Design, synthesis, biological evaluation and molecular docking studies of novel pleuromutilin derivatives containing nitrogen heterocycle and alkylamine groups

Abstract

A series of pleuromutilin derivatives containing alkylamine and nitrogen heterocycle groups were designed and synthesised under mild conditions. The in vitro antibacterial activity of these semisynthetic derivatives against four strains of Staphylococcus aureus (MRSA ATCC 43300, S.aureus ATCC 29213, S.aureus AD3, and S.aureus 144) were evaluated by the broth dilution method. Compound 13 was found to have excellent antibacterial activity against MRSA (MIC = 0.0625 μg/mL). Furthermore, compound 13 was further studied by the time-killing kinetics and the post-antibiotic effect approach. In the mouse thigh infection model, compound 13 exhibited superior antibacterial efficacy than that of tiamulin. Meanwhile, compound 13 showed a lower inhibitory effect than that of tiamulin on RAW264.7 and 16HBE cells at the concentration of 10 μg/mL. Molecular docking study revealed that compound 13 can effectively bind to the active site of the 50S ribosome (the binding free energy = -9.66 kcal/mol).

Keywords: MRSA; Pleuromutilin; alkylamine; antibacterial activity; molecular docking; nitrogen heterocycles.

Figure 1.

Structure of pleuromutilin (1), tiamulin (2), valnemulin (3), retapamulin (4), lefamulin (5).

Scheme 1.

Reagent and conditions: (i) dichloromethane, 4-tosyl chloride, NaOH, rt, 3 h (ii) ethyl acetate, 2-aminobenzenethiol, NaOH, 70 °C, 3 h; (iii) acetonitrile, chloroacetyl chloride, triethylamine, rt, 4 h; (iv) acetonitrile, secondary amine, K₂CO₃, 78 °C, 12 h.

Figure 2.

Time-kill curves for MRSA ATCC 43300 with different concentrations of compounds 10 (a), 13 (b), 18 (c), tiamulin (d), retapamulin (e).

Figure 3.

The bacterial growth kinetic curves for MRSA ATCC 43300 exposed to compound 10 (a), compound 13 (c) and compound 18 (e) for 1 h, compound 10 (b), compound 13 (d) and compound 18 (f) for 2 h.

Figure 4.

Efficacy of tiamulin, vancomycin, compound 13 against MRSA ATCC 43300 in murine neutropenic thigh models: black triangle: growth control; red triangle: tiamulin (20 mg/kg); green circular: vancomycin (20 mg/kg), blue circular: compound 13 (20 mg/kg).

Figure 5.

The cytotoxicity assay of compounds 10, 13, 18 and tiamulin to RAW264.7 (a) and 16HBE (b) cells at the concentration of 5 μg/mL and 10 μg/mL.

Figure 6.

Docking mode of compound 13 to 1XBP (a). Docking mode of tiamulin (yellow) and compound 13 (green) to 1XBP. (b) 3 D representation of docking poses for compound 13 in the 50S ribosome residues. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).