Antibacterial Activity of Fluorobenzoylthiosemicarbazides and Their Cyclic Analogues with 1,2,4-Triazole Scaffold

Abstract

The development of drug-resistant bacteria is currently one of the major challenges in medicine. Therefore, the discovery of novel lead structures for the design of antibacterial drugs is urgently needed. In this structure-activity relationship study, a library of ortho-, meta-, and para-fluorobenzoylthiosemicarbazides, and their cyclic analogues with 1,2,4-triazole scaffold, was created and tested for antibacterial activity against Gram-positive bacteria strains. While all tested 1,2,4-triazoles were devoid of potent activity, the antibacterial response of the thiosemicarbazides was highly dependent on substitution pattern at the N4 aryl position. The optimum activity for these compounds was found for trifluoromethyl derivatives such as 15a, 15b, and 16b, which were active against both the reference strains panel, and pathogenic methicillin-sensitive and methicillin-resistant Staphylococcus aureus clinical isolates at minimal inhibitory concentrations (MICs) ranging from 7.82 to 31.25 μg/mL. Based on the binding affinities obtained from docking, the conclusion can be reached that fluorobenzoylthiosemicarbazides can be considered as potential allosteric d-alanyl-d-alanine ligase inhibitors.

Keywords: 1,2,4-triazoles; S. aureus clinical isolates; SAR/QSAR analysis; antibacterial activity; thiosemicarbazides.

Scheme 1

Synthetic route for 1-fluorobenzoyl-4-aryl(alkyl)thiosemicarbazides (sets I–III) and 1,2,4-triazole-3-thiones (sets IV–VI). R¹ = o-F (set I-a, set IV-at), m-F (set II-b, set V-bt), p-F (set III-c, set VI-ct). R² = Pr (1), Bu (2), Ph (3), 1-naph (4), m-tol (5), p-tol (6), m-F (7), p-F (8), m-Cl (9), p-Cl (10), m-Br (11), p-Br (12), m-I (13), p-I (14), m-CF₃ (15), p-CF₃ (16).

Figure 1

Plot of values of log(1/MIC) for bacterial strains determined experimentally for the compounds of set I with meta electron-withdrawing substitution (7a, 9a, 11a, 13a, 15a), versus the appropriate values of the σ_m Hammett electronic substituent constant.

Figure 2

Time-kill kinetics of 15a, 15b, and 16b against S. aureus ATCC 25923 over a period of 24 h. Control—S. aureus ATCC 25923 growth without tested compounds.

Figure 3

Thione-thiole tautomeric forms of the thiosemicarbazides.

Figure 4

Predicted vs. experimental log₁₀(1/MIC) values for the respective species for all active compounds (red squares) and leave-one-out model validation results (blue circles). Note: S. a.^*—S. aureus ATCC 6538, S. a.^**—S. aureus ATCC 25923, S. e.—S. epidermidis ATCC 12228, B. s.—B. subtilis ATCC 6633, B. c.—B. cereus ATCC 10876, M. l.—M. luteus ATCC 10240.

Figure 4

Predicted vs. experimental log₁₀(1/MIC) values for the respective species for all active compounds (red squares) and leave-one-out model validation results (blue circles). Note: S. a.^*—S. aureus ATCC 6538, S. a.^**—S. aureus ATCC 25923, S. e.—S. epidermidis ATCC 12228, B. s.—B. subtilis ATCC 6633, B. c.—B. cereus ATCC 10876, M. l.—M. luteus ATCC 10240.

Figure 5

Interactions of previously reported Ddl inhibitor PR, our representative model thiosemicarbazide (9a), and native ligand NI in the allosteric binding site of Ddl.

Figure 5

Interactions of previously reported Ddl inhibitor PR, our representative model thiosemicarbazide (9a), and native ligand NI in the allosteric binding site of Ddl.