1 Institute of Medical Microbiology, National Reference Centre for Streptococci, University Hospital (RWTH Aachen), D-52057 Aachen, Germany; 2 Wockhardt Research Centre, Aurangabad, India
Received 18 August 2005; returned 30 August 2005; revised 1 September 2005; accepted 9 September 2005
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Abstract |
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Results: MIC50/MIC90 values (mg/L) of WCK 771, WCK 1152 and WCK 1153 for quinolone-susceptible S. pneumoniae (n = 119; 54 penicillin G-susceptible, 53 penicillin G-intermediate, and 12 penicillin G-resistant strains) were 0.25/0.5, 0.03/0.06 and 0.016/0.03, respectively. MIC50/MIC90 values (mg/L) for quinolone-resistant pneumococci (n = 40) increased to 4/16, 0.25/1 and 0.125/0.5, respectively. Against S. pyogenes, WCK 771, WCK 1152 and WCK 1153 were also highly active with MIC50/MIC90 values (mg/L) of 0.25/0.25, 0.03/0.06 and 0.03/0.03, respectively.
Conclusions: Overall, WCK 771 was highly active against quinolone-susceptible, but not against quinolone-resistant S. pneumoniae, whereas WCK 1152 and WCK 1153 were more potent and were able to overcome quinolone resistance in both S. pneumoniae and S. pyogenes.
Keywords: fluoroquinolones , resistance , Germany , pneumococcus
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Introduction |
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Bacterial resistance to quinolones occurs mainly by alteration of their intracellular drug targets, the DNA topoisomerase IV and DNA gyrase enzymes. Genetic and biochemical studies have shown that fluoroquinolones target primarily topoisomerase IV and secondarily DNA gyrase in S. pneumoniae.2,3 Moreover, resistance mutations are localized in the quinolone resistance-determining regions (QRDRs) of parC, parE and gyrA. Low-level quinolone-resistant strains usually harbour mutations altering the QRDR of one of the two subunits of topoisomerase IV: (i) S79 or D83 of parC or (ii) D435 of parE. This study examined the in vitro activity of the three quinolones WCK 771 [a novel arginine salt of the tricyclic fluoroquinolone S-()-nadifloxacin], WCK 1152 and WCK1153 (prepared by condensing 4-amino- or 4-hydroxy piperidines with known fluoroquinolone cores and subsequent optional derivatization) against selected clinical isolates of S. pneumoniae and S. pyogenes possessing different antibiotic resistance profiles (Figure 1).
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Materials and methods |
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One hundred and fifty-nine isolates of S. pneumoniae and 52 isolates of S. pyogenes were chosen from the collection of the German National Reference Centre for Streptococci. Pneumococcal strains were isolated from blood (n = 68, 42.8%), CSF (n = 36; 22.6%), bronchoalveolar lavages (n = 8, 5.0%), other normally sterile body sites (n = 12; 7.5%), and the respiratory tract (n = 35; 22.0%). Of the S. pyogenes isolates, 50 (96.2%) were isolated from the nasopharynx of patients with tonsillopharyngitis and 2 (3.8%) isolates were from wound infections. The strain collection included strains with different resistance profiles. Strains were isolated between 1999 and 2004.
Susceptibility testing
MIC testing was performed using the broth microdilution method as recommended by the Clinical Laboratory Standards Institute (CLSI; formerly NCCLS).4 Microtitre plates containing WCK 771, WCK 1152, WCK 1153 (all from Wockhardt Ltd, India) and comparators with cation-adjusted MuellerHinton broth (Oxoid, Wesel, Germany) plus 5% lysed horse blood (Oxoid) were used. S. pneumoniae ATCC 49619 was used as control strain.
Determination of resistance phenotypes and genotypes
For determination of macrolide-resistant phenotypes, discs (Oxoid Ltd, Basingstoke, UK) of erythromycin (15 µg) and clindamycin (2 µg) were placed 15 to 20 mm apart on MuellerHinton agar (BBL Microbiology Systems, Cockeysville, MD, USA) with 5% sheep blood (Oxoid, Wesel, Germany). Determination of macrolide-resistance genotypes was performed by a light cycler protocol as described previously.5,6 Nineteen pneumococcal isolates and one fluoroquinolone-resistant S. pyogenes isolate were randomly selected and analysed for alterations in the QRDRs. Prepared chromosomal DNA was used as a template for PCR amplification of target QRDRs. The primers and PCR conditions were those previously defined.7
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Results |
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Discussion |
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New fluoroquinolones have been developed to overcome resistance. WCK 771 is an arginine salt of the S-() isomer of nadifloxacin. Because the S-() isomer is primarily responsible for antibacterial activity, the potency of WCK 771 is two to four times higher than that of racemic nadifloxacin. The comparative assessment of WCK 771 with other fluoroquinolones demonstrated that this new agent is a highly potent antistaphylococcal fluoroquinolone with improved potency against even fluoroquinolone-resistant strains of S. aureus and coagulase-negative staphylococci.8 In this study, an excellent level of in vitro activity against both S. pneumoniae and S. pyogenes streptococci was found for WCK 771 with low MIC90 values (0.250.5 mg/L).
However, WCK 771 was less potent against highly ciprofloxacin-resistant S. pneumoniae, confirming findings of Appelbaum and co-workers who reported WCK 771 MIC50/MIC90 values for 25 quinolone-resistant pneumococcal isolates of 4/8 mg/L, compared with 0.5/1 mg/L for clinafloxacin, 2/4 mg/L for gatifloxacin and moxifloxacin, 8/16 mg/L for levofloxacin and 16/>32 mg/L for ciprofloxacin.9
Data on the in vitro activity of WCK 1152 and WCK 1153 are scarce. Both compounds were primarily developed for treatment of staphylococcal infections, including those by vancomycin- and fluoroquinolone-resistant isolates.8,10 This study demonstrates that both compounds also showed excellent in vitro activity against antibiotic-resistant streptococci. Moreover, in contrast to WCK 771, WCK 1152 and WCK 1153 were also highly active against fluoroquinolone-resistant S. pneumoniae. Of note, based on the MIC90 values, both WCK 1152 (MIC90 1 mg/L) and WCK 1153 (MIC90 0.5 mg/L) were up to eight times more active than moxifloxacin (MIC90 4 mg/L) against ciprofloxacin-resistant pneumococcal isolates. The analysis of the in vitro activity of WCK 1152 and WCK 1153 against fluoroquinolone-resistant streptococci showed that the primary target seems to be DNA gyrase.
In summary, WCK 771 was potent against quinolone-susceptible S. pneumoniae in vitro, but not quinolone-resistant S. pneumoniae, regardless of penicillin G and macrolide susceptibility. WCK 1152 and WCK 1153 showed potency superior even to that of newer quinolones in clinical use against streptococci. Therefore, both are promising new agents having high potency against streptococci. If clinical studies yield a favourable safety profile, and if human pharmacokinetic studies support a susceptibility breakpoint of 2 mg/L, both compounds will be active against both quinolone-susceptible and quinolone-resistant streptococci, features not achieved by currently available quinolones.
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Acknowledgements |
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References |
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