Synthesis of Novel Thiazole/Thiadiazole Conjugates of Fluoroquinolones as Potent Antibacterial and Antimycobacterial Agents

Twenty azole-fluoroquinolone hybrids were designed and synthesized by conjugating thiazole and thiadiazole structures to ciprofloxacin and norfloxacin via a 2-oxoethyl bridge. The structures and purities of the synthesized compounds were proven by spectral techniques. The antimycobacterial effects of target compounds 21-40 were tested against Mycobacterium tuberculosis H37Rv strain. Among the 20 synthesized compounds, 12 exhibited minimal inhibition concentration (MIC) values in the range of 1.56-25 μg/mL. Among the molecules screened for antimycobacterial effects, the most effective was compound 35, a thiadiazole-ciprofloxacin hybrid. The cytotoxic effect of this molecule was found to be lower than the reference drugs, and it was also determined to be a more effective inhibitor than ciprofloxacin and norfloxacin in the DNA-gyrase supercoiling test. The antimicrobial effects of compounds 21-40 were screened by agar-well diffusion and microdilution tests against Gram-positive/negative bacteria, a fast-growing mycobacterium, and two yeast strains. While most of the compounds tested showed antibacterial effects, the most effective fluoroquinolone derivative appeared to be compound 31 with an MIC value of < 0.63 μg/mL against all Gram-negative bacteria tested. Azole-fluoroquinolone hybrids 21-40 did not show any activity against non-pathogenic Lactobacillus species and yeast-like fungi, indicating that they have selective antibacterial and antimycobacterial activity, particularly against Gram-negative bacteria. In silico molecular docking studies were conducted to uncover the interactions between lead compound 35 and the DNA gyrase proteins of M. tuberculosis and S. aureus. Additionally, a 100 ns molecular dynamics simulation was carried out to assess the stability of the complexes formed between compound 35 and both proteins.