In Vitro Activity of Delafloxacin against Contemporary Bacterial Pathogens from the United States and Europe, 2014

ABSTRACT The in vitro activities of delafloxacin and comparator antimicrobial agents against 6,485 bacterial isolates collected from medical centers in Europe and the United States in 2014 were tested. Delafloxacin was the most potent agent tested against methicillin-susceptible Staphylococcus aureus (MSSA), methicillin-resistant S. aureus, Streptococcus pneumoniae, viridans group streptococci, and beta-hemolytic streptococci and had activity similar to that of ciprofloxacin and levofloxacin against certain members of the Enterobacteriaceae. Overall, the broadest coverage of the tested pathogens (Gram-positive cocci and Gram-negative bacilli) was observed with meropenem and tigecycline in both Europe and the United States. Delafloxacin was shown to be active against organisms that may be encountered in acute bacterial skin and skin structure infections, respiratory infections, and urinary tract infections.

T he fluoroquinolone class of antibiotics is currently used as standard empirical therapy in health care-associated infections and community-acquired infections; specifically, antibiotics of this class are indicated for the treatment of urinary tract infections (UTI), respiratory tract infections (RTI), acute bacterial skin and skin structure infections (ABSSSI), and intra-abdominal infections (1)(2)(3)(4)(5)(6). A recent point-prevalence study of antimicrobial use in U.S. acute care hospitals found levofloxacin to be the third most common antimicrobial agent prescribed to treat both community-acquired infections and health care-acquired infections (7). In the face of such broad utilization, the emergence of fluoroquinolone resistance has been observed in both Gram-positive cocci (GPC) and Gram-negative bacilli (GNB) (1,6,8).
Fluoroquinolones are the only class of antibiotics in clinical use that directly target two essential bacterial enzymes in DNA replication: DNA gyrase and topoisomerase IV (1,9). Resistance to fluoroquinolones is primarily caused by target mutations (e.g., mutations in chromosomal genes that encode the subunits of DNA gyrase and topoisomerase IV), efflux pumps, and reduced target expression (9). These mechanisms may occur in various combinations in resistant strains of staphylococci, Pseudomonas aeruginosa, and Enterobacteriaceae (1,6). Efforts to combat this resistance to the fluoroquinolone class have focused on improving activity against multidrug-resistant bacteria and providing a lower potential for the development of bacterial resistance (1,4,5,8).
Delafloxacin is an anionic investigational fluoroquinolone with documented efficacy in phase 2 trials for the treatment of RTI and ABSSSI and has recently completed phase 3 trials for the treatment of ABSSSI (1,10). Unlike other quinolones, which usually have a binding affinity for either DNA gyrase or topoisomerase IV, delafloxacin is equally potent against both enzymes (1,(11)(12)(13). This dual targeting is believed to help reduce the selection of resistant mutants in vitro and in vivo (11,12,14). Unlike other fluoroquinolones, the mutant prevention concentration for delafloxacin is within 1-to Ϫ2-log 2 dilutions of the MIC value (13). Additionally, the anionic structure of delafloxa-      calcoaceticus (MIC 90 s, Ͼ4 g/ml for all isolates) ( Table 1). The activity of delafloxacin was considerably greater against strains of E. coli of the non-extended-spectrum ␤-lactamase [ESBL]-producing phenotype (non-ESBL phenotype) than strains of E. coli of the ESBL-producing phenotype (ESBL phenotype) (MIC 50 , 0.03 g/ml versus 2 g/ml, respectively), non-ESBL-phenotype and ESBL-phenotype strains of K. pneumoniae (MIC 50 , 0.06 g/ml versus 4 g/ml, respectively), and non-ESBL-phenotype and ESBLphenotype strains of P. mirabilis (MIC 50 , 0.06 g/ml versus 2 g/ml, respectively). Delafloxacin retained potent activity against ESBL-phenotype strains of K. oxytoca (MIC 50 and MIC 90 , 0.06 and 0.12 g/ml, respectively) and was more active against ceftazidime-susceptible than ceftazidime-nonsusceptible strains of P. aeruginosa (MIC 50 , 0.25 g/ml versus 4 