Eric Desarbre

In Vitro Properties of BAL30072, a Novel Siderophore Sulfactam with Activity against Multiresistant Gram-Negative Bacilli

ABSTRACT BAL30072 is a new monocyclic β-lactam antibiotic belonging to the sulfactams. Its spectrum of activity against significant Gram-negative pathogens with β-lactam-resistant phenotypes was evaluated and was compared with the activities of reference drugs, including aztreonam, ceftazidime, cefepime, meropenem, imipenem, and piperacillin-tazobactam. BAL30072 showed potent activity against multidrug-resistant (MDR) Pseudomonas aeruginosa and Acinetobacter sp. isolates, including many carbapenem-resistant strains. The MIC90s were 4 μg/ml for MDR Acinetobacter spp. and 8 μg/ml for MDR P. aeruginosa, whereas the MIC90 of meropenem for the same sets of isolates was >32 μg/ml. BAL30072 was bactericidal against both Acinetobacter spp. and P. aeruginosa, even against strains that produced metallo-β-lactamases that conferred resistance to all other β-lactams tested, including aztreonam. It was also active against many species of MDR isolates of the Enterobacteriaceae family, including isolates that had a class A carbapenemase or a metallo-β-lactamase. Unlike other monocyclic β-lactams, BAL30072 was found to trigger the spheroplasting and lysis of Escherichiacoli rather than the formation of extensive filaments. The basis for this unusual property is its inhibition of the bifunctional penicillin-binding proteins PBP 1a and PBP 1b, in addition to its high affinity for PBP 3, which is the target of monobactams, such as aztreonam.

In Vitro and In Vivo Properties of BAL30376, a β-Lactam and Dual β-Lactamase Inhibitor Combination with Enhanced Activity against Gram-Negative Bacilli That Express Multiple β-Lactamases

ABSTRACT BAL30376 is a triple combination comprising a siderophore monobactam, BAL19764; a novel bridged monobactam, BAL29880, which specifically inhibits class C β-lactamases; and clavulanic acid, which inhibits many class A and some class D β-lactamases. The MIC90 was ≤4 μg/ml (expressed as the concentration of BAL19764) for most species of the Enterobacteriaceae family, including strains that produced metallo-β-lactamases and were resistant to all of the other β-lactams tested. The MIC90 for Stenotrophomonas maltophilia was 2 μg/ml, for multidrug-resistant (MDR) Pseudomonas aeruginosa it was 8 μg/ml, and for MDR Acinetobacter and Burkholderia spp. it was 16 μg/ml. The presence of the class C β-lactamase inhibitor BAL29880 contributed significantly to the activity of BAL30376 against strains of Citrobacter freundii, Enterobacter species, Serratia marcescens, and P. aeruginosa. The presence of clavulanic acid contributed significantly to the activity against many strains of Escherichia coli and Klebsiella pneumoniae that produced class A extended-spectrum β-lactamases. The activity of BAL30376 against strains with metallo-β-lactamases was largely attributable to the intrinsic stability of the monobactam BAL19764 toward these enzymes. Considering its three components, BAL30376 was unexpectedly refractory toward the development of stable resistance.

