Novel 8-trifluoromethylquinobenzothiazines—Synthesis and Evaluation for Antiproliferative and Antibacterial Activity

Background: Phenothiazine derivatives bearing trifluoromethyl substituents have attracted increasing interest as multifunctional scaffolds in drug repositioning strategies, particularly in cancer and infectious diseases. Structural modification of classical phenothiazines by incorporation of a quinoline moiety has previously been shown to enhance biological activity. Objectives: The present study aimed to develop an efficient synthesis of 8-trifluoromethylquinobenzothiazines and to evaluate the anticancer and antibacterial potential of their N-substituted analogues inspired by triflupromazine, trifluoperazine, and fluphenazine. Methods: 6H-8-Trifluoromethylquinobenzothiazine was synthesized by cyclization of 2-amino-4-trifluoromethylbenzenethiol and 3-bromo-2-chloroquinoline. The resulting quinobenzothiazine, unsubstituted at the nitrogen atom, was subjected to N-alkylation reactions to afford eleven new 6-dialkylaminoalkyl derivatives. Structural elucidation was performed using NMR and HRMS techniques. Anticancer activity was evaluated by MTT assay against human breast (MDA-MB-231), pancreatic (Mia-PaCa-2), and lung (A-549) carcinoma cell lines, as well as normal HaCaT keratinocytes. Antibacterial activity was assessed by MIC/MBC determination against selected Gram-positive and Gram-negative reference strains and clinical isolates. Results: Among the synthesized compounds, derivatives 8 and 12 exhibited the most favorable anticancer profiles, showing micromolar cytotoxicity (IC50 ≈ 4–10 µM) against lung and pancreatic cancer cells combined with moderate selectivity toward cancer cells over normal keratinocytes. Compound 6 displayed lower cytotoxic potency but a notably high selectivity index due to minimal toxicity toward normal cells. In antibacterial assays, compound 3 exhibited activity against Gram-positive bacteria, including a methicillin-resistant Staphylococcus aureus isolate, with MIC values ranging from 7.8 to 15.6 µg/mL. The corresponding MBC values were equal to or twofold higher than the MICs (MBC/MIC = 1–2), fulfilling commonly accepted criteria for bactericidal activity (MBC/MIC ≤ 4). OD-based growth kinetics confirmed concentration-dependent inhibition of S. aureus growth. Conclusions: The obtained results identify 8-trifluoromethylquinobenzothiazines as a promising class of multifunctional compounds. Selected derivatives combine anticancer activity with acceptable selectivity or display potent antibacterial effects against clinically relevant Gram-positive pathogens.