# Anticancer Efficacy of Antibacterial Quinobenzothiazazines
## 4 Conclusions  
A series of antibacterial active quinobenzothiazazines have been synthesized by the recently described method and tested in vitro for their anticancer activity against human glioblastoma SNB-19, lung adenocarcinoma A549 and breast cancer T47D cell lines. Three of the tested compounds showed remarkable activity on cancer lines, namely, 9-propoxy-5-methyl-12\(H\)-quino[3,4-\(b\)][1,4]benzothizinium chloride (**4a**), 9-allyoxy- 5-methyl-12\(H\)-quino[3,4-\(b\)][1,4]benzothizinium chloride (**4d**) and 11-benzyl-5-methyl- 12\(H\)-quino[3,4-\(b\)][1,4]benzothizinium chloride (**4l**), which had IC\({}_{50}\) values against all three cell lines ranging from 5.3 to 9.3 \(\upmu\)M. At the same time, the compounds showed no cytotoxic effect on normal human dermal fibroblasts (NHDFs). Based on the observed influence of the type/nature of substitution and the position of individual substituents on activity, it can be concluded that substitutions of C\({}_{(9)}\) with propoxy and/or allyoxy tails or C\({}_{(11)}\) with a benzyloxy moiety are crucial for anticancer activity. The least favorable is a propragyl group in all positions, as well as substitution with a benzyloxy moiety in the C\({}_{(9)}\) and C\({}_{(10)}\) positions. An intercalation mechanism of action can probably be assumed; however, all three compounds deserve the attention of further detailed investigation. Figure 1: Planar ligand interaction diagram for 4-androstene-3-17-dione (**a**) and the most potent inhibitors, 9-allyoxy derivative **4d** (**b**) and 11-benzyl derivative **4l** (**c**), generated by the Schrödinger Maestro program. Figure 2: Spatial comparison of binding interactions for ASD (**a**) and the most potent inhibitors, 9-allyoxy derivative **4d** (**b**) and 11-benzyl derivative **4l** (**c**), generated by the PLIP tool. Hence, the molecular docking approach was employed for the most potent agents against the human breast cancer line T47D in order to gain a comprehensive insight into the aromatase-inhibitor binding mode, which might be potentially valid for the antitumor characteristics of new quinobenzothiazazines. Despite some structural variations between androstenedione (ASD) and quinobenzothiazazine derivatives **4d** and **4l**, some regularities in the spatial atomic distribution and nonbonding interactions (e.g., hydrophobic patterns) can be observed. Basically, the resulting binding geometries determined for the most active molecules, **4d** and **4l**, resemble that of the reference ASD compound. Evidently, the oxygen atoms of the allyoxy substituent (**4d**) and benzyloxy moiety (**4l**) are oriented similarly to the 3-keto oxygen of the six-membered ASD ring, which is positioned near the Asp309 amino acid residue. Interestingly, the faces of the electron-rich aromatic rings of **4d** and **4l** molecules could contribute to \(\pi\)-\(\pi\) stacking interactions with protoporphyrin IX (HEM) as well as form \(\pi\)-cation interactions with the adjacent iron cofactor. The observation of anticancer activity is partially consistent with our recent findings on antimicrobial activity, where substitution at the C(\({}_{9}\)) position with a propoxy or allyoxy moiety was favorable in terms of antibacterial activity. Thus, compounds **4a** and **4h** can be considered interesting anti-invasive agents with dual (anticancer and antibacterial) activity, while compound **4l** is the most interesting purely anticancer agent. Conceptualization, A.Z. and J.J.; methodology, A.Z. and M.L. ; investigation, E.K.-N., M.L., V.K. and A.B. ; writing--original draft preparation, A.Z. and J.J.; writing--review and editing, J.J. and A.B. ; funding acquisition, A.Z. and J.J. All authors have read and agreed to the published version of the manuscript. This work was supported by the Medical University of Silesia in Katowice, Grant No. PCN-1-039/K/2/F.