In the quest for discovering potent antimicrobial agents with lower toxicity, we envisioned the design and synthesis of nalidixic acid-D-(+)-glucosamine conjugates. The novel compounds were synthesized and evaluated for their in vitro antimicrobial activity against Gram positive bacteria, Gram negative bacteria and fungi. Cytotoxicity using MTT assay over L6 skeletal myoblast cell line, ATCC CRL-1458 was carried out. In vitro antimicrobial assay revealed that 1-ethyl-7-methyl-4-oxo-N-(1,3,4,6-tetra-O-acetyl-2-deoxy-D-glucopyranose-2-yl)-[1,8]-naphthyridine-3-carboxamide (5) and 1-ethyl-7-methyl-4-oxo-N-(2-deoxy-D-glucopyranose-2-yl)-[1,8]-naphthyridine-3-carboxamide(6) possess growth inhibitory activity against resistant Escherichia coli NCTC, 11954 (MIC 0.1589 mM) and Methicillin resistant Staphylococcus aureus ATCC, 33591 (MIC 0.1589 mM). Compound (5) was more active against Listeria monocytogenes ATCC 19115 (MIC 0.1113 mM) in comparison with the reference nalidixic acid (MIC 1.0765 mM). Interestingly, compound (6) had potential antifungal activity against Candida albicans ATCC 10231 (MIC <0.0099 mM). Remarkably, the tested compounds had low cytotoxic effect. This study indicated that glucosamine moiety inclusion into the chemical structure of the marketed nalidixic acid enhances antimicrobial activity and safety.
Herein we report the design, synthesis and biological evaluation of structurally modified ciprofloxacin, norfloxacin and moxifloxacin standard drugs, featuring amide functional groups at C-3 of the fluoroquinolone scaffold. In vitro antimicrobial testing against various Gram-positive bacteria, Gram-negative bacteria and fungi revealed potential antibacterial and antifungal activity. Hybrid compounds 9 (MIC 0.2668 ± 0.0001 mM), 10 (MIC 0.1358 ± 00025 mM) and 13 (MIC 0.0898 ± 0.0014 mM) had potential antimicrobial activity against a fluoroquinolone-resistant Escherichia coli clinical isolate, compared to ciprofloxacin (MIC 0.5098 ± 0.0024 mM) and norfloxacin (MIC 0.2937 ± 0.0021 mM) standard drugs. Interestingly, compound 10 also exerted potential antifungal activity against Candida albicans (MIC 0.0056 ± 0.0014 mM) and Penicillium chrysogenum (MIC 0.0453 ± 0.0156 mM). Novel derivatives and standard fluoroquinolone drugs exhibited near-identical cytotoxicity levels against L6 muscle cell-line, when measured using the MTT assay.