Pixantrone Sensitizes Gram-Negative Pathogens to Rifampin

Abstract

The emergence of bacterial drug resistance poses a severe threat to global public health. In particular, antimicrobial-resistant pathogens lead to a high rate of treatment failure and significantly increase mortality. Repurposing FDA-approved compounds to sensitize superbugs to conventional antibiotics provides a promising strategy to alleviate such crises. Pixantrone (PIX) has been approved for treating aggressive B-cell non-Hodgkin's lymphoma. By high-throughput drug screening, we profiled the synergistic activity between PIX and rifampin (RFP) against Gram-negative extensively drug-resistant isolates by checkerboard assay. Mechanistic studies demonstrated that PIX impacted the flagellum assembly, induced irreversible intracellular reactive oxygen species accumulation and disrupted proton motive force. In addition, the combination of PIX with RFP possesses effective antimicrobial activity against multidrug-resistant strains in vivo without detected toxicity. Collectively, these results reveal the potential of PIX in combination with RFP as a therapy option for refractory infections caused by Gram-negative pathogens. IMPORTANCE Bacterial resistance has become increasingly serious because of the widespread use and abuse of antibiotics. In particular, the emergence of multidrug-resistant bacteria has posed a serious threat to human public health. Drug repurposing, the process of finding new uses for existing drugs, provide a promising pathway to solve antimicrobial resistance. Compared to the development of novel antibiotics, this strategy leverages well-characterized pharmacology and toxicology of known drugs and is more cost-effective.

Keywords: Gram-negative pathogen; drug repurposing; pixantrone; proton motive force; reactive oxygen species.

FIG 1

Effects of PIX on the antimicrobial activity of RFP against Gram-negative bacteria. (A) Combinational antimicrobial effects between PIX and RFP against K. pneumoniae ATCC 700603, A. baumannii ATCC 19606, P. aeruginosa PAO1 and E. coli ATCC 25922 by checkerboard assay. The MICs of RFP for four type strains are 64, 4, 64 and 32 μg/mL, respectively (B) The time-growth curve of four type strains treated with PIX and RFP alone or in combination (16 μg/mL + 1/2 ×MIC for ATCC 700603; 32 μg/mL + 1/8 ×MIC for ATCC 19606; 16 μg/mL + 1/2 ×MIC for PAO1; 16 μg/mL + 1/4 ×MIC for ATCC 25922). (C) Viable cell counting of bacteria after exposure to PIX (32 μg/mL) and RFP (1/4 × MIC or 1/2 × MIC) alone or in combination for 16 h. Data are presented as mean ± SD and the significances were determined by one-way ANOVA (*, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001). Representative CLSM images of (D) E. coli ATCC 25922 and (E) K. pneumoniae ATCC 700603 treated with PIX (32 μg/mL) and RFP (1/4 × MIC) alone or in combination and stained with SYTO9/PI to detect cell viability. Scale bar: 20 μm. (F) Resistance development during serial passaging of bacterial strains in the presence of RFP alone or RFP coupled with PIX (32 μg/mL).

FIG 2

The interaction of PIX on bacterial cell outer membranes. (A) Outer membrane permeability was evaluated by NPN after treatment with either increasing concentrations of PIX or SPR741 (positive control, 16 μg/mL). (B) Effects of exogenous Mg²⁺ (10 mM) and EDTA (1.25 mM) on the synergy between PIX and RFP against E. coli ATCC 25922. The top panel shared the same image as the rightmost panel of Fig. 1A due to the same treatment. (C) AFM and (D) TEM images of E. coli ATCC 25922 and K. pneumoniae ATCC 700603 incubated with PIX (32 μg/mL) or 0.1% DMSO (control). Red triangles indicate bacteria forming extracellular vesicles.

FIG 3

Effect of PIX on bacterial redox homeostasis. (A) K. pneumoniae ATCC 700603, A. baumannii ATCC 19606, P. aeruginosa PAO1 and E. coli ATCC 25922 were treated with varying concentrations of PIX or positive control (PB, polymyxins B, 16 μg/mL), and the ROS level was measured by DCFH-DA. (B) DCFH-DA staining of E. coli ATCC 25922 visualized by CLSM. (C) RNS levels of bacteria treated with PIX or PB (16 μg/mL) were quantified by DAF-FM DA. (D) Intracellular ATP level detection in four type strains treated with varying concentrations of PIX (8-32 μg/mL). (E) Representative CLSM images of E. coli ATCC 25922 and K. pneumoniae ATCC 700603 following treatment with PIX (32 μg/mL) or CIP (10 × MIC). Scale bar: 20 μm. (F) Checkerboard assays of PIX and RFP against E. coli ATCC 25922 and K. pneumoniae ATCC 700603 under anaerobic conditions. (G) The antimicrobial susceptibility of GSH and L-Cys against E. coli ATCC 25922. (H) The growth of E. coli ATCC 25922 supplemented with either PIX (32 μg/mL), RFP (1/4 × MIC), GSH (1 mg/mL) or L-Cys (0.5 mg/mL). (I) E. coli ATCC 25922 was treated with a combination of PIX and RFP supplemented with or without antioxidants (GSH, 1 mg/mL or L-Cys, 0.5 mg/mL), and the cultures were plated after incubation for 6 h. (J) The SOS-associated gene recA expression of E. coli ATCC 25922 in the presence of PIX (16 μg/mL) was determined by qRT-PCR.

FIG 4

The effect of PIX on bacterial PMF. (A) Detection of the intracellular pH of E. coli ATCC 25922 by monitoring the fluorescence intensity of BCECF-AM. (B) The growth of E. coli in pH-adjusted broth. (C) Decreased antibacterial activity of the combination of RFP and PIX in H⁺-reduced medium.

FIG 5

Transcriptome alterations in E. coli ATCC 25922 treated with PIX. (A) KEGG enrichment analysis of the DEGs in E. coli after exposure to PIX (16 μg/mL) for 1 h. (B) Scheme of genes involved in flagellum assembly, quoted from the KEGG database. Yellow represents genes affected by PIX.

FIG 6

In vivo antimicrobial efficacy of combination therapy. The combination of PIX and RFP increased the survival rate of mice infected with (A) E. coli ATCC 25922 or (E) XDR E. coli Y9395 during 7 days postinfection in a peritonitis-sepsis model. The related abscess area (B and F), bacterial load quantification (C and G) and H&E staining of the abscess (D and H) are shown. Scales, 200 μm. Untreated (saline); RFP (20 mg/kg); PIX + RFP (30 + 20 mg/kg).

FIG 7

Schematic diagram of the possible mechanism employed by PIX to enhance the antimicrobial effect of RFP. PIX interact with bacterial outer membrane facilitating entrance of RFP, influence the assembly of flagella, induce ROS accumulation and disrupt proton motive force.