Novel phenylthiazoles with a tert-butyl moiety: promising antimicrobial activity against multidrug-resistant pathogens with enhanced ADME properties

Abstract

The structure-activity relationship of a new tert-butylphenylthiazole series, with a pyrimidine linker, was investigated. We wished to expand knowledge of this novel class of antibiotics by generating 21 new derivatives bearing ≥2 heteroatoms in their side chains. Their activity was examined against isolates of methicillin-resistant Staphylococcus aureus (MRSA), Clostridium difficile, Escherichia coli, Neisseria gonorrhoeae, and Candida albicans. Two compounds with 1,2-diaminocyclohexane as a nitrogenous side chain showed promising activity against the highly infectious MRSA USA300 strain, with a minimum inhibitory concentration (MIC) of 4 μg mL^-1. One of these two compounds demonstrated potent activity against C. difficile, with a MIC of 4 μg mL^-1. Moderate activities against a C. difficile strain with a MIC of 8 μg mL^-1 were noted. Some new compounds possessed antifungal activity against a wild fluconazole-resistant C. albicans strain, with MIC values of 4-16 μg mL^-1. ADME and metabolism-simulation studies were performed for the most promising compound and compared with lead compounds. Our results revealed that one compound possessed greater penetration of bacterial membranes and metabolic resistance, which aided a longer duration of action against MRSA.

Fig. 1. Developmental progress of phenylthiazole antibiotics and a new approach to improve metabolic stability and activity.

Scheme 1. Reagents and conditions: (a) absolute EtOH, 3-chloropentane-2,4-dione, heat at reflux, 12 h; (b) DMF–DMA heat at 80 °C, 8 h; (c) thiourea, KOH, EtOH, heat at reflux, 8 h; (d) dimethyl sulfate, KOH, H₂O, stirring at 23 °C, 2 h; (e) MCPBA, dry DCM, stirring at 23 °C, 16 h; (f) appropriate amine, dry DMF, heat at 80 °C for 0.5–8 h.

Fig. 2. Analyses of the toxicity of compound 20 (tested in triplicates at 8, 16, 32 and 64 μg mL⁻¹) against human breast cancer (MCF-7) cells using the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay. Results are presented as percent viable cells relative to DMSO (negative control) to determine a baseline measure for the cytotoxic impact of each compound. Absorbance values represent an average of three samples analyzed for each compound. Error bars represent standard deviation values. Data were analyzed via two-way ANOVA with post hoc Dunnett's test for multiple comparisons. * denotes a significant difference (P < 0.05) between values obtained for compounds and DMSO.

Fig. 3. Radar map of the ADME calculations for lead compound 1a, lead compound 1c, and compound 20.

Fig. 4. Count of CYP inhibition of five strains by lead compound 1a, lead compound 1c, and compound 20.

Fig. 5. Drug-likeness model obtained from calculations of molecular properties of lead compound 1a, lead compound 1c, and compound 20.

Fig. 6. Pharmacokinetic analysis of lead compound 1a and compound 20.

Fig. 7. Half-life of lead compound 1a and compound 20.