In vitro activity of BAL30072 against Burkholderia pseudomallei

Burkholderia pseudomallei is an intrinsically antibiotic-resistant Category B priority pathogen and the aetiological agent of melioidosis. Treatment of B. pseudomallei infection is biphasic and lengthy in order to combat the acute and chronic phases of the disease. Acute-phase treatment preferably involves an intravenous cephalosporin (ceftazidime) or a carbapenem (imipenem or meropenem). In this study, the anti-B. pseudomallei efficacy of a new monosulfactam, BAL30072, was tested against laboratory strains 1026b and 1710b and several isogenic mutant derivatives as well as a collection of clinical and environmental B. pseudomallei strains from Thailand. More than 93% of the isolates had minimal inhibitory concentrations (MICs) in the range 0.004-0.016 μg/mL. For the laboratory strain 1026b, the MIC of BAL30072 was 0.008 μg/mL, comparable with the MICs of 1.5 μg/mL for ceftazidime, 0.5 μg/mL for imipenem and 1 μg/mL for meropenem. Time-kill curves revealed that BAL30072 was rapidly bactericidal, killing >99% of bacteria in 2 h. BAL30072 activity was not significantly affected by efflux, it was only a marginal substrate of PenA β-lactamase, and activity was independent of malleobactin production and transport and the ability to transport pyochelin. In summary, BAL30072 has superior in vitro activity against B. pseudomallei compared with ceftazidime, meropenem or imipenem and it is rapidly bactericidal.

Combined effects of the siderophore monosulfactam BAL30072 and carbapenems on multidrug-resistant Gram-negative bacilli

OBJECTIVES Carbapenem resistance in Gram-negative bacteria, mediated by restricted net influx and carbapenem-hydrolysing β-lactamases, is a growing problem. The monosulfactam antibiotic BAL30072 is stable to most carbapenemases, suggesting that it could be complementary to carbapenems. We have investigated the antimicrobial activity of BAL30072 combined with imipenem, meropenem and doripenem. METHODS The in vitro activities of the combinations were evaluated using broth microdilution susceptibility and agar disc diffusion tests, broth dilution chequerboard titration and time-kill studies, using strains of Enterobacteriaceae, Pseudomonas aeruginosa and Acinetobacter with carbapenem MICs ≥ 2 mg/L. RESULTS The combinations were effective against 70%-80% of the isolates tested in the presence of 1 mg/L of each antibiotic, whereas the carbapenems were ineffective and BAL30072 alone was effective against 20%-40% of the strains. Synergistic effects were observed with many Enterobacteriaceae and P. aeruginosa, but were less common among the Acinetobacter, although additive effects, where the activity of one partner compensated for lack of activity of the other, were common. None of the combinations exhibited an antagonistic effect in all tests, in contrast to other β-lactams where negative interactions were frequently observed. Animal models of septicaemia demonstrated that the synergy observed in vitro with BAL30072 and meropenem can translate into greater in vivo efficacy. CONCLUSIONS BAL30072/carbapenem combinations were effective against a broader range of multidrug-resistant Gram-negative bacteria than either of the single agents. Additive and synergistic effects were observed in Enterobacteriaceae and P. aeruginosa, and this enhanced activity was frequently associated with suppression of resistance development. The in vitro activity translated into improved in vivo efficacy.

A kinetic analysis of the inhibition of FOX-4 β-lactamase, a plasmid-mediated AmpC cephalosporinase, by monocyclic β-lactams and carbapenems

OBJECTIVES Class C β-lactamases are prevalent among Enterobacteriaceae; however, these enzymes are resistant to inactivation by commercially available β-lactamase inhibitors. In order to find novel scaffolds to inhibit class C β-lactamases, the comparative efficacy of monocyclic β-lactam antibiotics (aztreonam and the siderophore monosulfactam BAL30072), the bridged monobactam β-lactamase inhibitor BAL29880, and carbapenems (imipenem, meropenem, doripenem and ertapenem) were tested in kinetic assays against FOX-4, a plasmid-mediated class C β-lactamase (pmAmpC). METHODS The FOX-4 β-lactamase was purified. Steady-state kinetics, electrospray ionization mass spectrometry (ESI-MS) and ultraviolet difference (UVD) spectroscopy were conducted using the β-lactam scaffolds described. RESULTS The K(i) values for the monocyclic β-lactams against FOX-4 β-lactamase were 0.04 ± 0.01 μM (aztreonam) and 0.66 ± 0.03 μM (BAL30072), and the Ki value for the bridged monobactam BAL29880 was 8.9 ± 0.5 μM. For carbapenems, the Ki values ranged from 0.27 ± 0.05 μM (ertapenem) to 2.3 ± 0.3 μM (imipenem). ESI-MS demonstrated the formation of stable covalent adducts when the monocyclic β-lactams and carbapenems were reacted with FOX-4 β-lactamase. UVD spectroscopy suggested the appearance of different chromophoric intermediates. CONCLUSIONS Monocyclic β-lactam and carbapenem antibiotics are effective mechanism-based inhibitors of FOX-4 β-lactamase, a clinically important pmAmpC, and provide stimulus for the development of new inhibitors to inactivate plasmidic and chromosomal class C β-lactamases.

