Design, Synthesis and Biological Evaluation of Novel Heterocyclic Fluoroquinolone Citrate Conjugates as Potential Inhibitors of Topoisomerase IV: A Computational Molecular Modeling Study
Authors: Allaka Tejeswara Rao , Katari Naresh Kumar , Jonnalagadda Sreekantha B. , Malkhed Vasavi , Anireddy Jaya Shree 
Published: November 2020

Background & Objective:
A facile and efficient method for the synthesis of novel derivatives of FQ citrate
conjugates with 1,2,4-triazoles and 1,3,4-oxadiazole scaffolds 8-11 using conventional, as well as microwave
irradiation methods, was reported. Based on these original building blocks, the new derivatives of 3, 7-disubstituted
fluoroquinolones bearing the oxadiazolyl-triazole groups were obtained. These invaluable derivatives are of great
interest in medicinal and pharmaceutical studies because of their important biological properties.

Methods:
All the reactions were examined under conventional as well as microwave mediated conditions. The
structures of obtained compounds were confirmed by 1H NMR, 13C NMR, IR HRMS spectroscopy, and elemental
analysis. The antibacterial and antifungal activities of these compounds were screened against Gram-positive,
Gram-negative bacteria, and fungal stains by the agar well diffusion method. Cytotoxic assay of the title compounds
was evaluated against cervical carcinoma cell line (HeLa) by using the MTT assay. The crystal structure of
the Quinolone-DNA cleavage complex of type IV topoisomerase from S. pneumoniae (PDB ID: 3RAE) complex
was obtained from the Protein Database (PDB, http:// www.rcsb.org). Molecular properties prediction-drug likeness
was studied by Molinspiration and Molsoft software, while lipophilicity and solubility parameters were studied
using the Osiris program.

Results:
A novel approach for the synthesis of benzylthio-1,2,4-triazole and 1,3,4-oxadiazoles core with regioisomeric
norfloxacin citrate conjugates was developed. Among the title compounds, 11b, 10a reveal pronounced
activity against S. pneumoniae with minimum inhibitory concentrations of 0.89, 0.96 mg/mL and MBCs of
2.95, 2.80 mg/mL, respectively. Minimum Fungicidal Concentration (MFC) has been determined for each
compound against two fungal strains. Compound 11b showed maximum anti-cancer activity against HeLa cell
line with IC50 value 11.3 ± 0.41 comparable to standard drug DXN. For binding mode, active site residues and
docking energies (ΔG =-7.9 Kcal/mol) for ligand 9b exhibited the highest hydrogen bonding (3.59274 A˚), Pi–
Alkyl (5.14468 A˚) interactions with amino acid LEU479 of 3RAE protein. The compounds following the
Lipinski ‘Rule of five’ were synthesized for antimicrobial and anti-cancer screening as oral bioavailable
drugs/leads. Maximum drug likeness model score 1.52, 1.41 was found for compounds 10d, 11b.

Conclusion:
The present work, through simple synthetic approaches, led to the development of novel hybrids
of fluoroquinolone containing citrate-triazole-oxadiazole pharmacophores that exhibited remarkable
biological activities against different microorganisms and cell lines. The compounds showed suitable druglike
properties and are expected to present good bioavailability profile. An efficient combination of molecular
modeling and biological activity provided an insight into QSAR guidelines that could aid in further development
and optimization of the norfloxacin derivatives.