In Vitro and In Vivo Biological Activities of Iron Chelators and Gallium Nitrate against Acinetobacter baumannii

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b Multidrug-resistant Acinetobacter baumannii poses a tremendous challenge to traditional antibiotic therapy. Due to the crucial role of iron in bacterial physiology and pathogenicity, we investigated iron metabolism as a possible target for anti-A. baumannii chemotherapy using gallium as an iron mimetic. Due to chemical similarity, gallium competes with iron for binding to several redox enzymes, thereby interfering with a number of essential biological reactions. We found that Ga(NO 3 ) 3 , the active component of an FDA-approved drug (Ganite), inhibits the growth of a collection of 58 A. baumannii strains in both chemically defined medium and human serum, at concentrations ranging from 2 to 80 M and from 4 to 64 M, respectively. Ga(NO 3 ) 3 delayed the entry of A. baumannii into the exponential phase and drastically reduced bacterial growth rates. Ga(NO 3 ) 3 activity was strongly dependent on iron availability in the culture medium, though the mechanism of growth inhibition was independent of dysregulation of gene expression controlled by the ferric uptake regulator Fur. Ga(NO 3 ) 3 also protected Galleria mellonella larvae from lethal A. baumannii infection, with survival rates of >75%. At therapeutic concentrations for humans (28 M plasma levels), Ga(NO 3 ) 3 inhibited the growth in human serum of 76% of the multidrug-resistant A. baumannii isolates tested by >90%, raising expectations on the therapeutic potential of gallium for the treatment of A. baumannii bloodstream infections. Ga(NO 3 ) 3 also showed strong synergism with colistin, suggesting that a colistin-gallium combination holds promise as a last-resort therapy for infections caused by pan-resistant A. baumannii.

Section: Resultsmentioning

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See 1 more Smart Citation

In Vitro and In Vivo Antimicrobial Activities of Gallium Nitrate against Multidrug-Resistant Acinetobacter baumannii

Antunes

Imperi

Minandri

et al. 2012

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142

“…cobacteria and other bacteria (6,38). It is possible that the bacteriostatic effect of Ga on mycobacteria is related in part to the inactivation of bacterial Fe-centered enzymes such as aconitase and RR.…”

Section: Fig 5 Gallium Inhibits M Tuberculosis Aconitase Activity Amentioning

confidence: 99%

Gallium Nitrate Is Efficacious in Murine Models of Tuberculosis and Inhibits Key Bacterial Fe-Dependent Enzymes

Olakanmi

Kesavalu

Pasula

et al. 2013

f Acquiring iron (Fe) is critical to the metabolism and growth of Mycobacterium tuberculosis. Disruption of Fe metabolism is a potential approach for novel antituberculous therapy. Gallium (Ga) has many similarities to Fe. Biological systems are often unable to distinguish Ga 3؉ from Fe 3؉ . Unlike Fe 3؉ , Ga 3؉ cannot be physiologically reduced to Ga 2؉ . Thus, substituting Ga for Fe in the active site of enzymes may render them nonfunctional. We previously showed that Ga inhibits growth of M. tuberculosis in broth and within cultured human macrophages. We now report that Ga(NO 3 ) 3 shows efficacy in murine tuberculosis models. BALB/c SCID mice were infected intratracheally with M. tuberculosis, following which they received daily intraperitoneal saline, Ga(NO 3 ) 3 , or NaNO 3 . All mice receiving saline or NaNO 3 died. All Ga(NO 3 ) 3 -treated mice survived. M. tuberculosis CFU in the lungs, liver, and spleen of the NaNO 3 -treated or saline-treated mice were significantly higher than those in Ga-treated mice. When BALB/c mice were substituted for BALB/c SCID mice as a chronic (nonlethal) infection model, Ga(NO 3 ) 3 treatment significantly decreased lung CFU. To assess the mechanism(s) whereby Ga inhibits bacterial growth, the effect of Ga on M. tuberculosis ribonucleotide reductase (RR) (a key enzyme in DNA replication) and aconitase activities was assessed. Ga decreased M. tuberculosis RR activity by 50 to 60%, but no additional decrease in RR activity was seen at Ga concentrations that completely inhibited mycobacterial growth. Ga decreased aconitase activity by 90%. Ga(NO 3 ) 3 shows efficacy in murine M. tuberculosis infection and leads to a decrease in activity of Fe-dependent enzymes. Additional work is warranted to further define Ga's mechanism of action and to optimize delivery forms for possible therapeutic uses in humans.

“…The fact that the amidoxime moiety plays a prominent role in the activity in our compounds and the well-known property of amidoxime functional groups to form Fe(III) complexes gave rise to the presumption that the amidoximes display their antibacterial effect due to such a complexation (27,28). Consequently, we examined this hypothesis.…”

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New Insights into the Bacterial RNA Polymerase Inhibitor CBR703 as a Starting Point for Optimization as an Anti-Infective Agent

Zhu

Haupenthal

Groh

et al. 2014

c CBR703 was reported to inhibit bacterial RNA polymerase (RNAP) and biofilm formation, considering it to be a good candidate for further optimization. While synthesized derivatives of CBR703 did not result in more-active RNAP inhibitors, we observed promising antibacterial activities. These again correlated with a significant cytotoxicity toward mammalian cells. Furthermore, we suspect the promising effects on biofilm formation to be artifacts. Consequently, this class of compounds can be considered unattractive as antibacterial agents. Bacterial RNA polymerase (RNAP) is essential for bacterial growth and survival and is thus an attractive target for drug development (1, 2). Along with the recently FDA-approved fidaxomycin (3), the rifamycins, applied as first-line antituberculosis drugs, are the only RNAP inhibitors that are in clinical use (2). However, similarly to other anti-infectives, the use of rifamycins resulted in the occurrence of resistant bacterial strains (1, 4-7), which represents a remarkable threat to public health (8, 9). Consequently, there is a need to focus on novel promising inhibitors. Recently, interesting peptidic and peptidomimetic (10-12) as well as nonpeptidic (13-18) small-molecule RNAP inhibitors have been described. Another example is CBR703 (Fig. 1), whose mechanism of action is reported to be different from that of the rifamycins (19,20). This compound has been identified in a highthroughput screening searching for small-molecule inhibitors of RNAP (19). Two more-potent analogs in that report reveal the potential of optimizing CBR703 by structural enlargement. Furthermore, pursuing the hypothesis that RNAP is of particular importance for bacterial survival in biofilms, Villain-Guillot et al. showed CBR703 to significantly reduce Staphylococcus epidermidis biofilm mass (21). We therefore considered CBR703 to be a promising starting point for drug development. Consequently, we focused on CBR703 to perform systematic modifications on its core structure, aiming to obtain a more appropriate starting point for further structural optimization.Detailed information concerning the materials and methods used in synthesis and biology can be found in the supplemental material.In total, 30 final compounds and 24 intermediates were obtained and tested for Escherichia coli RNAP inhibition and their ability to inhibit the growth of E. coli TolC (see Table S1 to S3 in the supplemental material). According to their structures, the synthesized derivatives can be divided into three groups with modifications in part A, B, or C (Fig. 1). Compounds 1 to 25 (see Scheme S1 in the supplemental material) with introduction of substituents into the aromatic moieties (part A or B) were prepared by condensation of an intermediate amide with hydroxylamine (22, 23). In order to ensure an appropriate coverage of lipophilic and electronic properties, the substituents were chosen rationally from all quadrants of a Craig plot (e.g., Hansch-Fujita versus constant) (24). The results (see Table S1) showed that compound...