Contrasting Effects of Acidic pH on the Extracellular and Intracellular Activities of the Anti-Gram-Positive Fluoroquinolones Moxifloxacin and Delafloxacin againstStaphylococcus aureus

Abstract

In contrast to currently marketed fluoroquinolones, which are zwitterionic, delafloxacin is an investigational fluoroquinolone with an anionic character that is highly active against Gram-positive bacteria. We have examined the effect of acidic pH on its accumulation in Staphylococcus aureus and in human THP-1 cells, in parallel with its activity against extracellular and intracellular S. aureus. Moxifloxacin was used as a comparator. Delafloxacin showed MICs 3 to 5 log 2 dilutions lower than those of moxifloxacin for a collection of 35 strains with relevant resistance mechanisms and also proved to be 10-fold more potent against intracellular S. aureus ATCC 25923. In medium at pH 5.5, this difference was further enhanced, with the MIC decreasing by 5 log 2 dilutions. In infected cells incubated in acidic medium, the relative potency was 10-fold higher than that at neutral pH and the maximal relative efficacy reached a bactericidal effect at 24 h. These results can be explained by a 10-fold increase in delafloxacin accumulation in both bacteria and cells at acidic pH, making delafloxacin one of the most efficient drugs tested in this model. Opposite effects were seen for moxifloxacin with respect to both activity and accumulation. As reported for zwitterionic fluoroquinolones, delafloxacin was found associated with the soluble fraction in homogenates of eukaryotic cells. Taken together, these properties may confer to delafloxacin an advantage for the eradication of S. aureus in acidic environments, including intracellular infections.Fluoroquinolones are one of the first families of fully synthetic antibiotics with large clinical use, and they represent a wide range of molecules, which permits the establishment of in-depth structure-activity relationships (16,17). Over the last 20 years, most fluoroquinolone research efforts have been focused on new molecules with improved activity toward Grampositive cocci because of the increasing spread of multiresistant staphylococci and streptococci. The 8-methoxy fluoroquinolone moxifloxacin is the first example of this new generation to reach the commercial market. It combines many desirable microbiological and pharmacokinetic properties (rapid bactericidal activity, a spectrum covering pertinent pathogens, excellent oral bioavailability, a large tissue distribution, and a propensity to accumulate within eukaryotic cells [see reference 45 for a review]). Moxifloxacin shows extensive activity toward bacteria thriving in the cytosol (Listeria monocytogenes [14]), phagosomes (Legionella pneumophila, Mycobacterium avium, and Mycobacterium tuberculosis [5,9,46]), and phagolysosomes (Staphylococcus aureus [6]), suggesting that it easily diffuses between different subcellular compartments. Yet moxifloxacin activity is partially defeated by the acidic pH prevailing in vacuolar subcellular compartments (6).Delafloxacin [RX-3341; 1-(6-amino-3,5-difluoropyridin-2-yl)-8-chloro-6-fluoro-7-(3-hydroxyazetidin-1-yl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid] is an investigational fluoro...

Pharmacodynamic Evaluation of the Activity of Antibiotics against Hemin- and Menadione-Dependent Small-Colony Variants of Staphylococcus aureus in Models of Extracellular (Broth) and Intracellular (THP-1 Monocytes) Infections

“…Conversely, we can offer no explanation for moxifloxacin at this stage, as this will require more extensive studies and detailed comparisons with other fluoroquinolones and other strains. Notably, however, a biphasic shape was also observed when studying the intracellular activity of delafloxacin, a very potent investigational fluoroquinolone (30), against S. aureus. In this case, as in the present study, a biphasic response was observed when extending the range of concentrations to high multiples of the MIC (typically 1,000ϫ, which was possible for delafloxacin in view of its very low MICs, but explains why it escaped our attention in previous studies with other fluoroquinolones).…”

Renaissance of antibiotics against difficult infections: Focus on oritavancin and new ketolides and quinolones

“…Selection of resistance in S. aureus is infrequent (10 29 to 10 211 ), and concentrations preventing the selection of mutations (MPC (126)) range from 1 to 4 times the initial MIC, with values 8-to 32-fold lower than for other quinolones (127). Delafloxacin also proved active in in vitro models of biofilm or intracellular infection by S. aureus (124,128), despite the fact it mainly localizes in the cytoplasmic compartment of cells (124). This may be due to the high diffusibility of fluoroquinolones, which may help them freely to cross biological membranes within the cells to gain access to the infected compartment.…”

“…This chemical feature rationalizes why it accumulates much more in both bacteria and eukaryotic cells at acidic pH (124). Delafloxacin shows very low MICs against Gram-positive pathogens, with values typically 4 dilutions lower than those of moxifloxacin, even against strains showing elevated MIC to the reference drug (124). At acidic pH, the difference in potency between the two antibiotics can reach 7 dilutions (124).…”

Delafloxacin: A Novel Fluoroquinolone Introduced in Clinical Trials

“…: skin and soft tissue infections). This feature makes delafloxacin special among fluoroquinolones as ciprofloxacin and moxifloxacin lose potency in acidic environment [7,8]. Delafloxacin is a broad-spectrum agent, as it targets both DNA gyrase and topoisomerase IV enzymes [9].…”