g/ml, respectively). More than 90% of FQ r GNB showed decreased susceptibility (MIC, Ն2 g/ml) to delafloxacin. Susceptibilities of European and U.S. Gram-positive isolates to delafloxacin and comparator agents. The activities of delafloxacin and comparator agents tested against European (250 isolates) and U.S. (1,100 isolates) isolates of S. aureus are shown in Table 2. Delafloxacin was the most potent antimicrobial agent tested against isolates of MSSA (MIC 50 and MIC 90 , Յ0.004 and 0.008 g/ml, respectively) and on the basis of the MIC 90 s was 8-to at least 64-fold more potent than ceftaroline and at least 64-fold more potent than levofloxacin ( Table 2). Tigecycline (MIC 50 and MIC 90 , 0.06 and 0.06 g/ml, respectively), delafloxacin (MIC 50 and MIC 90 , 0.06 and 0.5 g/ml, respectively), and daptomycin (MIC 50 and MIC 90 , 0.25 and 0.5 g/ml, respectively) were the most potent agents tested against MRSA (Table 2). Delafloxacin was at least 64-fold more potent than levofloxacin (according to the MIC 50 s) and at least 8-fold more potent than ceftaroline against MRSA. MRSA strains exhibited high levels of resistance against levofloxacin (68.9 and 68.9% according to Clinical and Laboratory Standards Institute [CLSI] and European Committee on Antimicrobial Susceptibility Testing [EUCAST] criteria, respectively) and erythromycin (79.9 and 83.8% according to CLSI and EUCAST criteria, respectively) ( Table 2). The greatest coverage of all S. aureus isolates (MSSA and MRSA isolates from both Europe and the United States) was provided by linezolid, tigecycline, and vancomycin (to which 100.0% of isolates were susceptible). Isolates from both Europe and United States also exhibited high levels of susceptibility to daptomycin (99.8% of isolates were susceptible), ceftaroline (98.0%), and trimethoprimsulfamethoxazole (98.5%) ( Table 2).
The antibiogram results for MR-CoNS isolates from both Europe (67 isolates) and the United States (58 isolates) showed higher MIC values for all tested drugs except daptomycin (to which 99.2% of isolates were susceptible), linezolid (to which 100.0% of isolates were susceptible), and vancomycin (to which 100.0% of isolates were susceptible). Tigecycline (MIC 50 and MIC 90 , 0.06 and 0.12 g/ml, respectively), delafloxacin (MIC 50 and MIC 90 , 0.06 and 0.5 g/ml, respectively), linezolid (MIC 50 and MIC 90 , 0.5 and 0.5 g/ml, respectively), and ceftaroline (MIC 50 and MIC 90 , 0.5 and 1 g/ml, respectively) were the most potent antimicrobials tested against both European and U.S. strains of MR-CoNS. Levofloxacin, clindamycin, erythromycin, and trimethoprim-sulfamethoxazole all showed limited activity against MR-CoNS isolates from both regions.  All isolates of E. faecalis from Europe and the United States were susceptible to ampicillin ( Table 2). A small number of E. faecalis strains were resistant to vancomycin (2.2%). Delafloxacin (MIC 50 and MIC 90 , 0.06 and 1 g/ml, respectively) and linezolid (MIC 50 and MIC 90 , 1 and 1 g/ml, respectively) were the most potent antimicrobials tested ( Table 2).
The activities of delafloxacin and comparator antimicrobial agents against a total of 790 isolates of beta-hemolytic streptococci (433 isolates of Streptococcus pyogenes, 225 of Streptococcus agalactiae, and 132 of Streptococcus dysgalactiae) were tested (Tables  1 and 2). Delafloxacin was highly potent against these organisms (Table 1). All delafloxacin MIC values for S. pyogenes and S. dysgalactiae were Յ0.03 g/ml. The highest delafloxacin MIC value for S. agalactiae was 0.5 g/ml, and 97.3% of S. agalactiae isolates were inhibited by delafloxacin at Յ0.03 g/ml (Table 1). All beta-hemolytic streptococcal isolates were susceptible to ceftaroline, ceftriaxone, meropenem, penicillin, and vancomycin ( Table 2). The rates of resistance to levofloxacin were 0.2% for S. pyogenes, 2.2% for S. agalactiae, and 0.8% for S. dysgalactiae ( Table 2). The rate of resistance to erythromycin was higher among isolates of S. agalactiae (46.7%) and S. dysgalactiae (29.5%) than among isolates of S. pyogenes (14.1%). The rate of resistance to clindamycin among isolates of beta-hemolytic streptococci ranged from 8.1% to 27.6% ( Table 2).