Klebsiella pneumoniae carbapenemase-2 (KPC-2), substitutions at ambler position Asp179, and resistance to ceftazidime-avibactam: Unique antibiotic-resistant phenotypes emerge from β-lactamase protein engineering

ABSTRACT The emergence of Klebsiella pneumoniae carbapenemases (KPCs), β-lactamases that inactivate “last-line” antibiotics such as imipenem, represents a major challenge to contemporary antibiotic therapies. The combination of ceftazidime (CAZ) and avibactam (AVI), a potent β-lactamase inhibitor, represents an attempt to overcome this formidable threat and to restore the efficacy of the antibiotic against Gram-negative bacteria bearing KPCs. CAZ-AVI-resistant clinical strains expressing KPC variants with substitutions in the Ω-loop are emerging. We engineered 19 KPC-2 variants bearing targeted mutations at amino acid residue Ambler position 179 in Escherichia coli and identified a unique antibiotic resistance phenotype. We focus particularly on the CAZ-AVI resistance of the clinically relevant Asp179Asn variant. Although this variant demonstrated less hydrolytic activity, we demonstrated that there was a prolonged period during which an acyl-enzyme intermediate was present. Using mass spectrometry and transient kinetic analysis, we demonstrated that Asp179Asn “traps” β-lactams, preferentially binding β-lactams longer than AVI owing to a decreased rate of deacylation. Molecular dynamics simulations predict that (i) the Asp179Asn variant confers more flexibility to the Ω-loop and expands the active site significantly; (ii) the catalytic nucleophile, S70, is shifted more than 1.5 Å and rotated more than 90°, altering the hydrogen bond networks; and (iii) E166 is displaced by 2 Å when complexed with ceftazidime. These analyses explain the increased hydrolytic profile of KPC-2 and suggest that the Asp179Asn substitution results in an alternative complex mechanism leading to CAZ-AVI resistance. The future design of novel β-lactams and β-lactamase inhibitors must consider the mechanistic basis of resistance of this and other threatening carbapenemases. IMPORTANCE Antibiotic resistance is emerging at unprecedented rates and threatens to reach crisis levels. One key mechanism of resistance is the breakdown of β-lactam antibiotics by β-lactamase enzymes. KPC-2 is a β-lactamase that inactivates carbapenems and β-lactamase inhibitors (e.g., clavulanate) and is prevalent around the world, including in the United States. Resistance to the new antibiotic ceftazidime-avibactam, which was designed to overcome KPC resistance, had already emerged within a year. Using protein engineering, we uncovered a mechanism by which resistance to this new drug emerges, which could arm scientists with the ability to forestall such resistance to future drugs. IMPORTANCE Antibiotic resistance is emerging at unprecedented rates and threatens to reach crisis levels. One key mechanism of resistance is the breakdown of β-lactam antibiotics by β-lactamase enzymes. KPC-2 is a β-lactamase that inactivates carbapenems and β-lactamase inhibitors (e.g., clavulanate) and is prevalent around the world, including in the United States. Resistance to the new antibiotic ceftazidime-avibactam, which was designed to overcome KPC resistance, had already emerged within a year. Using protein engineering, we uncovered a mechanism by which resistance to this new drug emerges, which could arm scientists with the ability to forestall such resistance to future drugs.