented structure/property distribution was related with the experimental lipophilic data.

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.