Susceptibilities of European and U.S. Gram-negative isolates to delafloxacin and comparator agents. Delafloxacin was active against the majority of the Enterobacteriaceae, exhibiting MIC 50 and MIC 90 values of 0.06 and 4 g/ml, respectively, and with 80.9% of isolates being inhibited by delafloxacin at Յ1 g/ml ( Table 1). The rates of susceptibility to fluoroquinolones, as measured by the use of ciprofloxacin and levofloxacin, for the Enterobacteriaceae were 81.6% and 83.8%, respectively (Table 3). More than 90% of FQ r Enterobacteriaceae isolates showed decreased susceptibility (MIC, Ͼ1 g/ml) to delafloxacin (data not shown). The rates of susceptibility to aztreonam, ceftriaxone, cefepime, and ceftazidime ranged from 80.3% to 90.8% (Table 3). Meropenem (MIC 50 and MIC 90 , 0.03 and 0.06 g/ml, respectively; 97.5 and 97.9% of isolates were susceptible according to CLSI and EUCAST criteria, respectively) and tigecycline (MIC 50 and MIC 90 , 0.25 and 1 g/ml, respectively; 99.2 and 95.2% of isolates were susceptible according to CLSI and EUCAST criteria, respectively) were the most active agents (Table 3).   the most active agents against GNB. Delafloxacin was active against MRSA, MR-CoNS, viridans group streptococci, beta-hemolytic streptococci, and penicillin-and macrolideresistant S. pneumoniae strains (Tables 1 and 2). Isolates of E. faecium, ESBL-phenotype Enterobacteriaceae, ceftazidime-nonsusceptible P. aeruginosa, and Acinetobacter were considerably less susceptible to delafloxacin than the GPC and wild-type GNB. In contrast, delafloxacin showed activity comparable to that of the other fluoroquinolones tested against AmpC-producing strains of Enterobacteriaceae. These data build on reports by previous investigators (11,12,14,19,20) and indicate that delafloxacin merits further study for the treatment of ABSSSI, RTI, and urinary tract infections where an acid environment and mixed GPC and GNB infections are common.

Organisms.
A total of 6,485 nonduplicate bacterial isolates were collected prospectively from 69 medical centers located in the United States (4,410 isolates) and from 44 medical centers located in 25 European countries (2,075 isolates) in the year 2014. All organisms were isolated from hospitalized patients with bloodstream infections (1,373 isolates), RTI (1,368 isolates), ABSSSI (2,177 isolates), UTI (735 isolates), intra-abdominal infections (267 isolates), and other types of infections (565 isolates). Isolates were identified to the species level at each participating medical center, and the identity was confirmed by the monitoring laboratory (JMI Laboratories, North Liberty, IA, USA) using standard bacteriological algorithms and methodologies or matrix-assisted laser desorption ionization-time of flight mass spectrometry (Bruker, Billerica, MA, USA), when necessary.
Antimicrobial susceptibility testing. MICs were determined using the reference Clinical and Laboratory Standards Institute (CLSI) broth microdilution method (25). Quality control (QC) and interpretation of results were performed in accordance with the CLSI M100-S26 standard (26) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) 2016 guidelines (27). Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, and Proteus mirabilis were grouped as ESBL-phenotype strains on the basis of the CLSI screening criteria for potential ESBL production (i.e., a ceftazidime, ceftriaxone, or aztreonam MIC of Ն2 g/ml) (26). Isolates of P. aeruginosa were classified as ceftazidime susceptible (MIC, Յ 8 g/ml) and ceftazidime nonsusceptible (MICs, Ͼ8 g/ml). QC strains were tested concurrently and included E. coli ATCC 25922 and ATCC 35218, S. aureus ATCC 29213, P. aeruginosa ATCC 27853, Enterococcus faecalis ATCC 29212, and S. pneumoniae ATCC 49619. All QC results were within published ranges.