Evolution and Antibacterial Evaluation of 8-Hydroxy-cycloberberine Derivatives as a Novel Family of Antibacterial Agents Against MRSA

Abstract

Twenty-five new derivatives of 8-hydroxycycloberberine (1) were synthesized and evaluated for their activities against Gram-positive bacteria, taking 1 as the lead. Part of them displayed satisfactory antibacterial activities against methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant Staphylococcus aureus (MRSA), as well as vancomycin-intermediate Staphylococcus aureus (VISA). Especially, compound 15a displayed an excellent anti-MRSA activity with MICs (minimum inhibitory concentrations) of 0.25⁻0.5 μg/mL, better than that of 1. It also displayed high stability in liver microsomes and whole blood, and the LD₅₀ value of over 65.6 mg·kg^-1 in mice via intravenous route, suggesting a good druglike feature. The mode of action showed that 15a could effectively suppress topo IV-mediated decatenation activity at the concentration of 7.5 μg/mL, through binding a different active pocket of bacterial topo IV from quinolones. Taken together, the derivatives of 1 constituted a promising kind of anti-MRSA agents with a unique chemical scaffold and a specific biological mechanism, and compound 15a has been chosen for the next investigation.

Keywords: anti-MRSA; cycloberberine; structure–activity relationship; topoisomerase IV.

Figure 1

Structures of CBBR and 1 as well as modification strategies.

Scheme 1

Reagents and conditions: (a) NaBH₄, 5% NaOH/K₂CO₃, CH₃OH, room temperature, 3 h; (b) 40% glyoxal, HOAc/CH₃CN, reflux, 6 h; (c) CH₃OH/HCl (2/1 by vol.), room temperature, 24 h, 62% (over two steps); (d) 20–30 mmHg, 195–210 °C, 40 min; (e) HCl/EtOH (5/95 by vol.), 90% (over two steps).

Scheme 2

Reagents and conditions: (a) NaBH₄, 5% NaOH, K₂CO₃, CH₃OH, r.t., 3 h; (b) (i) 40% glyoxal, HOAc, CH₃CN, reflux, 6 h; (ii) CH₃OH/HCl (2/1 by vol.), room temperature, 24 h, 67% (over two steps); (c) benzene-1,3,5-triol, 60% H₂SO₄; (d) 20–30 mmHg, 195–210 ^oC, 40 min; (e) HCl/EtOH (5/95 by vol.), 39% (over two steps); (f) NaBH₄, K₂CO₃, CH₃OH, room temperature, 12 h; (g) substituted benzyl bromide, K₂CO₃ (2.0 eq.), CH₃CN, reflux, 3–24 h; (h) aliphatic aldehyde, HOAc, 80% EtOH, reflux, 8 h.

Figure 2

Time-kill curve assay of compound 15a over a 12 h incubation period at 37 °C against MRSA 18-4 (MIC = 0.5 μg/mL). Samples were taken at 2, 4, 8 and 12 h to determine viable bacterial numbers.

Figure 3

Inhibition of S. aureus topo IV. In all panels, enzyme and 15a are absent (-) in lane 1, enzyme present (+) and 15a absent in lane 2, and enzyme (arrow) and 15a present in lanes 3–10 with different concentration. Decatenation activity of topo IV was partially blocked by 15a (lanes 4, 5, 6) and completely blocked by 15a (lanes 7, 8, 9, 10).

Figure 4

(A) Solid surface map of the interaction pocket with compound 15a. Red, blue, and white colored regions correspond to negatively charged, positively charged, and neutral areas, respectively. (B) Binding modes within the receptor S. aureus topo IV active pocket. These figures were produced using the Discovery Studio 4.5 software. The receptor structure is shown in surface form. Key bonds are indicated by dashed lines between the atoms involved, and the colors of key bonds and residues are shown according to the interaction mode (van der Waals, light green; attractive charge, orange; hydrogen bond, green; carbon hydrogen bond, celadon; amide-π stacked, amaranth; alkyl and π-alkyl, mauve pink).