Quinolone Resistance in Staphylococci: Activities of New Nonfluorinated Quinolones against Molecular Targets in Whole Cells and Clinical Isolates

Abstract

The activity of three new, 8-methoxy-nonfluorinated quinolones (NFQs) against multiple-drug-resistant staphylococci was investigated. First, using Staphylococcus aureus strains containing point mutations in the serine 84-80 hot spots of the target genes (gyrA and grlA), cell growth inhibition potencies of the NFQs as a result of DNA gyrase and topoisomerase IV inhibition were estimated and compared with those of known fluoroquinolones. The NFQs and clinafloxacin showed higher affinities toward both the targets than ciprofloxacin, trovafloxacin and gatifloxacin. Furthermore, the ratio of the calculated affinity parameter for DNA gyrase to that for topoisomerase IV was lower in the case of the NFQs, clinafloxacin, and gatifloxacin than in the case of ciprofloxacin and trovafloxacin. These results suggest that the former group of quinolones is better able to exploit both the targets. Next, using clinical isolates of methicillin-resistant S. aureus (MRSA; n ‫؍‬ 34) and coagulasenegative staphylococci (CoNS; n ‫؍‬ 24), the NFQs and clinafloxacin were shown to be more potent (MIC at which 90% of the isolates are inhibited [MIC 90 ] ‫؍‬ 2 g/ml for MRSA and 0.5 g/ml for CoNS) than ciprofloxacin, trovafloxacin, and gatifloxacin (MIC 90 ‫؍‬ 16 to >64 g/ml for MRSA and 4 to >32 g/ml for CoNS). Bactericidal kinetics experiments, using two MRSA isolates, showed that exposure to the NFQs at four times the MIC reduced the bacterial counts (measured in CFU per milliliter) by >3 log units in 2 to 4 h. Overall, the NFQs and clinafloxacin were less susceptible than the other quinolones to existing mechanisms of quinolone resistance in staphylococci.

Dual Targeting of DNA Gyrase and Topoisomerase IV: Target Interactions of Garenoxacin (BMS-284756, T-3811ME), a New Desfluoroquinolone

“…The similar two-to fourfold increases in the MICs of garenoxacin for genetically defined gyrA or topoisomerase IV mutants suggested that garenoxacin targets both enzymes similarly. Other nonfluorinated quinolones have also been reported to target both enzymes more evenly than ciprofloxacin, as evident from MIC studies performed with some of the same genetically defined mutants used in this study (39). MICs of garenoxacin for first-step topoisomerase IV or gyrase mutants (0.064 to 0.128 g/ml) are substantially below the drug concentrations achievable in serum after single doses of 400 and 800 mg (6.4 and 10.9 g/ml, respectively) (18).…”

“…At twice the MIC of garenoxacin for ISP794, the highest concentration at which mutants could be selected, the selection frequency of resistant mutants was extremely low. Such extremely low frequencies of selection of mutants have previously been reported for clinafloxacin, which targets both gyrase and topoisomerase IV in S. pneumoniae (37), and for other nonfluorinated quinolones in S. aureus (39). Second, the characterization of single- on May 11, 2018 by guest http://aac.asm.org/ step mutants yielded interesting results in that two of the mutants had gyrA mutations, one had a grlA mutation, and one (B23) had mutations in both grlB and gyrA.…”

“…Quinolone resistance mutations at positions 426 and 447 in E. coli GyrB (53) and positions 437 and 458 in S. aureus GyrB are associated with resistance (26). Roychoudhury et al have selected another novel mutation in gyrB in codon 477 (Glu477Val), in addition to other novel mutations, with PGE 9509924, another nonfluorinated quinolone (39). The role of the these mutations in the resistance phenotype, however, has not yet been shown definitively by allelic exchange (39).…”

“…Roychoudhury et al have selected another novel mutation in gyrB in codon 477 (Glu477Val), in addition to other novel mutations, with PGE 9509924, another nonfluorinated quinolone (39). The role of the these mutations in the resistance phenotype, however, has not yet been shown definitively by allelic exchange (39). Because quinolones cause DNA damage in bacteria and because DNA repair responses can be error prone (1), additional mutations not contributing to the resistance phenotype can occur with quinolone exposure and selection, indicating the importance of genetic studies of novel mutations to confirm their role in resistance.…”

Staphylococcus aureus Mutants Isolated via Exposure to Nonfluorinated Quinolones: Detection of Known and Unique Mutations

“…On the basis of in vitro potency data, these compounds are more potent against MRSA and coagulase-negative staphylococci than several fluoroquinolones and have activities comparable to that of clinafloxacin (10). An apparent advantage of the NFQs against S. aureus lies in their ability to (i) better utilize both DNA gyrase and topisomerase IV as dual targets than certain quinolones, such as ciprofloxacin and trovafloxacin, and (ii) largely circumvent existing mutations in serine and glutamate "hot spots" of the target genes, gyrA and grlA, commonly associated with quinolone resistance (10,11). However, it is imperative to ascertain the potential for development of de novo resistance to the NFQs in these pathogens.…”