Activity of finafloxacin, a novel fluoroquinolone with increased activity at acid pH, towards extracellular and intracellular Staphylococcus aureus, Listeria monocytogenes and Legionella pneumophila

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Abstract

Finafloxacin, an 8-cyano-substituted fluoroquinolone, expresses enhanced activity at acidic pH and is less susceptible to several fluoroquinolone resistance determinants. In this study, we compared finafloxacin and ciprofloxacin for (i) activity against ciprofloxacin-susceptible and -resistant Staphylococcus aureus as well as wild-type and Lde efflux-positive (Lde+) Listeria monocytogenes, (ii) accumulation in THP-1 macrophages and (iii) intracellular activity towards phagocytised S. aureus, L. monocytogenes and Legionella pneumophila (developing in acidic, neutral and mildly acidic environments, respectively), using a pharmacological approach assessing drug potencies and maximal relative efficacies (Emax). Finafloxacin minimum inhibitory concentrations (MICs) were two-fold lower than those of ciprofloxacin against meticillin-susceptible S. aureus ATCC 25923, were only modestly increased in an isogenic strain overexpressing NorA and were ≤0.25 mg/L for community-acquired meticillin-resistant S. aureus. No loss of activity was seen in Lde+ L. monocytogenes. An acidic pH decreased the MIC of finafloxacin and increased that of ciprofloxacin both for S. aureus and L. monocytogenes, in parallel with corresponding changes in drug accumulation (tested with S. aureus ATCC 25923 only). Finafloxacin accumulated less than ciprofloxacin in THP-1 cells, but the situation was reversed by exposure of cells to acid pH. In S. aureus-infected cells, acid pH increased the potency of finafloxacin without change of Emax, whilst decreasing the potency and the maximal relative efficacy of ciprofloxacin (less negative Emax). Finafloxacin was more potent and showed larger Emax than ciprofloxacin against phagocytised L. pneumophila, but was less potent against phagocytised L. monocytogenes. Finafloxacin appears to be an acid-pH-favoured antibiotic that may find useful applications in infections where the local pH is low.

Introduction

Treating intracellular bacterial infections remains a challenge as the causative organisms are sheltered from many of the immune and innate defence mechanisms and show decreased susceptibility to many antibiotics (see [1], [2], [3], [4] for selected reviews), making it necessary to assess novel antibiotics in this context. Finafloxacin is an investigational broad-spectrum fluoroquinolone characterised by a 7-pyrrolo-oxazinyl moiety and an 8-cyano substituent (Fig. 1). It expresses markedly enhanced activity under acidic conditions where other fluoroquinolones are inactivated [5], [7], [8], [9]. This may confer advantages to finafloxacin for infections occurring not only in acidic body sites such as the skin, vagina and urinary tract or those rendered acidic by an inflammatory response to infection, but also against bacteria sojourning within acidic subcellular organelles (phagosomes and phagolysosomes).

Finafloxacin may be less susceptible than ciprofloxacin to several known fluoroquinolone resistance determinants (alone and in combination) in Escherichia coli [8]. Having a bulky substituent in position 7 somewhat similar to that of moxifloxacin, it could also be less susceptible to efflux by the bacterial multidrug transporter NorA [10] that affects the activity of ciprofloxacin but less so that of moxifloxacin [11], [12]. In this study, we examined the activity of finafloxacin against a panel of ciprofloxacin-susceptible and-resistant Staphylococcus aureus isolates and then studied its accumulation by THP-1 human macrophages and activity towards susceptible extracellular and intracellular S. aureus at neutral and acidic pH. In parallel, we also measured its activity against intracellular Listeria monocytogenes and Legionella pneumophila, representative of intracellular organisms sojourning and multiplying in neutral (cytosol [13]) and mildly acidic (phagosomes [14]) environments, respectively.

Section snippets

Antibiotics and main reagents

Finafloxacin and ciprofloxacin were obtained as microbiological standards from MerLion Pharmaceuticals GmbH (Berlin, Germany) and Bayer HealthCare AG (Wuppertal, Germany), respectively. Cell culture media and sera were from Invitrogen Corp. (Carlsbad, CA) and other reagents from Sigma-Aldrich Inc. (St Louis, MO) or Merck KGaA (Darmstadt, Germany).

Bacterial strains and susceptibility testing

Table 1, Table 2 show the strains used in the present study. Unless indicated otherwise, minimum inhibitory concentration (MIC) determinations were

Susceptibility testing

Table 1 shows the MICs of finafloxacin and ciprofloxacin against a panel of laboratory and clinical isolates of S. aureus and against laboratory strains of L. monocytogenes and L. pneumophila. Finafloxacin was twice as active as ciprofloxacin against the meticillin-susceptible S. aureus (MSSA) strain ATCC 25923 and its MIC was increased by only 2–3 log2 dilutions against the isogenic strain SA-1 overexpressing NorA (5 log2 dilutions increase for ciprofloxacin). For the community-acquired

Discussion

Developed and introduced in clinics since the mid 1980s, fluoroquinolones have represented a milestone in the chemotherapy of bacterial infections thanks to their wide spectrum, intense bactericidal activity and favourable pharmacokinetics. Fluoroquinolones rapidly accumulate in eukaryotic cells [25], [26], [27] and display significant activity towards susceptible bacteria present in various subcellular compartments, including S. aureus (phagolysosomes [21], [28]), L. monocytogenes (cytosol [23]

Acknowledgments

We thank P.C. Appelbaum (Hershey Medical Center, Hershey, PA), Y. Glupczynski (Cliniques universitaires de Mont-Godinne, Yvoir, Belgium), L.Y. Hsu (National University of Singapore, Singapore), Y.C. Huang (Chang Gung Children's Hospital, Taiwan), C. Quentin (Université Victor Ségalan, Bordeaux, France) and P. Courvalin (Institut Pasteur, Paris, France) for the kind gift of bacterial isolates. M.C. Cambier and C. Misson provided dedicated technical assistance throughout this work.

Funding: SL is

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  • Cited by (0)

    Part of this work has been presented as a poster at the 49th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC), 12–15 September 2009, San Francisco, CA [A1-1940].

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